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The effect of past changes in inter‐annual temperature variability on tree distribution limits
Abstract
Aim The northern limits of temperate broadleaved species in Fennoscanndia are controlled by their requirements for summer warmth for successful regeneration and growth as well as by the detrimental effects of winter cold on plant tissue. However, occurrences of meteorological conditions with detrimental effects on individual species are rare events rather than a reflection of average conditions. We explore the effect of changes in inter‐annual temperature variability on the abundances of the tree species Tilia cordata , Quercus robur and Ulmus glabra near their distribution limits using a process‐based model of ecosystem dynamics.
Location A site in central Sweden and a site in southern Finland were used as examples for the ecotone between boreal and temperate forests in Fennoscandia. The Finnish site was selected because of the availability of varve‐thickness data.
Methods The dynamic vegetation model LPJ‐GUESS was run with four scenarios of inter‐annual temperature forcing for the last 10,000 years. In one scenario the variability in the thickness of summer and winter varves from the annually laminated lake in Finland was used as a proxy for past inter‐annual temperature variability. Two scenarios were devised to explore systematically the effect of stepwise changes in the variance and shape parameter of a probability distribution. All variability scenarios were run both with and without the long‐term trend in Holocene temperature change predicted by an atmospheric general circulation model.
Results Directional changes in inter‐annual temperature variability have significant effects on simulated tree distribution limits through time. Variations in inter‐annual temperature variability alone are shown to alter vegetation composition by magnitudes similar to the magnitude of changes driven by variation in mean temperatures.
Main conclusions The varve data indicate that inter‐annual climate variability has changed in the past. The model results show that past changes in species abundance can be explained by changes in the inter‐annual variability of climate parameters as well as by mean climate. Because inter‐annual climatic variability is predicted to change in the future, this component of climate change should be taken into account both when making projections of future plant distributions and when interpreting vegetation history.
... In central Spain, dry periods of longer than a month are seen as detrimental (Rossignoli & Génova, 2003). In Sweden, the northern range of U. glabra is limited by a mean temperature of the coldest month lying between −9.5°C and −15°C (Giesecke et al., 2010;Prentice & Helmisaari, 1991). An investigation of the presence of U. glabra and other broadleaves in two localities near their distribution limits in Sweden (Holtjärnen) and Finland (Nautajärvi), found a continuous fluctuation in abundance of U. glabra over the last 8,000 years and a particularly strong decline in the last 1,000 years in terms of biomass and pollen accumulation rate under several temperature scenarios (Giesecke et al., 2010). ...
... In Sweden, the northern range of U. glabra is limited by a mean temperature of the coldest month lying between −9.5°C and −15°C (Giesecke et al., 2010;Prentice & Helmisaari, 1991). An investigation of the presence of U. glabra and other broadleaves in two localities near their distribution limits in Sweden (Holtjärnen) and Finland (Nautajärvi), found a continuous fluctuation in abundance of U. glabra over the last 8,000 years and a particularly strong decline in the last 1,000 years in terms of biomass and pollen accumulation rate under several temperature scenarios (Giesecke et al., 2010). Ulmus glabra is considered to be the least sensitive of the elms to low temperature, capable of withstanding winter temperatures down to −50°C (Larcher, 1981). ...
... It can also withstand mean monthly temperatures of the coldest month down to −15°C, similar to Quercus robur, Fraxinus excelsior and Corylus avellana (Sykes, Prentice, & Cramer, 1996). However, Giesecke, Miller, Sykes, Ojala, Seppä, and Bradshaw (2010) calculated that U. glabra is present when the growing degree-day sum (above 5°C) is as little as 850 (compared to 1,100 in Tilia cordata and Quercus robur; Giesecke, Miller, Sykes, Ojala, Seppä, and Bradshaw, 2010). At its upper altitudinal limit in the Caucasus, the winters are mild and snowy (average January temperature −3°C) and the summers are warm and wet (average July temperature 19°C, annual precipitation >1,200 mm) with a frost-free period of about 160 days (Akatov, 2009). ...
This account presents information on all aspects of the biology of Ulmus glabra Hudson (wych elm) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles : distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation.
Ulmus glabra is a large forest tree, and often an important canopy tree in ancient and semi‐natural woodlands. It is primarily native to the north and west of Britain and much of mainland Europe. It is the only elm native to Ireland. It is the most distinct of the British elms in that it rarely suckers and sets abundant viable seed. Although found on limestone screes and cliffs, and hedgerows, it is primarily a woodland tree, especially on moist, basic soils. In many secondary woodlands, it often co‐occurs with Acer pseudoplatanus and has ecological needs that are similar to Fraxinus excelsior .
Ulmus glabra has clusters of c . 25 hermaphrodite flowers appearing before the leaves on previous year’s growth. Seeds are wind‐dispersed, falling in April to July, but remain viable for only a few days. Nevertheless, seedling establishment can be abundant. Hybridisation with other northern European elms is common but hybrids are notoriously difficult to identify and therefore probably under‐recorded.
The health and survival of wych elm in Europe has been seriously compromised since the 1970s due to Dutch elm disease caused by the fungus Ophiostoma novo‐ulmi , transmitted by elm bark beetles ( Scolytus spp.). To the south of its Scottish stronghold, many elms are reduced to small trees regrowing from basal sprouts or seeds. These trees tend to be reinfected once trunk diameter exceeds 10 cm. Fortunately for its long‐term survival, seed production usually begins a number of years before they are reinfected.
... Studies range from simulations of single sites (e.g. Bugmann, 2001;Giesecke et al., 2010) to spatially explicit simulations with and without spatial linkage of the simulated grid cells (e.g. Lischke et al., 2013;Hickler et al., 2012). ...
... From such proxy data climate anomalies can be derived, ranging from 1000-year time periods (e.g. Miller et al., 2008;Giesecke et al., 2010) or approximately 250-year time periods (e.g. Lischke, 2005) to, at the best, around 10 to 20-year time periods (e.g. ...
... often need to be interpolated or extrapolated. Due to the influence of climate variability on tree population dynamics, the most simplistic approaches, such as linear interpolation, or extrapolation by steadily applying mean values, are not appropriate (Giesecke et al., 2010;Nabel et al., 2013). More sophisticated inter-and extrapolation methods use selected base periods of available climate time series to generate climatic fluctuations. ...
Simulations of tree population dynamics under past and future climatic changes with time- and space-discrete models often suffer from a lack of detailed long-term climate time series that are required to drive these models. Inter- and extrapolation methods which are applied to generate long-term series differ in terms of whether they do or do not account for spatial correlation of climatic fluctuations. In this study we compared tree species abundance and migration outcomes from simulations using extrapolation methods generating spatially correlated (SC) and spatially independent (SI) climatic fluctuations. We used the spatially explicit and linked forest-landscape model TreeMig and a simple cellular automaton to demonstrate that spatial correlation of climatic fluctuations affects simulation outcomes. We conclude that methods to generate long-term climate time series should account for the spatial correlation of climatic fluctuations found in available climate records when simulating tree species abundance and migration.
... However, the climate of an area is not simply its long-term average, but rather the array of conditions that are possible, and how often those conditions occur (McGregor, 2006). For example, two locations with the same long-term average summer maximum temperature and rainfall may vary according to how frequently extremely hot or dry conditions occur, and extreme conditions can have an important influence on species distributions (Mitikka et al., 2008; Adler et al., 2009; Beever et al., 2010; Giesecke et al., 2010). One method to cater for this variability is to use bioclimatic predictors such as BioClim (Houlder et al., 2003) and WorldClim (Hijmans et al., 2005), which include some additional grids that estimate daily and seasonal variability. ...
... Our results confirm those of Ashcroft et al. (2008), where mild maximum and minimum temperatures were well correlated with elevation, but extreme temperatures were not. Given that rare, extreme climatic events can have a strong influence on species distributions (Mitikka et al., 2008; Adler et al., 2009; Beever et al., 2010; Giesecke et al., 2010) more attention should be given to climatic processes and factors driving extreme maximum and minimum temperatures. ...
The development of fine-resolution climate grids is an important priority in explaining species' distributions at the regional scale and predicting how species may respond to variable and changing climates. Recent studies have demonstrated advantages of producing these grids using large networks of inexpensive climate loggers, as the resulting grids can capture local climatic variations over a range of environments. In this study we extend these methods to develop innovative fine-resolution (25 m) climate grids for a large region (∼200 × 300 km) of New South Wales, Australia. The key aspects of these grids is that they: (1) are based on near-surface (5 cm) observations to better reflect where many species live; (2) cover a wide variety of habitats including forests, woodlands and grasslands so that they are broadly applicable; (3) include both temperature and humidity, the latter of which has often been neglected in similar studies; (4) are developed using a variety of climate-forcing factors rather than relying only on elevation and geographic location; and (5) they focus on the extreme temperatures and humidities regardless of when these occur. Analyses showed that elevation was the dominant factor explaining mild temperatures (low maximums, high minimums), but cold air drainage, distance from coast, canopy cover and topographic exposure had more effect on the extreme maximum and minimum temperatures. Humidities were predominately determined by distance to coast, elevation, canopy cover and topography; however, the relationships were nonlinear and varied in both shape and effect size between dry and moist extremes. Extreme climates occur under specific weather conditions, and our results highlight how averaging climates over seasons or periods of consecutive days will include different weather patterns and obscure important trends. Regional-scale climate grids can potentially be further improved through a better understanding of how the effects of different climate-forcing factors vary under different weather conditions. Copyright © 2011 Royal Meteorological Society
... This could be emphasized by the fact that trees from an oak-mixed forest, such as Quercus robur, Fraxinus excelsior and Corylus avellana, can withstand mean monthly temperatures of the coldest months down to −15 °C [162]. This is similar to the way in which Ulmus glabra was limited by the mean temperature of the coldest months between −9.5 °C and −15 °C [231,232]. On the contrary, Fagus populations are frequent nowadays in areas wherein the January temperature is between ca. −2.5 and 1.5 °C, and Abies alba grows in the January range ca. ...
The aim of this study was to reconstruct the vegetation changes, fire history and local landscape dynamics of central Croatia (the western part of southeastern Europe) from 9800 cal yr BP to the beginning of the Common Era. Pollen, non-pollen palynomorphs and charcoal were analyzed for the first time in the aforementioned area by modern palynological methods. Three different assemblage (sub)zones were identified: "Pinus-Fagus-Quercetum mixtum" (Preboreal), "Fagus-Corylus" (Boreal) and "Alnus-Fagus" (Atlantic, Subboreal and older Subatlantic). Additionally, the oldest observation (~9800 cal yr BP) of beech pollen for continental Croatia was confirmed by radi-ocarbon dating. Our results indicated a possibly milder climate with less extreme temperatures and higher precipitation during the Preboreal chronozone, alongside intensive flooding, a transition from a mosaic of wetland/wet grassland communities to alder carr during the Boreal, and an unusually long multi-thousand-year period, the annual presence of alder on the mire itself. An increase in the number of secondary anthropogenic indicators can be tracked from the 6th century BC to the beginning of the Common Era. Although regional vegetation changes are insufficiently clear, our results fill a gap in the interpretation of vegetation/palaeoenvironmental changes before the Common Era in in this part of Europe.
... The number of studies that use temperature variability as a limiting factor of species distribution remains low, particularly for the marine domain. However, modeling experiments on tree distribution over longtime scales have already demonstrated that changes in temperature variability affect simulated tree distribution limits in a manner comparable to the effects of changes in mean temperatures for terrestrial ecosystems (Giesecke et al., 2010). ...
Understanding how the performance, fitness, and distribution of species are impacted by changes in temperature is crucial for forecasting the ecological effects of climate change. Increasing mean sea surface temperature may alter the distribution of abalone species in the northeastern Pacific, although no apparent pattern has yet emerged. Ecophysiological observations (energy budget and the temperature performance curve based on scope
for growth), satellite-derived daily sea surface temperature data, and observations of the population density distribution of wild abalones were integrated. The highest temperature-dependent scope for growth (physio-logical optimum) was discovered between 20.5 and 28.5 ◦ C, peaking at 24 ◦ C, with the upper thermal limit about 30 ◦C. Off the west coast of Baja California, the distribution of abalone appears to be dictated by the tradeoff
between a mean temperature close to the physiological optimum and exposure to a highly variable environment (i.e., more stress and the risk of reaching suboptimal and lethal warm temperatures at the daily scale). The findings suggest that future models of the distribution of marine species in relation to thermal habitat should
contain time-domain variability in addition to spatial-domain variability.
... Calibrating migration kernels in the way proposed here further supports the overall consensus that tree range shifts are likely to be rather slow and that substantial movement will take generations (Corlett & Westcott, 2013 Long-distance dispersal has long been thought to explain the rapid spread of trees at the end of the Pleistocene Delcourt & Delcourt, 1991;Giesecke et al., 2010). While LDD values ...
Aim
Species distribution models typically project climatically suitable habitat for trees in eastern North America to shift hundreds of kilometres this century. We simulated potential migration, accounting for various traits that affect species' ability to track climatically suitable habitat.
Location
Eastern Canada, covering ~3.7 million km².
Methods
We simulated migration‐constrained range shifts through 2100 using a hybrid approach combining projections of climatically suitable habitat based on two Representative Concentration Pathways (RCP4.5, RCP8.5) for three time periods and two species distribution modelling approaches with process‐based models parameterized using data related to dispersal ability and generation time. We developed a unique “migration kernel” that uses seed dispersal traits and observed migration velocities to obtain kernel shape and dispersal probabilities for each tree species. We then calculated lags between the migration‐constrained range limits obtained through simulations and limits of climatically suitable habitat.
Results
All species demonstrated northward range shifts at the leading edge of their simulated distribution through 2100, but the magnitude and rate of that shift varied by species and time period. Climatically suitable habitat limits were found to be north of simulated distribution limits across both RCPs, with lags increasing through time. On average, simulated distribution that remained within climatically suitable habitat declined more under RCP8.5 than RCP4.5, with large areas of the rear edge of the simulated distribution becoming partially or completely climatically unsuitable for many species.
Main conclusions
Climatically suitable habitat limits projected for 2100 far exceeded migration‐constrained range limits for all 10 tree species, particularly for temperate species. This study underlines the limited extent to which tree species will track climate change via natural migration. Integrating observed migration velocities, seed dispersal and generation time with SDM outputs allows for more realistic evaluations of tree migration ability under climate change and may help orient forest conservation and restoration efforts.
