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Abstract

AimSouthern temperate tree lines are found at low elevations compared with their Northern Hemisphere counterparts. They are also regarded as forming at warm temperatures, which has been attributed to taxon-specific limitations. Using New Zealand tree lines as an example, we assess whether these tree lines are anomalously warm compared with the global mean.LocationNew Zealand.Methods Soil and air temperatures were measured over 2 years at six New Zealand tree line sites, and compared with other local and global growing season temperature data. In New Zealand and other oceanic regions, the long, variable seasonal transitions make calculations of mean growing season temperatures highly sensitive to how the growing season is defined. We used both the conventional (wide) definition (from when mean weekly root-zone temperature exceeds 3.2 °C in spring, to when it first falls below 3.2 °C in autumn) and a narrow definition (the period during which temperatures are continuously above 3.2 °C). Application of these criteria leads to similar mean growing season temperatures in continental regions, but different ones in oceanic regions. We tested whether growing season temperatures differ between northern and southern temperate tree lines.ResultsNew Zealand tree lines had a mean root-zone temperature during the wide growing season of 7.0 °C ± 0.4 SD, not significantly different from those at northern temperate tree lines. The mean temperature of the narrow growing season was 7.8 °C, warmer than tree lines elsewhere, but still within the range reported for temperate tree lines (7–8 °C).Main conclusionsWhilst they are found at lower elevations, New Zealand tree lines form at temperatures similar to those at Northern Hemisphere temperate tree lines. Together with similar recent evidence from Chile, these results refute the previously postulated taxon-specific limitation hypothesis, and suggest these southern temperate tree lines are not climatically depressed, but are governed by the same thermal threshold as other tree lines worldwide.

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... In general, however, I could show that island treeline elevations are lower than on the mainland as postulated by Leuschner (1996) because of the missing mass elevation effect, the negative effect of high cloudiness associated with oceanic climates, the isolation-induced absence of adapted tree species and specific local island climates such as high elevation drought produced by the trade wind inversion (Manuscript 1). However, it has been recently shown that although island treelines are generally lower, they still behave in congruence to the lower thermal limit for tree growth, i.e. a minimum root zone temperature during the vegetation period of 7-8°C as proposed by Körner & Paulsen (2004), and are not taxon-specific as previously assumed (Cieraad et al. 2014). ...
... All these features of oceanic climates influence treeline elevation and are contrasting to continental climates, which have been argued to increase treeline elevation (Holtmeier 2009. More specifically, the effect of high cloudiness and high precipitation, especially in peak areas (all islands; Flenley 1995, Paulsen & Körner 2014, and low temperature seasonality (only non-tropical islands) in higher elevations of oceanic islands compared to continental mainland areas of similar latitude has been suggested as a possible cause of reduced tree growth suitability and thus lower treeline elevations on islands (Körner 1998, Cieraad et al. 2014. High cloudiness and high precipitation are likely to be important on islands of the equatorial tropics (e.g. ...
... The low phylogenetic and low ecological variability of high-elevation floras ) likely leads to a reduced probability of tree species suitable for high-elevation conditions. However, a recent study showed that some treelines on islands are lower due to thermal limitations as suggested by Körner and Paulsen (2004), and not necessarily for taxon-specific reasons (Cieraad et al. 2014). ...
Thesis
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In the seven manuscripts presented in this dissertation I contribute to our understanding of the drivers of plant diversity, endemism and speciation by using empirical, experimental and theoretical approaches. I introduce and define the concept of high elevation islands (HEI), which – in my opinion – are ideal research objects to address ecological and biogeographic questions. I intend to bridge the gap between island biogeography (i.e. by focusing on the global scale) and island ecology (i.e. by addressing the within-island scale). As a model HEI the mountainous island of La Palma (Canary Islands) is used and all field-related research was conducted there. HEIs are found in all major oceans from mid to low latitudes. Characteristic features are elevational range reaching more than 1000m a.s.l. and possessing ecosystems ranging from coastal to alpine systems. The alpine treeline, as the borderline between forest and treeless alpine systems, is therefore fundamental in describing HEIs. Interestingly, I demonstrate that globally treeline elevation on HEIs, which is generally lower on HEIs than on the mainland, is mainly driven by island elevation, and not, as expected, by latitude. Another characteristic feature of HEIs is the phenomenon described by the elevation-driven ecological isolation hypothesis, which suggests increasing speciation rates with elevation due to geographical and environmental isolation. Although island climates are generally considered to be buffered by the surrounding ocean, the reviewed literature indicates that global climate change poses a considerable threat to HEIs, especially to systems adapted to climatic stability (e.g. cloud or laurel forests) and alpine ecosystems. La Palma is a typical trade wind-dominated subtropical HEI hosting a variety of different environmental gradients, subsequent vegetation zones and a rich endemic flora. Topography and climate (including different measures of precipitation variability) express varying importance in explaining the distribution of species richness, endemic richness and endemicity (i.e. floristic uniqueness) on La Palma. Besides environmental gradients, I show that island ecological processes and patterns on La Palma are in a large measure shaped by human-mediated disturbances. Harsh environmental conditions, a high degree of endemism and the presence of several introduced herbivores, especially the European rabbit Oryctolagus cuniculus and the feral goat Capra hircus, characterize the high elevation ecosystem of La Palma. In a 11-yr exclosure experiment I am able to show that introduced herbivores have likely led to a vegetation shift in the high elevation ecosystem, which changed from a diverse shrub community to the mono-dominance of a single shrub species (Adenocarpus viscosus subsp. spartioides). In addition, introduced herbivores selectively browse rare endemics (some even on the brink of extinction) and reduce endemic seedling establishment to nearly zero, making a recuperation of the natural system impossible without substantial herbivore control measures and conservation efforts. Although fire frequencies have increased due to human interference, fire seems to have a positive effect on species richness and seedling establishment in the high elevation ecosystem. Contrary to our expectations, roads have a positive effect on endemic species on La Palma. Many rupicolous endemics profit from roadside cliffs because these cliffs function as ‘safe sites’ and protect them from introduced herbivores and fire. As a result of this dissertation several intriguing research questions have arisen in HEI science. These questions focus on within-island patterns of plant species diversity and especially endemism, the novel field of disturbance-driven island ecology, and global and macroecological patterns. HEI science is a promising research field with the potential to substantially advance our knowledge of ecology and biogeography in the future.
... Treeline varies from ca. 1500 m in the North Island to around 900 m in the far southern South Island; in the central South Island, eastern abrupt treelines are about 200 m higher than the gradual treelines on west- ern mountains. While these elevations are much lower than at similar latitudes on continents, the growing season mean temperature ranges from 6.6 to 7.8 °C ( Cieraad et al. 2014), similar to other temperate regions (Körner & Paulsen 2004). However, New Zealand treeline winters are relatively mild (minimum temperatures rarely below −10 °C), and growing sea- sons are 6-9 months long ( Benecke et al. 1981;Cieraad et al. 2012Cieraad et al. , 2014). ...
... While these elevations are much lower than at similar latitudes on continents, the growing season mean temperature ranges from 6.6 to 7.8 °C ( Cieraad et al. 2014), similar to other temperate regions (Körner & Paulsen 2004). However, New Zealand treeline winters are relatively mild (minimum temperatures rarely below −10 °C), and growing sea- sons are 6-9 months long ( Benecke et al. 1981;Cieraad et al. 2012Cieraad et al. , 2014). Local topographic features may depress treeline elevations by several hundred meters (Case & Hale 2015). ...
... latitude has been suggested as a possible cause of reduced tree growth suitability and thus lower treeline elevations on islands (K ö rner 1998, Cieraad et al. 2014). High cloudiness and high precipitation are likely to be important on islands of the equatorial tropics (e.g. ...
... Th e low phylogenetic and low ecological variability of high-elevation fl oras (K ö rner 2012) likely leads to a reduced probability of tree species suitable for high-elevation conditions. However, a recent study showed that some treelines on islands are lower due to thermal limitations as suggested by K ö rner and Paulsen (2004), and not necessarily for taxon-specifi c reasons (Cieraad et al. 2014). ...
Article
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Treeline research has strongly focused on mountain systems on the mainland. However, island treelines offer the opportunity to contribute to the global framework on treeline elevation due to their island-specific attributes such as isolation, small area, low species richness and relative youth. We hypothesize that, similar to the mainland, latitude-driven temperature variation is the most important determinant of island treeline elevation on a global scale. To test this hypothesis, we compared mainland with island treeline elevations, then we focused (1) on the global effects of latitude, (2) on the regional effects of island type (continental vs. oceanic islands) and (3) the local effects of several specific island characteristics (age, area, maximum island elevation, isolation and plant species richness). We collected a global dataset of islands (n = 86) by applying a stratified design using GoogleEarth and the Global Island Database. For each island we extracted data on latitude and local characteristics. Treeline elevation decreased from the mainland through continental to oceanic islands. Island treeline elevation followed a hump-shaped latitudinal distribution, which is fundamentally different from the mainland double-hump. Higher maximum island elevation generated higher treeline elevation and was found the best single predictor of island treeline elevation, even better than latitude. Lower island treeline elevation may be the result of a low mass elevation effect (MEE) influencing island climates and an increasingly impoverished species pool but also trade wind inversion-associated drought. The maximum island elevation effect possibly results from an increasing mass elevation effect (MEE) with increasing island elevation but also range shifts during climatic fluctuations and the summit syndrome (i.e. high wind speeds and poor soils in peak regions). Investigating islands in treeline research has enabled disentangling the global effect of latitude from regional and local effects and, at least for islands, a comprehensive quantification of the MEE.This article is protected by copyright. All rights reserved.
