Structure of Anogeissus leiocarpa Guill., Perr. natural stands in relation to anthropogenic pressure within Wari‐Maro Forest Reserve in Benin

The present study focused on the analysis of the structure of the Anogeissus leiocarpa dominated natural stands in the Wari‐Maro forest reserve which are under high and minimal anthropogenic pressures. These stands were considered for forest inventories after carrying out a random sampling scheme of 40 sample units of 30 m × 50 m. In each level pressure stand, the dbh and tree‐height of identified tree‐species were measured in each plot. Data analyses were based on the computation of structural parameters, establishment of diameter and height distributions and the floristic composition of the two types of stands. Results obtained showed higher values for the overall basal area (9.78 m² ha^?1), mean height (22.37 m) and mean diameter (36.92 cm) for A. leiocarpa in low‐pressure stands. In high‐pressure stands, some species like Afzelia africana had lower Importance Value Index and the frequency of A. leiocarpa trees in the successive diameter classes dropped rapidly and the value of the logarithmic slope of the height–diameter relationship was lower (9.77) indicating a lanky shape. Results obtained suggest that effective conservation is needed for A. leiocarpa stands under high pressure by limiting human interference and developing appropriate strategy for restoration purposes.     

Vegetation analysis and tree population structure of tropical wet evergreen forests in and around Namdapha National Park,northeast India

Species composition, diversity and tree population structure were studied in three stands of the tropical wet evergreen forest in and around Namdapha National Park, Arunachal Pradesh, India. Three study stands exposed to different intensities of disturbances were identified, viz., undisturbed (2.4 ha) in the core zone of the park, moderately disturbed (2.1 ha) in the periphery of the park and highly disturbed (2.7 ha) outside the park area. In total 200 plant species belonging to 73 families were recorded in three stands. Tree density and basal area showed a declining trend with the increase in disturbance intensity. The densities of tree saplings and seedlings were lower in the disturbed stands than in the undisturbed stand. Species like Altingia excelsa, Olea dioica, Terminalia chebula, Mesua ferrea and Shorea assamica in the undisturbed stand and Albizia procera alone in the moderately disturbed stand contributed more than 50% of the total tree density in respective stands. The undisturbed stand contained young tree population. In the highly disturbed stand, the tree density was scarce, but had uncut trees of higher girth class (>210 cm GBH). Low shrub density was recorded in both disturbed stands due to frequent human disturbances; the broken canopy and direct sunlight enhanced the abundance of herbs in these stands. With a species rarity (species having <2 individuals) of ca. 50%, the tropical wet evergreenforests of the Namdapha National Park and its adjacent areas warrant more protection from human intervention and also eco-development to meet the livelihood requirements of the local inhabitants in the peripheral areas of the Namdapha National Park in order to reduce the anthropogenic pressure on the natural resources of the park.     

Effects of anthropogenic disturbance on tree population structure and diversity of a rain forest biosphere reserve in Ghana,West Africa

We evaluated the impacts of anthropogenic disturbance on community structure and diversity along three management zones of the Bia biosphere reserve in Ghana. Sixty sample plots were distributed among the core, buffer and transition zones. We estimated the degree of disturbances from discernible indicators on the field and satellite images. All tree species ≥10 cm dbh (diameter at breast height) were identified and enumerated. Inventory data were compared across the zones and related to intensity of disturbances. A total of 1176 individual trees from 108 species and 33 families were encountered. Number of species varied from 27 in the highly disturbed (HD) to 61 in the least disturbed (LD) zone. Mean basal area (BA) varied from 11.71 in the HD to 28.26 in the LD. Both Margalef's species richness and Shannon‐Weiner's α‐diversity were highest in the moderately disturbed (MD) than either the least and most disturbed zones. Our study revealed significant differences in tree abundance, stem density, BA and species diversity, attributable to differences in degree of anthropogenic disturbances among zones. Given the different levels of anthropogenic disturbance and corresponding impacts across the reserve, we recommend an integrated management strategy for the conservation of biodiversity in the Bia biosphere reserve.     