... The effects of moisture on growth were also more frequent in the first year than in the second year, possibly due to stressful transplant effects, and in the last year, which was exceptionally dry and hot. This supports the hypothesis that extreme and rare events have the potential to affect range margins and that only multi-year experiments have the chance to capture a wide range of weather conditions (Camarero et al., 2015;Giesecke et al., 2010;Hoffmann et al., 2019;Lee-Yaw et al., 2016). The moss reacted negatively to warmer minimum temperatures in the first year, which is the only support we found for our first hypothesis on temperature limitation. ...
Species at their warm range margin are potentially threatened by higher temperatures, but may persist in microrefugia. Whether such microsites occur due to more suitable microclimate or due to lower biotic pressure from, for example competitive species, is still not fully resolved.
We examined whether boreal bryophytes and lichens show signs of direct climate limitation, that is whether they perform better in cold and/or humid microclimates at their warm range margin. We transplanted a moss, a liverwort and a lichen to 58 boreal forest sites with different microclimates at the species' southern range margin in central Sweden. Species were grown in garden soil patches to control the effects of competitive exclusion and soil quality. We followed the transplanted species over three growing seasons (2016–2018) and modelled growth and vitality for each species as a function of subcanopy temperature, soil moisture, air humidity and forest type. In 2018, we also recorded the cover of other plants having recolonized the garden soil patches and modelled this potential future competition with the same environmental variables plus litter.
Species performance increased with warmer temperatures, which was often conditional on high soil moisture, and at sites with more conifers. Soil moisture had a positive effect, especially on the moss in the last year 2018, when the growing season was exceptionally hot and dry. The lichen was mostly affected by gastropod grazing. Recolonization of other plants was also faster at warmer and moister sites. The results indicate that competition, herbivory, shading leaf litter and water scarcity might be more important than the direct effects of temperature for performance at the species' warm range margin.
Synthesis. In a transplant experiment with three boreal understorey species, we did not find signs of direct temperature limitation towards the south. Forest microrefugia, that is habitats where these species could persist regional warming, may instead be sites with fewer competitors and enemies, and with sufficient moisture and more conifers in the overstorey.
... Additionally, we used a trait-based approach to characterise long distance dispersal (LDDmin and LDDmax), which were then used to define the probability of such an event. Compared to some studies where LDDmax is very large (Prasad et al. 2013, Miller and Long distance dispersal events have long been thought to explain the rapid post glaciation spread of trees (Delcourt and Delcourt 1991, Giesecke et al. 2010 at the end of the Pleistocene, especially if seed fall distances have a long 'fat' tail (Clark 1998). In most cases, our method to define LDD resulted in lower limits that exceeded the mean of the dispersal kernel, a pattern we would expect to see. ...
... The SDM predictions were based on occurrence data (presences), which reflect environmental conditions over several decades, while estimated population growth rates Figure 5. Relationship of population growth rate, lambda, derived from an integral projection model (IPM) and habitat suitability predicted by a species distribution model (SDM) for 58 transplanted populations growing in either standard garden soil (light-blue triangles) or original site soil (red circles) in otherwise natural habitats with contrasting microclimate, moisture and soil conditions. in the transplant experiment reflect current conditions, measured during a single year. The SDM is thus more likely to have captured responses to long-term variation in important environmental drivers and rare events, like cold or dry years (but see Camarero et al. 2015), which can play an important role in determining cold range margins (Giesecke et al. 2010, Hargreaves et al. 2014, Lee-Yaw et al. 2016, Hoffmann et al. 2019). On the other hand, the long lifespan and poor dispersal of the study species implies that distributions might not yet have tracked recent changes in habitat suitability, such as climate warming. ...
The role of climate in determining range margins is often studied using species distribution models (SDMs), which are easily applied but have well‐known limitations, e.g. due to their correlative nature and colonization and extinction time lags. Transplant experiments can give more direct information on environmental effects, but often cover small spatial and temporal scales. We simultaneously applied a SDM using high‐resolution spatial predictors and an integral projection (demographic) model based on a transplant experiment at 58 sites to examine the effects of microclimate, light and soil conditions on the distribution and performance of a forest herb, Lathyrus vernus, at its cold range margin in central Sweden. In the SDM, occurrences were strongly associated with warmer climates. In contrast, only weak effects of climate were detected in the transplant experiment, whereas effects of soil conditions and light dominated. The higher contribution of climate in the SDM is likely a result from its correlation with soil quality, forest type and potentially historic land use, which were unaccounted for in the model. Predicted habitat suitability and population growth rate, yielded by the two approaches, were not correlated across the transplant sites. We argue that the ranking of site habitat suitability is probably more reliable in the transplant experiment than in the SDM because predictors in the former better describe understory conditions, but that ranking might vary among years, e.g. due to differences in climate. Our results suggest that L. vernus is limited by soil and light rather than directly by climate at its northern range edge, where conifers dominate forests and create suboptimal conditions of soil and canopy‐penetrating light. A general implication of our study is that to better understand how climate change influences range dynamics, we should not only strive to improve existing approaches but also to use multiple approaches in concert.
... Regional paleoclimate simulations with a higher spatial resolution might be required to obtain better landscape-scale model forcing. Another approach may be to modify the climate model derived forcing based on proxy data, such as increasing inter-and intraannual variability within reasonable ranges (e.g., Giesecke et al., 2010). ...
Vegetation is crucial for modulating rates of denudation and landscape evolution, as it stabilizes and protects hillslopes and intercepts rainfall. Climate conditions and the atmospheric CO2 concentration, hereafter [CO2], influence the establishment and performance of plants; thus, these factors have a direct influence on vegetation cover. In addition, vegetation dynamics (competition for space, light, nutrients, and water) and stochastic events (mortality and fires) determine the state of vegetation, response times to environmental perturbations and successional development. In spite of this, state-of-the-art reconstructions of past transient vegetation changes have not been accounted for in landscape evolution models. Here, a widely used dynamic vegetation model (LPJ-GUESS) was used to simulate vegetation composition/cover and surface runoff in Chile for the Last Glacial Maximum (LGM), the mid-Holocene (MH) and the present day (PD). In addition, transient vegetation simulations were carried out from the LGM to PD for four sites in the Coastal Cordillera of Chile at a spatial and temporal resolution adequate for coupling with landscape evolution models.
A new landform mode was introduced to LPJ-GUESS to enable a better simulation of vegetation dynamics and state at a sub-pixel resolution and to allow for future coupling with landscape evolution models operating at different spatial scales. Using a regionally adapted parameterization, LPJ-GUESS was capable of reproducing PD potential natural vegetation along the strong climatic gradients of Chile, and simulated vegetation cover was also in line with satellite-based observations. Simulated vegetation during the LGM differed markedly from PD conditions. Coastal cold temperate rainforests were displaced northward by about 5° and the tree line and vegetation zones were at lower elevations than PD. Transient vegetation simulations indicate a marked shift in vegetation composition starting with the past glacial warming that coincides with a rise in [CO2]. Vegetation cover between the sites ranged from 13% (LGM: 8%) to 81% (LGM: 73%) for the northern Pan de Azúcar and southern Nahuelbuta sites, respectively, but did not vary by more than 10% over the 21000 year simulation. A sensitivity study suggests that [CO2] is an important driver of vegetation changes and, thereby, potentially landscape evolution. Comparisons with other paleoclimate model drivers highlight the importance of model input on simulated vegetation.
In the near future, we will directly couple LPJ-GUESS to a landscape evolution model (see companion paper) to build a fully coupled dynamic-vegetation/landscape evolution model that is forced with paleoclimate data from atmospheric general circulation models.
... Another approach could be to modify the climate-model-derived forcing based on proxy-data, such as increasing inter-and intra-annual variability within reasonable ranges (e.g. Giesecke et al., 2010). 15 LPJ-GUESS explicitly simulates the soil moisture available for vegetation through a simple hydrological cycle (Gerten et al., 2004), and it is thus possible to assess changes in runoff due to changes in transpiration, evaporation and percolation. ...
Vegetation is crucial for modulating rates of denudation and landscape evolution as it stabilizes and protects hillslopes and intercepts rainfall. Climate conditions and atmospheric CO2 concentration ([CO2]) influence the establishment and performance of plants and thus have a direct influence on vegetation cover. In addition, vegetation dynamics (competition for space, light, nutrients and water) and stochastic events (mortality and fires) determine the state of vegetation, response times to environmental perturbations, and the successional development. In spite of this, state-of-art reconstructions of past transient vegetation changes have not been accounted for in landscape evolution models. Here, a widely used dynamic vegetation model (LPJ-GUESS) was used to simulate vegetation composition/ cover and surface runoff in Chile for the Last Glacial Maximum (LGM), Mid Holocene (MH) and present day (PD). In addition, we conducted transient vegetation simulations from LGM to PD for four sites of the Coastal Cordillera of Chile at a spatial and temporal resolution adequate for coupling with landscape evolution models.
Using a regionally-adapted parametrization, LPJ-GUESS was capable of reproducing present day potential natural vegetation along the strong climatic gradients of Chile and simulated vegetation cover was also in line with satellite-based observations. Simulated vegetation during the LGM differed markedly from PD conditions. Coastal cold temperate rainforests where displaced northward by about 5° and the tree line and vegetation zones were at lower elevations than at PD. Transient vegetation simulations indicate a marked shift in vegetation composition starting with the past-glacial warming that coincides with a rise in [CO2]. Vegetation cover between the sites ranged from 13 % (LGM: 8 %) to 81 % (LGM: 73 %) for the northern Pan de Azúcar and southern Nahuelbuta sites, respectively, but did not vary by more than 10 % over the 21,000 yr simulation. A sensitivity study suggests that [CO2] is an important driver of vegetation changes and, thereby, potentially landscape evolution. Comparisons with other paleoclimate model driver highlight the importance of model input on simulated vegetation.
In the near future, we will directly couple LPJ-GUESS to a landscape evolution model (see companion paper) to build a fully-coupled dynamic-vegetation/ landscape evolution model that is forced with paleoclimate data from atmospheric general circulation models.
... Palaeorecords and modern meteorological observations also indicate that there have been changes in the inter-annual variability of climate as well as in mean annual climate during the Holocene (e.g. Prentice et al. 1992, Giesecke et al. 2010). ...
Understanding the full range of natural climate variability is a fundamental basis for palaeoclimate reconstruction and for estimating the magnitude of the anthropogenic influence on global change. Earth's climate varies on many different time scales, and the magnitude and timing of temperature fluctuations has varied substantially between regions. Earth's orbital changes are responsible for the past glacial-interglacial cycles, but the global climate has also constantly fluctuated throughout the present interglacial (the Holocene). This review presents current understanding of the recent glacial-interglacial cycles in the Eurasian region. In addition, general characteristics of the Holocene climate fluctuations in Northern Europe are introduced, focusing on two different aspects. The first is longer-term (millennial) cooling and warming trends and the magnitude of the variability at regional scales, whereas the second is a review of the evidence for shorter-term climate oscillations, such as the 8.2 ka cooling event in Northern Europe during the Holocene. Special attention is paid to historically documented decadal or centennial climate episodes, namely the Medieval Climate Anomaly and Little Ice Age, which have been reported from numerous palaeoproxy records globally, especially in the Northern Hemisphere.
... Small-leaved lime is characteristic for subcontinental forest Tilio- Carpinetum, while large-leaved lime, which northern range limit extends through Poland, is typical for lowland slope forests (Aceri platanoidis–Tilletum platyphylli) (Boraty´nskaBoraty´ Boraty´nska and Dolatowski, 1991; Wysocki and Sikorski, 2009; Matuszkiewicz, 2012 ). Taking into account thermal requirements of lime and phenomenon of climate warming observed over last decades, it may be supposed that the geographical range and stand density of lime trees in Europe would increase ( Huntley, 1978, 1981; Giesecke et al., 2010). According to the European Pharmacopoeia, both species, as well as their hybrid Tilia × vulgaris, provide valuable medicinal raw material—inflorescences with bracts, commonly named 'lime flower' (Tiliae flos), characterized by an aromatic smell and a sweet mucilaginous taste (European Pharmacopoeia 8th). ...
... In order to obtain reliable long-term climatic data for the study area (see Giesecke et al., 2010), we extrapolated the short-term data from three local weather stations (Arber, Waldhäuser, Kligenbrunn) based on long-term data from Zwieselberg using monthly linear regressions (Fig. A.1). To consider the effect of elevation on climate and the seasonal changes in the thermal gradient, monthly linear regressions were also used to build monthly climatic data for each stratum based on their respective elevation (Table 1 and Fig. A.2). To represent the differences in aspect and slope among the strata, we used the model parameter kSlAsp [À2 to +2] that affects potential evapotranspiration (PET; Bugmann, 1994). ...
... Following model evaluation, further analysis on how temperature range is affecting growth was necessary. Here, we used Pearson's product-moment correlation to assess the relationship between growth and the distribution of the daily maximum and minimum temperatures, both the symmetry (skew) and standard deviation (amplitude), around the annual seasonal mean maximum and minimum temperatures [32]. These two measures provide an indication of whether the effect of temperature range results from the degree of variability (standard deviation) or the occurrence of extreme values (skewed symmetry). ...
Campbell Island, an isolated island 600 km south of New Zealand mainland (52°S, 169°E) is oceanic (Conrad Index of Continentality = -5) with small differences between mean summer and winter temperatures. Previous work established the unexpected result that a mean annual climate warming of c. 0.6°C since the 1940's has not led to upward movement of the forest limit. Here we explore the relative importance of summer and winter climatic conditions on growth and age-class structure of the treeline forming species, Dracophyllum longifolium and Dracophyllum scoparium over the second half of the 20th century. The relationship between climate and growth and establishment were evaluated using standard dendroecological methods and local climate data from a meteorological station on the island. Growth and establishment were correlated against climate variables and further evaluated within hierarchical regression models to take into account the effect of plot level variables. Winter climatic conditions exerted a greater effect on growth and establishment than summer climatic conditions. Establishment is maximized under warm (mean winter temperatures >7 °C), dry winters (total winter precipitation <400 mm). Growth, on the other hand, is adversely affected by wide winter temperature ranges and increased rainfall. The contrasting effect of winter warmth on growth and establishment suggests that winter temperature affects growth and establishment through differing mechanisms. We propose that milder winters enhance survival of seedlings and, therefore, recruitment, but increases metabolic stress on established plants, resulting in lower growth rates. Future winter warming may therefore have complex effects on plant growth and establishment globally.
... We therefore simulated forest dynamics with shallow soils and/or reduced precipitation (soil and precipitation scenario). This approach of testing competing scenarios with a dynamic vegetation model followed by a comparison with paleobotanical data has previously been successfully applied to investigate the effects of climate, soil or disturbance on the vegetation in the Swiss and New Zealand Alps as well as the in Mediterranean (Colombaroli et al., 2010;Giesecke et al., 2010;Henne et al., 2011Henne et al., , 2013McGlone et al., 2011). ...
