An analytical model of stand dynamics as a function of tree growth, mortality and recruitment: the shade tolerance-stand structure hypothesis revisited

Light competition and interspecific differences in shade tolerance are considered key determinants of forest stand structure and dynamics. Specifically two main stand diameter distribution types as a function of shade tolerance have been proposed based on empirical observations. All-aged stands of shade tolerant species tend to have steeply descending, monotonic diameter distributions (inverse J-shaped curves). Shade intolerant species in contrast typically exhibit normal (unimodal) tree diameter distributions due to high mortality rates of smaller suppressed trees. In this study we explore the generality of this hypothesis which implies a causal relationship between light competition or shade tolerance and stand structure. For this purpose we formulate a partial differential equation system of stand dynamics as a function of individual tree growth, recruitment and mortality which allows us to explore possible individual-based mechanisms--e.g. light competition-underlying observed patterns of stand structure--e.g. unimodal or inverse J-shaped equilibrium diameter curves. We find that contrary to expectations interspecific differences in growth patterns can result alone in any of the two diameter distributions types observed in the field. In particular, slow growing species can present unimodal equilibrium curves even in the absence of light competition. Moreover, light competition and shade intolerance evaluated both at the tree growth and mortality stages did not have a significant impact on stand structure that tended to converge systematically towards an inverse J-shaped curves for most tree growth scenarios. Realistic transient stand dynamics for even aged stands of shade intolerant species (unimodal curves) were only obtained when recruitment was completely suppressed, providing further evidence on the critical role played by juvenile stages of tree development (e.g. the sampling stage) on final forest structure and composition. The results also point out the relevance of partial differential equations systems as a tool for exploring the individual-level mechanisms underpinning forest structure, particularly in relation to more complex forest simulation models that are more difficult to analyze and to interpret from a biological point of view.     

Spatial pattern and process in forest stands within the Virginia piedmont

Abstract. Question: Underlying ecological processes have often been inferred from the analysis of spatial patterns in ecosystems. Using an individual‐based model, we evaluate whether basic assumptions of species’life‐history, drought‐susceptibility, and shade tolerance generate dynamics that replicate patterns between and within forest stands. Location: Virginia piedmont, USA. Method: Model verification examines the transition in forest composition and stand structure between mesic, intermediate and xeric sites. At each site, tree location, diameter, and status were recorded in square plots ranging from 0.25 to 1.0 ha. Model validation examines the simulated spatial pattern of individual trees at scales of 1–25 m within each forest site using a univariate Ripley's K function. Results: 7512 live and dead trees were surveyed across all sites. All sites exhibit a consistent, significant shift in pattern for live trees by size, progressing from a clumped understorey (trees ± 0.1 m in diameter) to a uniform overstorey (trees > 0.25 m). Simulation results reflect not only the general shift in pattern of trees at appropriate scales within sites, but also the general transition in species composition and stand structure between sites. Conclusions: This shift has been observed in other forest ecosystems and interpreted as a result of competition; however, this hypothesis has seldom been evaluated using simulation models. These results support the hypothesis that forest pattern in the Virginia piedmont results from competition involving species’life‐history attributes driven by soil moisture availability between sites and light availability within sites.     

Capturing juvenile tree dynamics from count data using Approximate Bayesian Computation

The juvenile life stage is a crucial determinant of forest dynamics and a first indicator of changes to species' ranges under climate change. However, paucity of detailed re-measurement data of seedlings, saplings and small trees means that their demography is not well understood at large scales, and rarely represented in forest models in detail. In this study we quantify the effects of climate and density dependence on recruitment and juvenile growth and mortality rates of thirteen species measured in the Spanish Forest Inventory. Single-census sapling count data is used to constrain demographic parameters of a simple forest juvenile dynamics model based on the perfect plasticity approximation model (PPA) within a likelihood-free parameterisation method, Approximate Bayesian Computation. Our results highlight marked differences between species, and the important role of climate and stand structure, in controlling juvenile dynamics. Recruitment had a hump-shaped relationship with conspecific density, and for most species conspecific competition had a stronger negative effect than heterospecific competition. Mediterranean species showed on average higher mortality and lower growth rates than temperate species, and in low density stands recruitment and mortality rates were positively correlated. Under climate change our model predicted declines in recruitment rates for almost all species. Reliable predictive models of forest dynamics should include realistic representation of critical early life-stage processes and our approach demonstrates that existing coarse count data can be used to parameterise such models. Approximate Bayesian Computation may have wide application in many fields of ecology to unlock information about past processes from single survey observations.     