... Biological rates respond nonlinearly to temperature, so that at a given mean temperature, fl uctuating temperatures should lead to different mean biological rates than constant temperatures. On the other hand, mean growing season temperatures are the most consistent thermal parameter at treelines worldwide ( Körner and Paulsen, 2004 ; Cieraad et al., 2014 ; Paulsen and Körner, 2014 ) and experimental results from Hoch and Körner (2009) showed similar growth rates in two conifer species at constant and variable temperatures. Although these observations still await a physiological explanation, they suggest that mean temperature really has a biological meaning. ...
... treelines have long been regarded as unusual, occurring at higher temperatures than most northern-hemisphere treelines, which was explained by genusspecifi c limitations ( Körner and Paulsen, 2004 ; Wardle, 2008 ). However, several recent studies suggest that mean temperatures in the growing season at these treelines are actually quite comparable to those at other treelines ( Mark et al., 2008 ; Cieraad et al., 2014 ; Fajardo and Piper, 2014 ). Another argument against genus-specifi c limitations is presented in the study discussed here ( Fajardo and Piper, 2014 ), where Pinus contorta seedlings did not outperform Nothofagus pumilio at 50 m above the treeline. ...
Article
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At alpine treeline, trees give way to low-stature alpine vegetation. The main reason may be that tree canopies warm up less in the sun and experience lower average temperatures than alpine vegetation. Low growth temperatures limit tissue formation more than carbon gain, but whether this mechanism universally determines potential treeline elevations is the subject of debate. To study low-temperature limitation in two contrasting treeline tree species, Fajardo and Piper (American Journal of Botany 101: 788–795) grew potted seedlings at ground level or suspended at tree-canopy height (2 m), introducing a promising experimental method for studying the effects of alpine-vegetation and tree-canopy microclimates on tree growth. On the basis of this experiment, the authors concluded that lower temperatures at 2 m caused carbon limitation in one of the species and that treeline-forming mechanisms may thus be taxon-dependent. Here we contest that this important conclusion can be drawn based on the presented experiment, because of confounding effects of extreme root-zone temperature fluctuations and potential drought conditions. To interpret the results of this elegant experiment without logistically challenging technical modifications and to better understand how low temperature leads to treeline formation, studies on effects of fluctuating vs. stable temperatures are badly needed. Other treeline research priorities are interactions between temperature and other climatic factors and differences in microclimate between tree canopies with contrasting morphology and physiology. In spite of our criticism of this particular study, we agree that the development of a universal treeline theory should include continuing explorations of taxon-specific treeline-forming mechanisms.
... Significance level was set at p  0.05. latitude has been suggested as a possible cause of reduced tree growth suitability and thus lower treeline elevations on islands (Körner 1998, Cieraad et al. 2014. High cloudiness and high precipitation are likely to be important on islands of the equatorial tropics (e.g. ...
... The low phylogenetic and low ecological variability of high-elevation floras likely leads to a reduced probability of tree species suitable for high-elevation conditions. However, a recent study showed that some treelines on islands are lower due to thermal limitations as suggested by Körner and Paulsen (2004), and not necessarily for taxon-specific reasons (Cieraad et al. 2014). ...
... (Körner and Paulsen 2004). Treelines in the southern hemisphere are situated at overall lower elevations compared to their northern equivalent latitudes, and where treelines at around 35°N are found at approximately 3000-3500 m elevation the southern equivalent treeline (≈35°S) is located at an elevation of about 2000 m (Cieraad et al. 2014). There is, however, no clear evidence that the thermal threshold is lower in the southern hemisphere, as treelines across the world (including the Southern hemisphere) appear at around the same annual mean growing season temperature (Körner and Paulsen 2004;Cieraad et al. 2014) It is not only the average temperatures that can influence treeline position; the timing of the onset and end of the growing season is also critical (Coops et al. 2013). ...
... Treelines in the southern hemisphere are situated at overall lower elevations compared to their northern equivalent latitudes, and where treelines at around 35°N are found at approximately 3000-3500 m elevation the southern equivalent treeline (≈35°S) is located at an elevation of about 2000 m (Cieraad et al. 2014). There is, however, no clear evidence that the thermal threshold is lower in the southern hemisphere, as treelines across the world (including the Southern hemisphere) appear at around the same annual mean growing season temperature (Körner and Paulsen 2004;Cieraad et al. 2014) It is not only the average temperatures that can influence treeline position; the timing of the onset and end of the growing season is also critical (Coops et al. 2013). Several additional biotic and abiotic factors have been identified to limit the growth of trees near the treeline, including both inadequate snow protection (Esper and Schweingruber 2004) and too heavy snow loads (Autio 2006), insufficient effective temperature sum and length of the growing season (Autio 2006), lack of precipitation (Aune et al. 2011), competition (Treml and Chuman 2015), incoming solar radiation (Bader et al. 2007), permafrost and depth of the active soil layer (Wilmking et al. 2012), extreme soil temperatures (Autio 2006), moisture availability (Andersen and Baker 2006;Autio 2006), fires (Butler and Dechano 2001), frost events occurring during the growing season (Autio 2006;Coop and Givnish 2007), high wind speeds (Autio 2006;Dinca et al. 2017), and insect outbreaks (Hofgaard et al. 2013). ...
Article
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Numerous studies have reported that treelines are moving to higher elevations and higher latitudes. Most treelines are temperature limited and warmer climate expands the area in which trees are capable of growing. Hence, climate change has been assumed to be the main driver behind this treeline movement. The latest review of treeline studies was published in 2009 by Harsch et al. Since then, a plethora of papers have been published studying local treeline migration. Here we bring together this knowledge through a review of 142 treeline related publications, including 477 study locations. We summarize the information known about factors limiting tree-growth at and near treelines. Treeline migration is not only dependent on favorable growing conditions but also requires seedling establishment and survival above the current treeline. These conditions appear to have become favorable at many locations, particularly so in recent years. The review revealed that at 66% of these treeline sites forest cover had increased in elevational or latitudinal extent. The physical form of treelines influences how likely they are to migrate and can be used as an indicator when predicting future treeline movements. Our analysis also revealed that while a greater percentage of elevational treelines are moving, the latitudinal treelines are capable of moving at greater horizontal speed. This can potentially have substantial impacts on ecosystem carbon storage. To conclude the review, we present the three main hypotheses as to whether ecosystem carbon budgets will be reduced, increased or remain the same due to treeline migration. While the answer still remains under debate, we believe that all three hypotheses are likely to apply depending on the encroached ecosystem. Concerningly, evidence is emerging on how treeline migration may turn tundra landscapes from net sinks to net sources of carbon dioxide in the future.
... Thanks to the availability of low-cost temperature loggers since the mid 1990s, the understanding of treeline temperature regimes and how tree growth near treeline responds to temperature has improved substantially (e.g. Körner and Paulsen 2004;Gehrig-Fasel et al. 2008;Cieraad et al. 2014;Noroozi and Körner 2018;Choler 2018). Among temperature characteristics affecting tree growth, air temperatures directly influence apical and cambial meristems, and thus, affect wood formation (Gričar et al. 2006;Rossi et al. 2007;Kašpar et al. 2018). ...
... The variance in growing season duration was intermediate, being a result of variance in the onset and termination of the growing season. The least variable metric was mean growing season temperature, which has also been commonly used as treeline temperature metric (Körner 2012;Cieraad et al. 2014;Kašpar and Treml 2016). Mean values averaged over a longer seasonal period might decrease the effects of a single extremely cold or warm month. ...
Article
Treeline isotherms are used in comparative and modelling studies to predict treeline positions. However, how representative local short-term temperature records are for a given region remains poorly understood. Furthermore, the predictive value of on-site temperatures for explaining tree growth requires further validation. Here we present temperature records and tree growth datasets from treeline ecotone sites differing in elevation and slope direction in the High Sudetes (Czechia and Poland). The goal was to determine the spatial and temporal variability of soil and air temperatures and to describe the relationship of various temperature metrics with tree growth. Our results demonstrate that, because of spatial and temporal variability, major temperature metrics used in comparative studies should be provided with an uncertainty range between 0.6 and 0.8 K for seasonal mean soil and air temperature. While soil temperatures exhibit high spatial variability, air temperatures vary more with time. Elevation is the most important driver of temperature patterns in treeline ecotones. Differences related to slope direction were important mainly for soil temperatures in lower parts of treeline ecotones. Tree growth is tightly related to June–September air temperature, with a modulating role of the onset date of soil temperature-defined growing season. In this study, we describe patterns of temperature variation in the treeline ecotones of two mountain ranges and demonstrate the extremely strong dependence of tree stem growth on air temperature, with very limited remaining space for other potentially limiting factors.
... latitude has been suggested as a possible cause of reduced tree growth suitability and thus lower treeline elevations on islands (K ö rner 1998, Cieraad et al. 2014). High cloudiness and high precipitation are likely to be important on islands of the equatorial tropics (e.g. ...
... Th e low phylogenetic and low ecological variability of high-elevation fl oras (K ö rner 2012) likely leads to a reduced probability of tree species suitable for high-elevation conditions. However, a recent study showed that some treelines on islands are lower due to thermal limitations as suggested by K ö rner and Paulsen (2004), and not necessarily for taxon-specifi c reasons (Cieraad et al. 2014). ...