Mountain vegetation is strongly affected by temperature and is expected to shift upwards with climate change. Dynamic vegetation models are often used to assess the impact of climate on vegetation and model output can be compared with paleobotanical data as a reality check. Recent paleoecological studies have revealed regional variation in the upward shift of timberlines in the Northern and Central European Alps in response to rapid warming at the Younger Dryas / Preboreal transition ca. 11 700 years ago, probably caused by a climatic gradient across the Alps. This contrasts with previous studies that successfully simulated the early Holocene afforestation in the (warmer) Central Alps with a chironomid-inferred temperature reconstruction from the (colder) Northern Alps. We use LandClim, a dynamic landscape vegetation model to simulate mountain forests under different temperature, soil and precipitation scenarios around Iffigsee (2065 m a.s.l.) a lake in the Northwestern Swiss Alps, and compare the model output with the paleobotanical records. The model clearly overestimates the upward shift of timberline in a climate scenario that applies chironomid-inferred July-temperature anomalies to all months. However, forest establishment at 9800 cal. BP at Iffigsee is successfully simulated with lower moisture availability and monthly temperatures corrected for stronger seasonality during the early Holocene. The model-data comparison reveals a contraction in the realized niche of Abies alba due to the prominent role of anthropogenic disturbance after ca. 5000 cal. BP, which has important implications for species distribution models (SDMs) that rely on equilibrium with climate and niche stability. Under future climate projections, LandClim indicates a rapid upward shift of mountain vegetation belts by ca. 500 m and treeline positions of ca. 2500 m a.s.l. by the end of this century. Resulting biodiversity losses in the alpine vegetation belt might be mitigated with low-impact pastoralism to preserve species-rich alpine meadows.
... Several factors may have played a significant role in limiting its postglacial abundance and distribution. Climate could have limited the spread of the trees, for example through influence on reproductive success (Giesecke et al., 2010). Spreading between islands and against the prevailing winds may have also slowed the process. ...
The southern fringes of the South American landmass provide a rare opportunity to examine the development of moorland vegetation with sparse tree cover in a wet, cool temperate climate of the Southern Hemisphere. We present a record of changes in vegetation over the past 17,000 years, from a lake in extreme southern Chile (Isla Santa Inés, Magallanes region, 53°38.97S; 72°25.24W), where human influence on vegetation is negligible. The western archipelago of Tierra del Fuego remained treeless for most of the Lateglacial period; Lycopodium magellanicum, Gunnera magellanica and heath species dominated the vegetation. Nothofagus may have survived the last glacial maximum at the eastern edge of the Magellan glaciers from where it spread southwestwards and established in the region at around 10,500 cal. yr BP. Nothofagus antarctica was likely the earlier colonizing tree in the western islands, followed shortly after by Nothofagus betuloides. At 9000 cal. yr BP moorland communities expanded at the expense of Nothofagus woodland. Simultaneously, Nothofagus species shifted to dominance of the evergreen Nothofagus betuloides and the Magellanic rain forest established in the region. Rapid and drastic vegetation changes occurred at 5200 cal. yr BP, after the Mt Burney MB2 eruption, including the expansion and establishment of Pilgerodendron uviferum and the development of mixed Nothofagus-Pilgerodendron-Drimys woodland. Scattered populations of Nothofagus, as they occur today in westernmost Tierra del Fuego may be a good analogue for Nothofagus populations during the Lateglacial in eastern sites.
... Recent studies suggest that the inclusion of these multi-scale measures of climatic variability can improve our understanding of species geographic limits (Jackson et al., 2009;Giesecke et al., 2010;Reside et al., 2010;Jiguet et al., 2011). However, these efforts, while important, do not provide clear guidelines for environmental variable selection for SDMs addressing questions related to modeling range expansions or shifts. ...
Our understanding of how species will respond to global change is still limited. Reasons hindering our ability to forecast species range shifts and expansions are the mismatch between realized climate niches in species’ native and invasive ranges, and the lack of available climatic datasets offering multiple scales of climatic variability (e.g., monthly and inter-annual climatic variability). Here we present a multi-taxon analysis of invasive species niche transferability that considers multi-scale climatic variability using ten noxious terrestrial invasive species. We compare native versus invasive ranges in geographic space as estimated using the species distribution modeling (SDM) algorithm Maxent, and the comparative index Hellinger's I with three possible climatic layer configurations representing natural climatic variability: (1) inter-annual, (2) monthly and (3) a combination of inter-annual and monthly climatic variability. We assess model performance using the area under the receiver characteristic curve (AUC). Results show that combined scales of climatic variability improved performance of the models for 60% of the species in the native range and 70% of the species in the invaded range. Contrasting native and invaded range SDM performance showed that the same climate layer configuration produced the best models only in 70% of the species. For 90% of the species the most similar niches were obtained based on monthly climatic variability. The divergence in our findings between higher performing SDMs and the most transferable SDMs, suggest some species range limits might be constrained by one scale of climatic variability in the native range and a different one in the invaded range. Where sufficient occurrence data in the invaded range is available, the inclusion of an additional scale of climatic variability can enhance ecological understanding of the invasion events. However, when invaded range occurrence data is not available, the most conservative approach would use only monthly climatic variability. If these finding are extrapolated to niche transferability in time, we suggest that historical collection records should be analyzed to understand species’ response to multiple scales of climate variability in the past, thereby informing the selection of appropriate scales of climate variability in the future.
... Evidence from paleoecological research has pointed to the importance of climatic tracking (i.e., changes in the realized climatic niche, defi ned as the projection of the geographical distribution of a taxon in a multivariate climatic space without reference to the particular mechanism that limits a species fundamental niche; Colwell and Rangel, 2009 ;Veloz et al., 2012 ) as the most common response to changes in environmental conditions. Climatic-niche tracking is supported by a number of documented Quater nary distributional shifts of tens to hundreds of kilometers since the last glacial maximum (~18 kyr ago [ka]) in Europe ( Huntley, 1991 ;Svenning and Skov, 2004 ;Giesecke et al., 2010 ), and in North America ( Webb, 1988 ;Williams et al., 2004Williams et al., , 2012Ordonez and Williams, 2013 ) in response to changes in climatic conditions. The fossil records provide additional evidence that plants have both tracked climatically suitable conditions on continental scales and are in dynamic equilibrium with climate on time scales of several thousand years ( Huntley, 1991 ;Shuman et al., 2002 ;Williams et al., 2002 ;Birks and Birks, 2008 ;Ordonez and Williams, 2013 ). ...
Premise of the study:
Predicting species responses to climate change has become a dynamic field in global change research. A crucial question in this debate is whether-or-not species have been and will be able to respond quickly enough to keep up with changing climatic conditions.
Methods:
Focusing on fossil pollen records and paleoclimatic simulations, this work assesses the change in realized climatic niches (climatic temporal trajectories) of 20 plant taxa over the last 16000 yr, and whether this tracking has been the same for different climatic niche dimensions.
Key results:
Climatic factors showed a consistent trend toward warmer temperatures and higher precipitation. Although the response types varied across taxa, species' realized climatic niches lagged in response to changes in climatic conditions. Temperature niches responded to late Pleistocene (16000-11000 yr ago) climate change, but did so at slower rates than changes in climatic conditions during the same period. In contrast, precipitation niches were relatively stable from 16000 to 11000 yr ago, but still lagged behind changes in climatic conditions. Changes in temperature and precipitation niches eventually stabilized during the Holocene (11000-1000 yr ago).
Conclusions:
These results underscore how the climatic niche realized at any one moment represents a subset of the climate conditions in which a taxon can persist, particularly during times of fast climatic change. Variability in the rates of temporal trajectories across evaluated climatic variables showed taxa specific responses to changes in climatic conditions over time and emphasizes the need to incorporate variation, intensity, and duration of lag effects in assessments of the possible effects of climatic change.
... In an experiment that manipulated the temporal variation of stress, higher variability muted negative impacts of stress on some seaweed taxa, but generated negative impacts in others (Benedetti-Cecchi et al. 2006). Although changes in variability can drive important biological changes in other systems (e.g., agricultural crops and forests; Southworth et al. 2000, Giesecke et al. 2010, studies on the effects of different magnitudes and temporal patterns of environmental variability, alone or in combination with changes in mean conditions, are exceedingly rare for seaweeds. ...
Seaweeds are ecologically important primary producers, competitors, and ecosystem engineers that play a central role in coastal habitats ranging from kelp forests to coral reefs. Although seaweeds are known to be vulnerable to physical and chemical changes in the marine environment, the impacts of ongoing and future anthropogenic climate change in seaweed- dominated ecosystems remain poorly understood. In this review, we describe the ways in which changes in the environment directly affect seaweeds in terms of their physiology, growth, reproduction, and survival. We consider the extent to which seaweed species may be able to respond to these changes via adaptation or migration. We also examine the extensive reshuffling of communities that is occurring as the ecological balance between competing species changes, and as top-down control by herbivores becomes stronger or weaker. Finally, we delve into some of the ecosystem- level responses to these changes, including changes in primary productivity, diversity, and resilience. Although there are several key areas in which ecological insight is lacking, we suggest that reasonable climate-related hypotheses can be developed and tested based on current information. By strategically prioritizing research in the areas of complex environmental variation, multiple stressor effects, evolutionary adaptation, and population, community, and ecosystem-level responses, we can rapidly build upon our current understanding of seaweed biology and climate change ecology to more effectively conserve and manage coastal ecosystems.
... Young trees are able to grow at or above the upper elevational limit of adult trees Temperature can control the elevational limits of species in many ways and at all life stages. It has been suggested that low temperatures first constrain reproduction (seed quality and seed production) or seedling establishment (Woodward & Williams, 1987;Giesecke et al., 2010). In contrast, this study indicates that seed availability at the upper elevational limits of tree species does not pose a serious constraint for tree recruitment in any of those studied species under current climatic conditions. ...
Aim The physical and physiological mechanisms that determine tree‐line position are reasonably well understood, but explanations for tree species‐specific upper elevational limits below the tree line are still lacking. In addition, once these uppermost positions have been identified, questions arise over whether they reflect past expansion events or active ongoing recruitment or even upslope migration. The aims of this study were: (1) to assess current tree recruitment near the cold‐temperature limit of 10 major European tree species in the Swiss Alps, and (2) to rank species by the extent that their seedlings and saplings exceed the elevational limit of adult trees, possibly reflecting effects of the recent climate warming.
Location Western and eastern Alps of Switzerland.
Methods For each species, occurrences were recorded along six elevational transects according to three size classes from seedlings to adult trees in 25‐m‐elevation steps above and below their regional upper elevational limit. Two methods were used to compare upper elevational limits between seedlings, saplings and adults within species. First, we focused on the uppermost occurrence observed in each life stage for a given species within each studied region; and second, we predicted their upper distribution range using polynomial models fitted to presence/absence data.
Results Species exhibited a clear ranking in their elevational limit. Regional differences in species limits (western versus eastern Swiss Alps) could largely be attributed to regional differences in temperature. Observed and predicted limits of each life stage showed that all species were represented by young individuals in the vicinity of the limit of adult trees. Moreover, tree recruitment of both seedlings and saplings was detected and predicted significantly beyond adult tree limits in most of the species. Across regions, seedlings and saplings were on average found at elevations 73 m higher than adult trees.
Main conclusions Under current conditions, neither seed dispersal nor seedling establishment constitutes a serious limitation of recruitment at the upper elevational limits of major European trees. The recruits found beyond the adult limits demonstrate the potential for an upward migration of trees in the Alps in response to ongoing climate warming.
... Third, the size of the grid cells used in this analysis also means that it is possible that, within a grid cell, analogous conditions may occur even though the mean conditions of the grid cell are non-analogous. Similarly, inter-annual variability and extreme events may be a more important driver of species persistence and extinctions than the multiyear averages used here (Giesecke et al., 2010). This can be rectified when more fine-resolution LGM climate reconstructions become available. ...
Aim To identify potential source and sink locations for climate‐driven species range shifts in Europe since the Last Glacial Maximum (LGM).
Location Europe.
Methods We developed a new approach combining past‐climate simulations with the concept of analogous climate space. Our index gives a continuous measure of the potential of a location to have acted as a source or a sink for species that have shifted their ranges since the LGM. High glacial source potential is indicated by LGM climatic conditions that are widespread now; high post‐glacial sink potential is indicated by current climatic conditions that were widespread at the LGM. The degree of isolation of source and sink areas was calculated as the median distance to areas with analogous climate conditions.
Results We identified areas of high glacial source potential in the previously recognized refugial areas in the southern European peninsulas, but also in large areas in central‐western Europe. The most climatically isolated source areas were located in northern Spain, in north‐western Europe and in eastern Turkey. From here species would have had to cover substantial distances to find current climate conditions analogous to LGM conditions of these areas. Areas with high post‐glacial sink potential were mainly located in Fennoscandia and in central and south‐eastern Europe. Some of the most isolated sink areas were located in the Spanish highlands and around the Baltic Sea.
Main conclusions Our species‐independent approach successfully identified previously recognized glacial refugial areas with high source potential for species range shifts in southern Europe and in addition highlighted other potential source areas in central Europe. This study offers new insights into how the distribution of past and current climatic conditions may have influenced past species range shifts and current large‐scale biodiversity patterns.
... For TBS trees, it suggests that the control simulation with its current parameterization can somewhat represent the " average " output from a large number of simulations with different combinations of parameter values. In our simulations, climate input (e.g., air temperature) plays an important role in determining vegetation redistribution ; this has also been demonstrated in other recent studies (Kharuk et al. 2009; Shuman and Shugart 2009; Giesecke et al. 2010; Lantz et al. 2010; Wright and Fridley 2010). For example, the continuous increased summer temperature in the Arctic region potentially achieves the lowest requirement for CPG to survive, therefore stimulating northward CPG expansion in the region. ...