Effects of disturbance and size structure on the regeneration process in a sub-boreal coniferous forest,northern Japan

The stand structure and disturbance history in a sub-boreal coniferous forest dominated byPicea jezoensis, Picea glehnii andAbies sachalinensis were investigated in four study plots set up in Taisetsuzan National Park, Japan. The effect of stand characteristics on the growth and mortality rates of understory trees was examined. Although all the stands showed inverse J-shape d.b.h. (diameter at breast height) distributions, the age structure and disturbance history differed amongst the stands. The stands with wide d.b.h. distribution (i.e. large CV and skewness) were more uneven-aged than those with narrow d.b.h. distribution (i.e. small CV and skewness). The disturbance-return interval based on the model of Hett and Loucks was 31 to 65 years. The gap ratio in the canopy was also different among the stands. These suggest that the variations in stand structure represent different occurrences of natural disturbances. Furthermore, the structural features such as size structure, canopy gap ratio and density of canopy trees also affected the growth dynamics of understory trees (≥2 m in height and <10 cm in diameter at breast height). The growth and mortality rates of understory trees changed with the canopy gap ratio and canopy tree density. The understory trees of stands with wide canopy d.b.h. distribution had higher growth and canopy recruitment rates than those of stands with narrow canopy d.b.h. distribution, contributing to the maintenance of continuous stand stratification. The understory trees of stands with narrow canopy d.b.h. distribution showed lower growth and higher mortality rates than those of stands with narrow canopy d.b.h. distribution, leading to the formation of a single-canopy structure. It is suggested that natural disturbance governs the regeneration process in the future by affecting the growth and mortality patterns of understory trees through the stand structure (size and age structure, canopy tree density, canopy gap ratio).     

How stand productivity results from size- and competition-dependent growth and mortality

Background

A better understanding of the relationship between stand structure and productivity is required for the development of: a) scalable models that can accurately predict growth and yield dynamics for the world''s forests; and b) stand management regimes that maximize wood and/or timber yield, while maintaining structural and species diversity.

Methods

We develop a cohort-based canopy competition model (“CAIN”), parameterized with inventory data from Ontario, Canada, to examine the relationship between stand structure and productivity. Tree growth, mortality and recruitment are quantified as functions of diameter and asymmetric competition, using a competition index (CAI_h) defined as the total projected area of tree crowns at a given tree''s mid-crown height. Stand growth, mortality, and yield are simulated for inventoried stands, and also for hypothetical stands differing in total volume and tree size distribution.

Results

For a given diameter, tree growth decreases as CAI_h increases, whereas the probability of mortality increases. For a given CAI_h, diameter growth exhibits a humped pattern with respect to diameter, whereas mortality exhibits a U-shaped pattern reflecting senescence of large trees. For a fixed size distribution, stand growth increases asymptotically with total density, whereas mortality increases monotonically. Thus, net productivity peaks at an intermediate volume of 100–150 m³/ha, and approaches zero at 250 m³/ha. However, for a fixed stand volume, mortality due to senescence decreases if the proportion of large trees decreases as overall density increases. This size-related reduction in mortality offsets the density-related increase in mortality, resulting in a 40% increase in yield.

Conclusions

Size-related variation in growth and mortality exerts a profound influence on the relationship between stand structure and productivity. Dense stands dominated by small trees yield more wood than stands dominated by fewer large trees, because the relative growth rate of small trees is higher, and because they are less likely to die.     

Microsite variation and climate result in stand dynamics variation in pristine Pinus sylvestris mires: Evidence from a stand structure study