... Through the analysis of a high-resolution map, we assume that our quantile regression was mainly based on the few remaining forest patches at the climatic treeline. Indeed, the resulting pattern of global treeline elevation closely resembles previous observations derived by field measurements , showing well known patterns like the higher elevation of southern hemisphere treelines, when compared to the northern at the same distance from the equator (Cieraad et al. 2014, Karger et al. 2019. It also shows the general decreasing trend in treeline elevation close to the equator already reported by , despite some unexpectedly high afroalpine treelines (e.g. ...
Article
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Although there is a general consensus on the distribution and ecological features of terrestrial biomes, the allocation of alpine ecosystems in the global biogeographic system is still unclear. Here, we delineate a global map of alpine areas above the treeline by modelling regional treeline elevation at 30 m resolution, using global forest cover data and quantile regression. We then used global datasets to 1) assess the climatic characteristics of alpine ecosystems using principal component analysis, 2) define bioclimatic groups by an optimized cluster analysis and 3) evaluate patterns of primary productivity based on the normalized difference vegetation index. As defined here, alpine biomes cover 3.56 Mkm2 or 2.64% of land outside Antarctica. Despite temperature differences across latitude, these ecosystems converge below a sharp threshold of 5.9°C and towards the colder end of the global climatic space. Below that temperature threshold, alpine ecosystems are influenced by a latitudinal gradient of mean annual temperature and they are climatically differentiated by seasonality and continentality. This gradient delineates a climatic envelope of global alpine biomes around temperate, boreal and tundra biomes as defined in Whittaker's scheme. Although alpine biomes are similarly dominated by poorly vegetated areas, world ecoregions show strong differences in the productivity of their alpine belt irrespectively of major climate zones. These results suggest that vegetation structure and function of alpine ecosystems are driven by regional and local contingencies in addition to macroclimatic factors.
... For example, lower temperature values and more days had to be included in calculations under a lower threshold, leading to a lower seasonal mean of air and soil temperatures. Cieraad et al. (2014) reported that brief excursions of temperature below the threshold included in the calculation may influence seasonal means by up to 1°C at the oceanic New Zealand tree line. Those results remind us that it is not appropriate to directly compare indicators calculated using different thermal growing season definitions with global results Tang and Fang 2006). ...
Article
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Fully and accurately studying temperature variations in montane areas is conducive to a better understanding of climate modeling and climate-growth relationships on regional scales. To explore the spatio-temporal changes in air and soil temperatures and their relationship in montane areas, on-site monitoring over two years (2015 and 2016) was conducted at nine different elevations from 2040 m to 2740 m a.s.l. on Luya Mountain in the semiarid region of China. The results showed that the annual mean of air temperature lapse rate (ATLR) was 0.67C/100m. ATLR varied obviously in different months within a range of 0.57 ~ 0.79C/100m. The annual mean of the soil temperature lapse rate (STLR) was 0.48 C/100m. Seasonally, monthly mean soil temperature did not show a consistent pattern with regard to elevation. The relationships between air and soil temperatures showed piecewise changes. Soil was decoupled from the air temperature in cold winter and early spring. The parameters of the growing season based on the two temperature types had no corresponding relations, and seasonal mean of soil temperature showed the smallest value at mid-elevation rather than in the treeline ecotone. Based on these changes, our results emphasized that altitudinal and seasonal variability caused by local factors (such as snow cover and soil moisture) should be taken into full consideration in microclimate extrapolation and treeline prediction in montane areas, especially in relation to soil temperature.
... Oceanic tree lines, such as those that characterize New Zealand and southern South America, are among those that have failed to respond to recent warming. Why this may be so, and under what circumstances these tree lines may change, is, therefore, of great interest [4]. The sub-Antarctic islands lying several hundred kilometers to the south of New Zealand offer a unique opportunity to explore these questions. ...
Article
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Campbell Island, which is 600 km south of New Zealand, has the southernmost tree line in this ocean sector. Directly under the maximum of the westerlies, the island is sensitive to changes in wind strength and direction. Pollen records from three peat cores spanning the tree line ecotone provide a 17,000-year history of vegetation change, temperature, and site moisture. With postglacial warming, tundra was replaced by tussock grassland 12,500 years ago. A subsequent increase of shrubland was reversed at 10,500 years ago and wetland-grassland communities became dominant. Around 9000 years ago, trees spread, with maximum tree line elevation reached around 6500 to 3000 years ago. This sequence is out of step with Southern Ocean sea surface temperatures, which were warmer than 12,500 to 9000 years ago, and, subsequently, cooled. Campbell Island tree lines were decoupled from temperature trends in the adjacent ocean by weaker westerlies from 12,500 to 9000 years ago, which leads to the intrusion of warmer, cloudier northern airmasses. This reduced solar radiation and evapotranspiration while increasing atmospheric humidity and substrate wetness, which suppressed tree growth. Cooler, stronger westerlies in the Holocene brought clearer skies, drier air, increased evapotranspiration, and rising tree lines. Future global warming will not necessarily lead to rising tree lines in oceanic regions.
... Treelines in New Zealand occur within similar thermal ranges as in other sites worldwide (although at lower elevations; Cieraad et al., 2014). The high altitude tree limit in New Zealand's alpine regions is dominated by two species of evergreen Nothofagus trees ranging from $1000 m in the southern part of the Southern Alps to $1400 m on the North Island (Wardle, 2008). ...
Chapter
Alpine ecosystems in the Pacific Islands are isolated and unique, characterized by high levels of endemism. Only Hawai‘i and New Zealand have elevations high enough to contain substantial alpine climates, and about 11% of the land area of both island groups is located above treeline. Both of these volcanically active archipelagos are characterized by complex topography, with peaks over 3700 m. These alpine ecosystems have significant cultural, social, and economic value; however, they are threatened by invasion of exotic species, climate change, and human impacts. Nonnative ungulates reduce native shrubland and grassland cover, and threaten populations of endangered birds. Exotic plants alter water yields and increase fire risk, and increased recreational visitation to these remote areas facilitates the introduction of exotic plant seeds, pests, and pathogens. Both New Zealand and Hawai‘i have experienced strong warming at higher elevations, and future projections indicate that these robust warming trends will continue. Glacial retreat has been noted in the Southern Alps, with 34% ice volume lost since 1977, and New Zealand may lose 88% of its ice volume by 2100. Snowfall on Hawai‘i's mountain peaks is projected to almost entirely disappear by 2100. Changes are occurring rapidly, and additional monitoring and research are needed to conserve these uniquely sensitive, remote regions.
... Supporting evidence includes the relationships between treeline position and temperature isotherms (Grace et al. 2002;Körner and Paulsen 2004), fluctuations in treeline position due to temperature changes (Mayor et al. 2017;Kattge and Knorr 2007), and historical treeline limits consistent with observed rates of global warming (Peñuelas and Boada 2003;Peñuelas et al. 2007;Mayor et al. 2017). From a global perspective, prior studies have focused on the effect of low temperatures on the acquisition or use of carbon (C) by plants (Cieraad et al. 2014;Fajardo and Piper 2014) and the effect of nutrient limitation on the formation of treelines (Hertel & Wesche 2008;Fajardo and Piper 2017;Mayor et al. 2017). ...
Article
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Alpine treelines are thought to be controlled by low temperature, which affects tree physiology and limits growth. Irrespective of carbon and nutrient limitations are physiological mechanisms affecting the formation of alpine treelines still needs to be defined. We measured the rates of tree growth (basal area increment, BAI), nutrient concentrations in leaves and roots, foliar concentration of nonstructural carbohydrates (NSCs), and gas exchange in Pinus taiwanensis at five elevations (1200, 1400, 1600, 1800, and 2000 m) in the Wuyi Mountains, China. Leaves and roots were sampled twice (summer and winter). The soil nitrogen (N) and phosphorus (P) concentrations and BAI were measured during the summer. We analyzed the foliar traits in summer and winter. The N:P ratio was also analyzed. BAI decreased significantly as elevation increased, accompanied by increases in foliar NSC, N, and P concentrations in both summer and winter. The root P concentration increased with elevation in summer, but the foliar N:P ratio and root N and P concentrations were not affected by elevation in winter. Foliar photosynthesis and respiration did not change in winter, but increased in summer as elevation increased. These results suggest that C and nutrients may not be limiting resources in P. taiwanensis at this alpine treeline site, which instead may be controlled by temperature. P. taiwanensis at alpine treelines accumulates C and nutrient to increase its rates of biochemical reactions at low temperatures.
... Iconic examples of this are the tree limits formed by Nothofagus in southern Chile, which are located at lower elevations than those at comparable latitudes in the Northern Hemisphere (Cieraad, McGlone, & Huntley, 2014;Hertel, Therburg, & Villalba, 2008). ...
Article
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Aim To determine the global position of tree line isotherms, compare it with observed local tree limits on islands and mainlands, and disentangle the potential drivers of a difference between tree line and local tree limit. Location Global. Time period 1979–2013. Major taxa studied Trees. Methods We modelled the potential climatic tree line based on monthly temperatures and precipitation for the period 1979–2013. We then compared the potential tree line based on climate to observed tree limits at 26 oceanic islands, 55 continental islands and 382 mainland locations. The differences between potential tree line and observed tree limits was then analysed by regression with the islands’ maximum elevation, age, isolation, and area. Additionally, we estimated growing season temperature niches for 16,041 species known to occur in the vicinity of the studied tree lines, and compared them across mainlands, and islands of continental and oceanic origin. Results Observed local tree limits differ up to 2,066 m from the potential tree line at the mainland on oceanic islands. Climatic effects are responsible for a difference of up to 1,296 m between tree lines of mainland regions and oceanic islands (but only for 756 m for continental islands). On oceanic islands, a remaining difference of up to 829 m correlates with the isolation and the maximum elevation of an island. Floras of oceanic islands are however depauperate with respect to potential tree line species and species show an affinity to higher growing season temperatures. Main conclusions Climate can explain about half of the differences between observed local tree limits and potential tree lines between the mainland and continental and oceanic islands. The remaining difference can be attributed to the higher isolation of oceanic islands, especially in the tropics, and as a consequence, a more depauperate flora and a lack of tree species that are able to grow at the tree line.
