Tree Size Diversity Research Articles

A Climate-Sensitive Mixed-Effects Individual Tree Mortality Model for Masson Pine in Hunan Province, South–Central China

Accurately assessing tree mortality probability in the context of global climate changes is important for formulating scientific and reasonable forest management scenarios. In this study, we developed a climate-sensitive individual tree mortality model for Masson pine using data from the seventh (2004), eighth (2009), and ninth (2014) Chinese National Forest Inventory (CNFI) in Hunan Province, South–Central China. A generalized linear mixed-effects model with plots as random effects based on logistic regression was applied. Additionally, a hierarchical partitioning analysis was used to disentangle the relative contributions of the variables. Among the various candidate predictors, the diameter (DBH), Gini coefficient (GC), sum of basal area for all trees larger than the subject tree (BAL), mean coldest monthly temperature (MCMT), and mean summer (May–September) precipitation (MSP) contributed significantly to changes in Masson pine mortality. The relative contribution of climate variables (MCMT and MSP) was 44.78%, larger than tree size (DBH, 32.74%), competition (BAL, 16.09%), and structure variables (GC, 6.39%). The model validation results based on independent data showed that the model performed well and suggested an influencing mechanism of tree mortality, which could improve the accuracy of forest management decisions under a changing climate.

Diversity effects and compensatory dynamics drive productivity and stability in temperate old‐growth forests

Abstract Understanding mechanisms stabilizing ecosystem functions, such as primary production, is crucial for forecasting global environmental responses. While biological diversity is expected to enhance stability through compensatory reactions to environmental changes, empirical evidence is lacking, especially in old‐growth forests vital for biodiversity conservation and climate change mitigation. Moreover, whether increased niche complementarity and stronger intraspecific than interspecific competition are key mechanisms promoting compensatory dynamics and stabilizing ecosystem functions in diverse forests remains unexplored. This study investigates productivity and stability in temperate old‐growth forests over 20 years at community and individual levels. Analysing 4380 trees in a 4‐ha plot in northern Japan with over 35 tree species, structural equation models evaluated the effects of biodiversity and average asynchrony in species fluctuations (compensatory dynamics) on productivity stability across 100 m2 grid quadrats. Functional traits and taxonomic diversity represented species complementarity and the insurance effect. Temporal growth correlations between conspecific and heterospecific neighbours and neighbourhood effects on growth performance indicated intra‐ and interspecific interactions at the individual level. Communities with greater stability exhibited higher diversity and asynchronous species fluctuations, suggesting that compensatory dynamics buffer community productivity against environmental variability. The inverse relationship between tree size variation and stability indicates that communities with less pronounced size and abundance hierarchies have more efficient compensatory mechanisms, ensuring stable forest functioning. The absence of negative temporal correlations in biomass production among heterospecific neighbours suggests the limited significance of interspecific competition in compensatory dynamics. Conversely, positive correlations among conspecific neighbours and their suppressed growth in dense conspecific patches highlight the importance of conspecific negative density‐dependent mechanisms in sustaining tree species diversity and ensuring stable productivity. Synthesis: The study underscores the critical role of tree species richness in stabilizing ecosystem functioning via asynchronous growth in one of the world's most diverse temperate forests. Stronger intraspecific than interspecific competition helps prevent single‐species dominance, maintaining diversity and productivity stability. Despite occasional destabilization from size‐asymmetric interspecific competition, species‐trait complementarity enhances stability by promoting overall biomass production. This study highlights the importance of overall diversity for the stability of forest productivity, with implications for nature conservation and ecosystem functionality.

Forest structural heterogeneity positively affects bird richness and acoustic diversity in a temperate, central European forest