Hurricane Disturbance Alters Secondary Forest Recovery in Puerto Rico

Land-use history and large-scale disturbances interact to shape secondary forest structure and composition. How introduced species respond to disturbances such as hurricanes in post-agriculture forest recovery is of particular interest. To examine the effects of hurricane disturbance and previous land use on forest dynamics and composition, we revisited 37 secondary forest stands in former cattle pastures across Puerto Rico representing a range of exposure to the winds of Hurricane Georges in 1998. Stands ranged from 21 to>80 yr since agricultural abandonment and were measured 9 yr posthurricane. Stem density decreased as stands aged, while basal area and species richness tended to increase. Hurricane disturbance exerted contrasting effects on stand structure, contingent on stand age. In older stands, the basal area of large trees fell, shifting to a stand structure characteristic of younger stands, while the basal area of large trees tended to rise in younger stands with increasing hurricane disturbance. These results demonstrate that large-scale natural disturbances can alter the successional trajectory of secondary forest stands recovering from human land use, but stand age, precipitation and soil series were better predictors of changes in stand structure across all study sites. Species composition changed substantially between census intervals, but neither age nor hurricane disturbance consistently predicted species composition change. However, exposure to hurricane winds tended to decrease the abundance of the introduced tree Spathodea campanulata, particularly in smaller size classes. In all sites the abundance of the introduced tree Syzygium jambos showed a declining trend, again most strongly in smaller size classes, suggesting natural thinning through succession.     

Stand characteristics and distribution of a relict population of Persian ironwood (Parrotia persica C.A. Meyer) in northern Iran

Parrotia persica C.A. Meyer (Persian ironwood) is a deciduous tree of the family Hamamelidaceae, native to northern Iran and endemic to the Alborz Mountains. The study objectives were to assess the current status and distribution of Persian ironwood by characterizing four forest stands where the tree was either a dominant or co-dominant species. Species richness within the stands varied from 3 to 16 woody species and from 9 to 27 understory species. Basal area varied between 37 m²/ha and 77 m²/ha and tree density varied from 320 to 367 stems/ha. Parrotia persica represented 63-86% of the relative dominance and 41-100% of the relative density. In non-pure P. persica stands, other important tree species include Fagus orientalis and Carpinus betulus. Parrotia persica regenerates mainly by sprouts and coppicing. Conservation of relict forests, such as the Persian ironwood forests of the Alborz Mountains, is of particular concern because they represent the only natural occurrence of this species in the world. Anthropogenic disturbance, in the form of timber harvesting, livestock grazing, and clearing forest land for agriculture appear to be the largest threats to Parrotia persica's future.     

Tree mortality in the African Sahel indicates an anthropogenic ecosystem displaced by climate change

Aim Widespread reports of disappearing tree species and senescing savanna parklands in the Sahel have generated a vigorous debate over whether climate change or severe human and livestock pressure is principally responsible. Many of the tree taxa in decline are closely associated with human settlement and farming, suggesting that the parkland ecosystem may not be a natural vegetation assemblage. The aim of this study is to assess the possibility that human activities promoted the spread of taxa with edible fruit into dry Sudano‐Sahelian areas during high‐rainfall periods in the climate cycle. Location West African savannas (Mali, Burkina Faso, Ghana, Togo, Benin). Methods Cultivated savanna parklands and adjacent forests and transitional landscapes were inventoried at 27 sites in five countries. All trees with basal diameters > 10 cm were counted within 500‐m² belt transects. Species composition and abundance were contrasted between three landscape classes to assess the degree of influence exerted by traditional human management. Twentieth century rainfall data were averaged for two sets of weather stations encompassing the north–south range of typical parkland tree species. Rainfall trends were used to evaluate the putative impact of climate change on edible and/or succulent fruit species at the northern limit of the parkland savanna zone. Results Species composition and spatial distribution data indicate that the parkland ecosystem is significantly shaped by human activities. Indigenous land management favours edible‐fruit‐yielding taxa from the wetter Sudanian and Guinean vegetation zones over Sahelian species. Rainfall isohyets at the northern range limits of parkland species shifted southwards in the late 20th century, crossing the critical 600‐mm mean annual rainfall threshold for Sudanian flora. Relict vegetation and historical records indicate that the Sudanian parkland system extended in the past to near 15° N latitude in middle West Africa, compared with 13.5° N today. Main conclusions The current loss of mesic trees in the Sudano‐Sahel zone appears to be driven by the sharp drop in rainfall since the 1960s, which has effectively stranded anthropogenically distributed species beyond their rainfall tolerance limits.     

