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"forest dynamics"
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Functional trait and phylogenetic tests of community assembly across spatial scales in an Amazonian forest
Despite a long history of the study of tropical forests, uncertainty about the importance of different ecological processes in shaping tropical tree species distributions persists. Trait- and phylogenetic-based tests of community assembly provide a powerful way to detect community assembly processes but have seldom been applied to the same community. Both methods are well suited to testing how the relative importance of different ecological processes changes with spatial scale. Here we apply both methods to the Yasuní Forest Dynamics Plot, a 25-ha Amazonian forest with >1100 tree species. We found evidence for habitat filtering from both trait and phylogenetic methods from small (25 m
2
) to intermediate (10 000 m
2
) spatial scales. Trait-based methods detected even spacing of strategies, a pattern consistent with niche partitioning or enemy-mediated density dependence, at smaller spatial scales (25-400 m
2
). Simulation modeling of community assembly processes suggests that low statistical power to detect even spacing of traits at larger spatial scales may contribute to the observed patterns. Trait and phylogenetic methods tended to identify the same areas of the forest as being subject to habitat filtering. Phylogenetic community tests, which are far less data-intensive than trait-based methods, captured much of the same filtering patterns detected by trait-based methods but often failed to detect even-spacing patterns apparent in trait data. Taken together, it appears that both habitat associations and niche differentiation shape species co-occurrence patterns in one of the most diverse forests in the world at a range of small and intermediate spatial scales.
Journal Article
Lightning is a major cause of large tree mortality in a lowland neotropical forest
• The mortality rates of large trees are critical to determining carbon stocks in tropical forests, but the mechanisms of tropical tree mortality remain poorly understood. Lightning strikes thousands of tropical trees every day, but is commonly assumed to be a minor agent of tree mortality in most tropical forests.
• We use the first systematic quantification of lightning-caused mortality to show that lightning is a major cause of death for the largest trees in an old-growth lowland forest in Panama. A novel lightning strike location system together with field surveys of strike sites revealed that, on average, each strike directly kills 3.5 trees (> 10 cm diameter) and damages 11.4 more.
• Given lightning frequency data from the Earth Networks Total Lightning Network and historical total tree mortality rates for this site, we conclude that lightning accounts for 40.5% of the mortality of large trees (> 60 cm diameter) in the short term and probably contributes to an additional 9.0% of large tree deaths over the long term.
• Any changes in cloud-to-ground lightning frequency due to climatic change will alter tree mortality rates; projected 25–50% increases in lightning frequency would increase large tree mortality rates in this forest by 9–18%. The results of this study indicate that lightning plays a critical and previously underestimated role in tropical forest dynamics and carbon cycling.
Journal Article
Shared friends counterbalance shared enemies in old forests
Mycorrhizal mutualisms are nearly ubiquitous across plant communities. Yet, it is still unknown whether facilitation among plants arises primarily from these mycorrhizal networks or from physical and ecological attributes of plants themselves. Here, we tested the relative contributions of mycorrhizae and plants to both positive and negative biotic interactions to determine whether plant–soil feedbacks with mycorrhizae neutralize competition and enemies within multitrophic forest community networks. We used Bayesian hierarchical generalized linear modeling to examine mycorrhizal-guild-specific and mortality-cause-specific woody plant survival compiled from a spatially and temporally explicit data set comprising 101,096 woody plants from three mixed-conifer forests across western North America. We found positive plant–soil feedbacks for large-diameter trees: species-rich woody plant communities indirectly promoted large tree survival when connected via mycorrhizal networks. Shared mycorrhizae primarily counterbalanced apparent competition mediated by tree enemies (e.g., bark beetles, soil pathogens) rather than diffuse competition between plants. We did not find the same survival benefits for small trees or shrubs. Our findings suggest that lower largediameter tree mortality susceptibility in species-rich temperate forests resulted from greater access to shared mycorrhizal networks. The interrelated importance of aboveground and belowground biodiversity to large tree survival may be critical for counteracting increasing pathogen, bark beetle, and density threats.
Journal Article
Importance of topography for tree species habitat distributions in a terra firme forest in the Colombian Amazon
Aims
To test the relative importance of topography versus soil chemistry in defining tree species-habitat associations in a terra firme Amazonian forest.
Method
We evaluated habitat associations for 612 woody species using alternative habitat maps generated from topography and soil chemistry in the 25-ha Amacayacu Forest Dynamics Plot, Colombian Amazon. We assessed the ability of each habitat map to explain the community-level patterns of species-habitat associations using two methods of habitat randomization and different sample size thresholds (i.e., species’ abundance).
Results
The greatest proportion of species-habitat associations arose from topographically-defined habitats (55% to 63%) compared to soil chemistry-defined (19% to 40%) or topography plus soil chemistry-defined habitats (18% to 42%). Results were robust to the method of habitat randomization and to sample size threshold.
Conclusions
Our results demonstrate that certain environmental factors may be more influential than others in defining forest-level patterns of community assembly and that comparison of the ability of different environmental variables to explain habitat associations is a crucial step in testing hypotheses about the mechanisms underlying assembly. Our results point to topography-driven hydrological variation as a key factor structuring tree species distributions in what are commonly considered homogeneous Amazonian terra firme forests.