IntroductionForests managed for timber production can also be managed for biodiversity conservation by retaining structures typical of old-growth forests, which provide heterogenous structures for forest-dwelling species, including birds. Ecoacoustic monitoring of forest birds is now a well-studied field, however the extent to which acoustic indices can reflect bird assemblage responses to stands of lower or higher structural heterogeneity is disputed. MethodIn this study, we acoustically surveyed 75 plots over two years in mature, previously managed forests and computed acoustic indices from the recordings. We first identified an acoustic index that significantly correlated with bird richness over more than one spring season. Next, we tested the response of bird richness to individual forest structural elements using linear regressions. We then repeated this analysis but combined the individual structural elements into one structural composition variable using an NMDS and gam smooth overlay, to compare the effect of individual forest structures versus overall forest heterogeneity on bird richness. We then repeated this analysis using our selected acoustic index, the Normalized Difference Soundscape Index (NDSI), to see if it followed the same patterns as bird richness in response to individual and collective forest structural elements.ResultsOur results showed that plots with high bird richness were also associated with high NDSI values, and high values of both variables occurred in plots with high structural heterogeneity (tree species richness, tree size variability, and snag height variability) and low amounts of standing and lying deadwood. DiscussionOur findings suggest that once an acoustic index can be identified as a robust correlate of bird richness, it can serve as a proxy for the response of birds to differences in forest structural heterogeneity within a managed forest context. It is therefore possible for forest managers interested in conserving or increasing bird richness in their production forests to identify a robust acoustic index to track the response of bird richness to management decisions over time.

Landscape and habitat characteristics jointly drive the urban distribution of a nuisance species

Identifying environmental variables that affect the distribution of pest and/or nuisance species promotes effective management strategies by improving urban planning and the targeted application of direct interventions like population control. Australian white ibis Threskiornis moluccus are considered a nuisance species throughout Australia, having increased significantly in abundance in coastal urban landscapes following declines in their native inland habitats over the past three decades. Few management interventions thus far have successfully reduced conflicts between ibis and humans, necessitating a better understanding of conditions driving ibis distribution and abundance. We surveyed 26 ibis colonies throughout the Moreton Bay region in central eastern Australia and quantified the effects of a range of spatial (e.g. connectivity with waste management facilities, urban land and wetlands) and habitat (e.g. nesting tree species, density and size) variables on ibis and nest abundance. Ibis and their nests were most abundant at sites within 10 km of waste management facilities, and < 60 m from residential or industrial estates. Ibis were also more abundant at sites with tree canopy height < 10 m and lower tree species richness. Nests were most abundant on water bound islands with tree density > 0.5 trees/m2. Urbanisation had contrasting effects, with nest numbers increasing with higher connectivity (i.e. closer proximity) to urban land, while ibis numbers decreased (relative to waste management facilities). Multifaceted approaches are required for controlling ibis populations, including restricting access to anthropogenic food resources, managing the condition of remnant habitats, and considering population control at sites with high connectivity to foraging grounds.

Effects of species and tree size diversity on above-ground biomass in diverse heterogeneous tropical evergreen forests, Quangbinh Province, north central Vietnam

Effects of species and tree size diversity on above-ground biomass in diverse heterogeneous tropical evergreen forests, Quangbinh Province, north central Vietnam

Contrasting Regeneration Patterns in Abies alba-Dominated Stands: Insights from Structurally Diverse Mountain Forests across Europe

To maintain the ecosystem resilience to large-scale disturbances in managed forests, it is essential to adhere to the principles of close-to-nature silviculture, adapt practices to the traits of natural forest types, and utilize natural processes, including natural regeneration. This study examines the natural regeneration patterns in silver fir (Abies alba Mill.)-dominated forests, analyzing how the stand structure—tree size diversity, species composition, and stand density—affects the regeneration. We analyze the data from four sites in Poland, Germany, and Italy, employing generalized linear and zero-inflated models to evaluate the impact of the management strategies (even- vs. uneven-aged) and forester-controlled stand characteristics (structural diversity, broadleaf species admixture, and stand density) on the probability of regeneration, its density, and the developmental stages (seedling, small sapling, and tall sapling) across a climatic gradient. Our results indicate a significantly higher probability of regeneration in uneven-aged stands, particularly in areas with lower temperatures and lower overall regeneration density. The tree size diversity in the uneven-aged stands favors advancement from juveniles to more developed stages (seedling to sapling) in places with higher aridity. A denser stand layer (higher stand total basal area) leads to a lower density of natural regeneration for all the present species, except silver fir if considered separately, signifying that, by regulating the stand growing stock, we can selectively promote silver fir. A higher admixture of broadleaf species generally decreases the regeneration density across all the species, except in a water-rich site in the Bavarian Alps, where it had a strong positive impact. These findings underscore the complex interactions of forest ecosystems and provide a better understanding required for promoting silver fir regeneration, which is essential for a close-to-nature silviculture under climate change.

