No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Historical mixed-severity disturbances shape current diameter distributions of primary temperate Norway spruce mountain forests in Europe
Abstract
Natural disturbances strongly influence forest structural dynamics, and subsequently stand structural heterogeneity, biomass, and forest functioning. The impact of disturbance legacies on current forest structure can greatly influence how we interpret drivers of forest dynamics. However, without clear insight into forest history, many studies default to coarse assumptions about forest structure, for example, whether forests are even or unevenly aged. The aim of this study was to analyze the effects of past disturbances on the current diameter distributions of Norway spruce (Picea abies (L.) Karst.)-dominated landscapes throughout the Carpathian Mountains. Our dendroecological dataset comprises tree cores from 339 plots (7,845 total tree cores), nested within 28 primary forest stands, known to vary greatly in the severity of historical disturbances. Our analyses revealed that historical disturbances had a strong and significant effect on the current diameter distribution shapes at the plot level. We demonstrated that mixed-severity disturbance regimes were more frequent and create a complex pattern of diameter distributions at the plot and stand scale. Here, we show that high severity disturbance was associated with unimodal diameter distributions, while low and moderate severity was associated with the reverse J-shaped distribution. This is a result of complex disturbance patterns, with structural biological legacies. Our results will have important management implication in the context of tree size heterogeneity, biomass storage, and productivity as influenced by natural disturbances. Lastly, these results demonstrate that structural changes may arise as consequences of changing disturbance regime associated with global change.
... The DBH distribution of tree species exhibits variable patterns owing to site quality, regional climate, and disturbances (Chhin et al. 2008;Lin et al. 2016;Rodrigo et al. 2022). Generally, consistent with the results of previous studies fitting the similar DBH distribution of the corresponding tree species in Northeast China (Hao et al. 2022;Liu et al. 2014;Xu and Jin 2012), the overall DBH distribution of most tree species predicted in this study displayed a right-skewed unimodal pattern (Fig. 5), suggesting that at the regional scale, a continuous regeneration and relatively stable community structure occurred in most tree species in Northeast China (Fang et al. 2012). ...
Key message
We propose a coupled framework to combine the strengths of the Weibull function in modeling diameter distributions and the ability of the k -nearest neighbor ( k NN) method to impute spatially continuous forest stand attributes for the prediction of wall-to-wall tree lists (lists of stems per hectare by species and diameter at breast height (DBH)) at regional scales. The tree lists of entire Northeast China’s forests predicted by the above framework reasonably reflect the species-specific tree density and diameter distributions.
Context
Detailed tree lists provide information about forest stocks disaggregated by species and size classes, which are crucial for forest managers to accurately characterize the current forest stand state to formulate targeted forest management strategies. However, regional tree list information is still lacking due to limited forest inventory.
Aims
We aimed to develop a coupled framework to enable the prediction of wall-to-wall tree lists for the entire forest region of Northeast China, then analyze the species-specific diameter distributions and reveal the spatial patterns of tree density by species.
Methods
A two-parameter Weibull function was used to model the species-specific diameter distributions in the sample plots, and a maximum likelihood estimation (MLE) was used to predict the parameters of the Weibull distributions. The goodness-of-fit of the predicted species-specific Weibull diameter distributions in each plot was evaluated by Kolmogorov-Smirnov (KS) test and an error index. The k NN model was used to impute the pixel-level stand mean DBH.
Results
Weibull distribution accurately described the species-specific diameter distributions. The imputed stand mean DBH from the k NN model showed comparable accuracy with earlier studies. No difference was detected between predicted and observed tree lists, with a small error index (0.24–0.58) of diameter distributions by species. The fitted species-specific diameter distributions generally showed a right-skewed unimodal or reverse J-shaped pattern.
Conclusion
Overall, the coupled framework developed in this study was well-suited for predicting the tree lists of large forested areas. Our results evidenced the spatial patterns and abundance of tree species in Northeast China and captured the forest regions affected by disturbances such as fire.
... In this study, disturbances directly improved the nutrient resorption and changed forest nutrient cycling, and there was a reducing tendency for P-limitation. The disturbances could impact the forest structure and stability by changing REs in the natural forests [49]. In the future, the forest managers need to consider the effect of the disturbances on the nutrient utilization and tourism resources for protection of natural forests. ...
Nutrient resorption plays an important role in the nutrient conservation of plants and ecosystem nutrient cycling. Although community succession and nutrient addition could regulate plant nutrient resorption, how resorptions of foliar nutrients vary with human disturbances remains unclear. With the economic development, Phoebe bournei forests (PF) have suffered varying degrees of human disturbances in China. In this study, the leaf nutrient resorption efficiency (RE) of the PF under two disturbances (i.e., severe and mild disturbances) were investigated. Results showed that the phosphorus (P) contents of green leaf, senesced leaf, and soil were low under both disturbances, reflecting that the PF had a potential P limitation. Phosphorus and potassium (K) REs were higher under the severe disturbance than those under the mild disturbance. The potassium resorption efficiency was the highest among the three REs under both disturbances. In addition, nutrient resorption efficiencies increased with green leaf nutrient contents under both disturbances. However, there were negative significant relationships of specific leaf area and leaf dry matter content with nutrient resorption under both disturbances. These findings provide a new perspective of nutrient resorption and revealed the potential impact of human disturbances on the nutrient cycle in forest ecosystems.
Ecosystem services (ESs) of mountain areas may be impacted by cumulative and interactive effects of multiple disturbances including insect infestations, wildfires, timber harvesting, and building construction. However, the impacts of natural and human disturbances on ESs and the trade-offs/synergies among ESs are poorly known, particularly in mountain ecosystems with diverse landscapes. Here, we used the Qilian Nature Reserve in northwestern China as a case study, for which we quantified mountain disturbances with a BEAST algorithm and three critical ESs (carbon sequestration, water yield, and habitat quality) with the CASA and InVEST models. We then simulated ESs using the BN model, and estimated the impacts of disturbances on ESs and their trade-offs in different environment conditions through multi-scenario analysis. Our results suggested that BEAST could effectively capture the patterns and dynamics of small-scale disturbances, which were previously difficult to predict with normal land use/cover products. The established BN model could simulate the spatio-temporal dynamics of carbon sequestration, water yield, and habitat quality with an average classification error of 17.8, 12.7 and 4.5% for each ES, respectively. Significant synergy existed between carbon sequestration and habitat quality at the regional scale, while trade-off existed between water yield and the other two ESs. Specifically, these trade-offs/synergies among ESs tended to be weak at medium value of ESs, but stronger at higher and lower states. Thus, significant differences existed in the “win-lose” solutions between water yield and the other two ESs, further resulting the limited space to simultaneously improve three ESs. Disturbances at medium frequency and low-medium intensity were beneficial for the maintenance and improvement of three ESs. The BN model is a promising decision support tool to integrate small-scale disturbances into ES evaluation and identify the most suitable management solutions for mountain ecosystems; this could provide critical information for decision-makers and guidance for sustainable development.
