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Pathways and drivers of canopy accession across primary temperate forests of Europe
Abstract
Canopy accession strategies reveal much about tree life histories and forest stand dynamics. However, the protracted nature of ascending to the canopy makes direct observation challenging. We use a reconstructive approach based on an extensive tree ring database to study the variability of canopy accession patterns of dominant tree species (Abies alba, Acer pseudoplatanus, Fagus sylvatica, Picea abies) in temperate mountain forests of Europe and elucidate how disturbance histories, climate, and topography affect canopy accession. All four species exhibited high variability of radial growth histories leading to canopy accession and indicated varying levels of shade tolerance. Individuals of all four species survived at least 100 years of initial suppression. Fir and particularly beech, however, survived longer periods of initial suppression, exhibited more release events, and reached the canopy later on average, with a larger share of trees accessing the canopy after initially suppressed growth. These results indicate the superior shade tolerance of beech and fir compared to spruce and maple. The two less shade-tolerant species conversely relied on faster growth rates, revealing their competitive advantage in non-suppressed conditions. Additionally, spruce from higher-elevation spruce-dominated forests survived shorter periods of initial shading and exhibited fewer releases, with a larger share of trees reaching the canopy after open canopy recruitment (i.e. in absence of suppression) and no subsequent releases compared to spruce growing in lower-elevation mixed forests. Finally, disturbance factors were identified as the primary driver of canopy accession, whereby disturbances accelerate canopy accession and consequently regulate competitive interactions. Intensifying disturbance regimes could thus promote shifts in species composition, particularly in favour of faster-growing, more light-demanding species.
... The old-growth forest status provides unique features in terms of large tree dominance [67] and lifespan [68], deadwood and microhabitats [69], as well as structural complexity [70]. Furthermore, the development of MOG into more advanced structural stages implies the onset of rare and complex patterns [71] and processes [72], favouring habitat conditions fundamental for the survival of highly specialised taxa [73,74]. Again, more research is still needed to fully describe and understand the casual relationships sustaining the presence of highly specialised forest species and the most advanced structural traits. ...
Forest Orchidaceae are important for European temperate forests, yet their distribution and abundance have so far interested limited research. In three pure or mixed silver fir stands in the Foreste Casentinesi National Park (NP) (Northern Apennines, Italy) we analysed how structural traits in mature and old-growth forests affected orchid communities in terms of abundance of the main genera, trophic strategy and rarity in the NP. We established three 20 × 60 m plots to quantify the structure of living and dead tree community, including a set of old-growth attributes connected to large trees, deadwood, and established regeneration. In each plot, we measured the abundance of all orchid species and explored their behaviour according to the trophic strategy (autotrophy/mixotrophy, obligate mycoheterotrophy), rarity within the NP, and threatened status according to the IUCN Red List. We used multivariate ordination and classification techniques to assess plot similarities according to forest structure and Orchid Community and identify the main structural factors related to orchid features. The main structural factors were used as predictors of community traits. Forest composition (i.e., the dominance/abundance of silver fir) affected the presence of the main orchid genera: Epipactis were abundant in silver fir-dominated forests, Cephalanthera in mixed beech and fir forests. Interestingly, Cephalanthera could become limited even in beech-dominated conditions if fir regeneration was abundant and established. Old-growth attributes like the density of deadwood and large tree volume were important determinants of the presence of rare and mycoheterotrophic species. Our results provided a first quantitative description of forest reference conditions to be used in the protection and restoration of threatened and rare orchid species.
... In the eastern United States, Choi et al. (2023) found that moderate disturbances enhanced canopy structure complexity, supporting resistance and resilience to a disturbance. Higher severity disturbances (i.e., >60% CA) produce larger openings, and topography influences the degree of exposure to solar radiation, thus affecting canopy recruitment (Pavlin et al., 2024). Disturbance events can greatly influence tree size ranges and spatial scales and are a key characteristic of a forest shaped by natural disturbances (Lutz et al., 2018;Choi et al., 2023;Szwagrzyk and Gazda, 2007). ...
Primary forests are spatially diverse terrestrial ecosystems with unique characteristics, being naturally regenerative and heterogeneous, which supports the stability of their carbon storage through the accumulation of live and dead biomass. Yet, little is known about the interactions between biomass stocks, tree genus diversity and structure across a temperate montane primary forest. Here, we investigated the relationship between tree structure (variability in basal area and tree size), genus-level diversity (abundance, tree diversity) and biomass stocks in temperate primary mountain forests across Central and Eastern Europe. We used inventory data from 726 permanent sample plots from mixed beech and spruce across the Carpathian Mountains. We used nonlinear regression to analyse the spatial variability in forest biomass, structure, and genus-level diversity and how they interact with plot-level tree age, disturbances, temperature and altitude. We found that the combined effects of genus and structural indices were important for addressing the variability in biomass across different spatial scales. Local processes in disturbance regimes and uneven tree age support forest heterogeneity and the accumulation of live and dead biomass through the natural regeneration, growth and decay of the forest ecosystem. Structural complexities in basal area index, supported by genus-level abundance, positively influence total biomass stocks, which was modulated by tree age and disturbances. Spruce forests showed higher tree density and basal area than mixed beech forests, though mixed beech still contributes significantly to biomass across landscapes. Forest heterogeneity was strongly influenced by complexities in forest composition (tree genus diversity, structure). We addressed the importance of primary forests as stable carbon stores, achieved through structure and diversity. Safeguarding such ecosystems is critical for ensuring the stability of the primary forest, carbon store and biodiversity into the future.
... Although the young specimens of beech tolerate shading well, in the case of young beech stands, regenerated naturally under shelter, through repeated regeneration cuts, with an average regeneration period, it is necessary to apply specific maintenance cuts at the right time, for achieving balanced dendrometric elements, obtaining optimal growth, and achieving a stable dynamic balance [45]. ...
Having in view the sustainable management of forests, the intensity of silvotechnical interventions is analyzed and documented for young mixed beech stands regenerated naturally by taking into account the allometric relationships between some representative dendrometric elements, respectively, between the height of the trees and the corresponding diameter of their crowns. For this purpose, a functional dependence is established between the height and diameter of the tree crown from young beech stands within which silvotechnical interventions were or were not applied. A mathematical model based on cubic spline regression was developed, also taking into account the determination of the average diameter of the crown for each stand. This average diameter can be used to determine the optimal number of trees per surface unit (ha) at a given time. The research was carried out in three young beech stands, through statistical-mathematical inventories, on sample plots of 300 square meters with a tolerance of 10% and a coverage probability of 90%. The optimal diameter of the crown was obtained by applying the spline regression model, and the most appropriate functional dependence was established, resulting in a linear regression law with a positive slope.
Protecting structural features, such as tree‐related microhabitats (TreMs), is a cost‐effective tool crucial for biodiversity conservation applicable to large forested landscapes. Although the development of TreMs is influenced by tree diameter, species, and vitality, the relationships between tree age and TreM profile remain poorly understood. Using a tree‐ring‐based approach and a large data set of 8038 trees, we modeled the effects of tree age, diameter, and site characteristics on TreM richness and occurrence across some of the most intact primary temperate forests in Europe, including mixed beech and spruce forests. We observed an overall increase in TreM richness on old and large trees in both forest types. The occurrence of specific TreM groups was variably related to tree age and diameter, but some TreM groups (e.g., epiphytes) had a stronger positive relationship with tree species and elevation. Although many TreM groups were positively associated with tree age and diameter, only two TreM groups in spruce stands reacted exclusively to tree age (insect galleries and exposed sapwood) without responding to diameter. Thus, the retention of trees for conservation purposes based on tree diameter appears to be a generally feasible approach with a rather low risk of underrepresentation of TreMs. Because greater tree age and diameter positively affected TreM development, placing a greater emphasis on conserving large trees and allowing them to reach older ages, for example, through the establishment of conservation reserves, would better maintain the continuity of TreM resource and associated biodiversity. However, this approach may be difficult due to the widespread intensification of forest management and global climate change.