We assessed the uncertainty of the simulated vegetation coverage by the
Lund-Potsdam-Jena dynamic global vegetation model (LPJ-DGVM) to changes
of parameters and different climate inputs. The analysis was based on a
set of 10,000 Monte Carlo ensemble simulations for northern high
latitudes (45oN north). The LPJ-DGVM was run under contemporary and
future climates from four Special Report Emission Scenarios (SRES):
A1FI, A2, B1, B2 based on the Hadley Centre General Circulation Model
(GCM), and six climate scenarios (X901M, X902L, X903H, X904M, X905L,
X906H) from the Integrated Global System Model (IGSM) of Massachusetts
Institute of Technology (MIT). Although all parameters exert significant
effects on the simulations, some parameters are more important than
others. Parameters that control plant carbon uptake and light-use
efficiency have predominant influences on vegetation distribution of
both woody and herbaceous plant functional types. We found that the
relative importance of different parameters varies temporally and
spatially and is influenced by climate input. Parameter and climate both
play an important role in projecting vegetation redistribution. However,
the parameter-based uncertainty overwhelms the effects of climate
scenarios. The simulations using the MIT IGSM climate scenarios
indicated that the high, moderate and low climate responses always
correspond to the largest, moderate and smallest uncertainty range of
vegetation coverage, respectively. Temperate trees are sensitive and
boreal forest and C3 perennial grass are less sensitive to the climate
variability. Although the uncertainty is significantly large, we
projected a unanimous northward tree migration due to anomalous warming
in northern high latitudes. Temporally, boreal needleleaved evergreen
plants are projected to decline considerably and a large portion of C3
perennial grass will disappear by the end of the century. In contrast,
the area of temperate trees largely increases, especially under the most
extreme A1FI scenario.
... Because we apply the chironomid-inferred July temperature reconstruction to all months, and use the observed modern distribution of inter-annual climatic variation, we do not explicitly examine all climatic factors that constrain species distributions. For example, winter and summer temperatures do not necessarily change in concert, and inter-annual variation may have been greater in the past (Miller et al., 2008;Giesecke et al., 2010). Extreme winter cold and desiccation may limit spruce at high elevations (Kullman, 1997;Latalowa & van der Knaap, 2006;Kullman & Oberg, 2009), and may have limited spruce in the Central Alps during the Late Glacial and early Holocene. ...
Aim Forest communities in the European Central Alps are highly sensitive to climatic change. Palaeobotanical studies have demonstrated that forests rapidly expanded upslope during Holocene warm intervals and contracted when temperatures fell. However, temperature alone cannot account for important changes in tree species abundance. For example, population expansion by Norway spruce (Picea abies), a dominant subalpine species, lagged suitable temperatures by about 3000 years in eastern and by 6000 years in western Switzerland. We hypothesize that spruce expansion was delayed by limited water availability in weakly developed soils and/or by drier-than-present climatic conditions.
... This anomaly was probably due to the fact that the monthly climate data used in the model do not fully capture the very high variability of summer precipitation in this part of the Alps, where short thunderstorms often bring heavy precipitation followed by extended periods of drought. See also Giesecke et al. (2010) for the significance of inter-annual climate variability as driver in simulations of vegetation change. ...
Recent temperature observations suggest a general warming trend that may be causing the range of tree species to shift to
higher latitudes and altitudes. Since biotic interactions such as herbivory can change tree species composition, it is important
to understand their contribution to vegetation changes triggered by climate change. To investigate the response of forests
to climate change and herbivory by wild ungulates, we used the forest gap model ForClim v2.9.6 and simulated forest development
in three climatically different valleys in the Swiss Alps. We used altitudinal transects on contrasting slopes covering a
wide range of forest types from the cold (upper) to the dry (lower) treeline. This allowed us to investigate (1) altitudinal
range shifts in response to climate change, (2) the consequences for tree species composition, and (3) the combined effect
of climate change and ungulate herbivory. We found that ungulate herbivory changed species composition and that both basal
area and stem numbers decreased with increasing herbivory intensity. Tree species responded differently to the change in climate,
and their ranges did not change concurrently, thus causing a succession to new stand types. While climate change partially
compensated for the reductions in basal area caused by ungulate herbivory, the combined effect of these two agents on the
mix of the dominant species and forest type was non-compensatory, as browsing selectively excluded species from establishing
or reaching dominance and altered competition patterns, particularly for light. We conclude that there is an urgent need for
adaptive forest management strategies that address the joint effects of climate change and ungulate herbivory.
Quaternary (last 2.6 million years) botany involves studying plant megafossils (e.g. tree stumps), macrofossils (e.g. seeds, leaves), and microfossils (e.g. pollen, spores) preserved in peat bogs and lake sediments. Although megafossils and macrofossils have been studied since the late eighteenth century, Quaternary botany today is largely dominated by pollen analysis.
Quaternary pollen analysis is just over 100 years old. It started primarily as a geological tool for correlation, relative dating, and climate reconstruction. In 1950 a major advance occurred with the publication by Knut Fægri and Johs Iversen of their Text-book of Modern Pollen Analysis which provided the foundations for pollen analysis as a botanical and ecological tool for studying past dynamics of biota and biotic systems. The development of radiocarbon dating in the 1950s freed pollen analysis from being a tool for relative dating. As a result of these developments, pollen analysis became a valuable implement in long-term ecology and biogeography.
Selected contributions that Quaternary botany has made to ecology and biogeography since 1950 are reviewed. They fall into four general parts: (1) ecological aspects of interglacial and glacial stages such as location and nature of glacial-stage tree refugia and long-term soil development in glaciated and unglaciated areas; (2) biotic responses to Quaternary environmental change (spreading, extinction, persistence, adaptation); (3) ecological topics such as potential niches, the nature of vegetation, and tree and forest dynamics; and (4) its application to ecological topics such as human impact in tropical systems, conservation in a changing world, island palaeoecology, plant–animal interactions, and biodiversity patterns in time.
The future of Quaternary botany is briefly discussed and 10 suggestions are presented to help strengthen it and its links with ecology and biogeography. Quaternary botany has much to contribute to ecology and biogeography when used in conjunction with new approaches such as ancient-DNA, molecular biomarkers, and multi-proxy palaeoecology.
Increasing environmental variability could exacerbate the effects of climate change on ecological processes such as population dynamics, or positive and negative effects (favorable or unfavorable weather) could balance. Such a balance could depend on constraints of the processes. Biological and spatial constraints are represented in a spatially explicit individual based simulation of an ecotone reduced to two species on a single environmental gradient. The effects of climate amelioration are simulated from a plant's-eye-view by increasing the establishment and decreasing the mortality rates. Variability is introduced as a random multiplier of these rates, and the strength of the variation is increased through the period of climate change. The biological constraints limit change in the rates, and the extent of the simulation grid represents a spatial constraint. A small increase in environmental variation, multiplied through time with climate change, increases extinction rates. The biological and spatial constraints have little effect on the response of populations. Instead, competition, based on the form of the species response functions to the environmental gradient at the point where they intersect, determines differences in population responses. Positive and negative variations in the environment do not balance because the responses are hierarchical and asymmetric. Differences persist because extinction during a negative anomaly cannot be reversed by a later positive one.
Plant species distributions, broadly shaped by climate, may also be constrained by other species. The degree to which biotic factors affect range limits is unclear, however, and few experimental studies have investigated both biotic and abiotic factors across and beyond a species’ range. We examined seedling survival and net growth for three years in contrasting canopy type (closed canopy vs. gap) and neighbor density (clipped versus unclipped) environments for northern, central and southern populations of sugar maple (Acer saccharum) representing a climate-of-origin gradient, experimentally planted from Arkansas, USA to Ontario, Canada at ten forested sites along a 1,700 km transect spanning beyond the species’ range. We hypothesized that each population's highest survival and growth would occur in its region of origin, with poorer performance in areas cooler or warmer. Refuting this hypothesis, seedlings of all three populations had greater growth and survival in sites increasingly warmer than their point of origin, although they did show poorer growth and survival at increasingly colder sites. We also hypothesized that maple survival and net growth near and beyond range margins are constrained primarily by cold temperature limitation in the north, where we expected neighbors to facilitate survival, and by competition in the south, where we expected to enhance survival and growth by reducing neighbor density. Results partially supported the hypothesis concerning biotic interactions: in canopy gaps, understory neighbors enhanced maple growth at the coolest sites but did not suppress growth as expected at the warmest sites. As the northern population grew and survived reasonably well beyond the northern range limit, and as all populations performed best at warmer sites, including beyond the southern range limit, there was tepid, if any, support for the hypothesis that climate regulated the northern limit and absolutely no support for the hypothesis that competition regulated the southern limit. Together, these three-year findings with juvenile trees suggest that sugar maple range limits may instead be constrained by factors besides climate and competition, by those factors at another life stage, and/or by climate events such as heat waves, droughts, and cold snaps that occur at longer return intervals. This article is protected by copyright. All rights reserved.
After massive proliferation over the last decade, distribution modelling (DM) – research with the purpose of modelling the distribution of observable objects of a specific type – has grown into an independent branch of ecological science. There is consensus that this new discipline needs a stronger theoretical foundation. I describe DM as an inductive scientific process with 12 steps, organised into three composite steps: ecological model, data model, and statistical model. Step 8, modelling of the overall ecological response, places DM unambiguously among gradient analysis techniques and motivates for a gradient analytic (GA) perspective on DM. DM terminology is reviewed and revised accordingly.
Three fundamental insights of the GA perspective are described: (1) that external ‘factors’ do not influence the species one by one, but act on the species in concert; (2) that a few major complex-gradients normally account for most of the variation in species composition that can be explained environmentally; and (3) that species occur within a restricted interval along each major complex-gradient. These insights are developed into a theoretical platform for DM. General patterns of species performance variation along environmental complex-gradients and the structuring processes responsible for these patterns are reviewed. Three categories of ecoclines, i.e., gradients of variation in species composition and the environment, are recognised: regional ecoclines, local ecoclines, and condition or impact ecoclines. Causes and implications of the unimodal shape of species’ responses to environmental complex-gradients are reviewed. Structuring processes are divided into three categories: limited physiological tolerance, interspecific interactions, and demographic processes. Relationships between categories of ecoclines, the processes responsible for variation in species performance along them, and the spatial and temporal scale intervals in which variation is large, are reviewed.
The GA perspective forms the basis for discussions of important steps in the DM process. Initially, the controversial concepts of the habitat and the niche are reviewed and their role in the ecological model (Step 1) discussed. I conclude that neither of these concepts are necessary, nor useful, for DM. As an alternative to conceptual models based upon the niche concept, I propose a new conceptual modelling framework for DM, the HED framework, which is rooted in the gradient analytic perspective. I show how this new framework can be used, in initial phases of a DM study to formulate a meta-model for factors that influence distributions, and in the analytic phase to guide important choices of methods and options and to assist interpretation of modelling results. Important data model issues are: collection of data for the modelled target and preparation of raw response variables (Steps 2 and 6); collection of explanatory data (Step 3); conceptualisation of the study area (Step 4); collection of data for calibration and evaluation (Step 9); and transformation of explanatory variables to derived variables subjected to DM (Step 5,ii). Important statistical model issues are: statistical model formulation, i.e. choice of method (Step 7,i) and model specification (Step 7,ii); model selection and internal assessment of model performance (Steps 8,i and 8,ii); and model evaluation (Step 10). Two points are emphasised: (1) that modelling purpose should inform choice of methods and options; and (2) the importance of an independently collected presence/absence data set, which can be used to calibrate, evaluate and iteratively improve models.
Finally I list seven challenges of particular importance for progress in DM: (1) that more knowledge of patterns of natural variation is needed; (2) that a better mechanistic understanding of causes of patterns of natural variation is needed; (3) that the availability of relevant rasterised explanatory variables needs to be improved; (4) that more studies of patterns at local and micro spatial scales, in addition to multiple-scale studies using DM methods, are needed; (5) that evaluation by independent data should be established as a standard in DM; (6) that further insights into statistical modelling methods and their options, with particular reference to appropriateness for different types of data and DM purposes, are needed; and (7) that DM methods should be incorporated in studies with a broader scope. I conclude that there are considerable potentials for improvement of DM methods and practice. Increased return from DM in terms of contributions that improve our understanding of patterns of natural variation and their causes, should be expected.
In this study, we explored the maximal response of soil carbon in a part of China to climate change, including variations in climatology and climate variability, under the condition of global warming. A conditional nonlinear optimal perturbation (CNOP) approach was employed to discuss the above issue using the Lund–Potsdam–Jena (LPJ) model. The variation in the soil carbon was compared with those caused by a linear temperature or precipitation perturbation. The key difference between the CNOP-type and the linear perturbations depended on whether the perturbations brought the variation in the temperature or the precipitation variability in comparison with the reference temperature or the precipitation variability. The model results demonstrated that the variations in the soil carbon resulted from the CNOP-type and linear temperature perturbations in south of the study region, which was corresponding to part of South China, had different variations. We examined three components of the soil carbon in the LPJ model: fast-decomposing soil carbon, slow-decomposing soil carbon, and litter below the ground. The variations of these components derived by the two types of temperature perturbations were different in the chosen region. The reduction in the litter below the ground may be the main cause of decreased soil carbon in arid and semi-arid regions as a result of the two types of temperature perturbations. The different impacts of the two types of temperature perturbations in the south of the study region may be mainly caused by the variations in the fast-decomposing soil carbon. The variations in the soil carbon caused by the two types of precipitation perturbations were similar. In the arid and semi-arid regions, the soil carbon increased due to the two types of precipitation perturbations. This research implies that the variation in temperature variability plays a crucial role in the variations of the soil carbon and its components in the study region.
Assessments of future tree species’ distributions should account for time lags in the adaptation of their external range limits to climatic changes. In simulation experiments it is therefore necessary to capture processes that influence such time lags, in particular tree species’ migration. We hypothesise that directional processes such as migration are sensitive to the exact sequence of simulated climate influences, and that the uncertainty associated with a given interannual climate variability has to be accounted for when simulating migration explicitly. In this paper we used the intermediate-complexity multi-species model TreeMig to examine whether different realisations of future climate influences with the same temporal mean and the same interannual variability cause fundamental differences in simulated migration. We assume that the impact of interannual climate variability becomes most apparent in situations which critically influence regeneration and survival. Such situations arise, for example, when species’ sensitivities to climate, competition and spatial fragmentation interact. We therefore developed an illustrative and realistic simulation setup representing this situation. We simulated the northwards migration of the sub-Mediterranean tree species Ostrya carpinifolia Scop. (European Hop Hornbeam) through the highly fragmented and climatically heterogeneous landscape of the Swiss Alps.
This study examines the importance of climate variability when simulating forest succession using a process-based model of stand development. The FORSKA-2V forest gap model, originally developed for forcing with monthly mean climate data, was modified to accept daily weather data. The model's performance was compared using different temporal resolutions of forcing along a bioclimatic transect crossing the boreal region of central Canada, including the aspen-parkland and forest-tundra ecotones. Forcing the model with daily weather data improved the simulation of key attributes of present-day forest along the transect, particularly at the ecotones, compared to forcing with monthly data or long term averages. The results support the hypothesis that climatic variability at daily time-scales is an important determinant of present-day boreal forest composition and productivity. To simulate boreal forest response to climatic change it will be necessary to create climatic scenarios that include plausible projections of future daily scale variability.