Primary forests are critical for forest biodiversity and provide key ecosystem services. In Europe, these forests are particularly scarce and it is unclear whether they are sufficiently protected. Here we aim to: (a) understand whether extant primary forests are representative of the range of naturally occurring forest types, (b) identify forest types which host enough primary forest under strict protection to meet conservation targets and (c) highlight areas where restoration is needed and feasible.
We combined a unique geodatabase of primary forests with maps of forest cover, potential natural vegetation, biogeographic regions and protected areas to quantify the proportion of extant primary forest across Europe's forest types and to identify gaps in protection. Using spatial predictions of primary forest locations to account for underreporting of primary forests, we then highlighted areas where restoration could complement protection.
We found a substantial bias in primary forest distribution across forest types. Of the 54 forest types we assessed, six had no primary forest at all, and in two‐thirds of forest types, less than 1% of forest was primary. Even if generally protected, only ten forest types had more than half of their primary forests strictly protected. Protecting all documented primary forests requires expanding the protected area networks by 1,132 km2 (19,194 km2 when including also predicted primary forests). Encouragingly, large areas of non‐primary forest existed inside protected areas for most types, thus presenting restoration opportunities.
Europe's primary forests are in a perilous state, as also acknowledged by EU's “Biodiversity Strategy for 2030.” Yet, there are considerable opportunities for ensuring better protection and restoring primary forest structure, composition and functioning, at least partially. We advocate integrated policy reforms that explicitly account for the irreplaceable nature of primary forests and ramp up protection and restoration efforts alike.
It is prominently claimed that enhancing forest diversity would play a dual role of nature conservation and climate regulation. While the idea is intuitively appealing, studies show that species richness effects on aboveground carbon (AGC) are not always positive, but instead unpredictable especially across scales and complex terrestrial systems having large‐diameter and tall‐stature trees. Previous studies have further considered structural complexity and larger trees as determinants of AGC. Yet it remains unclear what drives differential diversity‐AGC relationships across vegetation types. Here, we test whether structural complexity and large‐sized trees play an influential role in explaining shifting diversity‐AGC relationships across vegetation types, using a 22.3 ha sampled dataset of 124 inventory plots in woodlands, gallery forests, tree/shrub savannahs and mixed plantations in West Africa. Natural vegetation had greater species richness and structural complexity than mixed plantations, as expected. In addition, AGC was highest in gallery forests and mixed plantations, which is consistent with favorable environmental conditions in the former and high stocking densities and presence of fast‐growing species in the latter. Significant interaction effects of species richness and vegetation on AGC revealed a vegetation‐dependent species richness‐AGC relationship: consistently, we found positive species richness‐AGC relationship in both mixed plantations and woodlands, and nonsignificant patterns in gallery forests and tree/shrub savannah. Further, there was a vegetation‐dependent mediation of structural complexity in linking species richness to AGC, with stronger positive structural complexity effects where species richness‐AGC relationships were positive, and stronger positive large‐sized trees’ effect where species richness‐AGC relationships were neutral. Our study provides strong evidence of vegetation‐dependent species richness‐AGC relationships, which operated through differential mediation by structural complexity of the species richness and large trees’ effects. We conclude that even higher species richness in diversified ecosystems may not always relate positively with AGC, and that neutral pattern may arise possibly as a result of larger dominant individual trees imposing a slow stand dynamic flux and overruling species richness effects.
The disturbance regime of mountain spruce–beech temperate forests has not yet been sufficiently elucidated. We hypothesized that spruce and beech express completely different disturbance histories and behavioural strategies, potentially causing exceptionally complex disturbance regimes. We further hypothesized that the spontaneous development of mountain forests can temporarily result in a simplification of the forest's spatial structure. We wanted to discover how the disturbance history and growth plasticity of the main tree species differ, and whether some old managed forests arose from primeval forest remnants. We compared dendrochronological records of the unmanaged Boubín Primeval Forest and 30 sites with current forestry records. Using this comparison, we categorized all sites into three categories. In the disturbance history of all evaluated forest sites, there was clear evidence of the presence of severe disturbances in the nineteenth century. However, the regeneration of beech was more continuous and less dependent on the presence of severe disturbances than the regeneration of spruce, which depended on the presence of severe disturbances of low frequency. Human-induced changes at some sites were manifested in changes in the initial growth of both species and disrupted their mutual competition and also led to a higher growth plasticity of beech. Despite human impacts in the region since the end of the nineteenth century, about 30% of analysed trees were older than the severe disturbances in the nineteenth century; therefore, some studies sites preserved the characteristics of primeval forest. Our results revealed three main forest development trajectories since the end of the nineteenth century.
Assessing diameter distribution is an approach of observing regeneration strategies and impacts of anthropogenic disturbances on forest structural attributes. In this study, we evaluated the differences in diameter distributions in highly disturbed, moderately disturbed, and undisturbed forest sites of Buffer Zone Community Forests (BZCFs), viz. Janajagaran BZCF, Musharni Mai BZCF, and Radha Krishna BZCF of Central Terai, Nepal. Weibull and gamma distributions were fitted to explain changes in frequency distribution of diameter data collected. We observed a significant decrease in species richness (overall: p < 0.001) and density of smallest diameter class (0–10 cm diameter at breast height) stems (overall: p < 0.001) with increasing disturbances. Similarly, increase in median diameters (p < 0.01) and parameters of Weibull's and gamma distributions along gradients from undisturbed to highly disturbed forest sites (except between moderately disturbed and undisturbed sites of Musharni Mai BZCF) were observed. Bowley coefficient of skewness was lower in all highly disturbed sites of BZCFs showing reduction in right skewness of the distribution curves with disturbances. These results indicate anthropogenic disturbances shift diameter structure of forests stands, negatively affecting abundance of small diameter–sized woody species and shifting Shorea robusta mixed forests towards even-aged stands. © 2018 National Science Museum of Korea (NSMK) and Korea National Arboretum (KNA)
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.