Accounting for the impact of tree size and soil spatial variability on leaching from orchards

Commercial agricultural production of orchards is based on water and fertilizer applications. Over-application and orchard spatial variability lead to water and nitrogen (N) losses through leaching and environmental damage. This work introduces a novel advanced method to quantify N and water leaching at the orchard scale by combining soil monitoring, unmanned aerial vehicle (UAV) imagery to estimate tree size, leaf N and canopy content, and water flow and N transport models. Four neighboring commercial orange orchards were selected, and 48 representative trees were instrumented with suction cups and tensiometers at a depth of 90 cm. The physical properties of the soil profiles in the vicinity of each tree were determined. During 2019–2021, UAV structure-from-motion (SfM) photogrammetry was used to classify the tree sizes into small, medium, and big categories. Soil porewater extraction and soil matric potential measurements were conducted every three weeks, while leaf N contents (LNC) were determined through bimonthly leaf tissue sampling. The LNC, N leaching, soil water state, and N use efficiency (NUE) (the ratio between the mass of plant N uptake and the mass of N applied per area) were strongly related to tree size classification. Results of the calibrated hydrological model illustrated that the 'big' trees exhibited minimal N leaching due to higher transpiration compared to the other tree size categories. An NUE map was established using the calibrated model and field measurements, demonstrating that soil spatial variability minimally affected N leaching compared to tree size distribution. The presented holistic approach can be used to identify N-leaching hotspots, improve orchard scale NUE estimates, and promote sustainable agricultural management practices.

Distance and T-square sampling for spatial measures of tree diversity

Distance sampling and its statistically improved variant, T-square sampling, are important sampling methods in plant ecology. They have often been applied in the context of plant density estimations and are comparatively easy to implement, since they intuitively follow the nearest-neighbour principle and thus do not require the layout of sample plots. Previous research studying distance sampling suggested that T-square sampling may also lead to an improved estimation of spatial tree diversity indices. We simulated distance and T-square sampling in six large fully mapped forest areas for seven tree diversity indices of which some competed for the same diversity aspect, i.e. tree location (dispersion), tree species and tree size diversity. Our results demonstrated that both distance and T-square sampling are indeed robust methods for sampling spatial measures of tree diversity. The sample size required for a sampling error of 10% does not exceed 20% of the total number of trees in a sampling area. T-square sampling has the ability to adapt to different spatial patterns of tree locations and this ability is key to the way the method controls estimation bias. The sample size required for species mingling and size differentiation clearly depends on the underlying spatial tree pattern in the sampling area. With most diversity indices, sample size reductions between 0.06% and 40% could be achieved by the application of T-square sampling compared to traditional distance sampling. All other conditions being equal, we could identify the uniform angle index, the species mingling index and the size differentiation index as those diversity indices achieving lower sampling error values than their competitors. For tree density estimations the Diggle and Byth estimators performed best. Based on our results, T-square sampling can be considered a robust sampling method for spatial tree diversity indices that is easy to apply in the field.

Larger and structurally complex woodland creation sites provide greater benefits for woodland plants

Abstract Reforestation initiatives are underway across the world. However, we know relatively little about the ecological consequences of creating and restoring forest ecosystems, and there is a lack of studies examining the drivers of species colonisation and establishment across appropriate temporal and spatial scales to inform conservation practice. Using data from a long‐term natural experiment (the WrEN project), we explore ground plant species occurrence and community composition in 102 woodland creation sites (10–160 years since planting), and 27 old growth woodlands (&gt;250 years). We conducted field surveys to collect data on occurrence of plant species (classified into woodland specialist, woodland generalist, or non‐woodland) and used Structural Equation Modelling to investigate the influence of local (age, size, woodland structure) and landscape‐level (amount of surrounding woodland) attributes on species richness. Woodland generalists are readily colonising woodland creation sites to similar levels found in old growth woodlands. However, there were fewer woodland specialist and more non‐woodland plants in creation sites than in old growth. Specialists and generalists were more likely to be present in larger woodlands and those with higher variation in tree size (which was higher in older woodlands) and did not appear to be influenced by features of the surrounding landscape. Some plant communities in older creation sites (80–160 years) were similar to old growth, suggesting colonisation of a typical old growth flora over time; however, some sites were shifting away from this trajectory. Specialists are slow to colonise woodland creation sites and their occurrence was low relative to old growth woodlands even after &gt;80 years. However, woodland management to increase structural complexity may enhance the establishment of woodland plants. The lack of influence of the surrounding landscape on species occurrence is likely due to most of the study sites being relatively isolated resulting in limited colonisation. This suggests that new woodlands need to be adjacent or very near to existing woodland to receive the benefits of increased colonisation. Our results highlight the importance of creating large and structurally complex woodlands, close to existing woodlands to facilitate the colonisation and establishment of woodland plants.