Aim We play the role of an ice age ecologist (IAE) charged with conserving biodiversity during the climate changes accompanying the last deglaciation. We develop reserve-selection strategies for the IAE and check them against rankings based on modern data.
Location Northern and eastern North America.
Methods Three reserve-selection strategies are developed. (1) Abiotic: the IAE uses no information about species–climate relationships, instead maximizing the climatic and geographic dispersion of reserves. (2) Species distribution models (SDMs): the IAE uses boosted-regression trees calibrated against pollen data and CCSM3 palaeoclimatic simulations from 21 to 15 ka bp to predict modern taxon distributions, then uses these as input to the Zonation reserve-ranking program. (3) Rank-and-regress: regression models are used to identify climatic predictors of zonation rankings. All strategies are assessed against a Zonation ranking based on modern pollen distributions. Analysis units are ecoregions and grid cells.
Results The abiotic strategy has a negative or no correlation between predicted and actual rankings. The SDM-based strategy fares better, with a significantly positive area-corrected correlation (r= 0.474, P < 0.001) between predicted and actual rankings. Predictive ability drops when grid cells are the analysis unit (r= 0.217, P = 0.058). Predictive ability for the rank-and-regress strategy is similar to the SDM results.
Main conclusions For the IAE, SDMs improve the predictive ability of reserve-selection strategies. However, predictive ability is limited overall, probably due to shifted realized niches during past no-analogue climates, new species interactions as species responded individually to climate change, and other environmental changes not included in the model. Twenty-first-century conservation planning also faces these challenges, and is further complicated by other anthropogenic impacts. The IAE's limited success does not preclude the use of climate scenarios and niche-based SDMs when developing adaptation strategies, but suggests that such tools offer at best only a rough guide to identifying possible areas of future conservation value.
The influence of dispersal limitation on species ranges remains controversial. Considering the dramatic impacts of the last glaciation in Europe, species might not have tracked climate changes through time and, as a consequence, their present-day ranges might be in disequilibrium with current climate. For 1016 European plant species, we assessed the relative importance of current climate and limited postglacial migration in determining species ranges using regression modelling and explanatory variables representing climate, and a novel species-specific hind-casting-based measure of accessibility to postglacial colonization. Climate was important for all species, while postglacial colonization also constrained the ranges of more than 50 per cent of the species. On average, climate explained five times more variation in species ranges than accessibility, but accessibility was the strongest determinant for one-sixth of the species. Accessibility was particularly important for species with limited long-distance dispersal ability, with southern glacial ranges, seed plants compared with ferns, and small-range species in southern Europe. In addition, accessibility explained one-third of the variation in species' disequilibrium with climate as measured by the realized/potential range size ratio computed with niche modelling. In conclusion, we show that although climate is the dominant broad-scale determinant of European plant species ranges, constrained dispersal plays an important supplementary role.
The ecotone between the boreo-nemoral (hemiboreal) and the southern boreal vegetation zones constitutes the northern distributional limit of a number of thermophilous tree species in northern Europe and is, to a large extent, controlled by climatic conditions. We present a quantitative annual mean temperature reconstruction from a high-resolution pollen stratigraphy in southern boreal Finland, using a pollen-climate calibration model with a cross-validated prediction error of 0.9°C. Our model reconstructs low but steadily rising annual mean temperature from 10,700 to 9000calyr BP. At 8000–4500calyr BP reconstructed annual mean temperature reaches a period of highest values (Holocene thermal maximum) with particularly high temperatures (2.0–1.5°C higher than at present) at 8000–5800calyr BP. From 4500calyr BP to the present-day, reconstructed annual mean temperature gradually decreases by ca 1.5°C. Comparison of present results with palaeotemperature records from the Greenland ice cores, notably with the NorthGRIP δ18O record, shows marked similarities, suggesting parallel large-scale Holocene temperature trends between the North Atlantic and North European regions. The verification of the occurrence, timing, and nature of the short-term temperature fluctuations during the Holocene in the southern boreal zone in Europe requires replicate, high-resolution climate reconstructions from the region.
A model to predict global patterns in vegetation physiognomy was developed from physiological considerations influencing the distributions of different functional types of plant. Primary driving variables are mean coldest-month temperature, annual accumulated temperature over 5-degrees-C, and a drought index incorporating the seasonality of precipitation and the available water capacity of the soil. The model predicts which plant types can occur in a given environment, and selects the potentially dominant types from among them. Biomes arise as combinations of dominant types. Global environmental data were supplied as monthly means of temperature, precipitation and sunshine (interpolated to a global 0.5-degrees grid, with a lapse-rate correction) and soil texture class. The resulting predictions of global vegetation patterns were in good agreement with the mapped distribution of actual ecosystem complexes (Olson, J.S., Watts, J.A. & Allison, L.J. (1983) ORNL-5862, Oak Ridge Nat. Lab., 164 pp.), except where intensive agriculture has obliterated the natural patterns. The model will help in assessing impacts of future climate changes on potential natural vegetation patterns, land-surface characteristics and terrestrial carbon storage, and in analysis of the effects of past climate change on these variables.
The mid-Holocene ( 6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid-Holocene NAO and mean SLP show small changes compared to pre-industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO-negative phase during the mid-Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid-Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid-Holocene.
Atmospheric circulation is important in determining the surface climate and environment. To quantify its effect, circulation indices or classifications of circulation type are often used. In this study, the classification system developed by Lamb (1950. Quarterly Journal of the Royal Meteorological Society76: 393–438) is applied to obtain circulation information for Sweden on a monthly basis. For that purpose, monthly mean sea-level pressure (MSLP) data from 1873 to 1995 is used to derive six circulation indices and to provide a circulation catalogue with 27 circulation types. The frequency of circulation types over different periods is computed and described. Four major types (cyclonic, C; west, W; southwest, SW; anticyclonic, A) have been identified. The catalogue and the associated indices provide a tool for interpreting the regional climate and for developing statistical downscaling models to derive regional climate change scenarios for Sweden.
We investigated the potential drivers of Holocene vegetation changes recorded at four Scandinavian pollen sites, two in Sweden and two in Finland, at a time when they were largely free of anthropogenic influence.
We used the generalized dynamic vegetation model LPJ‐GUESS forced with climate anomaly output from an atmospheric general circulation model to simulate tree species dynamics from 10 000 years ago to the present. The model results were compared to high‐resolution pollen accumulation rates gathered at the sites.
Our results indicate that both the observed northern distributional limits of temperate trees, and the limits of Pinus sylvestris and Alnus incana at the tree line, are a result of millennial variations in summer and winter temperatures. The simulation of several distinct trends in species occurrence observed in the pollen record indicates that a time lag due to the slow spreading of species need not be invoked for most species.
Sensitivity studies indicate that competition, natural disturbance and the magnitude of interannual variability play key roles in determining the biomass, establishment and even the presence of species near their bioclimatic limits. However, neither disturbance due to fire nor limits on establishment due to drought were likely to have been major determinants of the observed trends on the timescales considered.
We were unable to limit the modelled occurrence of Picea abies at the study sites to the periods at which it was observed in the pollen records, indicating that we have still not completely understood the driving or limiting factors for Holocene changes in Picea abies abundance.
Synthesis . This study shows that by combining quantitative vegetation reconstructions with a modern, process‐based dynamic vegetation model, we may gain new insights into the potential biotic and abiotic drivers of Holocene vegetation dynamics, and their relative importance. This knowledge will be crucial in enabling us to assess more confidently the response of northern European vegetation to future climate change.
The variability of winter extreme low‐temperature events and summer extreme high‐temperature events was investigated using daily temperature series (1901–98) from 11 sites in central and southern Europe. An extreme temperature event (EXTE) is defined by various threshold values of daily temperature or daily temperature anomaly. Systematic changes in the frequencies of EXTEs are investigated by the Mann–Kendall test and a method based on the Wilcoxon test. The catalogue of macrocirculation types over central Europe (the Hess–Brezowsky classification) is applied to investigate the connections between EXTEs and large‐scale circulation. Circulation classes (HBC) are defined, and mostly spatial averages of EXTEs are examined.
There were large long‐term fluctuations in the frequencies of both winter extreme cold events (EXCEs) and summer extreme warm events (EXWEs) during the 20th century. The systematic changes referring to the entire period indicate a slight warming tendency, but only a few of the changes, mostly in the northernmost sites, are statistically significant. Strong connections are present between the frequencies of EXTEs and the large‐scale circulation on various time scales, particularly for EXCEs. The spatial differences of EXTE fluctuations and EXTE–HBC connections are small within the study area. Northerlies and easterlies, as well as meridional and anticyclonic situations, are favourable for EXCEs, whereas southerlies and persistent anticyclonic situations are favourable for EXWE occurrences. In the latest decades, a decline in the frequency of EXCEs and a sharp increase in the frequency of EXWEs happened, and the residence times of the circulation patterns over central Europe became longer both in winter and summer. Copyright © 2003 Royal Meteorological Society
Extreme events act as a catalyst for concern about whether the climate is changing. Statistical theory for extremes is used to demonstrate that the frequency of such events is relatively more dependent on any changes in the variability (more generally, the scale parameter) than in the mean (more generally, the location parameter) of climate. Moreover, this sensitivity is relatively greater the more extreme the event. These results provide additional support for the conclusions that experiments using climate models need to be designed to detect changes in climate variability, and that policy analysis should not rely on scenarios of future climate involving only changes in means.
Changes in the severity of extreme weather events under the influence of the enhanced greenhouse effect could have disproportionally
large effects compared to changes in the mean climate. Here, we explored the meteorological circumstances of extremes and
changes therein using two 49-member climate model ensembles for reference (1961–1990) and scenario (2051–2080) greenhouse-gas
concentrations. We have focused on daily-mean surface-air temperatures over the Northern Hemisphere in January. Over large
parts of the continents, changes in the one-in-10-year temperature events are influenced at least as much by changes in the
shape of the probability distribution functions (PDFs) as by shifts in the mean. In coastal areas, this is largely attributable
to changes in the large-scale circulation, for those types of extremes linked to infrequent wind directions. In other areas,
the inhomogeneous mean warming, increasing inland and polewards, affects the tails of the local temperature PDFs. Temperature
extremes in widely different regions were found to be linked by a large-scale circulation anomaly pattern, which resembles
the Arctic Oscillation. In the scenario ensemble, this anomaly pattern favors its positive phase, leading to enhanced probabilities
of westerly winds in a belt around the Northern Hemisphere.
Vegetation is known to interact with the other components of the climate system over a wide range of timescales. Some of these interactions are now being taken into account in models for climate prediction. This study is an attempt to describe and quantify the climate–vegetation coupling at the interannual timescale, simulated with a General Circulation Model (HadSM3) coupled to a dynamic global vegetation model (TRIFFID). Vegetation variability is generally strongest in semi-arid areas, where it is driven by precipitation variability. The impact of vegetation variability on climate is analysed by using multivariate regressions of boundary layer fluxes and properties, with respect to soil moisture and vegetation fraction. Dynamic vegetation is found to significantly increase the variance in the surface sensible and latent heat fluxes. Vegetation growth always causes evapotranspiration to increase, but its impact on sensible heat is less straightforward. The feedback of vegetation on sensible heat is positive in Australia, but negative in the Sahel and in India. The sign of the feedback depends on the competing influences, at the gridpoint scale, of the turbulent heat exchange coefficient and the surface (stomatal) water conductance, which both increase with vegetation growth. The impact of vegetation variability on boundary layer potential temperature and relative humidity are shown to be small, implying that precipitation persistence is not strongly modified by vegetation dynamics in this model. We discuss how these model results may improve our knowledge of vegetation–atmosphere interactions and help us to target future model developments.
Under the framework of the Palaeoclimate Modelling Intercomparison Project (PMIP), 17 climate models, 16 of which are atmospheric
general circulation models, have been run to simulate the climate of the Last Glacial Maximum (21 000 years ago) using the
same set of boundary conditions. Parallel to these numerical experiments, new, consistent, data bases have been developed
on a continental scale. The present work compares the range of the model responses to the large perturbation corresponding
to the conditions of the Last Glacial Maximum with consistently derived climate reconstructions from pollen records over Europe
and western Siberia. It accounts for the differences in the model results due to the models themselves and directly compares
this “error bar” due to the models to the uncertainties in the climate reconstructions from the pollen records. Overall the
Last Glacial Maximum climate simulated by the models over western Europe is warmer, especially in winter, and wetter than
the one depicted by the reconstructions. This is the region where the reconstructed increase in temperature, precipitation
and moisture index from the Last Glacial Maximum to the present conditions is largest. The same disagreement, but of smaller
amplitude, is found over Central Europe and the eastern Mediterranean Basin, while models and data are in broad agreement
over western Siberia. The numerous modelling results allow a study of the link between the changes in atmospheric circulation
and those in temperature, and an interpretation of the discrepancies in precipitation in terms of those in temperature.
Our central goal is to determine the importance of including both mean and variability changes in climate change scenarios in an agricultural context. By adapting and applying a stochastic weather generator, we first tested the sensitivity of the CERES-Wheat model to combinations of mean and variability changes of temperature and precipitation for two locations in Kansas. With a 2C increase in temperature with daily (and interannual) variance doubled, yields were further reduced compared to the mean only change. In contrast, the negative effects of the mean temperature increase were greatly ameliorated by variance decreased by one-half. Changes for precipitation are more complex, since change in variability naturally attends change in mean, and constraining the stochastic generator to mean change only is highly artificial. The crop model is sensitive to precipitation variance increases with increased mean and variance decreases with decreased mean. With increased mean precipitation and a further increase in variability Topeka (where wheat cropping is not very moisture limited) experiences decrease in yield after an initial increase from the 'mean change only case. At Goodland Kansas, a moisture-limited site where summer fallowing is practiced, yields are decreased with decreased precipitation, but are further decreased when variability is further reduced. The range of mean and variability changes to which the crop model is sensitive are within the range of changes found in regional climate modeling (RegCM) experiments for a CO2 doubling (compared to a control run experiment). We then formed two types of climate change scenarios based on the changes in climate found in the control and doubled CO2 experiments over the conterminous U. S. of RegCM: (1) one using only mean monthly changes in temperature, precipitation, and solar radiation; and (2) another that included these mean changes plus changes in daily (and interannual) variability. The scenarios were then applied to the CERES-Wheat model at four locations (Goodland, Topeka, Des Moines, Spokane) in the United States. Contrasting model responses to the two scenarios were found at three of the four sites. At Goodland, and Des Moines mean climate change increased mean yields and decreased yield variability, but the mean plus variance climate change reduced yields to levels closer to their base (unchanged) condition. At Spokane mean climate change increased yields, which were somewhat further increased with climate variability change. Three key aspects that contribute to crop response are identified: the marginality of the current climate for crop growth, the relative size of the mean and variance changes, and timing of these changes. Indices for quantifying uncertainty in the impact assessment were developed based on the nature of the climate scenario formed, and the magnitude of difference between model and observed values of relevant climate variables.