... , and it appears that the tree line in our site has been lower than the limit determined thermally (Daniels & Veblen 2004; Cieraad, McGlone & Huntley 2014). Even if abiotic conditions became otherwise increasingly suitable for regeneration, survival and growth, the influence of reindeer damaging seedlings and saplings could thus form a bottleneck that prevents recruitment of new P. sylvestris individuals. ...
Article
1.The elevational limit of trees (henceforth, the ‘tree line’) is widely considered to be a sensitive indicator of environmental change. Here, we document the 20th century tree line advance and increase in the tree population at the tree line ecotone, along a Pinus sylvestris-dominated slope in northeastern Finland, in conditions where growth and recruitment have generally been linked to temperature variation.2.Using tree recruitment ages (growth to 1.3 m height) along an elevational transect, we compared recruitment to variation in environmental conditions associated with tree line dynamics, seed and cone crops, and reindeer densities. We further investigated the relationships among temperature and tree-level growth variables.3.Results show the existence of a former tree line at approximately 400 m a.s.l., and an advance of trees that began in the1920s and reached the top of the fell (at 470 m a.s.l.) in the 1980s. During this time, the population density of P. sylvestris increased from 4 to 468 trees within the 5.6 ha study plot. Needle and shoot lengths were positively related to air temperature but recruitment was not.4.Average rate of P. sylvestris advance over the 20th century was consistent with earlier studies, but the temporal patterns of both upslope advance and population density increase were unexpected: both were characterised by a strongly stepwise pattern, culminating in a rapid advance and density increase in the 1970-80s, and returning to low levels in the 1990s.5.Despite a positive relationship between growth and temperature variables at the tree level, climatic variables, or seed and cone crops were inconsistent with recruitment patterns. For most part of the 20th century the increase corresponded to the gradual atmospheric CO2 increase, but of the variables screened only the changes in reindeer densities coincided with the stepwise pattern.6.Synthesis. Our results confirmed the connection between tree-level processes and temperature variability as expected from earlier studies. However, recruitment was not correlated with any of the environmental variables. Our findings point towards complex tree line dynamics, in which biotic agents may play a major role in mediating tree line response to environmental change.This article is protected by copyright. All rights reserved.
... For instance, in Mediterranean or some temperate biomes, drought, in addition to low temperatures, constrains tree growth and determines the treeline elevation [3][4][5]. In addition, based on new data from New Zealand and Chile, it was found that southern temperate treelines are driven by similar thermal thresholds as are northern treelines, thus refuting the postulated taxon-specific limitation hypothesis and confirming that southern treelines are not climatically depressed [6]. Therefore, more reliable climate data must be recorded in situ to determine the treeline thermal thresholds at regional and local scales. ...
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Little is known about the relationships between treeline elevation and climate at regional and local scales. It is compelling to fill this research gap with data from the Tibetan Plateau where some of the highest alpine treelines in the world are found. This research question partially results from the lack of in situ temperature data at treeline sites. Herein, treeline variables (e.g., elevation, topography, tree species) and temperature data were collected from published investigations performed during this decade on the Tibetan Plateau. Temperature conditions near treeline sites were estimated using global databases and these estimates were corrected by using in situ air temperature measurements. Correlation analyses and generalized linear models were used to evaluate the effects of different variables on treeline elevation including thermal (growing-season air temperatures) and non-thermal (latitude, longitude, elevation, tree species, precipitation, radiation) factors. The commonality analysis model was applied to explore how several variables (July mean temperature, elevation of mountain peak, latitude) were related to treeline elevation. July mean temperature was the most significant predictor of treeline elevation, explaining 55% of the variance in treeline elevation across the Tibetan Plateau, whereas latitude, tree species, and mountain elevation (mass-elevation effect) explained 30% of the variance in treeline elevation. After considering the multicollinearity among predictors, July mean temperature (largely due to the influence of minimum temperature) still showed the strongest association with treeline elevation. We conclude that the coupling of treeline elevation and July temperature at a regional scale is modulated by non-thermal factors probably acting at local scales. Our results contribute towards explaining the decoupling between climate warming and treeline dynamics.
... Because there was some variation in how close we could position plots to the desired elevation owing to topographic constraints, elevation relative to treeline was best represented as a continuous explanatory variable. Centring tran- sects on the treeline permitted direct comparisons of the same temperature ranges across regions, because treelines form where growing seasons greater than 3 months have average air temperatures between approximately 6.6-7 °C in both Northern and Southern Hemispheres 10,32 . Evidence for a remarkably similar average treeline temperature was derived from temperature sensors deployed across about 50 sites (including in several of the regions and four of the sites included in our study) for a period of 1-3 years (ref. ...
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SharedIt link: hFp://rdcu.be/oM9F for full text Temperature is a primary driver of the distribution of biodiversity as well as of ecosystem boundaries. Declining temperature with increasing elevation in montane systems has long been recognized as a major factor shaping plant community biodiversity, metabolic processes, and ecosystem dynamics. Elevational gradients, as thermoclines, also enable prediction of long-term ecological responses to climate warming. One of the most striking manifestations of increasing elevation is the abrupt transitions from forest to treeless alpine tundra. However, whether there are globally consistent above- and belowground responses to these transitions remains an open question. To disentangle the direct and indirect effects of temperature on ecosystem properties, here we evaluate replicate treeline ecotones in seven temperate regions of the world. We find that declining temperatures with increasing elevation did not affect tree leaf nutrient concentrations, but did reduce ground-layer community-weighted plant nitrogen, leading to the strong stoichiometric convergence of ground-layer plant community nitrogen to phosphorus ratios across all regions. Further, elevation-driven changes in plant nutrients were associated with changes in soil organic matter content and quality (carbon to nitrogen ratios) and microbial properties. Combined, our identification of direct and indirect temperature controls over plant communities and soil properties in seven contrasting regions suggests that future warming may disrupt the functional properties of montane ecosystems, particularly where plant community reorganization outpaces treeline advance.
... The elevation (hereafter called "altitude" to be consistent with the literature on alpine ponds) at which the transition from trees to herbaceous and shrubby, tundra-like vegetation varies considerably around the world depending on latitude, climate, soils, tree taxonomy, and northsouth aspect (Körner 2012 ). In general, tree line occurs at a lower altitude on southtemperate than north-temperate mountains at a given latitude (Körner 1998 ;Cieraad et al. 2014 ). ...
Chapter
Alpine ponds are small standing water bodies situated in mountainous regions at or above tree line. Hydrology is driven by snow and ice with harsh conditions comparable to that in shallow water bodies at high latitudes. Invertebrate communities are less diverse than at low altitudes and often dominated by “cold stenotherms” with arctic/boreal-alpine distributions. The unique assemblages in alpine ponds (many regional endemics) are of special conservation value. Species composition and diversity vary among basins of different size, substrate types, and permanence. Clusters of alpine ponds are excellent habitats for studying metacommunity dynamics and patterns of regional diversity. Alpine ponds are sentinel systems for, and especially vulnerable to, the effects of regional (e.g., acid precipitation) and global (climate change) human impacts.
... The elevation (hereafter called "altitude" to be consistent with the literature on alpine ponds) at which the transition from trees to herbaceous and shrubby, tundra-like vegetation varies considerably around the world depending on latitude, climate, soils, tree taxonomy, and northsouth aspect (Körner 2012 ). In general, tree line occurs at a lower altitude on southtemperate than north-temperate mountains at a given latitude (Körner 1998 ;Cieraad et al. 2014 ). ...
... The highest point is Mt Cook (Aoraki; 43°25ʹS, 170°08ʹE) which in 2014 reached 3724 m elevation. The treeline elevation ranges from about 1500 m in the north to 900 m in the far south (Cieraad et al. 2014). The permanent snowline similarly decreases north to south, being about 2400 m in the north and 2000 m in the south (Mark and Adams 1995). ...
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We produced the first national-scale quantitative classification of non-forest vegetation types, including shrubland, based on vegetation plot data from the National Vegetation Survey Databank. Semisupervised clustering with the fuzzy classification algorithm Noise Clustering was used to incorporate these new data into a pre-existing quantitative classification of New Zealand’s woody vegetation. Fuzzy classification allows plots to be designated as transitional when they are similar to multiple vegetation types; the Noise Clustering algorithm allows plots having unique composition to be designated as outliers. We combined plot data collected using two different methods by transforming abundances to relative ranks and showed our classification results were robust to this. Of the 6362 plots analysed, 505 were assigned to previously defined woody vegetation types. Using the remaining 5857 plots, we defined vegetation types at two hierarchical levels comprising 25 alliances and 56 associations. Ten of the alliances are tussocklands, six are grasslands, four are stonefields or gravelfields, two are herbfields, one is rushland, and two are newly defined woody alliances. The classification defined compositional differences among well-known widely distributed short and tall tussock grasslands of the South Island. Notably it distinguished Chionochloa pallens, C. crassiuscula and C. oreophila tussocklands in wetter western regions from those dominated by C. rigida and C. macra in the east, and the domination of eastern South Island short tussock grasslands by Festuca novae-zelandiae and Poa colensoi. We demonstrate the distinctiveness of the vegetation of four naturally uncommon ecosystems – coastal turfs, northern gumlands, granite sand plains and braided riverbeds. Insufficient plot data precluded the definition of North Island Chionochloa rubra grassland types and many wetland and coastal communities. The 1846 plots designated as outliers mainly occur on warmer, wetter and less invaded sites than classified plots. Semi-supervised clustering allowed us to progress the development of an extendable, plot-based, quantitative classification of all New Zealand’s vegetation despite data gaps.