We analyzed growth trajectories recorded in the tree-ring series of Norway spruce Picea abies (L.) H. Karst. and silver fir Abies alba Mill. co-occuring with European beech Fagus sylvatica L. in old-growth forests driven by natural stand dynamics. The study sites were localized in five old-growth stands in the Western Carpathians (Central Europe). We characterized the life histories of 218 individual trees by using 25 metrics which refer to the age of the trees, number of release events, reaction to releases, radial increment and basal area increment. We found a large variation in the dbh-age relationship. The life histories of most trees included two or three (in single cases up to seven) release events. We did not find tree age as an important factor in determining post-release growth response. The maximal basal area increment was frequently registered at the terminal stage of tree life. The life histories of spruces and firs did not significantly differ. We concluded that in natural forests driven by gap-phase dynamics, the history of release events is the primary driver of tree growth and that tree age plays a secondary role. After reaching the overstory, trees can continue growing without significant symptoms of aging until extrinsic forces cause their death. Our results indicate that in the studied ecosystem the persistence of less shade-tolerant spruce is not attributable to the niche differentiation and site partitioning mechanism. An alternative hypothesis based on shifts in stand densities, species composition and climatic factors is more probable.
Low mortality rates and slow growth differentiate shade-tolerant from shade-intolerant species and define the survival strategy of juvenile trees growing in deep shade. While radial stem growth has been widely used to explain mortality in juvenile trees, the leaf area ratio (LAR), known to be a key component of shade tolerance, has been neglected so far. We assessed the effects of LAR, radial stem growth and tree height on survival time and the age-specific mortality rate of juvenile Fagus sylvatica L. (European beech), Acer pseudoplatanus L. (sycamore maple) and Acer platanoides L. (Norway maple) in a primeval beech forest (Ukraine). Aboveground and belowground biomass and radial stem growth were analysed for 289 living and 179 dead seedlings and saplings. Compared with the other species, F. sylvatica featured higher LAR, slower growth and a lower mortality rate. The average survival time of F. sylvatica juveniles (72 years) allows it to reach the canopy more often than its competitors in forests with low canopy turnover rate. In contrast, a combination of lower LAR, higher growth rate and higher age-specific mortality rate of the two Acer species resulted in their shorter survival times and thus render their presence in the canopy a rare event. Overall, this study suggests that shade tolerance, commonly defined as a relationship between sapling mortality and growth, can alternatively be formulated as a relationship between survival time and the interplay of growth and LAR.
Primary forests, defined here as forests where the signs of human impacts, if any, are strongly blurred due to decades without forest management, are scarce in Europe and continue to disappear. Despite these losses, we know little about where these forests occur. Here, we present a comprehensive geodatabase and map of Europe’s known primary forests. Our geodatabase harmonizes 48 different, mostly field-based datasets of primary forests, and contains 18,411 individual patches (41.1 Mha) spread across 33 countries. When available, we provide information on each patch (name, location, naturalness, extent and dominant tree species) and the surrounding landscape (biogeographical regions, protection status, potential natural vegetation, current forest extent). Using Landsat satellite-image time series (1985–2018) we checked each patch for possible disturbance events since primary forests were identified, resulting in 94% of patches free of significant disturbances in the last 30 years. Although knowledge gaps remain, ours is the most comprehensive dataset on primary forests in Europe, and will be useful for ecological studies, and conservation planning to safeguard these unique forests.
Aims
We examined differences in lifespan among the dominant tree species (spruce (Picea abies (L.) H. Karst.), fir (Abies alba Mill.), beech (Fagus sylvatica L.), and maple (Acer pseudoplatanus L.)) across primary mountain forests of Europe. We ask how disturbance history, lifetime growth patterns, and environmental factors influence lifespan.
Locations
Balkan mountains, Carpathian mountains, Dinaric mountains.
Methods
Annual ring widths from 20,600 cores from primary forests were used to estimate tree life spans, growth trends, and disturbance history metrics. Mixed models were used to examine species-specific differences in lifespan (i.e. defined as species-specific 90th percentiles of age distributions), and how metrics of radial growth, disturbance parameters, and selected environmental factors influence lifespan.
Results
While only a few beech trees surpassed 500 years, individuals of all four species were older than 400 years. There were significant differences in lifespan among the four species (beech > fir > spruce > maple), indicating life history differentiation in lifespan. Trees were less likely to reach old age in areas affected by more severe disturbance events, whereas individuals that experienced periods of slow growth and multiple episodes of suppression and release were more likely to reach old age. Aside from a weak but significant negative effect of vegetation season temperature on fir and maple lifespan, no other environmental factors included in the analysis influenced lifespan.
Conclusions
Our results indicate species-specific biological differences in lifespan, which may play a role in facilitating tree species coexistence in mixed temperate forests. Finally, natural disturbances regimes were a key driver of lifespan, which could have implications for forest dynamics if regimes shift under global change.
Being able to persist in deep shade is an important characteristic of juvenile trees, often leading to a strong dominance of shade‐tolerant species in forests with low canopy turnover and a low disturbance rate. While leaf, growth, and storage traits are known to be key components of shade tolerance, their interplay during regeneration development and their influence on juveniles' survival time remains unclear. We assessed the ontogenetic effects of these three traits on the survival time of beech (Fagus sylvatica), and Norway and sycamore maples (Acer pseudoplatanus, Acer platanoides) in a primeval beech forest. Biomass allocation, age, and content of nonstructural carbohydrates (NSC) were measured in the stems and roots of 289 seedlings and saplings in high‐ and low‐vitality classes. Saplings experienced a trade‐off between absolute growth rate (AGR) and storage (NSC) as the leaf area ratio (LAR) decreases with biomass development. High LAR but low AGR and low NSC corresponded to beech with a marked ability to persist in deep shade while awaiting canopy release. In turn, a comparably small LAR in combination with a high AGR and higher storage (NSC), as observed in Norway maple and sycamore maple, reduced sapling survival time, thus offering an explanation for beech dominance and maple disappearance in the undergrowth of old‐growth beech forests. Can survival times of juvenile trees be inferred from traits underlying shade tolerance? This study analyzed the development of leaf, growth, and storage traits and their influence on the survival time of European beech, and Norway and sycamore maples in deep shade.
"Optimal cutpoints" for binary classification tasks are often established by testing which cutpoint yields the best discrimination, for example the Youden index, in a specific sample. This results in "optimal" cutpoints that are highly variable and systematically overestimate the out-of-sample performance. To address these concerns, the cutpointr package offers robust methods for estimating optimal cutpoints and the out-of-sample performance. The robust methods include bootstrapping and smoothing based on kernel estimation, generalized additive models, smoothing splines, and local regression. These methods can be applied to a wide range of binary-classification and cost-based metrics. cutpointr also provides mechanisms to utilize user-defined metrics and estimation methods. The package has capabilities for parallelization of the bootstrapping, including reproducible random number generation. Furthermore, it is pipe-friendly, for example for compatibility with functions from tidyverse. Various functions for plotting receiver operating characteristic curves, precision recall graphs, bootstrap results and other representations of the data are included. The package contains example data from a study on psychological characteristics and suicide attempts suitable for applying binary classification algorithms.