Life history scaling in a tropical forest

Abstract Both tree size and life history variation drive forest structure and dynamics, but little is known about how life history frequency changes with size. We used a scaling framework to quantify ontogenetic size variation and assessed patterns of abundance, richness, productivity and light interception across life history strategies from &gt;114,000 trees in a primary, neotropical forest. We classified trees along two life history axes: a fast–slow axis characterized by a growth–survival trade‐off, and a stature–recruitment axis with tall, long‐lived pioneers at one end and short, short‐lived recruiters at the other. Relative abundance, richness, productivity and light interception follow an approximate power law, systematically shifting over an order of magnitude with tree size. Slow saplings dominate the understorey, but slow trees decline to parity with rapidly growing fast and long‐lived pioneer species in the canopy. Like the community as a whole, slow species are the closest to obeying the energy equivalence rule (EER)—with equal productivity per size class—but other life histories strongly increase productivity with tree size. Productivity is fuelled by resources, and the scaling of light interception corresponds to the scaling of productivity across life history strategies, with slow and all species near solar energy equivalence. This points towards a resource‐use corollary to the EER: the resource equivalence rule. Fitness trade‐offs associated with tree size and life history may promote coexistence in tropical forests by limiting niche overlap and reducing fitness differences. Synthesis. Tree life history strategies describe the different ways trees grow, survive and recruit in the understorey. We show that the proportion of trees with a pioneer life history strategy increases steadily with tree size, as pioneers become relatively more abundant, productive, diverse and capture more resources towards the canopy. Fitness trade‐offs associated with size and life history strategy offer a mechanism for coexistence in tropical forests.

Stand age-driven tree size variation and stand type regulate aboveground biomass in alpine-subalpine forests, South Korea

Alpine and subalpine forests in mountains worldwide are ecologically significant because of their unique biodiversity and increased vulnerability to climate change. This study was conducted to explore the possibilities and ways to preserve the ecological diversity of alpine-subalpine forests and their function as important carbon sinks. In this study, data from 664 plots (400 m2) were collected in the alpine-subalpine zones above 1000 m elevation in South Korea, we divided 664 plots into four stand types: conifer, conifer-dominant mixed, broadleaved-dominant mixed, and broadleaved stands. Abiotic drivers and forest successional stage-related factor including topographic, climatic drivers and stand age class were used. Biotic drivers including taxonomic, phylogenetic, functional, stand structural diversity, and community-weighted mean of functional traits were used to find independent variables controlling aboveground biomass (AGB) for each stand type. We employed multi-model averaging approach as well as piecewise structural equation modeling (pSEM) for the identification of the most influential variables affecting AGB in each stand type of alpine-subalpine forests and to quantify their interrelationships and strengths. The main results showed that tree size variation (i.e., DBH STD) induced by stand age had direct effects on AGB, with varying degrees of significance (β) ranging from 0.146 to 0.241 across all stand types in alpine-subalpine forests. Following these results, as forest succession progresses, tree species adapted to the specific environmental conditions, such as topography and climate, become dominant by creating their own niche, which increases AGB in each stand type. Additionally, climatic and topographic conditions played an important role in controlling biotic drivers depending on the stand type. In this study, we suggest that AGB should be managed and conserved depending on forest stand types according to forest succession. Furthermore, increasing size variation among tree individuals through proper forest treatments is important for increasing AGB in alpine-subalpine forests.