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The New Zealand conifers (20 species of trees and shrubs in the Araucariaceae, Podocarpaceae, and Cupressaceae) are often regarded as ancient Gondwanan elements, but mostly originated much later. Often thought of as tall trees of humid, warm forests, they are present throughout in alpine shrublands, tree lines, bogs, swamps, and in dry, frost-prone regions. The tall conifers rarely form purely coniferous forest and mostly occur as an emergent stratum above evergreen angiosperm trees. During Maori settlement in the thirteenth century, fire-sensitive trees succumbed rapidly, most of the drier forests being lost. As these were also the more conifer-rich forests, ecological research has been skewed toward conifer dynamics of forests wetter and cooler than the pre-human norm. Conifers are well represented in the pollen record and we here we review their late Quaternary history in the light of what is known about their current ecology with the intention of countering this bias. During glacial episodes, all trees were scarce south of c. 40° S, and extensive conifer-dominant forest was confined to the northern third of the North Island. Drought- and cold-resistant Halocarpus bidwillii and Phyllocladus alpinus formed widespread scrub in the south. During the deglacial, beginning 18,000 years ago, tall conifers underwent explosive spread to dominate the forest biomass throughout. Conifer dominance lessened in favor of angiosperms in the wetter western lowland forests over the Holocene but the dryland eastern forests persisted largely unchanged until settlement. Mid to late Holocene climate change favored the more rapidly growing Nothofagaceae which replaced the previous conifer-angiosperm low forest or shrubland in tree line ecotones and montane areas. The key to this dynamic conifer history appears to be their bimodal ability to withstand stress, and dominate on poor soils and in cool, dry regions but, in wetter, warmer locations, to slowly grow thorough competing broadleaves to occupy an exposed, emergent stratum where their inherent stress resistance ensures little effective angiosperm competition.
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The alpine treeline ecotone is defined as a forest-grassland or forest-tundra transition boundary either between subalpine forest and treeless grassland, or between subalpine forest and treeless tundra. The alpine treeline ecotone serves irreplaceable ecological functions and provides various ecosystem services. There are three lines associated with the alpine treeline ecotone, the tree species line (i.e., the highest elevational limit of individual tree establishment and growth), the treeline (i.e., the transition line between tree islands and isolated individual trees) and the timber line (i.e., the upper boundary of the closed subalpine forest). The alpine treeline ecotone is the belt region between the tree species line and the timber line of the closed forest. The treeline is very sensitive to climate change and is often used as an indicator for the response of vegetation to global warming. However, there is currently no comprehensive review in the field of alpine treeline advance under global warming. Therefore, this review summarizes the literature and discusses the theoretical bases and challenges in the study of alpine treeline dynamics from the following four aspects: (1) Ecological functions and issues of treeline dynamics; (2) Methodology for monitoring treeline dynamics; (3) Treeline shifts in different climate zones; (4) Driving factors for treeline upward shifting.
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The growth limitation hypothesis (GLH) is the most accepted mechanistic explanation for treeline formation, although it is still uncertain whether it applies across taxa. The successful establishment of Pinus contorta--an exotic conifer species in the southern hemisphere--above the Nothofagus treeline in New Zealand may suggest a different mechanism. We tested the GLH in Nothofagus pumilio and Pinus contorta by comparing seedling performance and carbon (C) balance in response to low temperatures. At a southern Chilean treeline, we grew seedlings of both species 2 m above ground level, to simulate coupling between temperatures at the meristem and in the air (colder), and at ground level, i.e., decoupling air temperature (relatively milder). We recorded soil and air temperatures as well. After 3 yr, we measured seedling survival and biomass (as a surrogate of growth) and determined nonstructural carbohydrates (NSC). NOTHOFAGUS and Pinus did not differ in survival, which, as a whole, was higher at ground level than at the 2-m height. The root-zone temperature for the growing season was 6.6°C. While biomass and NSC decreased significantly for Nothofagus at the 2-m height compared with ground level (C limitation), these trends were not significant for Pinus. The treeline for Nothofagus pumilio is located at an isotherm that fully matches global patterns; however, its physiological responses to low temperatures differed from those of other treeline species. Support for C limitation in N. pumilio but not in P. contorta indicates that the physiological mechanism explaining their survival and growth at treeline may be taxon-dependent.
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In New Zealand, there are treelines of two main forms: abrupt southern beech treelines and gradual conifer-broadleaved treelines. At similar latitudes, abrupt treelines form at higher elevation than gradual treelines, but it is unclear whether this difference is also reflected in the climatic conditions experienced at the contrasting treeline ecotones. In this study, we measured soil and air temperatures across four gradual and two abrupt treelines ecotones in New Zealand for 2 years, and compared the climatic conditions between the treeline forms. Although gradual treelines form at lower elevations, they experience similar summer temperatures as the higher abrupt treelines. In contrast, temperatures in the shoulder season and during winter differed between sites of contrasting treeline forms. Soil scarcely froze and air temperature did not fall below -6°C at the gradual treeline sites, whereas freezing soils and snow were more common (extreme air frosts down to -9°C) at the abrupt treeline sites. Air and soil temperatures mirror the change in tree stature in the ecotone: with increasing altitude through the gradual treeline ecotone, temperature decreased gradually; whereas abrupt temperature changes were found at the abrupt treeline-grassland interface. These altitudinal patterns provide insights into potential mechanisms that drive treeline form and position, and their response to climatic change.
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The concept of mass elevation effect (massenerhebungseffect, MEE) was introduced by A. de Quervain about 100 years ago to account for the observed tendency for temperature-related parameters such as tree line and snowline to occur at higher elevations in the central Alps than on their outer margins. It also has been widely observed in other areas of the world, but there have not been significant, let alone quantitative, researches on this phenomenon. Especially, it has been usually completely neglected in developing fitting models of timberline elevation, with only longitude or latitude considered as impacting factors. This paper tries to quantify the contribution of MEE to timberline elevation. Considering that the more extensive the land mass and especially the higher the mountain base in the interior of land mass, the greater the mass elevation effect, this paper takes mountain base elevation (MBE) as the magnitude of MEE. We collect 157 data points of timberline elevation, and use their latitude, longitude and MBE as independent variables to build a multiple linear regression equation for timberline elevation in the southeastern Eurasian continent. The results turn out that the contribution of latitude, longitude and MBE to timberline altitude reach 25.11%, 29.43%, and 45.46%, respectively. North of northern latitude 32°, the three factors’ contribution amount to 48.50%, 24.04%, and 27.46%, respectively; to the south, their contribution is 13.01%, 48.33%, and 38.66%, respectively. This means that MBE, serving as a proxy indicator of MEE, is a significant factor determining the elevation of alpine timberline. Compared with other factors, it is more stable and independent in affecting timberline elevation. Of course, the magnitude of the actual MEE is certainly determined by other factors, including mountain area and height, the distance to the edge of a land mass, the structures of the mountains nearby. These factors need to be included in the study of MEE quantification in the future. This paper could help build up a high-accuracy and multi-scale elevation model for alpine timberline and even other altitudinal belts.
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Shoot and diameter growth of southern rata (Metrosideros umbellata Cav.) were measured at four south-aspect sites (1982–1983 and 1983–1984) covering the altitudinal range of the species in Camp Creek, and at two higher altitude north-aspect sites (1981–1982 to 1983–1984). Shoot growth in canopy trees at all sites was limited to the expansion of leaf primordia present in the overwintering bud. Where vigorous seedlings retained an apical meristem over winter, shoots grew continuously throughout the next growing season. Rata seedlings began growth earlier, grew more rapidly, and completed extension growth ahead of canopy foliage in mature trees. Shoot extension followed the pattern of prevailing temperature, with bud growth starting when mean weekly temperatures reached 5–6°C. The start of growth in spring was delayed by 3.5 to 4.0 days for each 100 m increase in altitude. Cooler temperatures in spring 1982 delayed bud break at high altitude sites by about 4 weeks, but had little effect at the lowest site. Shoots elongated rapidly after bud break and completed growth before temperatures became limiting for growth. Growth of new leaves continued well into autumn and, at high altitude sites, was subject to damage from early winter frosts when bud break was delayed. Diameter growth rates (≤2.0 mm per annum) decreased with altitude, with some trees at high altitude sites making no measurable growth (< 0.1 mm) over two growing seasons.