Journal Article
Mycorrhizal type influences plant density dependence and species richness across 15 temperate forests
Recent studies suggest that the mycorrhizal type associated with tree species is an important trait influencing ecological processes such as response to environmental conditions and conspecific negative density dependence (CNDD). However, we lack a general understanding of how tree mycorrhizal type influences CNDD strength and the resulting patterns of species abundance and richness at larger spatial scales. We assessed 305 species across 15 large, stem-mapped, temperate forest dynamics plots in Northeastern China and North America to explore the relationships between tree mycorrhizal type and CNDD, species abundance, and species richness at a regional scale. Tree species associated with arbuscular mycorrhizal (AM) fungi showed a stronger CNDD and a more positive relationship with species abundance than did tree species associated with ectomycorrhizal (ECM) fungi. For each plot, both basal area and stem abundance of AM tree species was lower than that of ECM tree species, suggesting that AM tree species were rarer than ECM tree species. Finally, ECM tree dominance showed a negative effect on plant richness across plots. These results provide evidence that tree mycorrhizal type plays an important role in influencing CNDD and species richness, highlighting this trait as an important factor in structuring plant communities in temperate forests.
Journal Article
Effects of light and topography on regeneration and coexistence of evergreen and deciduous tree species in a Chinese subtropical forest
1. Evergreen broad-leaved forests are widely distributed in eastern Asia with evergreen broad-leaved (EBL) and deciduous broad-leaved (DBL) tree species coexisting under the same climatic regime, raising questions as to the underlying mechanisms. Since EBL and DBL species differ in leaf life span, a key component of resource economic strategies, their coexistence might be attributed to regeneration niche partitioning across habitats varying in resource supply. 2. We investigated the effects of variation in insolation and topography on regeneration of EBL and DBL species in a subtropical EBL forest of eastern China after an ice storm that caused severe canopy disturbance. 3. Using a mixed-effects modelling framework and census data from 2011 to 2014 on 8,548 wild seedlings of 123 species, we quantified habitat preferences of EBL and DBL species during post-disturbance regeneration and how their survival and height relative growth rates varied among habitats. 4. The relative density of DBL seedlings (proportional to all seedlings) was greater in habitats with greater (canopy gaps) compared to habitats with lesser (understorey) insolation and increased with canopy gap size. However, DBL seedlings were not more frequent in higher (valleys) compared to lower (ridges) fertility habitats. Although DBL seedlings exhibited larger differences in growth between higher and lower resource habitats than EBL seedlings, their growth rates did not increase with canopy gap size. Seedlings of EBL species had high survival in all habitats, but larger DBL seedlings survived equally well on ridges. Consequently, the relative density of DBL seedlings declined in valleys, so that by 2014 it became more similar in valley and ridge habitats, whereas it remained higher in gaps than in the understorey, and especially in larger gaps. 5. Synthesis. Specialization on contrasting topographic habitats is considered the primary mechanism mediating coexistence between deciduous broad-leaved and evergreen broad-leaved species. Our results, however, suggest this may not always be true, since seedlings of deciduous broad-leaved and evergreen broad-leaved species partitioned regeneration niches based on light more so than topography. We propose that coexistence of deciduous broad-leaved and evergreen broadleaved species can strongly depend upon canopy disturbance to create a mosaic of habitat patches, including high light gaps favouring regeneration of deciduous broad-leaved species.
Journal Article
Tree by Tree
Tree by Tree is a warning and a toolkit for the future of forest recovery. Scott J. Meiners investigates the critical biological threats endangering tree species native to the forests of eastern North America, providing a needed focus on this plight. Meiners suggests that if we are to save our forests, the first step is to recognize the threats in front of us.
Meiners focuses on five familiar trees—the American elm, the American chestnut, the eastern hemlock, the white ash, and the sugar maple—and shares why they matter economically, ecologically, and culturally. From outbreaks of Dutch elm disease to infestations of emerald ash borers, Meiners highlights the challenges that have led or will lead to the disappearance of these trees from forests. In doing so, he shows us how diversity loss often disrupts intricately balanced ecosystems and how vital it is that we pay more attention to massive changes in forest composition.
With practical steps for the conservation of native tree species, Tree by Tree offers the inspiration and insights we need to begin saving our forests.
eBook
Capturing ecological processes in dynamic forest models: why there is no silver bullet to cope with complexity
Dynamic forest models are a key tool to better understand, assess, and project decadal‐ to centennial‐scale forest dynamics. Despite their success, many questions regarding appropriate model formulations remain unresolved, and few models have found widespread application, for example, across a whole continent. We aimed to scrutinize the representation of ecological processes in dynamic forest models so as to rigorously test core assumptions underlying forest dynamics and the consistency of their interplay, taking the ForClim model as a case study. We developed a set of alternative representations for the main ecological processes, that is, tree establishment, growth, and mortality, and light extinction through the canopy, based on diverse sources of empirical data. We applied a pattern‐oriented modeling (POM) approach to test all combinations of the standard and alternative formulations (>500 model versions) against a comprehensive set of patterns for diverse model applications across a wide range of site conditions. We found that adapting one process in isolation can improve model performance for one specific application. However, the best model versions typically included more than one alternative formulation. Importantly, the best version for an individual application was generally not the best across multiple applications, emphasizing the varying influences of ecological processes. We conclude that the behavior and performance of complex models should not be analyzed for a few specific applications only. Rather, multiple applications, system states, and dynamics of interest should be scrutinized across a wide range of site conditions. This allows for avoiding overfitting and detecting and eliminating structural shortcomings and parameterization problems. We thus propose to make use of the ever‐increasing data availability and the POM framework to challenge the core processes of dynamic models in a holistic manner. For model applications, we propose that a set of alternative formulations (ensemble simulations) should be used to quantify the impacts of structural uncertainty, rather than to rely on the projections from one single model version.
Journal Article