Natural disturbances are among the most important factors that shape forest dynamics and forest landscapes. However, the natural disturbance regime of Norway spruce (Picea abies (L.) Karst.) forests in Europe is not well understood. We studied the disturbance regimes in three forest reserves in Bulgaria (Parangalitsa, Bistrishko branishte, and Beglika), which are representative of the range of conditions typical for P. abies ecosystems in central and southern Europe. Our data indicated that large-scale disturbances were most numerous in forests that were between 120 and 160 years old, those with unimodal diameter at breast height (DBH) distributions, and especially those located in vulnerable topographic settings. Wind disturbances ranged up to 60 ha, followed in one case by a 200 ha Ips typographus (Linnaeus, 1758) outbreak. Older forests and those with more complex structures (i.e., reverse-J DBH) were characterized by numerous small gaps but were also affected by a few larger disturbances. In some old-growth forests at highly productive sites, gaps could be so numerous that the long-term existence of old trees may become an exception. Over the past centuries, the natural range of variability of these Norway spruce forests in Bulgaria appears to have been shaped mostly by wind and bark beetle disturbances of various sizes. © 2015, National Research Council of Canada. All rights Reserved.
Duncan, R.P. (1989). An evaluation of errors in tree age estimates based on increment cores in kahikatea (Dacrycarpus dacrydioides). New Zealand Natural Sciences 16: 31-37. Twelve kahikatea (Dacrycarpus dacrydioides) discs were used to assess the likely errors associated with estimating tree age from growth ring counts in increment cores. Two major sources of error were examined: (1) Failure of the increment core to pass through the tree's chronological centre. A geometric model is developed for estimating the distance to the chronological centre in cores where the arcs of the inner rings are visible. The mean percentage error from 84 cores that passed within 50 mm of the chronological centre was ± 35% corresponding to a mean absolute error of ± 21 years. The majority of this error is due to growth rate differences between the missing radius and the measured part of the core. (2) Missing rings. The average age underestimate from 48 cores due to missing rings was 13%. A significant correlation between radius length and age under estimate (r = 0.81) suggests that sampling along the longest radii will reduce this error. The average age underestimate due to missing rings from cores located along the longest radii of the twelve samples was 3%.
Maximum likelihood or restricted maximum likelihood (REML) estimates of the
parameters in linear mixed-effects models can be determined using the lmer
function in the lme4 package for R. As for most model-fitting functions in R,
the model is described in an lmer call by a formula, in this case including
both fixed- and random-effects terms. The formula and data together determine a
numerical representation of the model from which the profiled deviance or the
profiled REML criterion can be evaluated as a function of some of the model
parameters. The appropriate criterion is optimized, using one of the
constrained optimization functions in R, to provide the parameter estimates. We
describe the structure of the model, the steps in evaluating the profiled
deviance or REML criterion, and the structure of classes or types that
represents such a model. Sufficient detail is included to allow specialization
of these structures by users who wish to write functions to fit specialized
linear mixed models, such as models incorporating pedigrees or smoothing
splines, that are not easily expressible in the formula language used by lmer.
A methodology is proposed for developing a disturbance chronology in stands by identifying the probable date of canopy accession for each sample tree. Canopy disturbance intensity is defined as the percentage of sample trees with canopy accession events in each decade. Rotation periods for disturbances of various intensities are calculated from the chronology. The method was evaluated using 893 increment cores from 70 plots in northern hardwood stands of W Upper Michigan. Average disturbance rate for all plots and decades was 5.7-6.9% of land area per decade, with an implied average canopy tree residence time of 145-175 yr. -from Authors
Questions: How have the historical frequency and severity of natural disturbances
in primary Picea abies forests varied at the forest stand and landscape level
during recent centuries? Is there a relationship between physiographic attributes
and historical patterns of disturbance severity in this system?
Location: Primary P. abies forests of the Eastern Carpathian Mountains, Romania;
a region thought to hold the largest concentration of primary P. abies forests
in Europe’s temperate zone.
Methods: We used dendrochronological methods applied to many plots over a
large area (132 plots representing six stands in two landscapes), thereby providing
information at both stand and landscape levels. Evidence of past canopy disturbance
was derived from two patterns of radial growth: (1) abrupt, sustained
increases in growth (releases) and (2) rapid early growth rates (gap recruitment).
Thesemethods were augmented with non-metricmultidimensional scaling
to facilitate the interpretation of factors influencing past disturbance.
Results: Of the two growth pattern criteria used to assess past disturbance, gap
recruitment was the most common, representing 80% of disturbance evidence
overall. Disturbance severities varied over the landscape, including stand-replacing
events, as well as low- and intermediate-severity disturbances. More than
half of the study plots experienced extreme-severity disturbances at the plot
level, although they were not always synchronized across stands and landscapes.
Plots indicating high-severity disturbances were often spatially clustered
(indicating disturbances up to 20 ha), while this tendency was less clear for lowand
moderate-severity disturbances. Physiographic attributes such as altitude
and land form were only weakly correlated with disturbance severity. Historical
documents suggest windstorms as the primary disturbance agent, while the role
of bark beetles (Ips typographus) remains unclear.
Conclusions: The historical disturbance regime revealed in this multi-scale
study is characterized by considerable spatial and temporal heterogeneity,which
could be seen among plots within stands, among stands within landscapes and
between the two landscapes. When the disturbance regime was evaluated at
these larger scales, the entire range of disturbance severity was revealed within
this landscape.
For over a century, the negative exponential relationship of tree density to diameter has been observed and considered an indicator of equilibrium forest structure, especially at the stand level. More recent studies have suggested that other forms of diameter distributions should be considered “balanced”. Some of these seemingly contradictory findings can be resolved by understanding the impacts of scale and sampling area on the possibility of observing a negative exponential distribution. We calculated the minimum area required to sample the smallest number of trees necessary to produce hypothetical negative exponential distributions. The minimum area is influenced greatly by the maximum diameter and the exponential decrease rate of the distribution. It is also influenced by the width of diameter classes used to summarize the data, and the basal area per hectare of the sample. These differences in minimum required sampling area are important considerations because some recent studies that suggest a rotated sigmoid distribution, rather than negative exponential, is more characteristic of old-growth forests may have used sampling schemes that were insufficient to observe a negative exponential distribution even if one existed. Others have used criteria in selecting sampling locations that ensure a high density of large trees, thereby precluding them from observing a negative exponential distribution even if one existed. Our review suggests that within-stand sampling areas for testing diameter distribution in temperate forests typically need to exceed 1ha, especially in mature to old-growth stages, though mortality rate, diameter class width, and total stand basal area all affect the area or number of prism points needed. Diameter distributions have rarely been assessed at the landscape scale. Nonetheless, it is reasonable to expect that landscape distributions may generally be negative exponential and the slope of the line (i.e. the mortality rate) may be an important measure of landscape dynamics. Old-growth landscapes where natural disturbances are typically small patches will have a lower mortality rate than those where disturbances are more frequent and widespread.