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The development and maintenance of several types of visually striking vegetation patterns are controlled by positive feedback between pattern and process. These patterns are particularly common at ecotones, where the influence of positive feedback may affect the position and dynamics of the boundary between the adjacent biotic communities. In this study, I use dendrochronology to examine the role of feedback between existing trees and the establishment and survival of seedlings in the advancement of linear, finger-like strips of subalpine forest in Glacier National Park, Montana. A general upslope, windward to leeward pattern of older trees followed by progressively younger trees was evident in all sample transects, although in some cases this pattern repeated several times along the length of a transect, with each repetition originating leeward of boulders. Overall advancement rates varied from 0.28 to 0.62 m yr(-1). The oldest trees established in the early to mid-1700s, but establishment and advancement increased rapidly after 1850, and peaked in the early 1900s. In addition, almost all seedlings established within 5 m downwind of existing trees between 1700 and 1850, while establishment beyond this distance was common after 1850. These patterns suggest that existing trees facilitate leeward seedling establishment and survival, by depositing wind-blown snow. These seedlings in turn modify their leeward environment, thus allowing forest advancement in a linear pattern. Feedback was critical for the survival of seedlings before 1800, and strongly controlled advancement between about 1800 and 1850, but appears to have had little effect on establishment patterns since that time. The importance of feedback between pattern and process may change over time and space as a result of changes in climatic conditions or biotic surroundings.
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Background and Aims The most plausible explanation for treeline formation so far is provided by the growth limitation hypothesis (GLH), which proposes that carbon sinks are more restricted by low temperatures than by carbon sources. Evidence supporting the GLH has been strong in evergreen, but less and weaker in deciduous treeline species. Here a test is made of the GLH in deciduous–evergreen mixed species forests across elevational gradients, with the hypothesis that deciduous treeline species show a different carbon storage trend from that shown by evergreen species across elevations.
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Understanding the evolutionary history and biogeography of the New Zealand alpine flora has been impeded by the lack of an integrated model of geomorphology and climate events during the Late Miocene, Pliocene and Pleistocene. A new geobiological model is presented that integrates rock uplift age, rate of uplift and the resulting summit elevations in the Southern Alps (South Island) during the last 8.0 million years with a climate template using the natural gamma radiation pattern from the eastern South Island Ocean Drilling Program Site 1119 that covers the past 3.9 million years. This model specifically defines the average treeline in relation to mountain height, allowing predictions as to the timing of the formation of the alpine zone and other open habitats. This model predicts open habitats such as rock bluffs, tussock grasslands and riverbeds would have been available from about 4.0-3.0 Ma, coinciding with the initiation of summit uplift and a cooling climate providing an opportunity for the evolution of generalist alpine and open-habitat herbs and shrubs. Alpine habitats began to form at about 1.9 Ma and were a permanent feature of the Southern Alps from about 0.95 Ma. Specialist alpine plants confined to alpine habitats can have evolved only within this period once the alpine zone was persistent and widespread. Bog habitats are likely to date from the Late Miocene (c. 6.0 Ma), and the specialist bog species would have evolved from this time. Molecular-clock dates for DNA sequences from species of specialist alpine habitats, generalist open habitats, and bog habitats are consistent with predictions made on the basis of the model.
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The altitudinal zonation patterns of vegetation structure, vascular flora, and life/growth forms were comprehensively assessed in relation to temperature and soil factors from treeline (1040 m) to the high‐alpine summit of Mt Burns (1645 m) in southeastern Fiord Ecological Region. We tested Körner's hypothesis which stipulates that the physiognomic zonation pattern: treeline, shrub line, tussockline, and beyond, is driven mainly by increased decoupling between the ambient temperature and that experienced directly by plants in relation to proximity of their canopy to the ground. This hypothesis is generally supported, particularly with replacement of the tussock life form by dwarfed, mostly cushion species, at the low‐ to high‐alpine zone transition. The soil pattern appears to be more of a response to, rather than a driver of, the alpine vegetation pattern, including a localised area of frost‐active solifluc‐tion terraces. The Nothofagus menziesii treeline conformed to the “warmest month” model and also with a worldwide growing season mean (7.15°C) of 5.5–7.5°C. We stress the closer analogy in the overall alpine zonation pattern in this region of oceanic New Zealand to that of the tropical high mountains and other oceanic regions, than with the temperate Northern Hemisphere continental mountains.
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The altitudinal zonation patterns of vegetation structure, vascular flora, and life/growth forms were comprehensively assessed in relation to temperature and soil factors from treeline (1040 m) to the high-alpine summit of Mt Burns (1645 m) in southeastern Fiord Ecological Region. We tested Körner's hypothesis which stipulates that the physiognomic zonation pattern: treeline, shrub line, tussockline, and beyond, is driven mainly by increased decoupling between the ambient temperature and that experienced directly by plants in relation to proximity of their canopy to the ground. This hypothesis is generally supported, particularly with replacement of the tussock life form by dwarfed, mostly cushion species, at the low- to high-alpine zone transition. The soil pattern appears to be more of a response to, rather than a driver of, the alpine vegetation pattern, including a localised area of frost-active solifluction terraces. The Nothofagus menziesii treeline conformed to the "warmest month" model and also with a worldwide growing season mean (7.15°C) of 5.5-7.5°C. We stress the closer analogy in the overall alpine zonation pattern in this region of oceanic New Zealand to that of the tropical high mountains and other oceanic regions, than with the temperate Northern Hemisphere continental mountains.
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Results are presented from a present-day and a doubled CO{sub 2} experiment over the Alpine region with a nested regional climate model. The simulated temperature change signal shows a substantial elevation dependency, mostly during the winter and spring seasons, resulting in more pronounced warming at high elevations than low elevations. This is caused by a depletion of snowpack in doubled CO{sub 2} conditions and further enhanced by the snow-albedo feedback. This result is consistent with some observed temperature trends for anomalously warm years over the Alpine region and suggests that high elevation temperature changes could be used as an early detection tool for global warming. Changes in precipitation, as well as other components of the surface energy and water budgets, also show an elevation signal, which may have important implications for impact assessments in high elevation regions. 22 refs., 10 figs., 2 tabs.
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Many tropical alpine treelines lie below their climatic potential, because of natural or anthropogenic causes. Forest extension above the treeline depends on the ability of trees to establish in the alpine environment. This ability may be limited by different factors, such as low temperatures, excess solar radiation, competition, soil properties, dispersal ability, and fires. In this paper we address the following two questions: Do trees regenerate above the present treeline, and what are the inhibiting factors for tree establishment? To answer these questions we described the spatial pattern of recent tree establishment below and above the present treeline in northern Ecuador. Also, we experimentally transplanted seedlings into the alpine vegetation (páramo) and the forest, and investigated the effect of shade, neighboring plants, and substrate on their survival. The number of naturally occurring tree sprouts (seedlings, saplings and ramets) was highest just outside the forest, and decreased with distance to the forest edge. However, only two species that were radiation-tolerant made up these high numbers, while other species were rare or absent in the páramo. In the forest, the species diversity of sprouts was high and the abundance per species was relatively low. The transplanted seedlings survived least in experimental plots without artificial shade where neighboring plants were removed. Seedling survival was highest in artificially shaded plots and in the forest. This shade-dependence of most tree species can strongly slow down forest expansion toward the potential climatic treeline. Due to the presence of radiation-tolerant species, the complete lack of forest expansion probably needs to be ascribed to fire. However, our results show that natural processes can also explain both the low position and the abruptness of tropical treelines.
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In this review I first compile data for the worldwide position of climate-driven alpine treelines. Causes for treeline formation are then discussed with a global perspective. Available evidence suggests a combination of a general thermal boundary for tree growth, with regionally variable “modulatory” forces, including the presence of certain taxa. Much of the explanatory evidence found in the literature relates to these modulatory aspects at regional scales, whereas no good explanations emerged for the more fundamental global pattern related to temperature per se, on which this review is focused. I hypothesize that the life form “tree” is limited at treeline altitudes by the potential investment, rather than production, of assimilates (growth as such, rather than photosynthesis or the carbon balance, being limited). In shoots coupled to a cold atmosphere, meristem activity is suggested to be limited for much of the time, especially at night. By reducing soil heat flux during the growing season the forest canopy negatively affects root zone temperature. The lower threshold temperature for tissue growth and development appears to be higher than 3°C and lower than 10°C, possibly in the 5.5–7.5°C range, most commonly associated with seasonal means of air temperature at treeline positions. The physiological and developmental mechanisms responsible have yet to be analyzed. Root zone temperature, though largely unknown, is likely to be most critical.
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In recent research, both soil (root-zone) and air temperature have been used as predictors for the treeline position worldwide. In this study, we intended to (a) test the proposed temperature limitation at the treeline, and (b) investigate effects of season length for both heat sum and mean temperature variables in the Swiss Alps. As soil temperature data are available for a limited number of sites only, we developed an air-to-soil transfer model (ASTRAMO). The air-to-soil transfer model predicts daily mean root-zone temperatures (10cm below the surface) at the treeline exclusively from daily mean air temperatures. The model using calibrated air and root-zone temperature measurements at nine treeline sites in the Swiss Alps incorporates time lags to account for the damping effect between air and soil temperatures as well as the temporal autocorrelations typical for such chronological data sets. Based on the measured and modeled root-zone temperatures we analyzed the suitability of the thermal treeline indicators seasonal mean and degree-days to describe the Alpine treeline position. The root-zone indicators were then compared to the respective indicators based on measured air temperatures, with all indicators calculated for two different indicator period lengths. For both temperature types (root-zone and air) and both indicator periods, seasonal mean temperature was the indicator with the lowest variation across all treeline sites. The resulting indicator values were 7.0°C±0.4SD (short indicator period), respectively 7.1°C±0.5SD (long indicator period) for root-zone temperature, and 8.0°C±0.6SD (short indicator period), respectively 8.8°C±0.8SD (long indicator period) for air temperature. Generally, a higher variation was found for all air based treeline indicators when compared to the root-zone temperature indicators. Despite this, we showed that treeline indicators calculated from both air and root-zone temperatures can be used to describe the Alpine treeline position.