Aim: Distribution ranges of temperate tree species are shifting poleward and upslope
into cooler environments due to global warming. Successful regeneration is crucial
for population persistence and range expansion. Thus, we aimed to identify environmental
variables that affect germination and seedling establishment of Europe's
dominant forest tree, to compare the importance of plasticity and genetic variation
for regeneration, and to evaluate the regeneration potential at and beyond the southern
and northern distribution margins.
Location: Europe.
Time period: 2016–2018.
Major taxa studied: European beech (Fagus sylvatica (L.)).
Methods: We investigated how germination, establishment and juvenile survival
change across a reciprocal transplantation experiment using over 9,000 seeds of
beech from 7 populations from its southern to its northern distribution range margins.
Results: Germination and establishment at the seedling stage were highly plastic in
response to environmental conditions. Germination success increased with warmer and declined with colder air temperature, whereas establishment and survival were
hampered under warmer and drier conditions. Germination differed among populations
and was positively influenced by seed weight. However, there was no evidence
of local adaptation in any trait.
Main conclusions: The high plasticity in the early life-history
traits found irrespective
of seed origin may allow for short-term
acclimatization. However, our results also indicate
that this plasticity might not be sufficient to ensure the regeneration of beech in
the future due to the low survival found under dry and hot conditions. The future climatic
conditions in parts of the distribution centre and at the rear edge might thus become
limiting for natural regeneration, as the likelihood of extreme heat and drought
events will increase. By contrast, at the cold distribution margin, the high plasticity in
the early life-history
traits may allow for increasing germination success with increasing
temperatures and may thus facilitate natural regeneration in the future.
Wind is the leading disturbance agent in European forests, and the magnitude of wind impacts on forest mortality has increased over recent decades. However, the atmospheric triggers behind severe winds in Western Europe (large‐scale cyclones) differ from those in Southeastern Europe (small‐scale convective instability). This geographic difference in wind drivers alters the spatial scale of resulting disturbances and potentially the sensitivity to climate change. Over the 20th century, the severity and prevalence of cyclone‐induced windstorms have increased while the prevalence of atmospheric instability has decreased and thus, the trajectory of Europe‐wide windthrow remains uncertain. To better predict forest sensitivity and trends of windthrow disturbance we used dendrochronological methods to reconstruct 140 years of disturbance history in beech‐dominated primary forests of Central and Eastern Europe. We compared generalized linear mixed models of these disturbance time series to determine whether large‐scale cyclones or small‐scale convective storms were more responsible for disturbance severity while also accounting for topography and stand character variables likely to influence windthrow susceptibility. More exposed forests, forests with a longer absence of disturbance, and forests lacking recent high severity disturbance showed increased sensitivity to both wind drivers. Large‐scale cyclone‐induced windstorms were the main driver of disturbance severity at both the plot and stand scale (0.1–∼100 ha) whereas convective instability effects were more localized (0.1 ha). Though the prevalence and severity of cyclone‐induced windstorms have increased over the 20 century, primary beech forests did not display an increase in the severity of windthrow observed over the same period.
Aim
Understanding how natural forest disturbances control tree regeneration is key to predicting the consequences of globally accelerating forest diebacks on carbon stocks and forest biodiversity. Tropical cyclones (TCs) are important drivers of forest dynamics in Eastern Asia, and it is predicted that their importance will increase. However, little is known about the impact of TCs on forest regeneration.
Location
Latitudinal gradient from south Korea (33° N) to the Russian Far East (45° N).
Time period
Last 300 years.
Major taxa studied
Quercus mongolica, Abies nephrolepis and Pinus koraiensis.
Methods
We explored the effects of TC activity on canopy accession strategies derived from long‐term tree radial growth patterns along a 1,500‐km latitudinal gradient of decreasing TC activity. We analysed canopy accession strategies for > 800 trees of three widely distributed tree species by dividing them into gap trees (GTs), which established immediately after gap formation, and released trees (RTs), which accessed the upper canopy after a period of competitive suppression.
Results
We found a substantial decrease in GTs and increase in RTs proportionally along the gradient of decreasing TC activity. Pinus koraiensis and A. nephrolepis exhibited high variability in the proportions of the individual canopy accession strategies along the latitudinal gradient, whereas it was more stable for Q. mongolica. We identified the gradient of TC activity as the main driver influencing canopy dynamics and thus changes in life‐history traits for P. koraiensis and Q. mongolica, whereas maximal growth rate was the main driver for A. nephrolepis.
Main conclusions
Flexibility in growth strategies enabled the studied species to cover extensive areas and indicates that they will be able to cope with shifts in disturbance regimes induced by the poleward migration of TCs and increasing TC intensity. Our results highlight the canopy accession strategy as an ecological indicator of past disturbance activity.
Changes in forest disturbances can have strong impacts on forests, yet we lack consistent data on Europe’s forest disturbance regimes and their changes over time. Here we used satellite data to map three decades of forest disturbances across continental Europe, and analysed the patterns and trends in disturbance size, frequency and severity. Between 1986 and 2016, 17% of Europe’s forest area was disturbed by anthropogenic and/or natural causes. We identified 36 million individual disturbance patches with a mean patch size of 1.09 ha, which equals an annual average of 0.52 disturbance patches per km2 of forest area. The majority of disturbances were stand replacing. While trends in disturbance size were highly variable, disturbance frequency consistently increased and disturbance severity decreased. Here we present a continental-scale characterization of Europe’s forest disturbance regimes and their changes over time, providing spatial information that is critical for understanding the ongoing changes in Europe’s forests. Changes in forest disturbance affect their sustainability. This study finds that between 1986 and 2016, 36 million disturbances by humans or other causes affected 17% of Europe’s forest area.
Land vegetation is currently taking up large amounts of atmospheric CO 2 , possibly due to tree growth stimulation. Extant models predict that this growth stimulation will continue to cause a net carbon uptake this century. However, there are indications that increased growth rates may shorten trees′ lifespan and thus recent increases in forest carbon stocks may be transient due to lagged increases in mortality. Here we show that growth-lifespan trade-offs are indeed near universal, occurring across almost all species and climates. This trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment. Thus, current tree growth stimulation will, inevitably, result in a lagged increase in canopy tree mortality, as is indeed widely observed, and eventually neutralise carbon gains due to growth stimulation. Results from a strongly data-based forest simulator confirm these expectations. Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.
Estimates of historical disturbance patterns are essential to guide forest management aimed at ensuring the sustainability of ecosystem functions and biodiversity. However, quantitative estimates of various disturbance characteristics required in management applications are rare in longer‐term historical studies. Thus, our objectives were to (1) quantify past disturbance severity, patch size, and stand proportion disturbed and (2) test for temporal and subregional differences in these characteristics. We developed a comprehensive dendrochronological method to evaluate an approximately two‐century‐long disturbance record in the remaining Central and Eastern European primary mountain spruce forests, where wind and bark beetles are the predominant disturbance agents. We used an unprecedented large‐scale nested design data set of 541 plots located within 44 stands and 6 subregions. To quantify individual disturbance events, we used tree‐ring proxies, which were aggregated at plot and stand levels by smoothing and detecting peaks in their distributions. The spatial aggregation of disturbance events was used to estimate patch sizes. Data exhibited continuous gradients from low‐ to high‐severity and small‐ to large‐size disturbance events. In addition to the importance of small disturbance events, moderate‐scale (25–75% of the stand disturbed, >10 ha patch size) and moderate‐severity (25–75% of canopy disturbed) events were also common. Moderate disturbances represented more than 50% of the total disturbed area and their rotation periods ranged from one to several hundred years, which is within the lifespan of local tree species. Disturbance severities differed among subregions, whereas the stand proportion disturbed varied significantly over time. This indicates partially independent variations among disturbance characteristics. Our quantitative estimates of disturbance severity, patch size, stand proportion disturbed, and associated rotation periods provide rigorous baseline data for future ecological research, decisions within biodiversity conservation, and silviculture intended to maintain native biodiversity and ecosystem functions. These results highlight a need for sufficiently large and adequately connected networks of strict reserves, more complex silvicultural treatments that emulate the natural disturbance spectrum in harvest rotation times, sizes, and intensities, and higher levels of tree and structural legacy retention.