Information on historical disturbances is vital to our understanding of current forest conditions. Dendro chronological methods provide one means of reconstructing disturbance histories in temperate and boreal forests. In particular, the dates of significant growth releases recorded on surviving trees provide strong inferential evidence of past disturbance events. The most common method of detecting releases (the percent-increase method) expresses the postevent growth increase as a percentage of the preevent rate. Despite its widespread use, the method is known to be overly sensitive at low rates of prior growth and overly stringent at high rates. We present an alternative method that directly follows the percent-increase method, but instead of dividing the postevent growth rate by the preevent rate, we simply subtract the two. If the difference exceeds a predetermined species-specific threshold, the event is considered a release. This absolute-increase method has convenient properties that remedy the shortcomings of the percent-increase method. We tested the validity of the absolute-increase thresholds by binary logistic regressions, and we compared the absolute- and percent-increase methods by various methods. We conclude that for the species evaluated in this study, the absolute-increase method represents an improvement over the standard percent-increase method.
1Disturbance histories derived from old-growth forest remnants in Europe and eastern North America have shaped many of our current theories of forest dynamics and succession. Yet the small size typical of these remnants suggests they might not capture the full range of variability that may emerge at larger scales.2We investigated the frequency and severity of natural disturbance in a 2000-ha old-growth landscape (Big Reed Forest Reserve) in northern Maine, USA. Given its size, the Reserve provides an ideal opportunity to study, at multiple scales, natural forest processes in a region that has otherwise been dramatically altered by human activities. Using dendrochronological methods, we reconstructed disturbance histories for 37 randomly located plots stratified by five forest types (hardwood forests, mixed woods forests, red spruce forests, northern white-cedar seepage forests and northern white-cedar swamps).3We found no evidence of stand replacing disturbance on any plot during the last 120–280 years (depending on plot). The overall mean disturbance rate was 9.6% canopy loss per decade (median 6.5%, maximum 55%, plots pooled), yet the distribution was strongly skewed toward the lower rates.4We found little differences in disturbance rates between forest types, save a slightly lower rate in the northern white-cedar swamps. However, if we ignore forest-type classifications, we see that disturbance rates are clearly influenced by gradients in the relative abundance of component tree species, owing to species’ relative susceptibilities to particular disturbance agents.5Synthesis. Relatively low rates of canopy disturbance allow the accrual of shade-tolerant saplings. The abundance of this advance regeneration, coupled with the absence of stand-replacing disturbance, has maintained canopy dominance by shade-tolerant species in all plots, all forest types and throughout the entire landscape. Disturbance histories from individual plots coalesce to form a picture of a landscape in which pulses of moderate-severity disturbance are interposed upon a background of scattered small-scale canopy gaps. The landscape-level mosaic resulting from this disturbance regime consists of patches in various stages of structural development, not various stages of compositional succession.
This paper reviews the literature concerning forest and woodland structure at the scale of an individual stand. Stand structure is defined in terms of structural attributes and stand structural complexity. Stand structural complexity is considered to be a measure of the number of different attributes present and the relative abundance of each of these attributes. The review indicates there is no definitive suite of structural attributes; different authors emphasise subsets of different attributes, and relatively few studies provide quantitative evidence linking attributes to the provision of faunal habitat or other measures of biodiversity, although a number of studies identify attributes that distinguish between successional stages. A summary of key structural attributes identified in the literature is presented under the following stand elements: foliage arrangement, canopy cover, tree diameter, tree height, tree spacing, tree species, stand biomass, understorey vegetation, and deadwood. Indices of structural complexity are also reviewed. Three types of index framework are identified: indices based on the cumulative score of attributes; indices based on the average score of groups of attributes; and indices based on the interaction of attributes. The review identifies a variety of different indices under each of these frameworks with no single index preferred over the others. The most prominent of these indices are discussed in detail and the following guidelines suggested for the development of an index of structural complexity: (1) Start with a comprehensive set of structural attributes, in which there is a demonstrated association between attributes and the elements of biodiversity that are of interest. (2) Use a simple mathematical system to construct the index; this facilitates the use of multiple attributes and interpretation of the index in terms of real stand conditions. (3) Score attributes relative to the range of values occurring in stands of a comparable vegetation community. (4) Try different weightings of attributes in the index, adopting those weightings which most clearly distinguish between stands.
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 sub‐regional 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 dataset of 541 plots located within 44 stands and 6 sub‐regions. 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 sub‐regions, 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.
There is concern in the scientific community and among forest managers about potential reductions in the provisioning of forest ecosystem services due to the loss of tree species diversity. Many studies have shown how species diversity influences forest functioning, especially productivity, but the influence of structural diversity, such as tree size heterogeneity, has received much less attention. This study focused on understanding the relationship between stand productivity and several structural characteristics of spruce fir-beech mountain forest stands in Europe. We used a dataset of 89 long-term plots in spruce-fir-beech forests distributed along the European mountains where the three species, Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), represent at least 75% of the basal area. Site-dependent conditions were accounted for in a linear mixed-effect basic model, which related the stand productivity with the morphological, climatic and pedological characteristics. The influence of tree species diversity, tree size heterogeneity, species size dominance, and species overlapping in the size distribution on stand productivity was analysed by adding
variables to the basic model one by one and evaluating the change in the Akaike's Information Criterion (AIC). The variables that resulted in significant reductions in the AIC, and that were not correlated with each other, were used to build a model to estimate stand productivity. The model showed that in spruce-fir-beech mixed mountain forests (i) when Norway spruce, silver fir and European beech are evenly present within the size distribution (high evenness) the productivity decreases, (ii) the stand productivity increases when the diameter distribution is skewed to the right (higher numbers of smaller individuals), (iii) the stand productivity increases as the proportion of basal area that is spruce increases, and (iv) stand productivity increases with the variability
in diameter. We discuss the implications of our results for the management of spruce-fir beech mountain forest in Europe and for preserving and increasing the stand productivity of these mixed forests.
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.
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.