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The decrease in temperature with increasing elevation may determine the altitudinal tree distribution in different ways: affecting survival through freezing temperatures, by a negative carbon balance produced by lower photosynthetic rates, or by limiting growth activity. Here we assessed the relative importance of these direct and indirect effects of altitudinal decrease in temperature in determining the treeline in central Chile (33 degrees S) dominated by Kageneckia angustifolia. We selected two altitudes (2000 and 2200 m a.s.l.) along the treeline ecotone. At each elevation, leaf non-structural carbohydrates (NSC) and gas exchange parameters were measured on five individuals during the growing season. We also determined the cold resistance of K. angustifolia, by measuring temperatures that cause 50% seedling mortality (LT50) and ice nucleation (IN). No differences in net photosynthesis were found between altitudes. Although no differences were detected on NSC concentration on a dry matter basis between 2000 and 2200 m, when NSC concentration was expressed on a leaf area basis, higher contents were found at the higher elevation. Thus, carbon sink limitations may occur at the K. angustifolia's upper altitudinal limit. For seedlings derived from seeds collected at the 2200 m, LT50 of cold-acclimated and non-acclimated plants were -9.5 and -7 degrees C, respectively. However, temperatures as low as -10 degrees C can frequently occur at this altitude during the end of winter. Therefore, low temperature injury of seedlings seems also be involved in the treeline formation in this species. Hence, a confluence of global (carbon sink limitation) and regional (freezing tolerance) mechanisms explains the treeline formation in the Mediterranean-type climate zone of central Chile
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Recent hypotheses of timberline causation include the possibility that limitations to growth processes may be more limiting than restrictions on photosynthetic carbon gain, and that cold soil is a primary limiting factor at high altitude. However, almost all of the supporting data for timberline causation have come from studies on older trees, with little focus on the mechanisms of seedling establishment and the growth of saplings away from the forest edge into the treeline ecotone. We describe a conceptual model of timberline migration that invokes a strong dependence on ecological facilitation, beginning with seed germination and continuing through seedling establishment and sapling growth to the stage where trees with forest-like stature form new subalpine forest at a higher altitude. In addition to protection from severe mechanical damage, facilitation of photosynthetic carbon gain and carbon processing is enhanced by plasticity in plant form and microsite preference, enabling seedling survival and sapling growth inside and through the often severe boundary layer just above the ground cover. Several forms of facilitation (inanimate, interspecific, intraspecific and structural) result in substantial increases in photosynthetic carbon gain throughout the summer growth period, leading to enhanced root growth, subsequent amelioration of drought stress, and increased seedling survival. Avoidance of low temperatures and low-temperature photoinhibition of photosynthesis may be major benefits of the facilitation, enhancing photosynthetic carbon gain and respiratory-driven growth processes. We propose that the growth of vertical stems (flagged tree forms) from krummholz mats is analogous functionally to the facilitated growth of a seedling/sapling in and away from ground cover. Increasing abundance and growth of newly established trees in the treeline ecotone generates a structural and microsite facilitation characteristic of the subalpine forest below. This is followed by the formation of new subalpine forest with forest-like trees, and a new timberline at higher altitude.
Book
Mountain Timberlines is published as part of the broad area of research on the changing global climate and its impact on the environment. The upper timberline is the most conspicuous vegetation limit in high-mountain areas of all continents and islands, except for the Antarctic. The dynamics of timberline establishment and maintenance is being affected by global warming in a number of ways. From a global view point, the present timberline is far from being caused only by the current climate, but instead reflects also history of climate, human impact and local site conditions. It is the objective of the book to highlight the physiognomic and ecological variety of mountain timberlines as well as their regionally and locally varying heterogeneity and temporal dynamics thus giving a complex view of the global timberline pattern. After an introduction into the complexities of the subject, the history and present state of timberline research are outlined. Chapters on the tree species at timberline and on the relationship of timberline elevation to marcroclimate, climate character and the mass-elevation effect follow. The main chapter deals with the physiognomic and ecological differentiation of altitudinal timberlines, in particular with the timberline controlling physical and biological factors, their interactions and their influence on the spatial structures and temporal dynamics in the timberline ecotone. Also, the feedbacks of trees and tree stands on the timberline environment are considered. This is the base for understanding the response of timberlines to climatically driven changes, which are considered in the last chapters.
Article
Understanding the evolutionary history and biogeography of the New Zealand alpine flora has been impeded by the lack of an integrated model of geomorphology and climate events during the Late Miocene, Pliocene and Pleistocene. A new geobiological model is presented that integrates rock uplift age, rate of uplift and the resulting summit elevations in the Southern Alps (South Island) during the last 8.0 million years with a climate template using the natural gamma radiation pattern from the eastern South Island Ocean Drilling Program Site 1119 that covers the past 3.9 million years. This model specifically defines the average treeline in relation to mountain height, allowing predictions as to the timing of the formation of the alpine zone and other open habitats. This model predicts open habitats such as rock bluffs, tussock grasslands and riverbeds would have been available from about 4.0-3.0 Ma, coinciding with the initiation of summit uplift and a cooling climate providing an opportunity for the evolution of generalist alpine and open-habitat herbs and shrubs. Alpine habitats began to form at about 1.9 Ma and were a permanent feature of the Southern Alps from about 0.95 Ma. Specialist alpine plants confined to alpine habitats can have evolved only within this period once the alpine zone was persistent and widespread. Bog habitats are likely to date from the Late Miocene (c. 6.0 Ma), and the specialist bog species would have evolved from this time. Molecular-clock dates for DNA sequences from species of specialist alpine habitats, generalist open habitats, and bog habitats are consistent with predictions made on the basis of the model.
Book
Alpine treelines mark the low-temperature limit of tree growth and occur in mountains world-wide. Presenting a companion to his book Alpine Plant Life, Christian Körner provides a global synthesis of the treeline phenomenon from sub-arctic to equatorial latitudes and a functional explanation based on the biology of trees. The comprehensive text approaches the subject in a multi-disciplinary way by exploring forest patterns at the edge of tree life, tree morphology, anatomy, climatology and, based on this, modelling treeline position, describing reproduction and population processes, development, phenology, evolutionary aspects, as well as summarizing evidence on the physiology of carbon, water and nutrient relations, and stress physiology. It closes with an account on treelines in the past (palaeo-ecology) and a section on global change effects on treelines, now and in the future. With more than 100 illustrations, many of them in colour, the book shows alpine treelines from around the globe and offers a wealth of scientific information in the form of diagrams and tables.
Chapter
Publisher Summary This chapter discusses the positive-feedback switches in plant communities. A vegetation positive-feedback switch is a process in which a community modifies the environment, making it more suitable for that community. Positive-feedback switches operate by modifying any of several features of the environment, including water, pH, soil elements, light, temperature, wind, fire, or allelopathic toxins. The four types of switch can be distinguished as: (1) one-sided switch, where a single community modifies the environment of the patches it occupies, (2) reaction switch, where the community additionally modifies the patches it is not in, but in the opposite direction, (3) symmetric switch, where communities of both alternative states modify the same factor of their environment, but in opposite directions, and (4) two-factor switch, where the two communities both modify their environments, but in different factors. The positive-feedback switches producing four major vegetational effects (A–D): a stable vegetational mosaic may be produced in a previously uniform environment(situation A), or a vegetational gradient caused by environmental change can be intensified to give a sharp boundary (situation B). These mosaics and boundaries can occur at a wide variety of spatial scales, from landscape-scale to individual plant-scale. Switches can also sharpen or displace temporal boundaries: succession can be accelerated (situation C) or delayed (situation D). Not all of these effects can be produced by all types of switch; in particular, a one-sided (type 1) switch cannot produce a stable mosaic.
Article
Maximal resistance to winter freezing of trees of the South Temperature Zone, especially subalpine trees of Australasia, was assessed. Most of the tree species which grow in lower altitudes were marginally hardy to -10@?. Subalpine and alpine shrubby species such as Podocarpus nivalis, P. lawrencei and Dacrydium bidwillii were the hardiest conifers in New Zealand and Australia, resisting freezing to -20@? to -23@?. This hardiness was comparable to that of conifers native to the warm temperate or temperate parts of Japan. In Nothofagus, the deciduous, subalpine N. antarctica of South America was the hardiest, resisting freezing at -22@?. A New Zealand evergreen timberline species, N. solandri var. cliffortioides was marginally hardly to 15@?. Of the Eucalyptus species, E. pauciflora which forms the alpine tree limit on the mainland of Australia was the hardiest, resisting freezing to -15@? in the leaves. Other high-altitude angiosperm species tested mostly survived freezing to only -10@? or -15@?. Very hardy tree species that withstand freezing below -30@? seem not to have evolved in the Southern Hemisphere, because the mild, oceanic winters did not provide the stimulus, and because hardy northern genera, with minor exceptions, have been unable to cross the barrier formed by the tropics.
Article
Lodgepole pine (Pinus contorta, Dougl.) was introduced to New Zealand in about 1880. It is the most vigorous naturally regenerating introduced conifer, which has led to large areas of unwanted spread or ‘wildings’. Wildings threaten existing indigenous flora and fauna, visual landscape and land use values. The area affected by all conifer natural regeneration is estimated at 150,000 ha of which approximately two thirds is lodgepole pine. Control operations have been undertaken in New Zealand since the 1960s. The high ‘weed’ potential of lodgepole pine, coupled with its low grower and market acceptance in New Zealand, means that the species is seldom planted nowadays.