Tree size diversity is the major driver of aboveground carbon storage in dryland agroforestry parklands

Despite the importance of agroforestry parkland systems for ecosystem and livelihood benefits, evidence on determinants of carbon storage in parklands remains scarce. Here, we assessed the direct and indirect influence of human management (selective harvesting of trees), abiotic factors (climate, topography, and soil) and multiple attributes of species diversity (taxonomic, functional, and structural) on aboveground carbon (AGC) stocks in 51 parklands in drylands of Benin. We used linear mixed-effects regressions and structural equation modeling to test the relative effects of these predictors on AGC stocks. We found that structural diversity (tree size diversity, HDBH) had the strongest (effect size β = 0.59, R2 = 54%) relationship with AGC stocks, followed by community-weighted mean of maximum height (CWMMAXH). Taxonomic diversity had no significant direct relationship with AGC stocks but influenced the latter indirectly through its negative effect on CWMMAXH, reflecting the impact of species selection by farmers. Elevation and soil total organic carbon content positively influenced AGC stocks both directly and indirectly via HDBH. No significant association was found between AGC stocks and tree harvesting factor. Our results suggest the mass ratio, niche complementarity and environmental favorability as underlying mechanisms of AGC storage in the parklands. Our findings also highlight the potential role of human-driven filtering of local species pool in regulating the effect of biodiversity on AGC storage in the parklands. We conclude that the promotion of AGC stocks in parklands is dependent on protecting tree regeneration in addition to enhancing tree size diversity and managing tall-stature trees.

Diversity Patterns and Determinants of Soil Microorganisms and Nematodes along Elevation Gradients in a Temperate Forest in South Korea

The elevational patterns of soil microbial and nematodes diversity (SMND) and the determinants remain controversial. Moreover, how the SMND are modified simultaneously with an elevational gradient has not yet been established. In this study, we investigated the elevational patterns of the SMND and the relative importance among/within tree factors (i.e., tree diversity, identity, and quantity) and environmental factors (i.e., climate and soil) on the SMND. For this purpose, we analyzed datasets from 27 plots across nine elevation bands in the temperate forests of Mt. Gariwang, South Korea. We performed multimodel inference tests and subsequently conducted a variance partitioning to determine the most prominent factors controlling each SMND and compare the relative contribution of the trees and environmental effects. Our results revealed that bacterial and fungal diversity decreased along the elevation gradient. However, nematode diversity did not change significantly, indicating that site-specific environmental conditions may be more influential than the elevation per se. Moreover, this indicates that bacterial diversity was affected by the pH and functional dispersion of the leaf size, and that fungal diversity was governed only by the pH. However, nematode diversity was driven by aboveground biomass, ammonium-nitrogen, and tree size diversity. In summary, the soil microbial diversity was more strongly controlled by the environmental factors, whereas the tree factors were more important for nematodes. Our results show that the elevational patterns and determinants of SMND differed among the taxonomic groups in the common micro-food web. These findings provide new insights into the factors controlling the SMND in a temperate forest and expand the local knowledge of soil biodiversity which is necessary for promoting its mainstreaming. Thus, our results contribute to establishing a basis for more targeted and effective biodiversity conservation and management practices in forest ecosystems.

The relationship between forest structure and naturalness in the Finnish national forest inventory

Abstract There is considerable interest in identifying and locating natural forests as accurately as possible, because they are deemed essential in preventing biodiversity loss. In the boreal region, natural forests contain a substantial amount of dead wood and exhibit considerable variation in tree age, size, and species composition. However, it is difficult to define natural forests in a quantitative manner. This is an issue, for example, in the Finnish national forest inventory. If naturalness could be related to the metrics derived from tree measurements, it would be easier to locate natural forests based on the inventory data. In this study, we investigated the value of metrics computed from tree locations and tree sizes for the characterization of a key aspect of naturalness, namely, structural naturalness as defined in the Finnish national forest inventory. We used L-moments, Gini coefficient, Lorenz asymmetry, and interquartile range to quantify the variations in tree size at the plot level. We summarized the spatial pattern of trees with a spatial aggregation index. We compared the structural metrics, species proportions, and stand age using the classes of structural naturalness described in the Finnish national forest inventory, which have been determined in the field without strict numerical rules. These categories are ‘natural’, ‘near-natural’, and ‘non-natural’. We found that the forests evaluated as structurally natural had larger variations in tree size and species composition and showed a more clustered spatial pattern of trees on average, although the variation in the structural metrics was considerable in all three classes. In addition, we used the structural metrics to predict naturalness by employing a random forest algorithm. Based on the structural metrics, it was possible to obtain high precision in the classification only if we simultaneously accepted low recall, and vice versa; the link between the inspected metrics and naturalness evaluated in the field was weak. The stand age separated the three classes more clearly and it also improved the classification.