1) Understanding tree seedling responses to water, nutrient, and light availability is crucial to precisely predict potential shifts in composition and structure of forest communities under future climatic conditions. (2) We exposed seedlings of widespread Central European tree species with contrasting leaf habit, deciduous broadleaves (DB) and evergreen conifers (EC), to factorial combinations of manipulated precipitation (100% and 50% of ambient), shade (40% and 60% of full sunlight), and nutrient availability (low and high NPK), and measured specific leaf area, C/N ratio, soluble sugars, starch and non-structural carbohydrate concentration, and δ 13 C of the leaves. (3) We found contrasting effects of water and nutrient availability on foliar traits of the two species groups: EC exhibited higher tolerance to low resource availability but also less plasticity in foliar traits, which is congruent with a "slow" resource strategy. In contrast, foliage of DB reacted particularly to altered nutrient availability, corresponding to a "fast" resource strategy with high foliar plasticity and rapid adjustments to resource fluctuations, commonly adopted by species with high growth rates. (4) We conclude that DB will respond to environmental change with foliar acclimation, while EC will either tolerate, to some extent, or shift their distribution range in response to environmental change.
CRU TS (Climatic Research Unit gridded Time Series) is a widely used climate dataset on a 0.5° latitude by 0.5° longitude grid over all land domains of the world except Antarctica. It is derived by the interpolation of monthly climate anomalies from extensive networks of weather station observations. Here we describe the construction of a major new version, CRU TS v4. It is updated to span 1901–2018 by the inclusion of additional station observations, and it will be updated annually. The interpolation process has been changed to use angular-distance weighting (ADW), and the production of secondary variables has been revised to better suit this approach. This implementation of ADW provides improved traceability between each gridded value and the input observations, and allows more informative diagnostics that dataset users can utilise to assess how dataset quality might vary geographically.
The Colchic rainforest of the Western Caucasus is one of the few temperate rainforests dominated by broadleaf deciduous trees. Understanding natural dynamics of broadleaf-dominated temperate rainforests is essential for their conservation and management. Here, we investigate for the first time the structure, natural disturbance, and recruitment dynamics of a mixed Colchic old-growth rainforest, dominated by Fagus orientalis and Picea orientalis. We used forest inventories and dendrochronological analysis of tree growth in five 30-m-radius plots to quantify forest structure, growth, and disturbances. For the last 400 years, the forest experienced a mixed disturbance regime dominated by frequent small gaps superimposed onto medium disturbances with about a 25-year recurrence period, with no evidences of stand-replacing disturbances. This disturbance regime favored the dominance of shade-tolerant, late successional species with slow tree canopy access through multiple growth releases. These dynamics impose low growth rates and continuous recruitment of spruce and beech, and contributed to a high heterogeneity of tree ages and sizes that result in stable forest structure, as suggested by the low stand slenderness. Spruces were the oldest (up to 427 years) and fastest growing trees in the forest, suggesting that their low presence in the forest is due to low disturbance rates that limit their recruitment. Spring climate conditions that promoted beech growth were detrimental for spruce growth, suggesting that interspecies interactions may condition the effect of climate on forest growth and development. The dynamic equilibrium state we reconstructed in this old-growth forest could likely be disrupted by anthropogenic disturbances or management.
In this paper, the annual rainfall and temperature values, measured in the period 1951-2016 in a region of southern Italy (Calabria), have been spatially interpolated using deterministic and geostatistical techniques in an R environment. In particular, Inverse Distance Weighting (IDW), Ordinary Kriging (OK), Kriging with External Drift (KED) and Ordinary Cokriging (COK) were compared to evaluate the best suitability method in reproducing the actual surface. Then, the spatial variation of aridity in Calabria has been evaluated using the De Martonne aridity index (IDM), which is based on rainfall and temperature data. As a result, geostatistical methods incontrovertibly show a better estimate than the IDW. Specifically, the KED was identified as the best predictor method for both rainfall and temperature data. Moreover, the spatial distribution of the IDM evidenced that the majority of the study area can be classified as humid, with semi–arid conditions mainly identified in the coastal areas.
Species often respond to human‐caused climate change by shifting where they occur on the landscape. To anticipate these shifts, we need to understand the forces that determine where species currently occur. We tested whether a long‐hypothesised trade‐off between climate and competitive constraints explains where tree species grow on mountain slopes. Using tree rings, we reconstructed growth sensitivity to climate and competition in range centre and range margin tree populations in three climatically distinct regions. We found that climate often constrains growth at environmentally harsh elevational range boundaries, and that climatic and competitive constraints trade‐off at large spatial scales. However, there was less evidence that competition consistently constrained growth at benign elevational range boundaries; thus, local‐scale climate‐competition trade‐offs were infrequent. Our work underscores the difficulty of predicting local‐scale range dynamics, but suggests that the constraints on tree performance at a large‐scale (e.g. latitudinal) may be predicted from ecological theory.
Aim
Primary forests have high conservation value but are rare in Europe due to historic land use. Yet many primary forest patches remain unmapped, and it is unclear to what extent they are effectively protected. Our aim was to (1) compile the most comprehensive European‐scale map of currently known primary forests, (2) analyse the spatial determinants characterizing their location and (3) locate areas where so far unmapped primary forests likely occur.
Location
Europe.
Methods
We aggregated data from a literature review, online questionnaires and 32 datasets of primary forests. We used boosted regression trees to explore which biophysical, socio‐economic and forest‐related variables explain the current distribution of primary forests. Finally, we predicted and mapped the relative likelihood of primary forest occurrence at a 1‐km resolution across Europe.
Results
Data on primary forests were frequently incomplete or inconsistent among countries. Known primary forests covered 1.4 Mha in 32 countries (0.7% of Europe’s forest area). Most of these forests were protected (89%), but only 46% of them strictly. Primary forests mostly occurred in mountain and boreal areas and were unevenly distributed across countries, biogeographical regions and forest types. Unmapped primary forests likely occur in the least accessible and populated areas, where forests cover a greater share of land, but wood demand historically has been low.
Main conclusions
Despite their outstanding conservation value, primary forests are rare and their current distribution is the result of centuries of land use and forest management. The conservation outlook for primary forests is uncertain as many are not strictly protected and most are small and fragmented, making them prone to extinction debt and human disturbance. Predicting where unmapped primary forests likely occur could guide conservation efforts, especially in Eastern Europe where large areas of primary forest still exist but are being lost at an alarming pace.
Count data can be analyzed using generalized linear mixed models when observations are correlated in ways that require random effects. However, count data are often zero-inflated, containing more zeros than would be expected from the typical error distributions. We present a new package, glmmTMB, and compare it to other R packages that fit zero-inflated mixed models. The glmmTMB package fits many types of GLMMs and extensions, including models with continuously distributed responses, but here we focus on count responses. glmmTMB is faster than glmmADMB, MCMCglmm, and brms, and more flexible than INLA and mgcv for zero-inflated modeling. One unique feature of glmmTMB (among packages that fit zero-inflated mixed models) is its ability to estimate the Conway-Maxwell-Poisson distribution parameterized by the mean. Overall, its most appealing features for new users may be the combination of speed, flexibility, and its interface's similarity to lme4.