Article
CO2-assimilation and leaf conductance of Larix decidua Mill. were measured in the field at high (Patscherkofel, Austria) and low (Bayreuth, Germany) elevation in Europe, and outside its natural range along an altitudinal gradient in New Zealand.
Article
Cambial activity of a subalpine population of Olearia ilicifolia continued throughout the mild winter of 1984 with a subsequent resting period from September to November. Radial growth ceased from June to October in older trees at another stand at similar altitude some 400 m distant. It is suggested that mild winter temperatures allowed the continuation of wood increment in the younger trees.
Article
An experimental comparison of the native mountain beech (Nothofagus solandri var. cliffortioides) with exotic timberline species, and a study of the configuration of timberline in an alpine valley in relation to topography and microclimate are synthesised with the results of other timberline studies to develop general hypotheses about New Zealand timberlines.Although assimilation and growth in mountain beech decrease with increasing altitude, the abrupt nature and local altitudinal variations of beech timberlines mainly reflect the cold-tolerance limits of its seedlings. Winter death of beech twigs, that can lead to a krummholz form, is aligned with downslope winds, but sensitivity to low temperatures may also be involved. At least some of the tall shrubs that replace beech forest in cirque-form valley heads possess greater cold-tolerance. These shrubs also have endotrophic mycorrhizal associations, in contrast to the ectotrophic mycorrhizas of beech, which may have bearing on their lower stature and slower growth.On a regional scale, maximum tree limit, whether of beech or tall shrubs, is related to the regular altitudinal decrease in atmospheric temperatures, and not to microclimates as influenced by aspect and topography; and it is more likely related to how long temperatures lie within a range conducive to maturation and winter-hardening of shoots, than to annual carbon gain. Above tree limit, however, summer growth may not compensate for winter dieback. Low shrubs ascend far above tree limit, because the genetic restriction on their height growth protects them from repeated die-back.Species of northern hemisphere timberlines generally achieve far greater winter-hardiness than native trees, but many are limited in New Zealand because they are damaged by summer frosts. Pinus contorta, however, seems capable of forming trees 200 m above the natural tree limit. and krummholz at still higher altitudes.
Article
New Zealand high-altitude tree limits are formed either abruptly by evergreen Nothofagus or by low forest of more frost-tolerant small trees reaching similar maximum altitudes. Whereas most tree limits are contiguous with low-growing alpine vegetation, in New Zealand a belt dominated by tussock grasses intervenes that is vulnerable to invasion by hardy introduced trees and seems ecologically equivalent to fire-maintained high-altitude tropical grasslands. New Zealand tree limits coincide with warmer growing-season temperatures than other tree limits, including deciduous Nothofagus in the southern Andes. They also correlate with coldest-month mean temperatures around 0 degrees C, in accordance with the limits of broadleaved evergreen trees globally, unlike north temperate subalpine trees that withstand extreme winter cold. Adverse environments lead to krummholz that in temperate regions can form an attenuated belt above the forest limit, but in New Zealand Nothofagus krummholz develops only at or below the forest limit, in accordance with absence of Nothofagus seedlings beyond a few meters above the forest limit. The relatively low altitudes attained by New Zealand trees are related to isolation and the recent uplift of high mountains, and the differentiation between Nothofagus forest and low forest reflects historical and geological events.
Article
Aim Across all latitudes, high-elevation tree lines represent a drastic change in the dominant plant life-form, from upright trees to low-stature alpine plants. Although associated with low temperatures, the physiological mechanisms controlling this boundary are still not clear. The growth-limitation hypothesis assumes a direct low-temperature restriction of tissue formation at otherwise sufficient photoassimilation. In order to test this hypothesis, we present a global synthesis of previously published and new data on tree carbon supply status at high-elevation tree lines.
Article
Aim Two alternative hypotheses attempt to explain the upper elevation limit of tree lines world-wide, the carbon-limitation hypothesis (CLH) and the growth-limitation hypothesis (GLH); the altitudinal decrease of temperature is considered the driver constraining either carbon gain or growth. Using a widely distributed tree line species (Nothofagus pumilio) we tested whether tree line altitude is explained by the CLH or the GLH, distinguishing local from global effects. We elaborated expectations based on most probable trends of carbon charging with altitude according to both hypotheses, considering the alternative effects of drought.
Article
The sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change are increasingly discussed in terms of climate change, often forgetting that climate is only one aspect of environmental variation. As treeline heterogeneity increases from global to regional and smaller scales, assessment of treeline sensitivity at the landscape and local scales requires a more complex approach than at the global scale. The time scale (short-, medium-, long-term) also plays an important role when considering treeline sensitivity. The sensitivity of the treeline to a changing environment varies among different types of treeline. Treelines controlled mainly by orographic influences are not very susceptible to the effects of warming climates. Greatest sensitivity can be expected in anthropogenic treelines after the cessation of human activity. However, tree invasion into former forested areas above the anthropogenic forest limit is controlled by site conditions, and in particular, by microclimates and soils. Apart from changes in tree physiognomy, the spontaneous advance of young growth of forest-forming tree species into present treeless areas within the treeline ecotone and beyond the tree limit is considered to be the best indicator of treeline sensitivity to environmental change. The sensitivity of climatic treelines to climate warming varies both in the local and regional topo-graphical conditions. Furthermore, treeline history and its after-effects also play an important role. The sensitivity of treelines to changes in given factors (e.g. winter snow pack, soil moisture, temperature, evaporation, etc.) may vary among areas with differing climatic characteristics. In general, forest will not advance in a closed front but will follow sites that became more favourable to tree establishment under the changed climatic conditions.
Article
Aim At a coarse scale, the treelines of the world's mountains seem to follow a common isotherm, but the evidence for this has been indirect so far. Here we aim at underpinning this with facts.Location We present the results of a data-logging campaign at 46 treeline sites between 68° N and 42° S.Methods We measured root-zone temperatures with an hourly resolution over 1–3 years per site between 1996 and 2003.Results Disregarding taxon-, landuse- or fire-driven tree limits, high altitude climatic treelines are associated with a seasonal mean ground temperature of 6.7 °C (±0.8 SD; 2.2 K amplitude of means for different climatic zones), a surprisingly narrow range. Temperatures are higher (7–8 °C) in the temperate and Mediterranean zone treelines, and are lower in equatorial treelines (5–6 °C) and in the subarctic and boreal zone (6–7 °C). While air temperatures are higher than soil temperatures in warm periods, and are lower than soil temperatures in cold periods, daily means of air and soil temperature are almost the same at 6–7 °C, a physics driven coincidence with the global mean temperature at treeline. The length of the growing season, thermal extremes or thermal sums have no predictive value for treeline altitude on a global scale. Some Mediterranean (Fagus spp.) and temperate South Hemisphere treelines (Nothofagus spp.) and the native treeline in Hawaii (Metrosideros) are located at substantially higher isotherms and represent genus-specific boundaries rather than boundaries of the life-form tree. In seasonal climates, ground temperatures in winter (absolute minima) reflect local snow pack and seem uncritical.Main conclusions The data support the hypothesis of a common thermal threshold for forest growth at high elevation, but also reflect a moderate region and substantial taxonomic influence.
Article
In the oceans of the tropical and warm-temperate zone (40° N–40° S), only a small number of islands are high enough to show timberline and alpine vegetation. Excluding large islands with a more continental climate, only the following oceanic islands are relevant: Pico (Azores), Madeira, Tenerife, Gran Canaria and La Palma (Canary islands), Fogo (Cape Verde islands), Fernando Poo (Bioko) and Tristan da Cunha in the Atlantic Ocean, Réunion and Grande Comore (Ngazidja) in the Indian Ocean, Yakushima (Japan), Maui and Hawaii (Hawaiian islands), and Mas Afuera (Juan Fernandez islands) in the Pacific Ocean. Timberline and alpine vegetation exist here under a unique combination of a highly oceanic climate and a marked geographic isolation which contrasts with the tropical alpine vegetation in the extended mountains of South America, Africa and Southeast Asia. This review seeks to identify common physiognomic patterns in the high elevation vegetation that exist despite the fact that the islands belong to different floristic regions of the world. Based on the existing literature as well as personal observation, an overview of the elevation, physiognomy and floristics of the forest (and tree) line and the alpine vegetation on 15 island peaks is given. The forest line ecosystems are dominated either by conifers (Canary islands, Yakushima), heath woodland (Azores, Madeira, Réunion, Grande Comore, Fernando Poo) or broad-leaved trees (Hawaiian islands, Juan Fernandez islands, Tristan da Cunha). In the subalpine and alpine belts, dry sclerophyllous scrub occurs on island mountains that are exposed to the trade winds (Canary islands, Cape Verde islands, Hawaiian islands, Réunion, Grande Comore). These peaks are more or less arid above the forest line because a temperature inversion restricts the rise of humid air masses further upslope. In the summit regions of the remaining islands, which are located either in the wet equatorial and monsoonal regions or in the temperate westerly zones without an effective inversion layer, mesic to wet vegetation types (such as grassland, alpine heathland and fern scrub) are found. Compared to mountains at a similar latitude in continental areas, the forest line on the islands is found at 1000 to 2000 m lower elevations. The paper discusses four factors that are thought to contribute to this forest line depression: (1) drought on trade-wind exposed island peaks with stable temperature inversions, (2) the absense of well-adapted high-altitude tree species on isolated islands, (3) immaturity of volcanic soils, and (4) an only small mountain mass effect that influences the vertical temperature gradient.
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