Questions: How do climate conditions and the site's ecohy‐ drological properties affect the age and size structure of natural Pinus sylvestris stands on pristine boreal mires? How do the long‐term stand dynamics on mires proceed as stands age? Do the mire stands reach a balanced, old‐growth stage? Location: Boreal mire forests in southern and northern Finland. Methods: Tree age and diameter distributions were analysed in 52 stands in two climate areas and in two mire site types with different ecohydrological properties. Temporal stand dynamics were examined by (1) comparing the graphs of the stands’ mean tree ages by diameter at breast height (1.3 m) classes and (2) describing the changes in stand characteristics and stand age and size structures as a function of stand dominant age in a chronosequence. Results: In the south, the DBH distributions were mostly unimodal and bell‐shaped in both site type groups. Age distributions were multimodal and flat in fully‐stocked sites but more uneven in sparsely forested composite sites. In the north, both the age and size distributions were clearly uneven in both site type groups. Tree age and size variation increased with stand age, but levelled out in the long term. Particularly in the south, the abundance of small trees decreased as stand age increased. Conclusions: The pine stands on pristine boreal mires are more dynamic than anticipated and are generally not characterised by a balanced, self‐perpetuating structure. Their dynamics reflect differences in climate and ecohydrology: on stocked sites in favourable boreal conditions, the stands showed structures typically resultant of inter‐tree competition processes that control tree growth and regeneration, whereas in harsh boreal climates, the tree regeneration process is ongoing diversifying the stand structure.     

Fire disturbance and forest structure in old‐growth mixed conifer forests in the northern Sierra Nevada,California

Question: This study evaluates how fire regimes influence stand structure and dynamics in old‐growth mixed conifer forests across a range of environmental settings. Location: A 2000‐ha area of mixed conifer forest on the west shore of Lake Tahoe in the northern Sierra Nevada, California. Methods: We quantified the age, size, and spatial structure of trees in 12 mixed conifer stands distributed across major topographic gradients. Fire history was reconstructed in each stand using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing the fire history with the age, size, and spatial structure of trees in a stand. Results: There was significant variation in species composition among stands, but not in the size, age and spatial patterning of trees. Stands had multiple size and age classes with clusters of similar aged trees occurring at scales of 113 ‐ 254 m². The frequency and severity of fires was also similar, and stands burned with low to moderate severity in the dormant season on average every 9–17 years. Most fires were not synchronized among stands except in very dry years. No fires have burned since ca. 1880. Conclusions: Fire and forest structure interact to perpetuate similar stand characteristics across a range of environmental settings. Fire occurrence is controlled primarily by spatial variation in fuel mosaics (e.g. patterns of abundance, fuel moisture, forest structure), but regional drought synchronizes fire in some years. Fire exclusion over the last 120 years has caused compositional and structural shifts in these mixed conifer forests.     

A comparison of species composition and stand structure between planted and natural mangrove forests in Shenzhen Bay,South China

Aims For assisting faster restoration of damaged or severely disturbed coastal ecosystems, selected mangrove species have been planted on previously mangrove-inhabited sites of the tropical and subtropical coasts of southern China. The objective of this study was to understand the stand dynamics of the planted mangroves and their functional traits in comparison with natural mangrove forests under similar site conditions.Methods Species composition, stand density, tree size distribution, and aboveground production were investigated along three transects in a 50-year-old planted mangrove stand and three transects in an adjacent natural mangrove stand in Shenzhen Bay, South China. Measurements were made on tree distribution by species, stand structure, and aboveground biomass (AGB) distribution. Analyses were performed on the spatial patterns of tree size distribution and species association.Important findings We found that the planted and natural mangrove stands did not differ in stand density, average diameter at breast height (DBH), species composition, and AGB. Spatial distribution of AGB and frequency at species level were also similar between the planted and natural stands. However, the traits in stand structure were more variable in the planted stand than in the natural stand, indicating higher spatiotemporal heterogeneity in the development and succession of planted mangroves. Geostatistical analyses show that both DBH and AGB were spatially auto-correlated within a specific range in the direction perpendicular to coastline. More than 60% of the variance in these attributes was due to spatial autocorrelation. The Ripley's K -function analysis shows that the two dominant species, Kandelia obovata and Avicennia marina, clumped in broader scales in the natural stand than in the planted stand and displayed significant interspecific competition across the whole transect. It is suggested that interspecific competition interacts with spatial autocorrelation as the underlying mechanism shaping the mangrove structure. This study demonstrates that at age 50, mangrove plantations can perform similarly in stand structure, spatial arrangement of selected stand characteristics and species associations to the natural mangrove forests.     