Life-span growth dynamics and xylem anatomical patterns of diffuse-porous Afzelia africana Sm. (Fabaceae) in different ecological zones in Burkina Faso

Tropical West African savannas are exposed to high climatic variability with potential impacts on tree growth, forest dynamics and ecosystem productivity. In such context, understanding the long-term ecological responses of savanna trees to changing environmental conditions is of great relevance for taking appropriate conservation actions. We conducted the first study on tree-ring analysis and quantitative wood anatomy on Afzelia africana Sm. in Burkina Faso, to investigate the life-span growth trajectories and wood anatomical adjustment to site and to climate variations. A total of 24 stem discs was collected in four protected forests along the Sudano-sahelian and the Sudanian climatic zones. Wood samples were analyzed using standard dendrochronological methods and quantitative wood anatomy. The mean annual growth rates varied from 1.002 (± 0.249) mm. year⁻¹ in the Sudanian zone to 1.128 (± 0.436) mm. year⁻¹ in the Sudano-sahelian zone. Analysis of growth trajectories showed high variations within sites and between climatic zones. Wood anatomical traits significantly varied between sites. Principal Component Analysis revealed strong relationships between ring width, wood density and vessel traits, with 82.81 % of the total variance explained. Vessel size significantly increased from the pith to the bark, highlighting the ontogenetic effects on xylem anatomical variations. Inverse relationships were found between vessel size and vessel density across the driest site and the wettest site, suggesting that the higher the rainfall, the taller the tree, the larger vessel size, but the lower vessel density. By contrast, more arid conditions and high evapotranspiration lead to smaller vessel sizes and higher vessel density. Such anatomical adjustments highlight the trade-offs between water conductance efficiency and hydraulic safety, and emphasize physiological responses to climate variability. These variations on the long-term dynamics and xylem anatomical patterns underline complex interactions between ontogenetic effects and contrasting environmental factors that affect the eco-physiological functioning of A. africana throughout the Sudanian region.     

Forest degradation and invasive species synergistically impact Mimusops andongensis (Sapotaceae) in Lama Forest Reserve,Benin

Harvesting of Non‐Timber Forest Products (NTFPs) can threaten target species, especially those with limited distribution and density. Exploited species also face threats from habitat fragmentation, fire, and invasive species. We assessed the impact of human disturbances and invasive species on the population of a key multipurpose NTFP species, Mimusops andongensis, in Lama Forest reserve (Benin). The densities of adult trees and regenerative stems decreased with increasing degradation. Mimusops andongensis contributed less to total tree density with increasing human disturbance. There were significantly fewer M. andongensis recruits with increasing cover of invasive Chromolaena odorata. Smaller diameter individuals predominated in non‐degraded and moderately degraded sites while in degraded sites, the structure showed a negative exponential trend with the density of small diameter individuals being less than two trees/ha. Larger individuals were also rare in degraded sites. The low density of both mature trees and seedlings in degraded sites may undermine the long‐term viability of M. andongensis, despite existing protection against NTFP harvesting and other anthropogenic pressures. Management should emphasize facilitating recruitment subsidies and limiting the presence of C. odorata.     