Theoretical and empirical studies have suggested that climate and soils are the main drivers of biodiversity, stand structure and aboveground biomass in natural forests. Yet, the direct effects of climate and soils on aboveground biomass versus the indirect effects mediated by species diversity and stand structural complexity remain unclear in forest ecosystems across large-scale ecological gradients. Here, we hypothesized that 1) climate and soils would influence aboveground biomass through strong indirect effects; 2) stand structural complexity rather than species diversity would strongly mediate the response of aboveground biomass to climate and soils; and 3) species diversity and stand structural complexity would promote each other under the niche differentiation and facilitation effects, and that stand structural complexity would have positive effects on aboveground biomass across large-scale ecological gradients. To test these hypotheses, we quantified climatic water availability, soil total exchangeable bases, species diversity, stand structural complexity including tree DBH (diameter at breast height) diversity and height diversity, and aboveground biomass across 907 plots in tropical forests of Hainan Island, Southern China. We tested 126 structural equation models to examine the direct and indirect effects of climate and soils on aboveground biomass via species diversity and stand structural complexity. Climatic water availability and soil fertility did not affect aboveground biomass directly but did affect indirectly via increasing stand structural complexity rather than species diversity. Species diversity and stand structural complexity promoted each other, and both increased with increasing climatic water availability. Stand structural complexity increased aboveground biomass directly, whereas species diversity increased it indirectly via increasing stand structural complexity. The total effects of climatic water availability, soil fertility, stand structural complexity and species diversity on aboveground biomass were significantly positive. This study shows that climatic water availability exerts a strong direct effect on stand structural complexity, indicating that any decrease in climatic water availability (i.e. increasing atmospheric drought) may directly diminish stand structural complexity and hence indirectly reduce aboveground biomass and carbon storage. This study suggests that maintaining high stand structural complexity can enhance aboveground biomass under favourable climate and soils while maintaining the benefits of species diversity on stand structural complexity for better ecosystem services such as carbon storage.
Examines, in a variety of contexts, a number of theoretical and empirical relationships between disturbance (environmental fluctuations and destructive events, whether predictable and/or cyclical or not) and patch dynamics (where discrete spatial patterns possess internal characteristics and also inter-relate with surrounding patch and non-patch areas). The main sections are on: patch dynamics in nature; adaptations of plants and animals in a patch dynamic setting; and implications of patch dynamics for the organisation of communities and the functioning of ecosystems. A final chapter moves towards a general theory of disturbance. All 21 chapters are abstracted separately. -P.J.Jarvis
Norway spruce (Picea abies L.) is an important tree species with a remarkable natural range throughout Europe and Asia, ranging from the Balkan Peninsula to Siberia in the north and from the French Alps in the west to the Sea of Okhotsk in the east. Wherever it occurs, it is a key component of both natural and managed forests. Norway spruce is the most economically valuable conifer in Europe, producing high-quality timber and wood products.
This book presents a concise and comprehensive review of the biology, ecology, and management of Norway spruce. It integrates classic and contemporary literature (more than 2000 works cited in the text), highlighting basic research and forestry practices in central and eastern Europe. The topics include anatomy and morphology, physiology and nutrition, reproductive biology and genetics, and ecology.
In addition, it examines mycorrhiza, diseases and pests as well as silviculture and wood products. In the light of increasing threats to forest health from air pollution, climate change, and insects and disease, it provides an essential information source to those concerned with the ecology, conservation, and management of the species.
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.
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.
Mountain forests are among the most important ecosystems in Europe as they support numerous ecological, hydrological, climatic, social, and economic functions. They are unique relatively natural ecosystems consisting of long-lived species in an otherwise densely populated human landscape. Despite this, centuries of intensive forest management in many of these forests have eclipsed evidence of natural processes, especially the role of disturbances in long-term forest dynamics. Recent trends of land abandonment and establishment of protected forests have coincided with a growing interest in managing forests in more natural states. At the same time, the importance of past disturbances highlighted in an emerging body of literature, and recent increasing disturbances due to climate change are challenging long-held views of dynamics in these ecosystems. Here, we synthesize aspects of this Special Issue on the ecology of mountain forest ecosystems in Europe in the context of broader discussions in the field, to present a new perspective on these ecosystems and their natural disturbance regimes. Most mountain forests in Europe, for which long-term data are available, show a strong and long-term effect of not only human land use but also of natural disturbances that vary by orders of magnitude in size and frequency. Although these disturbances may kill many trees, the forests themselves have not been threatened. The relative importance of natural disturbances, land use, and climate change for ecosystem dynamics varies across space and time. Across the continent, changing climate and land use are altering forest cover, forest structure, tree demography, and natural disturbances, including fires, insect outbreaks, avalanches, and wind disturbances. Projected continued increases in forest area and biomass along with continued warming are likely to further promote forest disturbances. Episodic disturbances may foster ecosystem adaptation to the effects of ongoing and future climatic change. Increasing disturbances, along with trends of less intense land use, will promote further increases in coarse woody debris, with cascading positive effects on biodiversity, edaphic conditions, biogeochemical cycles, and increased heterogeneity across a range of spatial scales. Together, this may translate to disturbance-mediated resilience of forest landscapes and increased biodiversity, as long as climate and disturbance regimes remain within the tolerance of relevant species. Understanding ecological variability, even imperfectly, is integral to anticipating vulnerabilities and promoting ecological resilience, especially under growing uncertainty. Allowing some forests to be shaped by natural processes may be congruent with multiple goals of forest management, even in densely settled and developed countries.
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.
This study investigates temporal shifts in Norway spruce (Picea abies) mortality, stand structure characteristics, and stand complexity facilitated by a bark beetle (Ips typographus) outbreak that affected an unmanaged subalpine forest region in Tatra National Park, Poland in the late 2000s. Changes in survivorship and stand structure characteristics (diameter (DBH), basal area, height, age, and crown length ratio) of nearly 2500 spatially-referenced trees located in 64 long-term survey plots were compared over four time periods that spanned the duration of the outbreak disturbance event. Stand structure characteristics, topographic factors (slope, elevation, and aspect), and solar equinox radiation were tested as predictors of mortality for multiple stages in the outbreak using boosted regression tree modeling. Our findings showed that: (1) spatial synchrony was not reflective of mortality severity; (2) mortality rates increased significantly as the outbreak progressed; (3) the stand’s structure was altered significantly by the outbreak (larger trees were killed most frequently); (4) stand structure characteristics were the best predictors of mortality in all stages of the outbreak, though topographic factors and solar equinox radiation also exhibited moderate to strong predictive power in some stages; and (5) stand complexity decreased significantly as the outbreak progressed. This illustrates the inherently complex nature of bark beetle outbreaks on fine spatial scales and suggests that the extent and severity of spruce mortality during an outbreak event is largely dependent on the relative stage of the outbreak and the structure of the stand.