Contrasting patterns of tree mortality in late‐successional Picea abies stands in two areas in northern Fennoscandia

Question: What are tree mortality rates and how and why do they vary in late‐successional Picea abies‐dominated forests? Do observed tree mortality patterns allow comparative assessment of models of long‐term stand development? Location: Northern boreal Fennoscandia. Methods: We measured stand structure in 10 stands in two different areas. We determined age distributions and constructed a chronology of tree deaths by cross‐dating the years of death of randomly sampled dead trees. Results: The stands in the two areas had contrasting tree age distributions, despite similar live tree structure. In one area, stands were relatively even‐aged and originated following a stand‐replacing fire 317 years earlier. The stands in the second area had an uneven age structure and virtually no signs of past fires, suggesting a very long period since the last major disturbance. The younger stands were characterized by a high mortality rate and inter‐annual variation, which we attributed to senescence of the relatively even‐aged stands approaching the maximum age of P. abies. In contrast, the tree mortality rates in the older stands were low and relatively stable. Conclusions: Patterns of tree mortality were, to a large extent, dependent on the time since the last stand‐replacing disturbance, suggesting that northern boreal P. abies stands eventually reach a shifting mosaic state maintained through small‐scale dynamics, but the time needed to reach this state appears to be lengthy; even 300 years after a forest fire stands showed changes in patterns of tree mortality that were related to the developmental stage of the stands.     

Within‐stand spatial structure and relation of boreal canopy and understorey vegetation

Question: How do spatial patterns and associations of canopy and understorey vegetation vary with spatial scale along a gradient of canopy composition in boreal mixed‐wood forests, from younger Aspen stands dominated by Populus tremuloides and P. balsamifera to older Mixed and Conifer stands dominated by Picea glauca? Do canopy evergreen conifers and broad‐leaved deciduous trees differ in their spatial relationships with understorey vegetation? Location: EMEND experimental site, Alberta, Canada. Methods: Canopy and understorey vegetation were sampled in 28 transects of 100 contiguous 0.5 m × 0.5 m quadrats in three forest stand types. Vegetation spatial patterns and relationships were analysed using wavelets. Results: Boreal mixed‐wood canopy and understorey vegetation are patchily distributed at a range of small spatial scales. The scale of canopy and understorey spatial patterns generally increased with increasing conifer presence in the canopy. Associations between canopy and understorey were highly variable among stand types, transects and spatial scales. Understorey vascular plant cover was generally positively associated with canopy deciduous tree cover and negatively associated with canopy conifer tree cover at spatial scales from 5–15 m. Understorey non‐vascular plant cover and community composition were more variable in their relationships with canopy cover, showing both positive and negative associations at a range of spatial scales. Conclusions: The spatial structure and relation of boreal mixed‐wood canopy and understorey vegetation varied with spatial scale. Differences in understorey spatial structure among stand types were consistent with a nucleation model of patch dynamics during succession in boreal mixed‐wood forests.     

不同采伐强度对阔叶红松林主要树种空间分布格局和物种空间关联性的影响

分析采伐后森林群落中物种的空间格局有助于认识该格局形成的生态学过程、种群的生物学特性及其与环境因子之间的相互关系,并对制定可持续的森林经营方案具有重要意义。以长白山地区经历不同采伐方式形成的阔叶红松林次生林为研究对象,利用空间点格局分析的研究方法,探讨了采伐对阔叶红松林主要树种空间分布格局、种间关联性以及更新的影响。研究结果显示:较低强度的择伐对阔叶红松林主要树种的空间分布格局、种间关联性的改变较小,群落可以在较短时间内恢复。中等强度的择伐减少了成年树种对幼树的抑制作用,可以促进森林的天然更新。皆伐后,森林的群落结构,物种的空间分布格局、种间关联性都发生显著变化,尽管更新状况良好,但要恢复到伐前水平仍需要较长时间。择伐不仅通过改变主要树种的密度对阔叶红松林群落结构产生影响,还通过改变物种空间关联性改变群落的结构动态。因此,在制定森林生态系统经营管理方案时,不仅要选择适合的采伐强度,还要综合考虑采伐时物种的选择以及种间关系。     

Stand characteristics and biodiversity indicators along the productivity gradient in boreal forests: Defining a critical set of indicators for the monitoring of habitat nature quality