Native tree regeneration in native tree plantations: understanding the contribution of Araucaria angustifolia to biodiversity conservation in the threatened Atlantic Forest in Argentina

Deforestation is a global process that has strongly affected the Atlantic Forest in South America, which has been recognised as a threatened biodiversity hotspot. An important proportion of deforested areas were converted to forest plantations. Araucaria angustifolia is a native tree to the Atlantic Forest, which has been largely exploited for wood production and is currently cultivated in commercial plantations. An important question is to what extent such native tree plantations can be managed to reduce biodiversity loss in a highly diverse and vulnerable forest region . We evaluated the effect of stand age, stand basal area, as a measure of stand density, and time since last logging on the density and richness of native tree regeneration in planted araucaria stands that were successively logged over 60 years, as well as the differences between successional groups in the response of plant density to stand variables. We also compared native tree species richness in planted araucaria stands to neighbouring native forest. Species richness was 71 in the planted stands (27 ha sampled) and 82 in native forest (18 ha sampled) which approximate the range of variation in species richness found in the native forests of the study area. The total abundance and species richness of native trees increased with stand age and time since last logging, but ecological groups differed in their response to such variables. Early secondary trees increased in abundance with stand age 3–8 times faster than climax or late secondary trees. Thus, the change in species composition is expected to continue for a long term. The difference in species richness between native forest and planted stands might be mainly explained by the difference in plant density. Therefore, species richness in plantations can contribute to local native tree diversity if practices that increase native tree density are implemented.     

Current regeneration patterns at the tree line in the Pyrenees indicate similar recruitment processes irrespective of the past disturbance regime

Aim Impacts of global change, such as land‐use and climate changes, could produce significant alterations in the elevational patterns of alpine tree line ecotones and their adjacent vegetation zones. Because the responses of the tree line to environmental variations are directly related to successful tree regeneration, understanding recruitment dynamics is an indispensable step in tree line research. We aimed to compare potential ecological limitations on recent tree line regeneration in undisturbed and disturbed sites by analysing the demographic structure and spatiotemporal patterns of recruits and large trees. Location Alpine tree line ecotones comprising Pinus uncinata in the Catalan Pyrenees (north‐east Spain) and Andorra. Methods We assessed the demographic structure and spatial pattern of recent recruitment using techniques of point‐pattern and autocorrelation analyses. A total of 3639 P. uncinata individuals were mapped, measured and aged at 12 sites. To evaluate the effects of past disturbances on recent tree line response we compared tree lines that had either been recently affected by human‐induced disturbances or had remained undisturbed for many years. Results The age structure of the tree lines, together with the lack of an age gap between seedlings and saplings, did not indicate recent episodes of high seedling mortality and suggest that recruitment has been frequent under current climate conditions. Seedlings appeared highly aggregated at short distances (up to 3 m), irrespective of disturbance history, and were spatially segregated with respect to large trees. However, we found no evidence of patches of even‐aged seedlings, and our results suggest that dispersal events at intermediate distances (10–17 m) may be frequent. Autocorrelation analyses revealed different patterns of density and age of recruits between disturbed and undisturbed tree lines, but the strength and small‐scale clustering of seedlings and saplings were very similar between sites. Main conclusions We found no recruitment limitation on recent tree line dynamics in the Pyrenees. Furthermore, processes affecting tree recruitment seem to be similar among populations regardless of their past disturbance regime. Our results suggest that constraints on tree line dynamics causing differential responses between sites may operate on older life stages and not upon recruits, and that such constraints may be more contingent on local site conditions than on disturbance history.     

Spatial Distribution Patterns in the Very Rare and Species-Rich Picea chihuahuana Tree Community (Mexico)