Knowledge of the vegetation and environment of eastern North America during the Last Glacial Maximum (LGM) is important to understanding postglacial vegetational and biogeographic dynamics, assessing climate sensitivity, and constraining and evaluating earth-system models. Our understanding of LGM conditions in the region has been hampered by low site density, problems of data quality (particularly dating), and the possibility that LGM vegetation and climate lacked modern analogs. In order to generate improved reconstructions of LGM vegetation and environment, we assembled pollen and plant macrofossil data from 21 and 17 well-dated LGM sites, respectively. All sites have assemblages within the LGM timespan of 21,000±1500 calendar yr BP. Based on these data, we prepared maps of isopolls, macrofossil presence/absence, pollen-analogs, biomes, inferred mean January and July temperatures and mean annual precipitation for the LGM. Tundra and open Picea-dominated forest grew along the Laurentide ice sheet, with tundra predominantly in the west. In the east, Pinus-dominated vegetation (mainly P. banksiana with local P. resinosa and P. strobus) occurred extensively to 34°N and possibly as far south as 30°N. Picea glauca and a now-extinct species, P. critchfieldii, occurred locally. Picea-dominated forest grew in the continental interior, with temperate hardwoods (Quercus, Carya, Juglans, Liriodendron, Fagus, Ulmus) growing locally near the Lower Mississippi Valley at least as far north as 35°N. Picea critchfieldii was the dominant species in this region. The Florida peninsula was occupied by open vegetation with warm-temperate species of Pinus. Eastern Texas was occupied by open vegetation with at least local Quercus and Picea. Extensive areas of peninsular Florida and the continental interior had vegetation unmatched by any modern pollen samples. The paleovegetational data indicate more extensive cooling in eastern North America at the LGM than simulated by either the NCAR CCM0 or CCM1 climate models. The occurrence of cool-temperate conifers and hardwoods as far north as 34-35°N, however, indicates less severe cooling than some previous reconstructions. Paleoclimate inferences for the LGM are complicated by lowered atmospheric CO2 concentrations, which may be responsible for the open nature and dominance of conifers in LGM vegetation.
The variability in alkenone-derived sea surface temperatures (SSTs) in the North Atlantic realm shows that a continuous SST decrease in the northeast Atlantic from the early to the late Holocene was accompanied by a persistent warming over the western subtropical Atlantic, the eastern Mediterranean Sea and the northern Red Sea. Based on the analysis of the instrumental data and of atmospheric general circulation model experiments, we show that this variation in SSTs during the Holocene can be attributed to a continuous weakening of a Northern Hemisphere atmospheric circulation pattern similar to that of the Arctic/North Atlantic Oscillation.
A bioclimatic model based on physiological constraints to plant growth and regeneration is used here in an empirical way to describe the present natural distributions of northern Europe's major trees. Bioclimatic variables were computed from monthly means of temperature, precipitation and sunshine (%) interpolated to a 10' grid taking into account elevation. Minimum values of mean coldest-month temperature (T c ) and 'effective' growing degree days (GDD*) were fitted to species' range limits. GDD* is total annual growing degree days (GDD) minus GDD to budburst (GDD°). Each species was assigned to one of the chilling-response categories identified by Murray, Cannell & Smith (1989) to calculate GDD°. Maximum T c values were fitted to continental species' mild-winter limits and other deciduous species' warm-winter limits. Minimum values of relative growing-season moisture availability (α*) were estimated from silvics. Growth indices were calculated based on potential net assimilation (a quadratic in daily temperature) and α*. Growth can be rapid near a range limit, e.g. Picea abies (L.) Karsten in southern Sweden. Climate changes expected for CO 2 doubling were projected on to the grid. Simulated distribution changes reflected interspecific differences in response to changing seasonality. Chilling responses proved important, e.g. the predicted range limit of Fagus sylvatica L. contracts in the west while expanding northwards as winters warm more than summers. Transient responses to climate change can be modelled using the same information provided that fundamental and realized niche limitations are distinguished-a caveat that underlines the dearth of experimental information on the climatic requirements for growth, and especially regeneration, of many important trees.
With the rise of new powerful statistical techniques and GIS tools, the development of predictive habitat distribution models has rapidly increased in ecology. Such models are static and probabilistic in nature, since they statistically relate the geographical distribution of species or communities to their present environment. A wide array of models has been developed to cover aspects as diverse as biogeography, conservation biology, climate change research, and habitat or species management. In this paper, we present a review of predictive habitat distribution modeling. The variety of statistical techniques used is growing. Ordinary multiple regression and its generalized form (GLM) are very popular and are often used for modeling species distributions. Other methods include neural networks, ordination and classification methods, Bayesian models, locally weighted approaches (e.g. GAM), environmental envelopes or even combinations of these models. The selection of an appropriate method should not depend solely on statistical considerations. Some models are better suited to reflect theoretical findings on the shape and nature of the species’ response (or realized niche). Conceptual considerations include e.g. the trade-off between optimizing accuracy versus optimizing generality. In the field of static distribution modeling, the latter is mostly related to selecting appropriate predictor variables and to designing an appropriate procedure for model selection. New methods, including threshold-independent measures (e.g. receiver operating characteristic (ROC)-plots) and resampling techniques (e.g. bootstrap, cross-validation) have been introduced in ecology for testing the accuracy of predictive models. The choice of an evaluation measure should be driven primarily by the goals of the study. This may possibly lead to the attribution of different weights to the various types of prediction errors (e.g. omission, commission or confusion). Testing the model in a wider range of situations (in space and time) will permit one to define the range of applications for which the model predictions are suitable. In turn, the qualification of the model depends primarily on the goals of the study that define the qualification criteria and on the usability of the model, rather than on statistics alone.
An attempt is made to study, by statistical treatment of surface observations, the fluctuations in variability of winter temperatures during the past century at eight selected European stations, four of them being considered as maritime and four continental in their location.
Variability has been studied by three different parameters: interannual variability of winter months by consecutive decades from 1870–1980, intermonthly variability throughout the winters by consecutive decades from 1870–1980 and frequency of extreme values of monthly winter temperatures by consecutive decades from 1870–1980.
Both interannual and intermonthly variability are shown to have been at a minimum during the decades of a strong climate maritimity in the beginning of this century (1900–1920) and at a maximum during the decades of transition (1930–1950) from the mild era in the first four decades of the century to a colder period. There is no clear indication that variability increased during the most recent decades although Moscow, the most eastern of the used stations, shows high variability in recent time. Extreme values seem to be increasing in frequency during recent decades in both maritime and continental parts of Europe although the maximum in frequency of extreme values occurred in the transition period 1930–1950.
Short-term regeneration ability of a cold-marginal Picea abies forest was studied by analyzing spruce seed rain during nine years and by assessing resulting recruitment at the end of the studied period. Quantitatively large differences occurred between studied years, and seed viability was low during the whole study period. Only exceptionally did seed viability exceed 1 %. Sparse regeneration occurred in 1984, which is the year when most viable seeds/ha were recorded. In addition, the results are compared with recruitment data from the 1930s (from the same forest), a time period when regeneration peaked. It is suggested that the period 1984–1992, due to the sparse regeneration, retrospectively probably will appear as a regeneration trough or a decreasing regeneration trend. Furthermore, it is concluded that, in a long-term perspective, periodic regeneration is sufficient for maintenance of the forest because of the longevity of spruce. Additionally, climatically induced variations of regeneration success give rise to natural fluctuations in stand density.
summaryInstant stand-level dieback occurred in boreal coniferous forests of northern Sweden in 1987. A population of pine (Pinus sylvestris L.) and subdominant spruce (Picea abies (L.) Karst.) was analysed with respect to defoliation, age structure, radial increment and soil conditions. Defoliation and mortality were due to frost and drought in response to an early winter with abnormal cold and a thin snow cover. A thick mor humus layer preconditioned ground frost preservation. Annual radial increment had declined steadily over c. 40 years, in variance with climatic cooling and local exposure. Possibly, the growth decline depleted energy reserves and lowered the tolerance to climatic stress during extreme conditions. It is obvious that the structure of northern boreal forests responds perceivably to short-term thermal decline. The outlined mechanism of dieback may prevent the evolution of climax in the absence of other exogenous disturbances. The present event may be a small-scale analogue of forest responses to rapid cooling during, for example, the Younger Dryas period and the Little Ice Age.
The pollen spectra obtained from 127 lake sediment surface samples from the western interior of Canada are examined. Principal components biplots and discriminant analysis are used to investigate the correspondence between modern pollen rain and major vegetation zones in the study area. It is demonstrated that the major vegetation zones in the western interior of Canada can be distinguished on the basis of their modern pollen rain. Principal components biplots and discriminant analysis are also used to compare late Quaternary fossil pollen assemblages from lakes located near the southern and northern periphery of the modern boreal forest with the modern pollen spectra. The results suggest that the late Pleistocene and early Holocene pollen spectra from the southern and northern periphery of the boreal forest lack modern analogues in the western interior of Canada. The northern and southern boundaries of the boreal forest were located north of their modern positions during the mid Holocene. Vegetation developed and responded to mid Holocene climatic change at different rates at the two sites. Modern vegetation conditions were established at both sites between 5000 B.P. to 3000 B.P. Vegetation has remained relatively stable during the last three thousand years.
The question of which factors limit the occurrence of a plant species to a particular site is addressed by considering 53 cases in which the distribution of Pinus species has changed in the last century. The authors consider expansions of pines in and adjacent to their natural ranges in the Northern Hemisphere and the spread from sites of introduction in the Southern Hemisphere well outside the contemporary range of pines. They consider a neutral hypothesis (with respect to climate or biological interactions as determinants of invasion): invasion simply requires that a species is present in sufficient numbers, with sufficient propagules over sufficient time to invade. They then explore the relative importance of climatic changes, disturbance, competition, herbivory, pathogens, and other agents that might influence pine membership in communities. Environmental stresses created by moisture and temperature appear to exercise primary control on invasibility at xeric and high-elevation sites, respectively, but play a smaller role at intermediate locations. At these sites, range limits are determined principally by interactions between pine seedlings and the resident biota in adjacent communities. Pine invasions are most prevalent where there is limited competition in the regeneration niche and occur more easily in habitats where the dominant growth form is most different from that of pines, namely in grasslands. The disturbance regime in the receiving habitat is important and interacts directly and indirectly with the "inherent' susceptibility to invasion. Severe disturbances may initiate pine invasions at any latitude, but are essential for reducing the cover of vigorous plant growth (and hence competition in the regeneration niche) in tropical and near-tropical regions. -from Authors
. A high-altitude boreal Picea abies forest, with tree ages of up to 410 yr, was studied with respect to age structure, spatial regeneration patterns, and substrate. The results suggest that recruitment is primarily dependent on germination substrate but also negatively correlated with the density of the tree layer. 60 % of all spruces < 1.3 m high grew on substrates connected with tree-fall; ca. 40 % were found on decomposing logs and stumps, covering only ca. 6 % of the forest floor. Individual logs remain important as a regeneration substrate for ca. 150 yr. Continuous presence of decomposing coarse wood is a condition for the maintenance of the population structure under the prevailing climatic conditions. Peaks in the age distribution (the 1870's and the 1940's - 1950's) are probably climatically induced. The results challenge the previous assumptions that this kind of forest undergoes cyclic development. Long-term structural stability with climatically induced minor variations may be an alternative model.
A bioclimatic model based on physiological constraints to plant growth and regeneration is used here in an empirical way to describe the present natural distributions of northern Europe's major trees. Bioclimatic variables were computed from monthly means of temperature, precipitation and sunshine (%) interpolated to a 10′ grid taking into account elevation. Minimum values of mean coldest-month temperature (Tc) and 'effective' growing degree days (GDD*) were fitted to species' range limits. GDD* is total annual growing degree days (GDD) minus GDD to budburst (GDD⚬). Each species was assigned to one of the chilling-response categories identified by Murray, Cannell & Smith (1989) to calculate GDD⚬. Maximum Tc values were fitted to continental species' mild-winter limits and other deciduous species' warm-winter limits. Minimum values of relative growing-season moisture availability (α*) were estimated from silvics. Growth indices were calculated based on potential net assimilation (a quadratic in daily temperature) and α*. Growth can be rapid near a range limit, e.g. Picea abies (L.) Karsten in southern Sweden. Climate changes expected for CO2 doubling were projected on to the grid. Simulated distribution changes reflected interspecific differences in response to changing seasonality. Chilling responses proved important, e.g. the predicted range limit of Fagus sylvatica L. contracts in the west while expanding northwards as winters warm more than summers. Transient responses to climate change can be modelled using the same information provided that fundamental and realized niche limitations are distinguished-a caveat that underlines the dearth of experimental information on the climatic requirements for growth, and especially regeneration, of many important trees.
A detailed understanding of decadal to millennial-scale climate changes requires seasonal-scale (summer-winter) reconstructions of past precipitation and temperature fluctuations. Comparing seasonally resolved varve records with pollen-based sum of growing degree-days (GDD) reconstructions from Lake Nautajärvi, we examined the intra-annual nature of climate variability in central southern Finland during the Holocene. The organic varve record and the GDD reconstruction show roughly comparable trends supporting the interpretation that both proxies predominantly reflect summer temperatures in the study area. The records suggest low but rising early-Holocene (9500 to 8500 cal. yr BP) summer temperatures. The Holocene Thermal Maximum (HTM) in the GDD record dates to about 7500 to 4500 cal. yr BP, but the organic varve record along with reconstructed changes in vegetation composition, notably a peak of Tilia pollen percentages, indicate that during the HTM there was a trend towards a more continental climate with maximum mid-summer temperatures reached at 6500 to 4500 cal. yr BP. Both records reflect the start of the post-HTM cooling at about 4500 cal. yr BP, simultaneously with an increase of the amount of catchment erosion and mineral matter influx into the lake, suggesting gradually colder and/or longer winters with high net accumulation of snow. The organic varve record and the GDD record start to diverge at 2000 cal. yr BP, possibly owing to the human influence on catchment processes. The reconstructed mid-Holocene summer temperature peak deviates from the regional climate model outputs, which suggest highest summer temperatures during the early Holocene.