Abstract

We quantified the effects of anthropogenic disturbances on the structure and biodiversity of boreal forests on acidic soils and created a statistically supported rational set of indicators to monitor the stand “naturalness”. For that, we surveyed various traits of tree layer, understory, herb layer, forest floor and several widely accepted biodiversity epiphytic indicators in 252 old‐aged boreal stands in Estonia, mostly dominated by Scots pine or Norway spruce. Multifactorial general linear model analyses showed that many forest characteristics and potential indicators were confounded by the gradient of soil productivity (reflected by the forest site type), local biogeographic gradients and also by stand age. Considering confounding effects, boreal forests in a near‐natural state have more large‐diameter trees (diameter at breast height >40 cm) and larger variety of diameter classes, higher proportion of spruce or deciduous trees, a larger amount of coarse woody debris in various stages, a more closed tree canopy and denser understory than managed mature forests. By increasing light availability above the field layer, forest management indirectly increases the coverage of herbs and lichens on the forest floor but reduces the alpha‐ and beta‐diversity of herbs and the proportion of graminoids. Human disturbances reduce the relative incidence of many commonly accepted biodiversity indicators such as indicator lichens, woodpeckers, wood‐dwelling insects or fungi on trees. The test for the predictive power of characteristics reacting on disturbance revealed that only a fraction of them appeared to be included in a diagnostic easy‐to‐apply set of indicators to assess the nature quality of boreal forest: the amount of dead wood, the proportion of deciduous trees, the presence of specially shaped trees and woodpeckers and, as an indicator of disturbances, the forest herb Melampyrum pratensis. Many of these indicators have already been implemented in practice.     

A GIS-based simulation program to predict multi-species size-structure dynamics for natural forests in Hokkaido, northern Japan

A simulation program that runs on a geographic information system (GIS) was developed to predict the multi-species size-structure dynamics of forest stands. Because important characteristics of a forest stand, including woody biomass accumulation, carbon storage, commercial value of timber, and functions for environmental conservation, can be inferred from the size structures of the component populations, management plans can be made from the predictions of the size-structure dynamics. For example, the simulation can incorporate various forms of thinning; forest managers can then try several thinning plans in simulated forest stands and choose the appropriate plan that achieves the best results. Using GIS to predict the size-structure dynamics of forest stands is of practical importance, because GIS has been used widely in forest management and can easily handle spatial distributions of environmental information (e.g., climate, geology, soils) that may influence tree performance. To predict size-structure dynamics, the program numerically solves a continuum equation that describes size-structure dynamics based on growth and mortality rates of individual trees. When predicting size-structure dynamics of a forest stand, the program obtains the environmental information of the stand from a database stored in the GIS and calculates environmental factors such as warmth index and potential evapotranspiration/precipitation ratio that influence growth and mortality rates. The simulation program calculates growth and mortality rates using published growth and mortality models that incorporate the effects of size of the individual, competition between trees, and abiotic environmental factors. To demonstrate the effects of abiotic environmental factors on the multi-species size-structure dynamics, sensitivity analyses were conducted. The size-structure dynamics varied in a way that was predictable from the responses of the growth and mortality rates to variations in the abiotic environmental factors. To demonstrate the size-structure dynamics in different locations, five test runs of the simulation program were also performed using the same initial size-structure and five different sets of abiotic environmental conditions from five locations. At the end of the simulation, the predicted size structures differed because the growth and mortality rates differed among the five locations. Finally, the response of the size structure to thinning was clarified. The result showed how the size structure of a component species in a forest stand is dependent on the presence of other species.     

基于改进PSO的洞庭湖水源涵养林空间优化模型

以结构化森林经营思想为理论基础,从与水源林涵养水源、保持水土功能密切相关的林分物种组成(树种混交)、种内及种间竞争、空间分布格局、垂直结构4个方面选择混交度、竞争指数、角尺度、林层指数、空间密度指数、开阔比数作为水源林健康经营和林分空间结构优化的目标函数,建立洞庭湖水源林林分多目标空间优化模型,应用改进的群智能粒子群算法求解林分空间结构优化模型,并针对模型输出的目标树空间结构单元制定周密的经营策略.研究结果表明,优化模型能准确定位林分空间关系的薄弱环节,调控措施能显著改善林分空间结构,有利于促进森林生态系统的正向演替,为恢复洞庭湖水源林生态功能和健康经营提供理论依据和技术支撑.应用优化模型进行水源涵养林健康经营突破了传统森林经营模式,为智能信息技术在森林空间经营中的应用提供了新的思路.     