The very rare Mexican Picea chihuahuana tree community covers an area of no more than 300 ha in the Sierra Madre Occidental. This special tree community has been the subject of several studies aimed at learning more about the genetic structure and ecology of the species and the potential effects of climate change. The spatial distribution of trees is a result of many ecological processes and can affect the degree of competition between neighbouring trees, tree density, variability in size and distribution, regeneration, survival, growth, mortality, crown formation and the biological diversity within forest communities. Numerous scale-dependent measures have been established in order to describe spatial forest structure. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies. In this study, we examined the spatial distribution pattern of trees in the P. chihuahuana tree community in 12 localities, in relation to i) tree stand density, ii) diameter distribution (vertical structure), iii) tree species diversity, iv) geographical latitude and v) tree dominance at a fine scale (in 0.25 ha plots), with the aim of obtaining a better understanding of the complex ecosystem processes and biological diversity. Because of the strongly mixed nature of this tree community, which often produces low population densities of each tree species and random tree fall gaps caused by tree death, we expect aggregated patterns in individual Picea chihuahuana trees and in the P. chihuahuana tree community, repulsive Picea patterns to other tree species and repulsive patterns of young to adult trees. Each location was represented by one plot of 50 x 50 m (0.25 ha) established in the centre of the tree community. The findings demonstrate that the hypothesis of aggregated tree pattern is not applicable to the mean pattern measured by Clark-Evans index, Uniform Angle index and Mean Directional index of the uneven-aged P. chihuahuana trees and P. chihuahuana tree community and but to specific spatial scales measured by the univariate L-function. The spatial distribution pattern of P. chihuahuana trees was found to be independent of patches of other tree species measured by the bivariate L-function. The spatial distribution was not significantly related to tree density, diameter distribution or tree species diversity. The index of Clark and Evans decreased significantly from the southern to northern plots containing all tree species. Self-thinning due to intra and inter-specific competition-induced mortality is probably the main cause of the decrease in aggregation intensity during the course of population development in this tree community. We recommend the use of larger sampling plots (> 0.25 ha) in uneven-aged and species-rich forest ecosystems to detect less obvious, but important, relationships between spatial tree pattern and functioning and diversity in these forests.     

Water availability determines the richness and density of fig trees within Brazilian semideciduous forest landscapes

The success of fig trees in tropical ecosystems is evidenced by the great diversity (+750 species) and wide geographic distribution of the genus. We assessed the contribution of environmental variables on the species richness and density of fig trees in fragments of seasonal semideciduous forest (SSF) in Brazil. We assessed 20 forest fragments in three regions in Sao Paulo State, Brazil. Fig tree richness and density was estimated in rectangular plots, comprising 31.4 ha sampled. Both richness and fig tree density were linearly modeled as function of variables representing (1) fragment metrics, (2) forest structure, and (3) landscape metrics expressing water drainage in the fragments. Model selection was performed by comparing the AIC values (Akaike Information Criterion) and the relative weight of each model (wAIC). Both species richness and fig tree density were better explained by the water availability in the fragment (meter of streams/ha): wAIC_richness = 0.45, wAIC_density = 0.96. The remaining variables related to anthropic perturbation and forest structure were of little weight in the models. The rainfall seasonality in SSF seems to select for both establishment strategies and morphological adaptations in the hemiepiphytic fig tree species. In the studied SSF, hemiepiphytes established at lower heights in their host trees than reported for fig trees in evergreen rainforests. Some hemiepiphytic fig species evolved superficial roots extending up to 100 m from their trunks, resulting in hectare-scale root zones that allow them to efficiently forage water and soil nutrients. The community of fig trees was robust to variation in forest structure and conservation level of SSF fragments, making this group of plants an important element for the functioning of seasonal tropical forests.     

Effects of population density on male and female reproductive success in the wind-pollinated, wind-dispersed tree species Betula maximowicziana

The effects of population density on male and female reproductive success of Betula maximowicziana were evaluated in two mixed and two post-fire stands, with various population densities, ranging from 1.9 to 300.0 trees per ha, in central Hokkaido, Japan. First, we investigated ecological determinants of reproductive success (seed set and germination) of both seeds collected from the trees (tree seeds) and dispersed seeds collected from seed traps (dispersed seeds). We then evaluated the effects of population density on seed set and germination of tree seeds and dispersed seeds using a generalized linear mixed model (GLMM). Subsequently, we genotyped 950 seeds collected from mother trees and 940 seeds trapped after dispersal derived from tree seeds and dispersed seeds, respectively, using eleven microsatellite loci. Using the acquired data, we then evaluated the outcrossing rate and effective number of pollen donors (N _ep) of the tree seeds, and the genetic structure of both pollen pools and dispersed seed populations. The seed set and germination rate of dispersed seeds was significantly lower both in the lowest-density stand and in the highest-density stand. The GLMM revealed that seed set and germination rates of dispersed seeds may be maximal at approximately 120 trees per hectare (optimal density). Outcrossing rates were consistently high (t _m = 0.995), regardless of the population density. In contrast, N _ep was lower in the lowest-density stands. Significant genetic structure of the dispersed seed population was found in two low-density stands, probably due to the limitation of overlapping seed shadows.     