Forest structure is both a product and driver of ecosystem processes and biological diversity. It has become apparent in recent years that changes in forest structure as a result of management for timber production have undesirable consequences for other components of forest ecosystems. The objective of this paper is to provide an overview of what we have learned about the ecological roles of forest structure in the Pacific Northwest and how forest structure changes as a result of disturbance and succession. Forests are structurally diverse, but many structures derive from the same processes of disturbance and growth. Consequently, measurements on a few structural attributes can be used to estimate many other structural conditions. Particularly important components of forest structure include live-tree sizes, vertical foliage distributions, horizontal variation in canopy density, and coarse woody debris. Knowledge of the ecological roles of these structures has increased in recent years and we now have a general understanding of how these structures change during succession. Although the ecological values of forest structures are now more widely appreciated, we still have many significant knowledge gaps including the ecological roles of below-ground structure, woody debris, and landscape pattern.
Windthrow is all too often looked at as an exceptional, catastrophic phenomenon rather than a recurrent natural disturbance that falls within the spectrum of chronic and acute effects of wind on forests, and that drives ecosystem patterns and processes. This paper provides an integrative overview of the nature, contributing factors and impacts of wind-caused disturbance in forests, including its effects on trees, stands, landscapes and soils. Windthrow is examined through the integrating concepts of: the capacity of trees for acclimative growth, the limitation of acclimative growth under inter-tree competition, the recurrent nature of severe weather, how terrain and soil conditions affect local stand vulnerability and the effect of recurrent windthrow on stand dynamics and soils. Windthrow management should take place within a framework of general risk management, with evaluation of the likelihood, severity and potential impacts of wind damage considered with reference to the broad and specific aims of management. There is much to be gained from interdisciplinary communication about the nature and consequences of recurrent wind damage. There are opportunities for climatologists, engineers, ecologists, geomorphologists and others to develop integrative process models at the tree, stand and landscape scales that will improve our collective understanding, and inform management decision-making.
Disturbances from wind, bark beetles, and wildfires have increased in Europe's forests throughout the 20(th) century (1). Climatic changes were identified as a main driver behind this increase (2), yet how the expected continuation of climate change will affect Europe's forest disturbance regime remains unresolved. Increasing disturbances could strongly impact the forest carbon budget (3,4), and are hypothesized to contribute to the recently observed carbon sink saturation in Europe's forests (5). Here we show that forest disturbance damage in Europe has continued to increase in the first decade of the 21(st) century. Based on an ensemble of climate change scenarios we find that damage from wind, bark beetles, and forest fires is likely to increase further in coming decades, and estimate the rate of increase to +0.91·10(6) m(3) of timber per year until 2030. We show that this intensification can offset the effect of management strategies aiming to increase the forest carbon sink, and calculate the disturbance-related reduction of the carbon storage potential in Europe's forests to be 503.4 Tg C in 2021-2030. Our results highlight the considerable carbon cycle feedbacks of changing disturbance regimes, and underline that future forest policy and management will require a stronger focus on disturbance risk and resilience.
For many types of forest studies, it is essential to identify the exact years of formation of annual rings in increment cores taken from living trees. To accomplish this, dendrochronologists employ cross dating, which involves both ring counting and ring-width pattern matching, to ensure against counting error, or errors, caused by missing or false rings. To date, published accounts of the cross-dating process generally describe a graphical method for achieving cross dating, known as skeleton plotting. However, when working with cores from living trees, skeleton plotting is seldom necessary. Such cores can commonly be cross-dated more quickly and easily by listing the narrow rings that are present in each core in a laboratory notebook and then comparing core notes for shared narrow rings. The latter approach permits faster recognition of ring-width patterns because calendar-year, rather than relative-year, dates are assigned to rings in cores. It also allows cross-dating records to be stored in a more concise manner.
Disturbances influence forest dynamics across a range of spatial and temporal scales. In tropical forests most studies have focused on disturbances occurring at small spatial and temporal scales (i.e., gap dynamics). This is primarily due to the difficulty of reconstructing long-term disturbance histories of forests in which most tree species lack annual growth rings. Consequently, the role of past disturbances in tropical forests is poorly understood. We used a combination of direct and indirect methods to reconstruct the his- torical disturbance regime and stand development patterns in mature and regenerating seasonal dry evergreen forest (SDEF) in the Huai Kha Khaeng Wildlife Sanctuary in western Thailand. Direct estimates of long-term establishment and growth patterns were obtained from 12 tree species that form annual growth rings as a consequence of the region's strong intra-annual rainfall seasonality. Indirect estimates of establishment patterns were obtained from analyses of stand structure and individual tree architecture and application of age- estimation models to 10 dominant canopy-tree species using demographic data from a large- scale, permanent forest-dynamics plot. The combination of direct and indirect methodologies revealed a complex disturbance history in the seasonal evergreen forest over the past 250 years. In the mid-1800s, 200-300 ha of forest were destroyed by a catastrophic disturbance, which led to the synchronous es- tablishment of many of the trees that presently dominate the forest canopy. Since then wide- spread disturbances of variable intensity have occurred at least three times (1910s, 1940s, and 1960s). These disturbances created discrete temporal pulses of establishment in small to large gaps in the forest matrix across several square kilometers. Background mortality and gap formation were evident in every decade since 1790, but these varied in intensity and frequency. The SDEF retains a distinct structural and floristic legacy from the catastrophic dis- turbance of the mid-1800s. The single-age cohort that established after the disturbance has developed a complex three-dimensional structure as a consequence of differences in in- terspecific growth patterns of the canopy-tree species and subsequent disturbances of mod- erate and low intensity. While no single methodological approach provided a complete picture of the disturbance history and stand development patterns of the seasonal evergreen forest, taken together they offered new insights into the long-term dynamics of a primary tropical forest. In particular, the study highlighted the role of disturbance at multiple spatial and temporal scales and varying intensities in determining the structure and composition of a complex, species-rich tropical forest and raises important questions about the role of rare, catastrophic events on tropical forest dynamics.
The authors begin by outlining the role of the dendrochronologists both in the field and in the laboratory. The basic principles of tree-ring dating are then explained in detail, followed by a guide to the collection of archaeological and modern specimens from the field. The final section deals with the laboratory techniques used: the preliminary processing and preparation of archaeological and modern specimens; the process of dating specimens; and finally the compilation of a master chronology.