Disentangling stand and environmental correlates of aboveground biomass in Amazonian forests

Tropical forests contain an important proportion of the carbon stored in terrestrial vegetation, but estimated aboveground biomass (AGB) in tropical forests varies two‐fold, with little consensus on the relative importance of climate, soil and forest structure in explaining spatial patterns. Here, we present analyses from a plot network designed to examine differences among contrasting forest habitats (terra firme, seasonally flooded, and white‐sand forests) that span the gradient of climate and soil conditions of the Amazon basin. We installed 0.5‐ha plots in 74 sites representing the three lowland forest habitats in both Loreto, Peru and French Guiana, and we integrated data describing climate, soil physical and chemical characteristics and stand variables, including local measures of wood specific gravity (WSG). We use a hierarchical model to separate the contributions of stand variables from climate and soil variables in explaining spatial variation in AGB. AGB differed among both habitats and regions, varying from 78 Mg ha^?1 in white‐sand forest in Peru to 605 Mg ha^?1 in terra firme clay forest of French Guiana. Stand variables including tree size and basal area, and to a lesser extent WSG, were strong predictors of spatial variation in AGB. In contrast, soil and climate variables explained little overall variation in AGB, though they did co‐vary to a limited extent with stand parameters that explained AGB. Our results suggest that positive feedbacks in forest structure and turnover control AGB in Amazonian forests, with richer soils (Peruvian terra firme and all seasonally flooded habitats) supporting smaller trees with lower wood density and moderate soils (French Guianan terra firme) supporting many larger trees with high wood density. The weak direct relationships we observed between soil and climate variables and AGB suggest that the most appropriate approaches to landscape scale modeling of AGB in the Amazon would be based on remote sensing methods to map stand structure.     

A conceptual model of vegetation dynamics for the unique obligate‐seeder eucalypt woodlands of south‐western Australia

Models of vegetation dynamics framed as testable hypotheses provide powerful tools for predicting vegetation change in response to contemporary disturbances or climate change. Synthesizing existing information and applying new data, we develop a conceptual model of vegetation states and transitions for the previously overlooked woodlands dominated by obligate‐seeder eucalypts of dry to semi‐arid south‐western Australia. These comprise the largest extant temperate woodland globally, are uniquely dominated by a high diversity of obligate‐seeder eucalypts (55 taxa), but are under threat from wildfire. Our conceptual model incorporates four critical ecological processes that also distinguish obligate‐seeder woodlands from temperate woodlands dominated by resprouting eucalypts: (i) a lack of well‐protected epicormic buds results in major disturbances (prominently fire) being stand‐replacing. The pre‐disturbance tree cohort is killed, followed by dense post‐disturbance recruitment from seed shed from a serotinous seed bank; (ii) competition between saplings leads to self‐thinning over a multi‐century timeframe, with surviving individuals having great longevity (regularly >400 years); (iii) multiple processes limit recruitment in the absence of stand‐replacement disturbances, leading to frequent single‐cohort stands. However, unlike the few other obligate‐seeder eucalypt communities, trickle recruitment in very long‐unburnt stands can facilitate indefinite community persistence in the absence of stand‐replacement disturbances; and (iv) discontinuous fuels, relatively low understorey flammability (low grass and often high chenopod cover) and topographic barriers to fire (salt lakes) allow mature woodlands to persist for centuries without burning. Notably though, evidence suggests that flammability peaks at intermediate times since fire, establishing a ‘flammability bottleneck’ (or landscape fire trap) through which regenerating woodlands must pass. Our model provides a framework to support management to conserve obligate‐seeder eucalypt woodlands. Research into reasons for exceptional tree heights relative to ecosystem productivity, the evolution of diverse and dominant obligate‐seeder eucalypts, the paucity of grass, and the recent spatial distribution of fires, will further inform conservation management.     

Coupled effects of wind‐storms and drought on tree mortality across 115 forest stands from the Western Alps and the Jura mountains

Damage due to wind‐storms and droughts is increasing in many temperate forests, yet little is known about the long‐term roles of these key climatic factors in forest dynamics and in the carbon budget. The objective of this study was to estimate individual and coupled effects of droughts and wind‐storms on adult tree mortality across a 31‐year period in 115 managed, mixed coniferous forest stands from the Western Alps and the Jura mountains. For each stand, yearly mortality was inferred from management records, yearly drought from interpolated fields of monthly temperature, precipitation and soil water holding capacity, and wind‐storms from interpolated fields of daily maximum wind speed. We performed a thorough model selection based on a leave‐one‐out cross‐validation of the time series. We compared different critical wind speeds (CWSs) for damage, wind‐storm, and stand variables and statistical models. We found that a model including stand characteristics, drought, and storm strength using a CWS of 25 ms^?1 performed the best across most stands. Using this best model, we found that drought increased damage risk only in the most southerly forests, and its effect is generally maintained for up to 2 years. Storm strength increased damage risk in all forests in a relatively uniform way. In some stands, we found positive interaction between drought and storm strength most likely because drought weakens trees, and they became more prone to stem breakage under wind‐loading. In other stands, we found negative interaction between drought and storm strength, where excessive rain likely leads to soil water saturation making trees more susceptible to overturning in a wind‐storm. Our results stress that temporal data are essential to make valid inferences about ecological impacts of disturbance events, and that making inferences about disturbance agents separately can be of limited validity. Under projected future climatic conditions, the direction and strength of these ecological interactions could also change.     