Composition,structure and diversity of cove forest stands in the Great Smoky Mountains: a patch dynamics perspective

Abstract. Cove forests of the Great Smoky Mountains are North American examples of old-growth temperate forest. Ecological attributes of seven stands were studied using one 0.6 - 1.0 ha plot per stand. Stand basal area (39 - 55 m²/ha) and biomass (326 - 471 Mg/ha) were high for temperate deciduous forest. Density ranged from 577 to 1075 stems/ha. All stands had a mixture of deciduous canopy species. Only rarely did a single species comprise more than half of the stand by density, basal area or biomass. Shade-intolerant species were present at low levels (1 - 5 % of total stand density). A wide range of stem diameters was characteristic of most species. However, some species lacked small stems, indicating discontinuous regeneration. Stands tended to have 10 - 20 tree species per ha and at least five species had biomass levels > 10 Mg/ha, indicating high evenness. Canopy gaps covered 10 % of the total area (2 - 21 % by stand). Gaps and conspecific patches of canopy trees > 0.05 ha in size were infrequent. Spatial analyses revealed a variety of patterns among species at inter-tree distances of 1 to 25 m. When all species were combined, juveniles showed aggregation, and adults were often hyperdispersed. Analyses for individual species confirmed that the mosaic of canopy species is influenced by non-random spatial processes. Adults of several species were aggregated at distances > 10 m. Juveniles of all major species exhibited aggregation. Several species exhibited regeneration near conspecific adults. This pattern suggested limited mobility for such species within the shifting mosaic. A diverse patchwork resulted despite the fact that many species did not exhibit segregation of adults and juveniles. Further understanding of patch dynamics and the potential for compositional steady state in cove forests requires long-term study with spatial data.     

'Nucleating’ succession in recovering neotropical wet forests: The legacy of remnant trees

Question: What is the influence of remnant trees on secondary forest structure and composition in tropical pastures many years after abandonment? Location: Neotropical lowland wet forest, La Selva Biological Station, Costa Rica. Methods: Tree and sapling density, basal area, and species richness were quantified at three distances from remnant trees, 0–10 m (inner), 20–30 m (intermediate), and ca. 50 m (distal) zones. A total of 15 remnant trees were sampled in pastures ~23 years after abandonment. Results: Tree density decreased along a gradient from inner (1117 ± 377 individuals/ha) to distal (592 ± 282 individuals/ha) zones, and the number of large‐seeded individuals (seeds > 1 cm diameter) was significantly greater in the inner zone. Basal area of tree individuals was greater in the inner (25.6 ± 12 m²/ha) and intermediate (28.3 ± 15.6 m²/ha) zones than the distal zone (14.7 ± 7.2 m²/ha), but there were no differences between inner and intermediate zones. Similar patterns are reported for species richness. Additionally, saplings (1 ‐ 5 cm DBH) had higher density directly beneath and adjacent to remnants, suggesting that remnant trees can affect recruitment even many years after pasture abandonment and the formation of a surrounding secondary forest. Conclusions: Results indicate that remnant trees facilitate forest recovery over a broad temporal range, and appear to ‘nucleate’ forest regeneration by expanding their sphere of influence outward over time.     

Variation in structural diversity and regeneration potential of tree species in different tropical forest types of Similipal Biosphere Reserve,Eastern India