Determining the drivers of shifting forest disturbance rates remains a pressing global change issue. Large-scale forest dynamics are commonly assumed to be climate driven, but appropriately scaled disturbance histories are rarely available to assess how disturbance legacies alter subsequent disturbance rates and the climate sensitivity of disturbance. We compiled multiple tree-ring based disturbance histories from primary Picea abies forest fragments distributed throughout five European landscapes spanning the Bohemian Forest and the Carpathian Mountains. The regional chronology includes 11 595 tree cores, with ring dates spanning the years 1750 to 2000, collected from 560 inventory plots in 37 stands distributed across a 1000 km geographic gradient, amounting to the largest disturbance chronology yet constructed in Europe. Decadal disturbance rates varied significantly through time and declined after 1920, resulting in widespread increases in canopy tree age. Approximately 75% of current canopy area recruited prior to 1900. Long-term disturbance patterns were compared to an historical drought reconstruction, and further linked to spatial variation in stand structure and contemporary disturbance patterns derived from LANDSAT imagery. Historically, decadal Palmer drought severity index minima corresponded with higher rates of canopy removal. The severity of contemporary disturbances increased with each stand's estimated time since last major disturbance, increased with mean diameter and declined with increasing within-stand structural variability. Reconstructed spatial patterns suggest that high small-scale structural variability has historically acted to reduce large-scale susceptibility and climate sensitivity of disturbance. Reduced disturbance rates since 1920, a potential legacy of high 19th century disturbance rates, have contributed to a recent region-wide increase in disturbance susceptibility. Increasingly common high-severity disturbances throughout primary Picea forests of Central Europe should be reinterpreted in light of both legacy effects (resulting in increased susceptibility) and climate change (resulting in increased exposure to extreme events). This article is protected by copyright. All rights reserved.
The coefficient of determination R² quantifies the proportion of variance explained by a statistical model and is an important summary statistic of biological interest. However, estimating R² for generalized linear mixed models (GLMMs) remains challenging. We have previously introduced a version of R² that we called for Poisson and binomial GLMMs, but not for other distributional families. Similarly, we earlier discussed how to estimate intra-class correlation coefficients (ICCs) using Poisson and binomial GLMMs. In this paper, we generalize our methods to all other non-Gaussian distributions, in particular to negative binomial and gamma distributions that are commonly used for modelling biological data. While expanding our approach, we highlight two useful concepts for biologists, Jensen's inequality and the delta method, both of which help us in understanding the properties of GLMMs. Jensen's inequality has important implications for biologically meaningful interpretation of GLMMs, whereas the delta method allows a general derivation of variance associated with non-Gaussian distributions. We also discuss some special considerations for binomial GLMMs with binary or proportion data. We illustrate the implementation of our extension by worked examples from the field of ecology and evolution in the R environment. However, our method can be used across disciplines and regardless of statistical environments.
Background
The quantitative impact of forest management on forests’ wood resource was evaluated for Picea and Fagus mixed forests. The effects on the productivity of tendering operations, thinnings and rotation length have seldom been directly quantified on landscape scale. Methods
Two sites of similar fertility but subject to contrasted forest management were studied with detailed inventories: one in Germany, the other in Romania, and compared with the respective national forest inventories. In Romania, regulations impose very long rotations, low thinnings and a period of no-cut before harvest. In contrast, tending and thinnings are frequent and intense in Germany. Harvests start much earlier and must avoid clear cutting but maintain a permanent forest cover with natural regeneration. While Germany has an average annual wood increment representative for Central Europe, Romania represents the average for Eastern Europe. ResultsThe lack of tending and thinning in the Romanian site resulted in twice as many trees per hectare as in the German site for the same age. The productivity in Romanian production forests was 20 % lower than in Germany despite a similar fertility. The results were supported by the data from the national forest inventory of each country, which confirmed that the same differential exists at country scale. Furthermore, provided the difference in rotation length, two crops are harvested in Germany when only one is harvested in Romania. The losses of production due to a lower level of management in Romania where estimated to reach 12.8 million m3.y-1 in regular mountain production forests, and to 15 million m3.y-1 if managed protection forest is included. Conclusions
The productivity of Picea and Fagus mountain forests in Romania is severely depressed by the lack of tending and thinning, by overly long rotations and the existence of a 25-years no-cut period prior to harvest. The average standing volume in Germany was 50 % lower than in Romania, but the higher harvesting rate resulted in more than doubling wood production. Considering the mitigation effects of climate change by forests, it emerges that the increase in standing volume of forests in Romania is smaller than the additional harvest in Germany which serves fossil fuel substitution.
A within-species trade-off between growth rates and lifespan has been observed across different taxa of trees, however, there is some uncertainty whether this trade-off also applies to shade-intolerant tree species. The main objective of this study was to investigate the relationships between radial growth, tree size and lifespan of shade-intolerant mountain pines. For 200 dead standing mountain pines (Pinus montana) located along gradients of aspect, slope steepness and elevation in the Swiss National Park, radial annual growth rates and lifespan were reconstructed. While early growth (i.e. mean tree-ring width over the first 50 years) correlated positively with diameter at the time of tree death, a negative correlation resulted with lifespan, i.e. rapidly growing mountain pines face a trade-off between reaching a large diameter at the cost of early tree death. Slowly growing mountain pines may reach a large diameter and a long lifespan, but risk to die young at a small size. Early growth was not correlated with temperature or precipitation over the growing period. Variability in lifespan was further contingent on aspect, slope steepness and elevation. The shade-intolerant mountain pines follow diverging growth trajectories that are imposed by extrinsic environmental influences. The resulting trade-offs between growth rate, tree size and lifespan advance our understanding of tree population dynamics, which may ultimately improve projections of forest dynamics under changing environmental conditions.
Many studies report that intraspecific genetic variation in plants can affect community composition and coexistence. However, less is known about which traits are responsible and the mechanisms by which variation in these traits affect the associated community. Focusing on plant-plant interactions, we review empirical studies exemplifying how intraspecific genetic variation in functional traits impacts plant coexistence. Intraspecific variation in chemical and architectural traits promotes species coexistence, by both increasing habitat heterogeneity and altering competitive hierarchies. Decomposing species interactions into interactions between genotypes shows that genotype x genotype interactions are often intransitive. The outcome of plant-plant interactions varies with local adaptation to the environment and with dominant neighbour genotypes, and some plants can recognize the genetic identity of neighbour plants if they have a common history of coexistence. Taken together, this reveals a very dynamic nature of coexistence. We outline how more traits mediating plant-plant interactions may be identified, and how future studies could use population genetic surveys of genotype distribution in nature and methods from trait-based ecology to better quantify the impact of intraspecific genetic variation on plant coexistence. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
The consequences of global change for the maintenance of species diversity will depend on the sum of each species responses to the environment and on the interactions among them. A wide ecological literature supports that these species-specific responses can arise from factors related to life strategies, evolutionary history and intraspecific variation, and also from environmental variation in space and time. In the light of recent advances from coexistence theory combined with mechanistic explanations of diversity maintenance, we discuss how global change drivers can influence species coexistence. We revise the importance of both competition and facilitation for understanding coexistence in different ecosystems, address the influence of phylogenetic relatedness, functional traits, phenotypic plasticity and intraspecific variability, and discuss lessons learnt from invasion ecology. While most previous studies have focused their efforts on disentangling the mechanisms that maintain the biological diversity in species-rich ecosystems such as tropical forests, grasslands and coral reefs, we argue that much can be learnt from pauci-specific communities where functional variability within each species, together with demographic and stochastic processes becomes key to understand species interactions and eventually community responses to global change.