Climate Strengthens the Positive Effects of Stand Structure on Understory Plant Diversity in Chinese Temperate Forests

Stand structure, which links function and management, plays a crucial role in regulating forest ecosystems and influencing biodiversity. Nevertheless, knowledge of the effect of climate change on stand structure and plant diversity is still poorly understood on a large scale. To explore the effects of various climate conditions on stand structure–plant diversity, we conducted a comprehensive analysis of data from 1272 plots across China’s temperate and subtropical forests. Leveraging the structural equation model (SEM), we explored the direct and indirect effects of climate, topography, and tree diversity on understory woody and herbaceous plants with respect to stand structure. Furthermore, we evaluated the effect size of stand structure on understory vegetation diversity under different climatic zones. Our results showed that tree size variation (CV DBH) and stem density (SD) were the key drivers for understory woody plants, while the stand structure complexity index (SSCI) was more important for understory herbaceous diversity. Furthermore, the positive effects of stand structure differed across various climate zones and were enhanced with an increase in the climatic gradient. For instance, the impact of SD on understory woody plants, as well as the influence of the SSCI on the diversity of understory herbaceous vegetation, were both strengthened. These findings raise our awareness of the pressing need to manage stand structure heterogeneity differently across different climate zones, and different management also needs to be implemented among different understory plant types. It becomes evident that distinct forest management measures must be applied under future climate change and forest management practices in order to preserve biodiversity.

A Framework for Analyzing Individual-Tree and Whole-Stand Growth by Fusing Multilevel Data: Stochastic Differential Equation and Copula Network

In forestry, growth functions form the basis of research and are widely used for the mathematical modeling of stand variables, e.g., tree or stand basal area, stand height, stand volume, site index, and many more. In this study, to estimate five-dimensional dependencies between tree diameter at breast height, potentially available area, height, crown area and crown base height, we used a normal copula approach whereby the growths of individual variables are described using a stochastic differential equation with mixed-effect parameters. The normal copula combines the marginal distributions of tree diameter at breast height, potentially available area, height, crown area, and crown base height into a joint multivariate probability distribution. Copula models have the advantage of being able to use collected longitudinal, multivariate, and discrete data for which the number of measurements of individual variables does not match. This study introduced a normalized multivariate interaction information measure based on differential entropy to assess the causality between tree size variables. In order to accurately and quantitatively assess the stochastic processes of the tree size variables’ growth and to provide a scientific basis for the formalization of models, an analysis method of the synergetic theory of information entropy has been proposed. Theoretical findings are illustrated using an uneven-aged, mixed-species empirical dataset of permanent experimental plots in Lithuania.

Stem shape and structural complexity change in beech forests along a management gradient

For about half a century, attempts have been made to manage forests as close to nature as possible. This management targeted the creation of structures that resemble those found in primary forests. Laser scanning provides the opportunity to quantify such structural “naturalness” and allows to evaluate which management practices come closest to forest structures found in primary forests. In this paper, European beech (Fagus sylvatica L.) forests with different management intensity were compared to primary forests in terms of their structural complexity and the shape of tree stems. For this purpose, data from mobile and terrestrial laser scans (MLS and TLS) of managed forests, of forests whose management has been abandoned, and of primary forests was used. We found that management intensity influenced the distribution of plant material in the forest stand and thus the structural complexity on stand level. Also, management affected the shape of the stems. Here, it is important to consider the management history of the forest stands and the forest development phase in which management was abandoned. The stem shapes of trees in primary forests were significantly different (larger stem diameters and longer branch free boles) from those of the other investigated forests. Nevertheless, our results showed that it is possible to achieve old-growth-like structures such as high standing volumes, a multi layered canopy, high number of large trees and high variation in tree size and age through targeted silvicultural treatments in order to accelerate the close-to-nature development of forests. The present study illustrates the possibility of using mobile and terrestrial laser scanning to objectively compare and evaluate the structural effects of different silvicultural concepts.