Understanding the regeneration potential of tree species in natural forest ecosystems is crucial to deliver suitable management practices for conservation of biodiversity. We studied the variation in structural diversity and regeneration potential of tree species in three different tropical forest types, namely: Dry Deciduous forest (DDF), Moist Deciduous forest (MDF) and Semi-evergreen forest (SEF) of Similipal Biosphere Reserve (SBR), Eastern India. Random sample plots were laid for studying the diversity and distribution pattern of tree, sapling, and seedling stages of the tree species. A total of 84 species belong to 73 genera and 35 families were recorded from the study area. The highest species richness was reported for tree (54 species) in DDF, sapling (24 species) in MDF and seedling (22 species each) in SEF and DDF. The overall density of trees with GBH (Girth at Breast Height) ≥ 10 cm was 881 individuals/ha. The regeneration potential of tree species was poor in DDF (39%) where as it was fair in SEF (43%) and MDF (49%). Most of the dominant tree species at each forest type performed good regeneration. The species such as Ehretia laevis Roxb., Bridelia retusa (L.)A.Juss., Mitragyna parviflora (Roxb.) Korth., Terminalia tomentosa Wight & Arn., Terminalia chebula Retz., Terminalia bellirica (Gaertn.) Roxb.etc. had either no regeneration or poor regeneration potential need immediate attention for conservation measures. The diversity of standing trees did not correlate with seedling or sapling diversity in all the cases but there was significant correlation among seedling and sapling diversity found in DDF (r = 0.67, p ≤ 0.05) and SEF (r = 0.83, p ≤ 0.05). Further, the diversity of tree species increased with their age (trees > saplings > seedlings) and the stem density decreased with their age (trees < saplings < seedlings) in all three forest types. The results of our study would be helpful in understanding the structural attributes, diversity and regeneration potential of different tropical forest types of India for their better conservation and management.     

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.     

A general framework to describe the alteration of natural tree species composition as an indicator of forest naturalness

The alteration of natural tree species composition is defined as the deviation of the current tree species composition from that of the natural state. It can be used as a measure of human influence on forest vegetation, and thus as an indicator of the naturalness of forest vegetation. The aim of the study was to develop a standard procedure for estimating the alteration of natural tree species composition, to explain factors driving alteration and to examine its significance for susceptibility of forest stands to natural disturbances. The alteration of natural tree species composition was estimated for the Dinaric region (5556 km², Slovenia) by the Robič Index of Dissimilarity (RID), ranging from 0 (completely natural) to 100 (completely altered). The index was calculated on the compartment level (24 ha each on average) with data on current and potential natural forest vegetation. The influence of human activities on tree species alteration was examined by using topographic and accessibility variables. The susceptibility of forest stands to natural disturbances was analysed with data on sanitary felling. In the study area, the natural tree species composition of forest stands is moderately preserved; the average value of RID was 50.05, ranging from 1.76 to 100, and the coefficient of variation was 0.49. The alteration of the natural tree species composition of forest stands is primarily the result of forest management and past land use, conditioned either by topography or accessibility of forests. The degree of alteration of tree species composition decreased along the gradients of rockiness, inclination and elevation. A greater degree of alteration appeared on the slopes of intermediate and south facing aspects than on north facing slopes, and in areas that were closer to the forest edge. A higher level of alteration significantly increases the susceptibility of forest stands to natural disturbances. The procedure represents a novel approach in modelling the alteration (naturalness) of tree species composition of forest vegetation. It is applicable at different spatial scales and fosters an understanding of the patterns of tree species composition under the influence of human activity across forest landscapes.     

不同干扰强度下针阔混交林树种空间格局及种间关联性

为了探究不同干扰程度下针阔混交林树种空间格局变化及树种种间关联性,本研究以关帝山庞泉沟自然保护区不同干扰强度的针阔混交林为对象,选取郁闭度、林分密度、伐桩数量作为划分干扰程度的因子,采用相对影响法,将林分划分为未干扰、中度干扰和严重干扰3个等级。利用空间点格局K2函数分析了林分的空间分布格局及其不同树种的种间关联。结果表明: 未干扰样地直径分布呈倒“J”型,中度和严重干扰样地直径分布均为双峰曲线分布。同时,未干扰和中度干扰林分在小尺度上呈现聚集分布,严重干扰的林分随机分布更为显著。针阔树种种间关系表现为未干扰林分在所有尺度上无关联;小尺度上中度干扰表现为正相关,严重干扰林分表现为负相关;较大尺度上中度与严重干扰林分均表现为不相关。说明林分干扰程度越严重,林下小径级林木数量越少,导致小尺度上种内聚集程度越低。同时,适当的干扰程度有利于林分内物种间对环境资源的协同利用。研究揭示了干扰对林分结构的影响,可为林分结构调整及抚育经营提供理论依据。