Understanding temporal and spatial variations in historical disturbance regimes across intact, continuous, and altitudinally diverse primary forest landscapes is imperative to help forecast forest development and adapt forest management in an era of rapid environmental change. Because few complex primary forest landscapes remain in Europe, previous research has largely described disturbance regimes for individual forest types and smaller isolated stands. We studied the largest but still largely unprotected mountain primary forest landscape in temperate Europe, the Fagaraș Mountains of Romania. To describe historical disturbance regimes and synchronicity in disturbance activity and trends between two widespread forest community types, dominated by Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.), we established 191 permanent study plots (70 beech; 121 spruce) across 11 valleys, thereby providing information at both stand and landscape
levels. We used a dendrochronological approach to reconstruct and describe the spatiotemporal patterns of
historical disturbances. We observed a diverse spectrum of disturbance severities and timing across the forest landscape. High-severity disturbances created periods of synchrony in disturbance activity at the landscape scale, while moderate- and low-severity disturbances were asynchronous and random in both spruce- and beech-dominated primary forests. We detected a peak of canopy disturbance across the region at the end of the
nineteenth century, with the most important periods of disturbance between the 1890s and 1910s. At the stand scale, we observed periods of synchronised disturbances with varying severities across both forest types. The level of disturbance synchrony varied widely among the stands. The beta regression showed that spruce forests had significantly higher average synchrony and higher between-stand variability of synchrony than the beech-dominated forests. Synchronised disturbances with higher severity were infrequent, but they were critical as drivers of subsequent forest development pathways and dynamics across both forest types.
Our results provide valuable insight into future resilience to climate-driven alterations of disturbance regimes in spruce- and beech-dominated mountain temperate forests in the Carpathians. We suggest that conservation efforts should recognize strictly protecting large continuous and altitudinally diversified forest landscapes such as Fagaraș Mts. as a necessary measure to tackle climate change and ensure temporal and spatial structural heterogeneity driven by a wide range of disturbances. The diverse and synchronous disturbance activity among two interconnected forest vegetation types highlights the need for complex spatiotemporal forest management approaches that emulate disturbance synchronicity to foster biodiversity across multiple forest vegetation types within forest landscapes.
In a world of accelerating changes in environmental conditions driving tree growth, tradeoffs between tree growth rate and longevity could curtail the abundance of large, old trees (LOTs), with potentially dire consequences for biodiversity and carbon storage. However, the influence of tree‐level tradeoffs on forest structure at landscape scales will also depend on disturbances, which shape tree size and age distribution, and on whether LOTs can benefit from improved growing conditions due to climate warming. We analyzed temporal and spatial variation in radial growth patterns from ~ 5000 Norway spruce (Picea abies (L.) H. Karst) live and dead trees from the Western Carpathian primary spruce forest stands. We applied mixed‐linear modeling to quantify the importance of LOT growth histories and stand dynamics (i.e. competition and disturbance factors) on lifespan. Finally, we assessed regional synchronization in radial growth variability over the 20th century, and modelled the effects of stand dynamics and climate on LOTs recent growth trends. Tree age varied considerably among forest stands, implying an important role of disturbance as an age constraint. Slow juvenile growth and longer period of suppressed growth prolonged tree lifespan, while increasing disturbance severity and shorter time since last disturbance decreased it. The highest age was not achieved only by trees with continuous slow growth, but those with slow juvenile growth followed by subsequent growth releases. Growth trend analysis demonstrated an increase in absolute growth rates in response to climate warming, with late summer temperatures driving the recent growth trend. Contrary to our expectation that LOTs would eventually exhibit declining growth rates, the oldest LOTs (> 400 years) continuously increase growth throughout their lives, indicating a high phenotypic plasticity of LOTs for increasing biomass, and a strong carbon sink role of primary spruce forests under rising temperatures, intensifying droughts, and increasing bark beetle outbreaks.
Physiological response to rapid light exposure due to canopy disintegration in young beech, fir, Norway spruce and sycamore trees was measured in three consecutive years after the severe ice storm in 2014 and after windthrow in 2017. Nitrogen amount (Ntot), maximum assimilation response to light (Amax) and quantum yield (Φ) were measured in three categories of different light intensities under closed canopy with indirect site factor (ISF) < 15%, at the forest edge (15% <ISF < 25%) and in the open (ISF > 25%). Tree responses with number of seedlings per hectare were compared between damaged and undamaged sites, with young trees gradually adapting to light conditions in the two years following the two disturbance events.
Nitrogen levels were in the optimal range for all species studied. Rapid exposure to elevated light reduced efficiency in fir and increased efficiency in beech and especially sycamore. No differences in response were observed in spruce. Assimilation efficiency, where both tree species were equal, shifted towards shade. Recovery was similar for all species studied after both disturbances: better after the windstorm, compared to the ice storm, indicating the severity of the event. Reductions in seedling numbers followed the pattern observed for physiological traits.
Abiotic forest disturbances are an important driver of ecosystem dynamics. In Europe, storms and fires have been identified as the most important abiotic disturbances in the recent past. Yet, how strongly these agents drive local disturbance regimes compared to other agents (e.g., biotic, human) remains unresolved. Furthermore, whether storms and fires are responsible for the observed increase in forest disturbances in Europe is debated. Here, we provide quantitative evidence for the prevalence of storm and fire disturbances in Europe 1986‐2016. For 27 million disturbance patches mapped from satellite data we determined whether they were caused by storm or fire, using a random forest classifier and a large reference dataset of true disturbance occurrences. We subsequently analyzed patterns of disturbance prevalence (i.e., the share of an agent on the overall area disturbed) in space and time. Storm‐ and fire‐related disturbances each accounted for approximately 7 % of all disturbances recorded in Europe in the period 1986‐2016. Storm‐related disturbances were most prevalent in western and central Europe, where they locally account for >50% of all disturbances, but we also identified storm‐related disturbances in south‐eastern and eastern Europe. Fire‐related disturbances were a major disturbance agent in southern and south‐eastern Europe, but fires also occurred in eastern and northern Europe. The prevalence and absolute area of storm‐related disturbances increased over time, whereas no trend was detected for fire‐related disturbances. Overall, we estimate an average of 127,716 (97,680 – 162,725) ha of storm‐related disturbances per year and an average of 141,436 (107,353 – 181,022) ha of fire‐related disturbances per year. We conclude that abiotic disturbances caused by storm and fire are important drivers of forest dynamics in Europe, but that their influence varies substantially by region. Our analysis further suggests that increasing storm‐related disturbances are an important driver of Europe’s changing forest disturbance regimes.
Natural disturbances are key factors in the formation of forest ecosystem structure. Evaluation of the spatial and temporal extent of disturbance regimes is critical for understanding forest dynamics, forest structural hetero-geneity, and biodiversity habitats. Quantifying disturbance regimes is therefore imperative for appropriate management of forests and protected areas. However, natural disturbance regimes have rarely been assessed using dendrochronological methods on a regional scale across primary mixed beech-fir forest stands-one of the dominant forest vegetation types in Europe. To study the natural disturbance regimes of beech-dominated mixed-forest stands, we established 42 permanent study plots with an area of 0.1 ha across three primary forest stands in the Western Carpathians, a region that still contains large areas of primary forest. We reconstructed each stand-level disturbance history using a tree-ring based approach. The temporal synchronicity of disturbance events was then evaluated by delineating stand-level disturbance events using a kernel density function, and through the detection of plot-level disturbances with severities greater than 10 percent. The results obtained from the chronologies showed substantial variability in time and space, especially in the mid-19th century. Low-and moderate-severity plot-level disturbance events were most common, but high-and extremely high-severity plot-level disturbance events also occurred. The observed spatial and temporal variability suggests that the beech-dominated forests were primarily driven by mixed-severity disturbance regimes, with windstorms as the main disturbance agent. This reconstruction of the disturbance regime provided unique insight into the scale of mortality processes in these beech-dominated mixed forests. This information can help guide ecological forestry in areas where both wood production and biodiversity preservation are simultaneous goals, such as by employing more spatio-temporally-complex silvicultural systems that resemble natural disturbance patterns and facilitate heterogeneous forest structures.