Physical properties of varves formed over the last ca. 10000 years from Lake Nautajärvi, central southern Finland provide a potential proxy record of winter precipitation and temperature via catchment runoff and erosion. The seasonal-scale varved data indicate that the winter severity and duration of summer growing season has changed considerably during the Holocene in southern central Finland. Periods of increased catchment erosion occurred at 7590–7530 BC, 7450–7400 BC, 7220–7110 BC, 7000–6000 BC, 5400–5200 BC, 4400–4000 BC, 2700–2400 BC, ca. 1500 BC to AD 500, and 1400 AD onwards. The observed changes are likely related to severe winters conditions with high net accumulation of snow between ca. 7500 BC and AD 300, whereas AD 300 onwards an increasing agricultural input is seen in the varve and pollen data. Periods that can be interpreted as indicating attenuated spring floods caused by milder and wetter winters appeared around 7000 BC and AD 1000–AD 1200, the latter corresponding with the last historically recorded warm interval in Europe, known as the Medieval Climate Anomaly.
Quantitative reconstruction of the area cleared of forest in the past is essential to assess the possible indirect anthropogenic impacts on the past environment of Europe, including past climate. We apply a simul ation model of pollen dispersal and deposition (1) to re-examine the relationship between pollen and landscape openness, often uncritically inferred from non-arboreal pollen (NAP) percentages alone, and (2) to predict the relevant source area of pollen, the smallest spatial scale of vegetation that can be reconstructed from pollen records. The simulations use landscapes simplified from the modern open agricultural and semi-open forested regions in southern Sweden where traditional cultural landscapes still remain. The model is appropriate, because the simulated pollen assemblages resemble the pollen assemblages observed in each of the two landscape types, and because the simulated relationships between NAP percentages and percentage cover of open land within 1000 m agree with the empirical relationships. The simulated relevant source area of pollen is the area within 800–1000 m from both small hollows and 3-ha ponds. NAP percentages give only a rough first approximation of the percentage cover of open land. More comprehensive methods will be required to obtain quantitative estimates of open land from fossil pollen.
Holocene vegetation dynamics in the ecotone between the boreo-nemoral and boreal forest were reconstructed from sediment cores of two small lakes using pollen and macrofossil analyses. Competition, migration and changing climate parameters determine species dynamics in the ecotone. The spread of Picea abies was the most important event in the vegetation history of the region, probably shaping the present distribution limits of Corylus avellana, Tilia cordata and Quercus robur through competitive exclusion. The expansion of Picea abies is well documented by abundant macrofossil finds at one of the lakes, mirroring the rise in pollen percentage and confirming the presence of the tree before Picea abies pollen frequencies reached 1%. No Picea abies macrofossils were encountered before pollen was regularly found. Changing vegetation composition through migration affects both sites at different times, while a shift in atmospheric circulation pattern may be responsible for a concordant change in vegetation composition at both sites around 5700 cal. BP.
Aim Beringia, far north‐eastern Siberia and north‐western North America, was largely unglaciated during the Pleistocene. Although this region has long been considered an ice‐age refugium for arctic herbs and shrubs, little is known about its role as a refugium for boreal trees and shrubs during the last glacial maximum (LGM, c . 28,000–15,000 calibrated years before present). We examine mapped patterns of pollen percentages to infer whether six boreal tree and shrub taxa ( Populus , Larix , Picea , Pinus , Betula , Alnus/Duschekia ) survived the harsh glacial conditions within Beringia.
Methods Extensive networks of pollen records have the potential to reveal distinctive temporal–spatial patterns that discriminate between local‐ and long‐distance sources of pollen. We assembled pollen records for 149 lake, peat and alluvial sites from the Palaeoenvironmental Arctic Sciences database, plotting pollen percentages at 1000‐year time intervals from 21,000 to 6000 calibrated years before present. Pollen percentages are interpreted with an understanding of modern pollen representation and potential sources of long‐distance pollen during the glacial maximum. Inferences from pollen data are supplemented by published radiocarbon dates of identified macrofossils, where available.
Results Pollen maps for individual taxa show unique temporal‐spatial patterns, but the data for each taxon argue more strongly for survival within Beringia than for immigration from outside regions. The first increase of Populus pollen percentages in the western Brooks Ranges is evidence that Populus trees survived the LGM in central Beringia. Both pollen and macrofossil evidence support Larix survival in western Beringia (WB), but data for Larix in eastern Beringia (EB) are unclear. Given the similar distances of WB and EB to glacial‐age boreal forests in temperate latitudes of Asia and North America, the widespread presence of Picea pollen in EB and Pinus pollen in WB indicates that Picea and Pinus survived within these respective regions. Betula pollen is broadly distributed but highly variable in glacial‐maximum samples, suggesting that Betula trees or shrubs survived in restricted populations throughout Beringia. Alnus/Duschekia percentages show complex patterns, but generally support a glacial refugium in WB.
Main conclusions Our interpretations have several implications, including: (1) the rapid post‐glacial migration rate reported for Picea in western Canada may be over estimated, (2) the expansion of trees and shrubs within Beringia should have been nearly contemporaneous with climatic change, (3) boreal trees and shrubs are capable of surviving long periods in relatively small populations (at the lower limit of detection in pollen data) and (4) long‐distance migration may not have been the predominant mode of vegetation response to climatic change in Beringia.
Aim Understanding the driving forces and mechanisms of changes in past plant distribution and abundance will help assess the biological consequences of future climate change scenarios. The aim of this paper is to investigate whether modelled patterns of climate parameters 6000 years ago can account for the European distribution of Fagus sylvatica at that time. Consideration is also given to the role of non‐climatic parameters as driving forces of the Holocene spread and population expansion of F. sylvatica .
Location Europe.
Methods European distributions were simulated using a physiologically‐based bioclimatic model (STASH) driven by three different atmospheric general circulation model (AGCM) outputs for 6000 years ago.
Results The three simulations generally showed F. sylvatica to have potentially been as widespread 6000 years ago as it is today, which gives a profound mismatch with pollen‐based reconstructions of the F. sylvatica distribution at that time. The results indicate that drier conditions during the growing season 6000 years ago could have caused a restriction of the range in the south. Poorer growth conditions with consequently reduced competitive ability were modelled for large parts of France.
Main conclusions Consideration of the entire European range of F. sylvatica showed that no single driving force could account for the observed distributional limits 6000 years ago, or the pattern of spread during the Holocene. Climatic factors, particularly drought during the growing season, are the likely major determinants of the potential range. Climatic factors are regionally moderated by competition, disturbance effects and the intrinsically slow rate of population increase of F. sylvatica . Dynamic vegetation modelling is needed to account for potentially important competitive interactions and their relationship with changing climate. We identify uncertainties in the climate and pollen data, as well as the bioclimatic model, which suggest that the current study does not identify whether or not climate determined the distribution of F. sylvatica 6000 years ago. Pollen data are better suited for comparison with relative abundance gradients rather than absolute distributional limits. These uncertainties from a study of the past, where we have information about plant distribution and abundance, argue for extreme caution in making forecasts for the future using equilibrium models.
It is shown that the probability P that the annual maximum value of a meteorological element is less than a given value x can be expressed as exp
where a and k are constants such that ak is positive.
The parameters a and k can be evaluated from data of annual maximum values for a given period of years. Hence we derive the average highest and lowest in sets of T annual maxima, and also the maximum value to be expected once in T years.
Advances in dynamic ecosystem modelling have made a number of different approaches to vegetation dynamics possible. Here we compare two models representing contrasting degrees of abstraction of the processes governing dynamics in real vegetation.
Model (a) (GUESS) simulates explicitly growth and competition among individual plants. Differences in crown structure (height, depth, area and LAI) influence relative light uptake by neighbours. Assimilated carbon is allocated individually by each plant to its leaf, fine root and sapwood tissues. Carbon allocation and turnover of sapwood to heartwood in turn govern height and diameter growth.
Model (b) (LPJ) incorporates a ‘dynamic global vegetation model’ (DGVM) architecture, simulating growth of populations of plant functional types (PFTs) over a grid cell, integrating individual‐level processes over the proportional area (foliar projective cover, FPC) occupied by each PFT. Individual plants are not simulated, but are replaced by explicit parameterizations of their growth and interactions.
The models are identical in their representation of core physiological and biogeochemical processes. Both also use the same set of PFTs, corresponding to the major woody plant groups in Europe, plus a grass type.
When applied at a range of locations, broadly spanning climatic variation within Europe, both models successfully predicted PFT composition and succession within modern natural vegetation. However, the individual‐based model performed better in areas where deciduous and evergreen types coincide, and in areas subject to pronounced seasonal water deficits, which would tend to favour grasses over drought‐intolerant trees.
Differences in model performance could be traced to their treatment of individual‐level processes, in particular light competition and stress‐induced mortality.
Our results suggest that an explicit individual‐based approach to vegetation dynamics may be an advantage in modelling of ecosystem structure and function at the resolution required for regional‐ to continental‐scale studies.
Incidences of oak decline have occurred repeatedly during the past three centuries as well as in the most recent decades. On the basis of historical records and dendrochronological measurements, oak decline in Central Europe has been attributed to the single or combined effects of climatic extremes (winter frost, summer drought), defoliating insects, and pathogenic fungi. Starting from a literature review, we discuss the possible roles of various abiotic (air pollution, nitrogen eutrophication, soil chemical stress, climatic extremes, site conditions) and biotic factors (insect defoliation, borer attack, infection by pathogenic fungi, microorganisms) that have been related to oak decline. On the basis of investigations on Quercus petraea and Quercus robur at three different levels (from experiments with young trees to monitoring on a supraregional scale), we suggest a conceptual model of the interaction of abiotic and biotic factors responsible for the onset of oak decline. This model should be valid for Central European oak stands at more acidic sites (soil pH (H2O) ≤ 4.2; on soils with higher pH, pathogenic Phytophthora species may contribute to oak decline). The combination of severe insect defoliation in at least two consecutive years with climatic extremes is the most significant complex of factors in the incidence of oak decline. Combined with defoliation, summer drought or winter/spring frost or both have to occur within the same year or in consecutive years to trigger major outbreaks of decline. Important additional stress factors are the following: (1) hydromorphic site conditions which, particularly in the case of Q. robur, render the trees more susceptible to drought stress as a result of an impairment of root growth in the subsoil; and (2), possibly, excess nitrogen which, in combination with drought stress, results in distinct decreases in the foliar concentrations of allelochemicals in Q. robur, thereby probably making the trees more susceptible to insect defoliation. Air pollution, soil chemical stress (including excess manganese), and nitrogen-induced nutritional imbalance do not seem to be important causal factors in the complex of oak decline. On the basis of the model, the appearance of the most recent oak decline in North-western Germany can be adequately explained.
Facteurs abiotiques et biotiques et leurs interactions, comme causes du dépérissement des chênes dans le centre de l'Europe
Des dépérissements de chênes ont eu lieu périodiquement durant les trois siècles passés comme durant les récentes décades. D'après les mentions historiques et des observations dendrochronologiques, le dépérissement des chênes dans le centre de l'Europe a été attribuéà l'effet simple ou combiné d'extrêmes climatiques (froids hivernaux, sécheresses estivales), de défoliations par les insectes, et de champignons parasites. Dans le présent article, nous discutons du rôle possible de facteurs abiotiques (pollution atmosphérique, eutrophisation azotée, stress chimique du sol, extrêmes climatiques, conditions stationnelles) et de facteurs biotiques (défoliations entomologiques, attaques de xylophages, infection de champignons parasites, et autres microorganismes) qui ont été associés au dépérissement des chênes. Sur la base des recherches réalisées à trois niveaux différents sur Quercus petraea et Q. robur (depuis des expériences sur jeunes plants jusqu'au suivi supra régional), nous proposons un modèle conceptuel d'interactions entre les facteurs abiotiques et biotiques responsables de l'initiation du dépérissement. Ce modèle devrait être valide pour les chênaies du centre de l'Europe sur sols acides (pH(H2O) ≤ 4,2, des Phytophthora ssp. pouvant contribuer au dépérissement sur les sols à pH plus élevé). Les défoliations entomologiques sévères pendant au moins deux années consécutives et les extrêmes climatiques, constituent la combinaison de facteurs la plus significative dans l'apparition du dépérissement. La sécheresse estivale ou les gelées hivernales ou printanières, ou les deux, doivent avoir lieu la même année que la défoliation, ou les années suivantes, pour qu'un dépérissement majeur se développe. D'autres facteurs additionnels de stress sont: (1) les conditions d'hydromorphie qui rendent les arbres (particulièrement Q. robur) plus sensibles au stress hydrique à cause d'une moindre croissance racinaire dans le sol profond; (2) éventuellement l'excès d'azote qui, combiné au stress hydrique, induit une réelle diminution des concentrations foliaires en métabolites secondaires chez Q. robur, ce qui rend probablement les arbres plus sensibles aux défoliations entomologiques. La pollution atmosphérique, le stress chimique du sol (y compris l'excès en manganèse), et les déséquilibres nutritionnels induits par l'azote ne semblent pas intervenir de façon importante dans le processus de dépérissement. Sur la base du modèle, le dépérissement le plus récent apparu dans le nord-ouest de l'Allemagne peut être expliqué de façon satisfaisante.