The frequency of natural disturbances and their influence on the forest landscape mosaic were investigated on three large tracts of primary forest in Upper Michigan. Seventy 0.5-ha plots were randomly distributed in a total forest area of 23 000 ha dominated by sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis). Radial increment patterns were used to estimate canopy accession dates for each of a number of randomly selected overstory trees on each plot. From these data a disturbance chronology, representing the percentage of stand area occupied by cohorts originating during each decade over the last 130 yr, was compiled for each plot. Average rates of disturbance or canopy mortality are estimated at 5.7 to 6.9% per decade. The corresponding average canopy residence time of a tree is 145-175 yr. No significant differences were detected in average disturbance rates among the three study areas, between plots near the coast of Lake Superior and inland plots, among several different aspects, and among several different slope positions. Natural rotation periods increase exponentially with increasing disturbance intensity, which is defined as the approximate percentage of the plot area converted to gaps during a disturbance episode. Estimates of rotation periods range from 69 yr for greater-than-or-equal-to 10% canopy removal to 1920 yr for greater-than-or-equal-to 60% canopy removal. Spatial autocorrelation analysis indicated that plots with light and medium disturbances (< 40%) are randomly distributed over the landscape. Plots with heavy disturbances (greater-than-or-equal-to 40%) are clustered with a patch radius of almost-equal-to 2 km, consistent with the sizes of thunderstorm downbursts. The data indicate that light and medium disturbances dominate the disturbance regime. The majority of stands on the landscape are composed of several major and many minor age classes. Even-aged stands with one predominant age class are uncommon. The age distribution of individual patches or cohorts in the two larger study areas (14 500 and 6073 ha) follows a nearly uniform distribution. None of the three study areas had more than 15% of the forest area converted to gaps in a single decade. The two larger areas meet most of the criteria that have been proposed for equilibrium landscapes.
a b s t r a c t Correct knowledge of disturbance ecology is essential for understanding the characteristic behavior of forest ecosystems and for guiding appropriate management strategies. However, the role of natural dis-turbances in shaping European mountain forest ecosystems has not been adequately studied, possibly because of the perception that the development of most European forests is primarily shaped by human influences and/or fine-scale gap-phase dynamics. In the present study, we investigate the long-term disturbance history of old protected forest dominated by Norway spruce in the Parangalitsa Reserve, Bulgaria. We used aerial photo interpretation and dendro-ecological methods to reconstruct the history of wind, insect, and fire disturbances across a topographically complex landscape. Over the past 150 years wind has been the most important disturbance agent in this ecosystem and at least 18% of the forested area shows evidence of high-severity blowdowns. Windthrow patches ranged in size from <1 ha to >10 ha (minimum 0.11 ha, mean 0.16 ha, maximum 10 ha). Although small disturbances were much more frequent, few larger blowdowns accounted for most of the disturbed area. Pure coniferous and single-cohort coniferous forest patches were more affected by blowdowns than mixed coniferous–deciduous and multi-cohort coniferous forest patches. Although bark beetle (Ips typhog-raphus) populations were large enough to cause mortality of some live trees, the populations did not grow to epidemic proportions during recent decades. Fire disturbance was of limited importance in the last 200 years and only two patches (4% of the study area) showed evidence of fire. The present research indicates that wind disturbances have been characteristic of these ecosystems at least over the past decades to centuries. Thus, blowdowns appear integral to the normal function and struc-ture of the Picea-dominated mountain forests in the region and such events, in and of themselves, do not represent unhealthy forest conditions or environmental emergencies. Management strategies that aim to maintain these ecosystems within a natural range of variation should incorporate wind disturbances into the management strategy. The frequency and magnitude of future wind disturbances may be considered within the historical framework described in the current study to assess potential effects of climate change on altered disturbance regimes.
The Weibull probability density function is proposed as a diameter distribution model. Its advantages include flexibility in shape and simplicity of mathematical derivations. Estimation and interpretation of parameters are discussed and illustrated with published data. Forest Sci. 19:97-104.
• After storm disturbances, there is a risk for degradation of the quality of fallen trees, and for subsequent tree mortality caused by the spruce bark beetle Ips typographus (L.) (Coleoptera: Curculionidae). Models assessing the risk for bark beetle colonization of different kinds of storm gaps would be a valuable tool for management decisions. • The present study aimed to determine which gap and landscape characteristics are correlated with the probability of colonization of wind-felled Norway spruce trees by I. typographus. • The study included 36 storm gaps, varying in size from three to 1168 wind-felled spruces, created by the storm Gudrun in southern Sweden in January 2005. • In the first summer, on average, 5% of the wind-felled spruces were colonized by I. typographus. The percentage of colonized wind-felled trees per gap was negatively correlated with the total area of storm gaps within 2000 m in the surrounding forest landscape. • In the second summer, the proportion of colonized trees increased to 50%. Both gap (mean diameter of wind-felled trees and basal area of living spruce trees) and landscape variables (amount of spruce forest) were significantly correlated with colonization percentage and explained almost 50% of the variation between gaps. • There was no relationship between gap area and colonization percentage. This implies that landscapes with many large storm gaps, where logging resources will be most effectively used, should be salvaged first.
1Tree-size distributions are changing in many natural forests around the world, and it is important to understand the underlying processes that are causing these changes. Here we use a classic conceptual framework – the shifting mosaic of patches model – to explore the ways in which competitive thinning and disturbance influence tree-size distributions, and to consider the effects of temporal variability in disturbance frequency on the size structure of forests.2We monitored 250 stands of Nothofagus solandri var. cliffortiodes (mountain beech), randomly distributed over 9000 hectares, for 19 years. Mountain beech is a light-demanding species that forms monospecific forests in New Zealand mountains. For the purposes of our model, we assumed that each stand functions as an even-aged population: it is initiated by a pulse of recruitment, undergoes competitive thinning as it matures, and is eventually destroyed by a disturbance event. The tree-size distribution of the whole forest is driven partly by the frequency and temporal patchiness of disturbance events and partly by competitive processes within the constituent stands.3Temporal changes in stem density and mean tree size were observed to be remarkably similar in all young stands, indicating that a consistent packing rule operates during this phase of stand development. A popular idea in the self-thinning literature is that the maintenance of constant leaf area index (LAI) provides the mechanism for this packing rule, but our analyses suggest that LAI increased by about 30% during the thinning phase. We use leaf economic theory to develop a new packing rule based on light interception, and argue that LAI increases with stand age because of changes in canopy organisation.4Smaller trees were significantly more likely to die than larger trees within the young stands. Tree-diameter distributions within young stands were left skewed but those of older populations were normally distributed. These observations are consistent with asymmetric competition winnowing out small, suppressed trees from young stands but having less effect in older stands.5Large-scale disturbances created gaps of sufficient size to allow mass recruitment of seedlings in about 0.8% of stands each year. Older stands were most susceptible to such large-scale disturbance, but the trend was weak.6The diameter-distribution of the whole Nothofagus forest was found to be approximately exponential in form. Simulation models only produced realistic diameter distributions when competitive packing rules and disturbance were included. Therefore, the shifting mosaic model provides a general framework for understand the ways in which these mortality processes determine forest size structure.7The diameter distribution of the forest was not in equilibrium over the 19-year study. Using simulation models, we show that temporal variability in disturbance frequency can generate enormous deviations in tree-diameter distributions away from the long-term mean, leading us to conclude that modern-day disequilibrium in natural forests may be the legacy of past disturbance events.