Given the global intensification of forest management and climate change, protecting and studying forests that develop free of direct human intervention-also known as primary forests-are becoming increasingly important. Yet, most countries still lack data regarding primary forest distribution. Previous studies have tested remote sensing approaches as a promising tool for identifying primary forests. However, their precision is highly dependent on data quality and resolution, which vary considerably. This has led to underestimation of primary forest abundance and distribution in some regions, such as the temperate zone of Europe. Field-based inventories of primary forests and methodologies to conduct these assessments are inconsistent; incomplete or inaccurate mapping increases the vulnerability of primary forest systems to continued loss from clearing and land-use change. We developed a comprehensive methodological approach for identifying primary forests, and tested it within one of Europe's hotspots of primary forest abundance: the Carpathian Mountains. From 2009 to 2015, we conducted the first national-scale primary forest census covering the entire 49,036 km 2 area of the Slovak Republic. We analyzed primary forest distribution patterns and the representativeness of potential vegetation types within primary forest remnants. We further evaluated the conservation status and extent of primary forest loss. Remaining primary forests are small, fragmented, and often do not represent the potential natural vegetation. We identified 261 primary forest localities. However, they represent only 0.47% of the total forested area, which is 0.21% of the country's land area. The spatial pattern of primary forests was clustered. Primary forests have tended to escape anthropogenic disturbance on sites with higher elevations, steeper slopes, rugged terrain, and greater distances from roads and settlements. Primary forest stands of montane mixed and subalpine spruce forests are more abundant compared to broadleaved forests. Notably, several habitat types are completely missing within primary forests (e.g., floodplain forests). More than 30% of the remaining primary forests are not strictly protected, and harvesting occurred at 32 primary forest localities within the study period. Almost all logging of primary forests was conducted inside of protected areas, underscoring the critical status of primary forest distribution in this part of Europe. Effective conservation strategies are urgently needed to stop the rapid loss and fragmentation of the remaining primary forests. Our approach based on precise, field-based surveys is widely applicable and transferrable to other fragmented forest landscapes.
Climatic constraints on tree growth mediate an important link between terrestrial and atmospheric carbon pools. Tree rings provide valuable information on climate‐driven growth patterns, but existing data tend to be biased towards older trees on climatically extreme sites. Understanding climate change responses of biogeographic regions requires data that integrate spatial variability in growing conditions and forest structure. We analyzed both temporal (c. 1901‐2010) and spatial variation in radial growth patterns in 9 876 trees from fragments of primary Picea abies forests spanning the latitudinal and altitudinal extent of the Carpathian arc. Growth was positively correlated with summer temperatures and spring moisture availability throughout the entire region. However, important seasonal variation in climate responses occurred along geospatial gradients. At northern sites, winter precipitation and October temperatures of the year preceding ring formation were positively correlated with ring width. In contrast, trees at the southern extent of the Carpathians responded negatively to warm and dry conditions in autumn of the year preceding ring formation. An assessment of regional synchronization in radial growth variability showed temporal fluctuations throughout the 20th century linked to the onset of moisture limitation in southern landscapes. Since the beginning of the study period, differences between high and low elevations in the temperature sensitivity of tree growth generally declined, while moisture sensitivity increased at lower elevations. Growth trend analyses demonstrated changes in absolute tree growth rates linked to climatic change, with basal area increments in northern landscapes and lower altitudes responding positively to recent warming. Tree growth has predominantly increased with rising temperatures in the Carpathians, accompanied by early indicators that portions of the mountain range are transitioning from temperature to moisture limitation. Continued warming will alleviate large‐scale temperature constraints on tree growth, giving increasing weight to local drivers that are more challenging to predict. This article is protected by copyright. All rights reserved.
Disentangling the importance of developmental vs. environmental drivers of variation in forest biomass is key to predicting the future of forest carbon sequestration. At coarse scales, forest biomass is likely to vary along major climatic and physiographic gradients. Natural disturbance occurs along these broad biophysical gradients, and depending on their extent, severity and frequency, could either amplify or dampen spatial heterogeneity in forest biomass. Here we evaluate spatial variation in the basal area of late-successional Picea abies (L./Karst.) forests across the Carpathian Mountain Range of central Europe and compare the roles of coarse-scale biophysical gradients and natural disturbances in driving that variation across a hierarchy of scales (landscapes, stands, and plots). We inventoried forest composition and structure, and reconstructed disturbance histories using tree cores collected from 472 plots nested within 30 late-successional stands, spanning the Carpathian Mountains (ap-proximately 4.5 degrees of latitude). We used linear mixed-effects models to compare the effect of disturbance regimes and site conditions on stand basal area at three hierarchical scales. We found that the basal area of late-successional Picea abies forests varied across a range of spatial scales, with climatic drivers being most important at coarse scales and natural disturbances acting as the primary driver of forest heterogeneity at fine scales. For instance, the stand-level basal area varied among landscapes, with the highest values (48-68 m 2 ha −1) in the warmer southern Carpathian Mountains, and lower values (37-52 m 2 ha −1 on average) in cooler areas of the eastern and western Carpathians. Finer-scale variation was driven by local disturbances (mainly bark beetle and windstorms) and the legacies of disturbances that occurred more than a century ago. Our findings suggest that warming could increase the basal area of northern sites, but potential increasing disturbances could disrupt these environmental responses.
Functional diversity informs about biodiversity-ecosystem functioning relationships. The intraspecific component of functional diversity (i.e. the phenotypic space of each species) depicts individual differences in the resource use and fitness among conspecifics, and gives valuable information about the functional similarity (competition) or dissimilarity (complementarity) of coexisting species. Here, we quantified trait differences within tree species along local diversity gradients to shed light on the role that this intraspecific variability exerts on functional complementarity of tree species. We measured architectural traits in 5,036 individuals and leaf traits in 1,403 individuals from nine dominant tree species, surveyed in 92 plots located in three major European forest types (Mediterranean, temperate and boreal forests). In each forest type, plots were positioned along a canopy richness gradient, with every study species present in different species richness levels, including monocultures. Our results showed that the relative magnitude of intraspecific trait variability to community-level variability is high in these forests. At the species level, we found adjustments of species leaf traits (mean shifts) in response to neighbouring trees, suggesting the existence of processes that limit niche overlap. We also found higher variability in architectural traits of conspecific individuals in more diverse canopies, suggesting greater niche packing and a more efficient use of available space as the number of species in the canopy increases. Altogether, our results support the hypothesis that differential responses of individuals within a species promote species complementarity, suggesting that biodiversity-ecosystem functioning relationships cannot be properly estimated without accounting for the intraspecific level of functional variation.