The spatial characteristics of stand structure in Pinus torreyana

The arrangement of trees within a stand by location and age (stand structure) is in part determined by the life history strategy of the species and the disturbance history of the stand. In western North America such disturbances are often the product of wildfires and human management activities. The current study uses spatial analysis to characterize three stands of Pinus torreyana with known disturbance histories. Two stands are located at Torrey Pines State Reserve (TPSR). Of these, one stand has burned twice since 1972. Fire has been successfully excluded from the other stand since the early part of this century. A third stand, on Santa Rosa Island, Channel Islands National Park (SRI), has been grazed heavily since the mid-19th century and has not experienced fire since that time. One-hectare study plots were established in the interior of each stand. Considering the known life history attributes of P. torreyana, and the disturbance histories of the stands, predictions are made concerning the spatial characteristics of the respective stands. All trees within each study plot were sized by diameter breast height (DBH) and mapped. Three techniques of spatial analysis are applied to the resulting unweighted point pattern distributions and the distributions weighted by the square root of DBH as a surrogate for age. The results are consistent with predictions and confirm the following generalizations concerning patterns of aggregation in Torrey pine stands. Young trees tend to be more aggregated than old trees within the same stands. Young stands tend to be more aggregated than old stands on otherwise ecologically similar sites. On a periodically disturbed site there are clusters of trees that represent cohorts of post-disturbance recruitment.     

A spatial model of forest dynamics

Effects of spatial processes on temperate deciduous forest structure and dynamics were investigated with a spatial simulator derived from a forest gap model. The multi-species neighborhood model accounted for competitive interactions and endogenous disturbance in the form of small canopy gaps. Simulated and actual spatial pattern of old-growth stands were compared. The 400 yr simulations produced a pattern scale (0.07–0.2 ha patches) similar to that of an actual stand; simulated pattern intensity was greater than actual intensity, however. Distances to nearest neighbor were somewhat similar for trees in the simulated and actual stands; yet the frequency distributions of distance to nearest neighbor values differed substantially. The simulated stand patterns were generally less random than the actual patterns. Spatial pattern changed markedly during the course of simulated succession. Pattern approached a random dispersion in early succession. Intensity peaked at mid-succession (ca. 150 yr) with a hyperdispersed overstory and a strongly clumped understory. Pattern intensity diminished in late succession as a mixed size structure developed. Old-growth patch size was greater than the neighborhood (or gap) size, suggesting the gap-sized areas do not behave independently.     

Spatial patterns and competition of tree species in a Douglas-fir chronosequence on Vancouver Island

While the successional dynamics and large-scale structure of Douglas-fir forest in the Pacific Northwest region is well studied, the fine-scale spatial characteristics at the stand level are still poorly understood. Here we investigated the fine-scale spatial structure of forest on Vancouver Island, in order to understand how the three dominant species, Douglas-fir, western hemlock, and western redcedar, coexist and partition space along a chronosequence comprised of immature, mature, and old-growth stands. We quantified the changes in spatial distribution and association of the species along the chronosequence using the scale-dependent point pattern analyses pair-correlation function g(r) and Ripley's L-function. Evidence on intra- and inter-specific competition was also inferred from correlations between nearest-neighbor distances and tree size. Our results show that 1) the aggregation of Douglas-fir in old-growth was primarily caused by variation in local site characteristics, 2) only surviving hemlock were more regular than their pre-mortality patterns, a result consistent with strong intra-specific competition, 3) inter-specific competition declined rapidly with stand age due to spatial resource partitioning, and (4) tree death was spatially randomly distributed among larger overstory trees. The study highlights the importance of spatial heterogeneity for the long-term coexistence of shade-intolerant pioneer Douglas-fir and shade-tolerant western hemlock and western redcedar.