Effects of geo-climate factors on phenotypic variation in cone and seed traits of Pinus yunnanensis.

Evaluating variations in reproductive traits and the response of the variations to geo-climate conditions are essential for understanding the persistence, evolution, and range dynamics of plant populations. However, there are insufficient studies to attempt to analyze the importance of geo-climate factors in explaining within- or among-population variation in reproductive traits. We examined 14 traits for 2671 cones of Pinus yunnanensis collected from nine populations in the mountains of Southwest China to characterize the patterns of phenotypic variation of traits and estimate environmental effects on these trait performances and trait variation. We found the contribution of intrapopulation variation to the overall variation was greater than the interpopulation variation and the larger coefficients of variation for the populations lying at the edge of northern and southern regions. Climatic variables are more important than geographical and tree size variables in their relationships to cone and seed traits. Populations in more humid and warmer climate expressed greater cone and seed weight and seed number but lower seed abortion rate, while the larger coefficients of variation in seed weight and number were detected in northern and southern marginal regions with drier or colder climate. Our study illustrates that intraspecific trait variation should be considered when examining plant species response to changing climate and suggests that the high variability rather than high quality of seed traits in the marginal regions with drier or colder climate might foster plant-population persistence in stressful conditions.

Relative effects of climate, stand environment and tree characteristics on annual tree growth in subtropical Cunninghamia lanceolata forests

Tree growth is driven by various factors, including site climate, stand and tree variables. A thorough understanding of the respective roles of the main drivers is imperative to project or promote forest productivity in future scenarios. However, the complex relationships among the drivers and tree growth remained insufficiently understood. Using the structural equation modeling (SEM) approach, we examined the multiple and interactive relationships between annual tree growth and climate, stand- (site quality and competition) and tree-level (tree age and tree size) variables based on data from Chinese fir (Cunninghamia lanceolata) plantations, Southeastern China. To simplify the SEM structures, randomforest (RF) was employed initially to screen for factors with greater impacts on tree growth. SEMs showed that annual tree growth largely depended on climate and site quality, while competition had a smaller relative effect of less than 8%. Climate was the primary driver. Different climate factors influenced tree growth directly, and also indirectly via tree size and site quality. Tree growth increments decreased with the increase of maximum daily temperature (especially in current-year August), while increased with relative humidity (especially in previous-year October) and potential evapotranspiration (especially in current-year May). Site index (SI) and tree size were both of higher importance in predicting the annual tree increments and had a positive impact on tree growth, contributing about 1/3 and 15 % to the total effect, respectively. In contrast, both the direct and indirect effects of stand competition were negative. Basal area of larger trees (BAL) appeared to better capture competition than stand density. This study suggested that SEM combined with RF provided a promising way to understand the complexity of tree growth. Larger trees in forests with higher site quality and lower density grow faster, however, their annual increments seem to decrease with increasing temperature.

Recreating structurally realistic tree maps with airborne laser scanning and ground measurements

The estimation of forest attributes representing aspects of structural biodiversity from vast areas is difficult not only because of the lack of agreement on what constitutes a structurally diverse forest, but also due to difficulties in the detection of small trees. However, a structurally diverse forest should have large variation in tree size (i.e., height and diameter), as well as some clustering of trees, as regular tree pattern is typical for managed forests. In this work we developed a framework for building structurally representative tree maps using airborne laser scanning data and a limited number of ground measurements using data from two locations in Finland. In the proposed method, an individual tree detection algorithm is optimized so that the number of undetected trees and false detections is minimized. The ground measurements are then used to train models for predicting the number and location of undetected trees and false detections by resampling the attributes of undetected trees in the training data. This model can then be applied to other areas in the proximity of the ground measurements to build tree maps, with the location, height, and diameter for each tree. The methodology was shown to reproduce the number of trees, the height distribution, and the spatial pattern of the trees with sufficient accuracy for practical use in large-scale mapping of forest attributes. For example, the individual tree detection could find at best about 54% of the trees with an additional 7% of false detections, resulting in a bias of approximately −30%, while with the new method the bias was reduced to ±10% of the stem density. Further research is needed on how to account for tree species when building tree maps.