Mixed-severity disturbance regimes are prevalent in temperate forests worldwide, but key uncertainties remain regarding the variability of disturbance-mediated structural development pathways. This study investigates the influence of disturbance history on current structure in primary, unmanaged Norway spruce (Picea abies) forests throughout the Carpathian Mountains of central and eastern Europe, where windstorms and native bark beetle outbreaks are the dominant natural disturbances. We inventoried forest structure on 453 plots (0.1 ha) spanning a large geographical gradient (\>1,000 km), coring 15–25 canopy trees per plot for disturbance history reconstruction (tree core total n = 11,309). Our specific objectives were to: (1) classify sub-hectare-scale disturbance history based on disturbance timing and severity; (2) classify current forest structure based on tree size distributions (live, dead, standing, downed); (3) characterize structural development pathways as revealed by the association between disturbance history and current forest structural complexity. We used hierarchical cluster analysis for the first two objectives and indicator analysis for the third. The disturbance-based cluster analysis yielded six groups associated with three levels of disturbance severity (low, moderate, and high canopy loss) and two levels of timing (old, recent) over the past 200 years. The structure-based cluster analysis yielded three groups along a gradient of increasing structural complexity. A large majority of plots exhibited relatively high (53\%) or very high (26\%) structural complexity, indicated by abundant large live trees, standing and downed dead trees, and spruce regeneration. Consistent with conventional models of structural development, some disturbance history groups were associated with specific structural complexity groups, particularly low-severity/recent (very high complexity) and high-severity/recent (moderate complexity) disturbances. In other cases, however, the distribution of plots among disturbance history and structural complexity groups indicated either divergent or convergent pathways. For example, multiple disturbance history groups were significantly associated with the high complexity group, demonstrating structural convergence. These results illustrate that complex forest structure – including features nominally associated with old-growth – can be associated as much with disturbance severity as it is with conventional notions of forest age. Because wind and bark beetles are natural disturbance processes that can induce relatively high levels of tree mortality while simultaneously contributing to structural complexity and heterogeneity, we suggest that forest management plans allow for the stochastic occurrence of disturbance and variable post-disturbance development trajectories. Such applications are especially appropriate in conservation areas where biodiversity and forest resilience are management objectives, particularly given projections of increasing disturbance activity under global change.
Light availability in forests is a strong driver for forest development and diversity. Tree species develop differently under varying levels of light. Understanding the reasons for the individual growth strategies of tree species is crucial to understand dynamics of forest communities. This study aims at further disentangling aboveground biomass allocation patterns and growth variables for saplings of the tree species sycamore maple (Acer pseudoplatanus L.), European beech (Fagus sylvatica L.), and European ash (Fraxinus excelsior L.). Plants were destructively sampled along a light gradient. European beech allocated more biomass to its branches and less to its stem, in comparison to the other two species. The relative growth rate (RGR) and leaf mass area (LMA) of all species increased towards an asymptote along the increasing light gradient, whereas the leaf are ratio (LAR) decreased. The rate of increase and decrease differed among the tree species. Net assimilation rate (NAR) and absolute growth rate (AGR) both also increased with light availability, but the distinction among the species was not as clear. This study showed varying reactions of all three species to light and allowed a quantitative distinction among the species regarding their shade tolerance (ash < maple < beech), whereas the individual reaction was not always the typical reaction associated with shade tolerance.
In order to gauge ongoing and future changes to disturbance regimes, it is necessary to establish a solid baseline of historic disturbance patterns against which to evaluate these changes. Further, understanding how forest structure and composition respond to variation in past disturbances may provide insight into future resilience to climate-driven alterations of disturbance regimes.
Old‐growth studies commonly emphasize structural and age conditions, selecting proxy indicators of long‐term ecological processes. Transition dynamics from mature to old‐growth status reveal how natural legacies are progressively accumulated in forests after major disturbances, including human ones. In late‐successional, multi‐aged forests, the chronosequential ranking of developmental stages is a difficult task, as stand age provides little information, and time since last stand‐replacing disturbance cannot be easily determined.
Canopy age features, disturbance/suppression history and growth trajectories were reconstructed from ring‐width series of canopy trees in a network of 19 old‐growth and managed European beech forests in the eastern Alps and central Apennines. A set of tree‐ring metrics able to describe the intensity and time distribution of biological and ecological processes (e.g. understorey suppression, canopy accession age) were used to describe the advancement of old‐growth status and compared to established metrics of forest structure.
Tree‐ring metrics were site dependent, as biogeoclimate affects turnover rates and constrains the onset and recovery rate of old‐growth attributes. Under the same environmental conditions (high‐mountain, limestone‐bedrock beech forests), values of the best indicators (number/duration of growth suppression phases; synchronicity of first release; maximum and range of canopy tree age; canopy accession age of the slowest‐growing trees) increased monotonically (two to five times) from managed to secondary and primary old‐growth forests. Trees in well‐conserved primary old‐growth forests experienced several and long suppressions, showing the highest complexity in recruitment history, canopy accession and growth trajectories. The best tree‐ring metrics, condensed in a Naturalness Score to provide a synthetic functional ranking of forests, varied coherently with structural complexity, which represented stand dynamics more closely than biomass‐related metrics.
Synthesis and applications . We propose a synthetic ranking of forest functional naturalness based on the ecological processes experienced by trees. This ranking helps to overcome the limitations associated with the use of arbitrary size‐ or age‐related thresholds of old‐growth status and provides a functional approach to establish chronosequences in ecological studies. The quantitative description of complex processes underpinning the unique biological and ecological features (e.g. extreme tree longevity) found in primary old‐growth forests enhances their irreplaceable value in nature conservation. The proposed framework of tree‐ring indicators describes functional traits tightly related to forest naturalness and may thus become a tool to identify and protect old‐growth forests, benchmark the impact of silvicultural practices, prescribe targets or evaluate the effectiveness of restoration programmes.
Quantitative descriptions of natural disturbance regimes are lacking for temperate forest regions in Europe, primarily because a long history of intensive land-use has been the overriding driver of forest structure and composition across the region. The following contribution is the first attempt to comprehensively describe the natural disturbance regime of the dominant forest communities in the Dinaric Mountain range, with an emphasis on the range of natural variability of regime components for the main disturbance agents. Compared to other forest regions in Europe, the mountain range has a history of less intensive forest exploitation and provides a suitable record of natural disturbance processes. Our synthesis is based on multiple types of evidence, including meteorological information, historical documentation, evidence from old-growth remnants, and salvage logging data from National forest inventories. Taken together, the results show that no single disturbance agent dominates the regime in the dominant forest types (i.e. beech and mixed beech-fir forests), and any given agent exhibits remarkable variation in terms of severity and spatial extent both within and among individual disturbance events. Thunderstorm winds cause the most severe damage (i.e. near stand replacement), but blowdown patches are typically limited to stand-scales (e.g. 10s of ha). Ice storms and heavy snow typically cause intermediate severity damage and affect much larger areas (e.g. 100s of km2). A notable exception was the 2014 ice storm, which was nearly an order of magnitude larger and more severe than any other event recorded in the synthesis. Severe and prolonged periods of drought have occurred several times over the past century, and along with secondary insect damage (e.g. bark beetles), have caused episodes of forest decline. Overall, our synthesis indicates that on top of the background of relatively continuous gap dynamics, stand-scale intermediate severity events are an important part of the regime; these events likely have rotation periods that are less than the lifespan of a tree cohort (e.g. several centuries) and create canopy openings large enough to alter successional trajectories.
Disturbances shape forest structure and composition, but the temporal dynamics of disturbance patterns, their influence on dynamics of forest structural complexity, and the potential impacts of ongoing climate changes are not fully understood. We addressed these issues by focusing on (1) long-term, landscape level retrospective analysis of disturbance dynamics of mountain Norway spruce (Picea abies) forest; (2) testing for the prevailing disturbance agent; and (3) the detection of disturbance drivers, particularly site conditions, using a dendrochronological approach.
In spatial statistics the ability to visualize data and models superimposed with their basic social landmarks and geographic context is invaluable. ggmap is a new tool which enables such visualization by combining the spatial information of static maps from Google Maps, OpenStreetMap, Stamen Maps or CloudMade Maps with the layered grammar of graphics implementation of ggplot2. In addition, several new utility functions are introduced which allow the user to access the Google Geocoding, Distance Matrix, and Directions APIs. The result is an easy, consistent and modular framework for spatial graphics with several convenient tools for spatial data analysis.