The study presents an analysis of the diameter distributions of nine virgin beech forests (Fagus sylvatica) in south-eastern Europe. Data were collected from published and unpublished sources. We included predominantly full calliperings of coherent areas between 3.6 and 13.0 ha of size. The objective of the study was to compare and characterize the curve forms and to test systematically which mathematical function provides a better fit: the negative exponential function, third or seventh degree Weibull functions. The parameters were estimated applying the maximum likelihood method. To evaluate the goodness of fit, the absolute discrepancy was used. The residuals were examined with respect to systematic deviation.The nine virgin forests displayed a great variety of structures, only four out of 36 possible pairs of diameter distributions were found to be from the same population. The negative exponential model produced a good fit for four of the nine empirical distributions. However, the analysis of the residuals produced by the models exhibited systematic errors. Both the negative exponential and the third degree Weibull function underpredict to some extent the number of stems in the midsize diameter range in all nine stands. The higher number of trees frequently found in the midsize range of the empirical distributions as compared to the models indicates a common trend towards a rotated sigmoid diameter distribution. For some stands even a tendency towards two peaks in the distribution can be found. These results reveal that the reverse J-shaped curve form is not the only applicable model for describing diameter distributions in virgin beech forests. The systematic nature of the deviations from the negative exponential curve leads us to conclude that we might have found a general trend typical for virgin beech forests in south-eastern Europe.
Summary • Many theoretical models have been proposed to explain the empirical self-thinning relationship given by Yoda et al. in 1963 for even-aged, monospecific stands of plants, but the models are inadequate to allow consensus on the processes driving variation in density-dependent mortality and self-thinning. • Most non-individual based models (non-IBMs), and many IBMs, employ a common representation of competition in which a finite amount of potential crown area remains completely allocated throughout self-thinning, making stand density inversely proportional to mean projected crown area. • This representation entails four assumptions regarding the competition process: the population is adequately represented by the mean plant; total stand resource utilization is constant throughout self-thinning; competition is a horizontal packing process; and differences in initial stand conditions may affect the rate of competition but not the process itself. • Reviewing published empirical data, the competition literature and the logical implications for the self-thinning process shows that all four assumptions are untenable as generalizations. Unfortunately, their application provides neither a mortality-inducing mechanism nor insight into the relationship between stand growth and mortality. • Explaining the observed variation in self-thinning relationships therefore requires improved representation of the competition process. This improvement is likely to require IBMs that explicitly represent variation in plant size or resource acquisition, two-dimensional stand distribution, dynamic rather than static stand resource utilization, and, perhaps, explicit three-dimensional stand development. Most importantly, the requirement for explicitly modelling mortality mechanisms implies that whole plant models may be insufficient for insight into the self-thinning process. • The review reinforces the need to assess mechanistic models for more than their ability to reproduce a single, high-level pattern. Such models should be assessed for their ability to simultaneously reproduce multiple features selected from the levels of both the modelled mechanisms and the high-level patterns. • Progress in understanding the observed variation in self-thinning currently requires a shift from searching for universal insight into the modelling of specific mechanisms for specific plant types, eventually leading to a broader theory explaining how variation in plants affects the competition process. Journal of Ecology (2005) doi: 10.1111/j.1365-2745.2005.00976.x
Question: To what extent do tree growth, mortality, and long-term disturbance patterns affect stand structure and composition of an old-growth Picea abies forest?Location: Boreal Sweden.Methods: We linked data from three 50 m × 50 m permanent plots established in 1986 with dendrochronology data to evaluate tree growth and mortality over an 18-year period and to describe a several-hundred-year disturbance history for this forest type.Results: Averaged over all diameters, P. abies trees had an annual mortality rate of 0.60%; however, diameter had a striking effect on both growth and mortality, with trees of intermediate diameters (ca. 20–30 cm) showing faster growth and lower mortality. Their increased vigor gave rise to a diameter distribution resembling the ‘rotated sigmoid’(not reverse-J) proposed for such conditions, and it led to a deficit of snags of intermediate diameters. Slow-growing trees had an increased likelihood of dying. Although recruitment occurred in most decades over the past 400 years, two prominent recruitment peaks occurred (mid 1700s and 1800s), neither of which appeared to cause a shift in tree species composition. The lack of fire evidence suggests that fire was not responsible for these recruitment peaks.Conclusions: Taken together, these results depict a rather impassive system, where canopy trees die slowly over decades. Field observations suggest that fungal infections, mediated by wind, account for much of the mortality during these periods of relative quiescence. However, these periods are at times punctuated by moderate-severity disturbances that foster abundant recruitment.
The outbreaks of Ips typographus (L.) in Central Europe after severe storms in the 1990s triggered extensive research. Molecular techniques were used to analyze the relations and origins of European Ips species. The biological characteristics of I. typographus such as the influence of temperature on life history parameters and flight behavior have been analyzed in detail. The spruce bark beetle was found to disperse well beyond 500 m. However, new attacks mostly occurred in the vicinity of old ones.Many studies refer to invertebrate natural enemies of I. typographus such as predatory beetles and flies as well as various parasitoids. While the species assemblages of antagonists have been extensively investigated their dynamics and impact on I. typographus populations are controversial.The susceptibility and defense mechanisms of host trees are crucial for a successful attack by bark beetles. Newly attacked trees respond with preformed resin, local wound reactions and eventually with systemic changes in their physiology. Risk assessments have been performed at both tree and stand level. Risk of attack seems to be mainly related to the exposition, age, and nutrient and water supply of the trees. The dynamics of outbreaks largely depends on insect abundance, tree susceptibility, weather conditions, and human measures. This renders predictions difficult. Various control techniques are reviewed and the need for more sophisticated risk assessment tools is stressed.
We show that explicit mathematical and biological relationships exist among the scaling exponents and the allometric constants (α and β, respectively) of log - log linear tree-community size frequency distributions, plant density N T, and minimum, maximum and average stem diameters (D min, D max, and D̄, respectively). As individuals grow in size and D max increases, N T is predicted to decrease as reflected by a decrease in the numerical value of α and an increase in the value of β. Our derivations further show that N T decreases as D̄ increases even if D min or D max remain unchanged. Because D max and the age of the largest individuals in a community are correlated, albeit weakly, we argue that the interdependent relationships among the numerical values of α, β, N T, and D̄ shed light on the extent to which communities have experienced recent global disturbance. These predicted relationships receive strong statistical support using two large datasets spanning a broad spectrum of tree-dominated communities.