Abiotische und biotische Faktoren und ihre Wechselwirkungen als Ursachen für das Eichensterben in Mitteleuropa
Eichensterben-Episoden traten wiederholt in den letzten zweihundertfünfzig Jahren wie auch in den letzten Dekaden auf. Auf der Grundlage historischer Aufzeichnungen und dendrochronologischer Untersuchungen wurde das Eichensterben in Mitteleuropa auf einzelne oder kombinierte Auswirkungen klimatischer Extreme (Winterfrost, sommerliche Trockenheit), Entlaubung durch herbivore Insekten und Befall mit pathogenen Pilzen zurückgeführt. Im vorliegenden Beitrag wird auf der Basis einer Literaturübersicht die Rolle verschiedener abiotischer (Luftverschmutzung, Stickstoff-Eutrophierung, bodenchemischer Stress, Witterungsextreme, Standortsbedingungen) und biotischer Faktoren (fraßbedingte Entlaubung durch Insektenlarven, Borkenkäfer, pathogene Pilze, Mikroorganismen) diskutiert, die mit Eichensterben in Verbindung gebracht wurden. Vor dem Hintergrund von Untersuchungen, die auf drei unterschiedlichen Ebenen (von Experimenten mit Jungbäumen bis zum Monitoring im überregionalen Maßstab) an Quercus petraea und Q. robur durchgeführt wurden, wird ein Modell der Wechselwirkungen abiotischer und biotischer Faktoren bei der Entstehung des Eichensterbens vorgestellt. Dieses Modell gilt für mitteleuropäische Eichenbestände an stärker sauren Standorten (pH (H2O) ≤ 4,2; auf Böden mit höherem pH-Wert können Phytophthora-Arten zum Eichensterben beitragen). Eine Kombination von Kahlfraß in aufeinanderfolgenden Jahren und Witterungsextremen ist bei der Entstehung von Eichenschäden die bedeutendste. Von den drei Faktoren Kahlfraß, sommerliche Trockenheit und Winter- bzw. Spätfrost müssen mindestens zwei zeitgleich oder kurz nacheinander auftreten, um schwerwiegende Episoden von Eichensterben auszulösen. Schadverstärkende Stressfaktoren sind wechselfeuchte Standortsbedingungen, die, insbesondere bei Q. robur, die Anfälligkeit der Bäume für Trockenstress aufgrund der Beeinträchtigung des Wurzelwachstums im Unterboden erhöhen, sowie möglicherweise überschüssiger Stickstoff, der, in Kombination mit Trockenstress, bei Q. robur zu einer drastischen Abnahme der Konzentration sekundärer Pflanzenstoffe in den Blättern führt und somit die Bäume wahrscheinlich anfälliger für fraßbedingte Entlaubung macht. Luftverschmutzung, bodenchemischer Stress (einschließlich überschüssiges Mangan) und Stickstoff-induziertes Nährstoffungleichgewicht scheinen im Ursachenkomplex des Eichensterbens keine wesentliche Rolle zu spielen. Auf der Grundlage des vorgestellten Modells lässt sich die Entstehung der jüngsten Eichenschäden in Nordwestdeutschland angemessen erklären.
ABSTRACTA statistical analysis of the seasonal and interannual variations in the regional temperature anomalies of Sweden during 1861–1994 is performed. The study uses homogenized monthly temperatures averaged over 6 regions to minimize the non climatic and local-scale climatic effects. It is found that the temperature variability shows a clear regional and seasonal dependency. The topography, the influence of the sea and the synoptic climatology may have determined the dependency. The anomaly is related to variations in the North Atlantic Oscillation (NAO) expressed by an index (NAOI) and the extent to which the temperature anomaly can be explained by the NAO is investigated. The results show that the NAO has an important effect on the regional Swedish temperature on the monthly and interannual scales. The relationship between the temperature and NAOI over the period 1985–1994 are strong, implying that the NAOI may be a suitable candidate for a statistical downscaling model of the regional temperature. However, correlation analysis over different 31-year periods shows that the strength of the association varies with time and region. The further north the weaker the association. On the other hand, the temporal variations of the moving correlations for the 6 regions are similar. Part of the temporal variations may be explained by the averaged strength of NAO during different 31-year periods. This is especially evident for southern Sweden. At last, the coherency spectrums between the temperature anomalies and the NAO index is determined, which enables an examination of the association over the frequency domain. The result supports the idea that the NAO has an important effect on the Swedish temperature, though the strength of the association varied with time. These results have implications for statistical downscaling.
A database of monthly climate observations from meteorological stations is constructed. The database includes six climate elements and extends over the global land surface. The database is checked for inhomogeneities in the station records using an automated method that refines previous methods by using incomplete and partially overlapping records and by detecting inhomogeneities with opposite signs in different seasons. The method includes the development of reference series using neighbouring stations. Information from different sources about a single station may be combined, even without an overlapping period, using a reference series. Thus, a longer station record may be obtained and fragmentation of records reduced. The reference series also enables 1961–90 normals to be calculated for a larger proportion of stations.
The station anomalies are interpolated onto a 0.5° grid covering the global land surface (excluding Antarctica) and combined with a published normal from 1961–90. Thus, climate grids are constructed for nine climate variables (temperature, diurnal temperature range, daily minimum and maximum temperatures, precipitation, wet-day frequency, frost-day frequency, vapour pressure, and cloud cover) for the period 1901–2002. This dataset is known as CRU TS 2.1 and is publicly available (http://www.cru.uea.ac.uk/). Copyright
We present a dataset of daily resolution climatic time series that has been compiled for the European Climate Assessment (ECA). As of December 2001, this ECA dataset comprises 199 series of minimum, maximum and/or daily mean temperature and 195 series of daily precipitation amount observed at meteorological stations in Europe and the Middle East. Almost all series cover the standard normal period 1961–90, and about 50% extends back to at least 1925. Part of the dataset (90%) is made available for climate research on CDROM and through the Internet (at http://www.knmi.nl/samenw/eca).
A comparison of the ECA dataset with existing gridded datasets, having monthly resolution, shows that correlation coefficients between ECA stations and nearest land grid boxes between 1946 and 1999 are higher than 0.8 for 93% of the temperature series and for 51% of the precipitation series. The overall trends in the ECA dataset are of comparable magnitude to those in the gridded datasets.
The potential of the ECA dataset for climate studies is demonstrated in two examples. In the first example, it is shown that the winter (October–March) warming in Europe in the 1976–99 period is accompanied by a positive trend in the number of warm-spell days at most stations, but not by a negative trend in the number of cold-spell days. Instead, the number of cold-spell days increases over Europe. In the second example, it is shown for winter precipitation between 1946 and 1999 that positive trends in the mean amount per wet day prevail in areas that are getting drier and wetter.
Because of its daily resolution, the ECA dataset enables a variety of empirical climate studies, including detailed analyses of changes in the occurrence of extremes in relation to changes in mean temperature and total precipitation. Copyright
Treeline ecotones are regarded as sensitive monitors of the recent climatic warming. However, it has been suggested that their sensitivity depends more on changes in tree density than on treeline position. We study these processes and the effect of climate, mainly air temperature, on tree recruitment and recent treeline dynamics. We selected three relatively undisturbed sites in the Spanish Pyrenees, dominated by Pinusuncinata, and analyzed their recent dynamics at local spatial (0.3–0.5 ha) and short temporal scales (100–300 years). We wanted to establish whether higher temperature was the only climatic factor causing an upward shift of the studied alpine treelines. The data we report show that treelines were ascending until a period of high interannual variability in mean temperature started (1950–95). During the late twentieth century, treeline fluctuation was less sensitive to climate than was the change in tree density within the ecotone. Tree recruitment and treeline position responded to contrasting climatic signals; tree recruitment was favored by high March temperatures whereas treeline position ascended in response to warm springs. We found a negative relationship between mean treeline-advance rate and March temperature variability. According to our findings, if the interannual variability of March temperature increases, the probability of successful treeline ascent will decrease.
The question of what signal, if any, appears in the pollen record when trees are present in the vegetation without producing pollen or with no pollen being recorded, is addressed. Four scenarios are envisaged: (i) the number of trees in the landscape are very few and scattered, (ii) the trees are too young to produce pollen, (iii) climate conditions are unfavourable for the trees to produce pollen and (iv) the trees are cut or damaged so that they do not flower. Each of these is considered in terms of pollen accumulation rates (PARs) and present theories and models of pollen dispersal. Examples are provided for the forest limit areas of the northern boreal trees in Finnish Lapland using data of pollen deposition monitored by pollen traps and results from the high temporal resolution (near annual) analyses of peat profiles. The relevance of the results to questions such as finds of spruce macrofossils in the Swedish mountains, the 8200 cal b.p. cold event, the migration of species/vegetation succession, and widespread damage to trees are all considered. It is concluded that although these situations are sometimes ‘invisible’ or misrepresented when pollen assemblages are expressed in the traditional percentage manner, they are often revealed by PARs. The fact that the pollen assemblage reflects a much wider regional area than is often understood can strengthen signals which have a regional impact, such as those which are climate induced, but may obscure events which affect only a limited spatial area or occur as small patches in the landscape.
The impact of interannual variability in temperature and precipitation on global terrestrial ecosystems is investigated using
a dynamic global vegetation model driven by gridded climate observations for the twentieth century. Contrasting simulations
are driven either by repeated mean climatology or raw climate data with interannual variability included. Interannual climate
variability reduces net global vegetation cover, particularly over semi-arid regions, and favors the expansion of grass cover
at the expense of tree cover, due to differences in growth rates, fire impacts, and interception. The area burnt by global
fires is substantially enhanced by interannual precipitation variability. The current position of the central United States’
ecotone, with forests to the east and grasslands to the west, is largely attributed to climate variability. Among woody vegetation,
climate variability supports expanded deciduous forest growth and diminished evergreen forest growth, due to difference in
bioclimatic limits, leaf longevity, interception rates, and rooting depth. These results offer insight into future ecosystem
distributions since climate models generally predict an increase in climate variability and extremes.
The location and survival of trees in the coldest stages of the last full-glacial has long been of interest to palaeoecologists, biogeographers, archaeologists and geneticists alike. In particular, where species survived in isolated refugia and the influence that this has had upon the long-term ancestry of the populations, remain key research questions. However, the exact location of refugia during the coldest stages of the full-glacial still remains illusive for many species of fauna and flora, with different lines of evidence often being at odds. This is particularly true for Europe. Emerging evidence from various fossil proxies, palaeoclimatic modelling and genetic research is starting to suggest that the traditional paradigm that trees were restricted to southern Europe and in particular the three southern peninsulas (Balkan, Italian and Iberian) during the full-glacial is questionable. This is backed by increasing evidence, including 151 14C-dated and identified pieces of macrofossil charcoal wood from 40 localities in central and eastern Europe to indicate that during the last full-glacial populations of coniferous and some deciduous trees grew much further north and east than previously assumed. This paper reviews the fossil evidence and considers it alongside genetic and palaeoclimatic evidence in order to contribute towards a newly emerging synthesis of the full-glacial refugial localities in Europe and their influence upon the ancestry of European species. Plotted against a new high-resolution millennial time-scale for the interval ∼32–∼16 ka BP in Greenland our evidence shows that coniferous as well as some broadleaf trees were continuously present throughout those interstadial/stadial cycles for which there are adequate data.
With global warming, the distribution of warmth through the year is likely to change in the future and comparable changes may have occurred over the course of the Holocene. Its effect on vegetation composition and species distribution can be compared to that of a continental versus an oceanic climate. The distribution of five major tree species along a continentality gradient was studied in Fennoscandia based on distribution maps and on their proportions of pollen in surface-sediment samples. Both analyses indicate that the five arboreal species show similar patterns of response to a continentality index in the order Ulmus glabra, Corylus avellana, Quercus robur, Tilia cordata, and Picea abies from the most oceanic to most continental. Continentality, growing degree days above 5 °C, and January temperature were reconstructed quantitatively from four pollen diagrams using transfer functions based on a combined Fennoscandian pollen surface-sample data-set. Quantitative reconstructions indicate that the climate in Fennoscandia has become increasingly more continental over the last 7000 years, and this is largely an effect of winter cooling. Early Holocene vegetation composition has poor analogues to the present vegetation in Fennoscandia, which hampers quantitative reconstructions. Qualitative reconstructions suggest that the early Holocene in Fennoscandia was the most oceanic period, but probably with a high variability in temperature.
This study concerns the investigation of how changes in climate variability could affect agricultural production. To date very few studies of this effect have been made, whereas changes in climate variability, in addition to changes in mean conditions could have serious effects on crops. Currently there is very little certainty regarding how climate variability will change in a greenhouse-gas-warmed world. In view of this uncertainty, it is desirable to perform sensitivity analyses of how crop-climate models (e.g. the CERES-Wheat model) respond to changes in climate variability. We apply the winter wheat model to two climate stations in the wheat growing region of Kansas. Goodland in the western region, and Topeka in the moister eastern region. Time series of temperature and precipitation, are simulated, systematically changing the interannual variance of the series (from one quarter to four times the actual variance), and determining how the crop climate model responds to these changes. In addition, these variability changes are combined with mean changes of both temperature and precipitation. We then also apply a GCM-derived climate change scenario. Results indicate that it is the relative magnitude of change of the mean and variance of the climate that determines their relative importance to changes in wheat yields. In general increases in variability of temperature and precipitation resulted in significant increases in yield variability and crop failures in the sensitivity experiments, but precipitation changes had a more pronounced effect. In the GCM-derived scenario, mean temperature effects dominate, resulting in increased yield variability and crop failures, because the magnitude of mean change is much greater than the magnitude of variance change. Over all experiments, the importance of considering not only mean but also variance changes of climate variables, when investigating the effect of climate change on wheat yields, is confirmed.
A series of nine simulations has been made with the Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ-DGVM) in order to explore the impacts of climate variability and Holocene changes in variability (as simulated by the Fast Ocean-Atmosphere Model, FOAM) on vegetation in three forest-dominated regions of China and in the semi-arid Sahelian region of northern Africa. The simulations illustrate that changes both in the magnitude of climate variability and in the persistence of above/below average conditions have the potential to modify the vegetation response to changes in mean climate. Simulated changes in moisture availability affect vegetation through drought stress or through changing the fuel availability in semi-arid regions where lack of fuel often limits the incidence of fire. Increasing moisture availability causes trees to replace grasses in China by reducing drought stress; increasing moisture availability in the Sahel increases the available fuel and hence reduces fire return times, favouring grasses. The modelling results imply that climate variability is important to vegetation dynamics; that not only the magnitude, but also the temporal structure of variability is important; and that correctly simulating vegetation changes in response to climate variability requires a realistic “baseline” simulation of plant community composition. They further indicate that the impacts of climate change on ecosystems can sometimes derive as much from changes in variability as from changes in mean climate.
This short introductory paper illustrates some key issues concerning extremes by focusing on daily temperature extremes defined using quantiles and threshold exceedances. The examples include both a low- and a high-elevation site in the Swiss Alps where long records of homogenous daily data are readily available. The analysis of extremes highlights several features, some of them taken from the 2003 heat wave that affected Europe, in particular significant changes in the trends of quantiles in the course of the 20th century, differences in the altitudinal behavior of maximum or minimum temperatures, and close links between means and the extreme quantiles of daily temperatures.
The presence of trees in central and southern Europe during the last full-glaciation has long been a matter of debate. A low but persistent presence of fossil tree pollen in central and southern European full-glacial paleoecological sequences has been interpreted either as representing long-distance pollen transport from southerly refuges or as representing in situ refugial populations. Here we present macroscopic charcoal results from 31 sequences located throughout Hungary that provide unequivocal evidence for the presence of at least seven different tree types between approximately 32,500 and 16,500 14C yr B.P. This evidence is presented in conjunction with molluscan and pollen analyses to indicate that during the last full-glaciation, trees grew as far north as Hungary, probably in microenvironmentally favorable sites. These areas provided an important cold-stage refugium for the European flora and fauna.