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  1. Free, publicly-accessible full text available May 1, 2024
  2. Abstract

    Successful management of fire‐prone woody ecosystems is challenging and requires knowledge of the spatial arrangement of the trees and how the tree distribution patterns influence the nature and consequences of subsequent fires.

    In open tree landscapes, trees are often aggregated, and the ability of trees within the clumps to survive fires plays a significant role in determining subsequent landscape dynamics. If positive interactions exist among neighbouring trees, this will help maintain the patterns of clumped trees. However, the tree‐aggregated landscape will continue to exist only if the positive neighbour interactions persist consistently over time. In cases where disturbances are episodic, detecting these interactions is only possible through long‐term studies.

    Data reported here are from a 25‐year study involving the annual tree censusing of a large grid‐plot in a frequently burned open oak landscape dominated byQuercus macrocarpaandQuercus ellipsoidallis. The results showed that while having neighbours reduced tree growth, neighbours consistently facilitated survival, irrespective as to whether the neighbours were conspecifics or heterospecifics. Trees of all sizes in close proximity to neighbours were considerably more likely to survive fire throughout the study. This neighbour facilitation is likely the result of a reduction of both herbaceous and woody fuel within clumps.

    Synthesis. This is the first study to document consistent neighbour facilitation among trees experiencing repeated stressors over an extended time period. Our findings support the literature documenting positive neighbour effects among plants in stressful and highly disturbed environments, in accordance with the stress‐gradient hypothesis. While aggregated tree regeneration is typically viewed as the primary cause for the development of tree clumps in fire‐prone ecosystems, our study showed that aggregated tree survival, by itself, can also be an important driver of post‐fire tree clumping. Our results support the growing literature emphasizing the importance of landscape heterogeneity as a driver of resilience in fire‐prone tree ecosystems, and the value of maintaining or creating this heterogeneity during forest management.

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  3. null (Ed.)
  4. Understanding tropical forest dynamics and planning for their sustainable management require efficient, yet accurate, predictions of the joint dynamics of hundreds of tree species. With increasing information on tropical tree life histories, our predictive understanding is no longer limited by species data but by the ability of existing models to make use of it. Using a demographic forest model, we show that the basal area and compositional changes during forest succession in a neotropical forest can be accurately predicted by representing tropical tree diversity (hundreds of species) with only five functional groups spanning two essential trade-offs—the growth-survival and stature-recruitment trade-offs. This data-driven modeling framework substantially improves our ability to predict consequences of anthropogenic impacts on tropical forests. 
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  5. Simova, Irena (Ed.)
  6. Abstract

    Environmental gradients act as potent filters on species distributions driving compositional shifts across communities. Compositional shifts may reflect differences in physiological tolerances to a limiting resource that result in broad distributions for tolerant species and restricted distributions for intolerant species (i.e. a nested pattern). Alternatively, trade‐offs in resource use or conflicting species' responses to multiple resources can result in complete turnover of species along gradients.

    We combined trait (leaf area, leaf mass per area, wood density and maximum height) and distribution data for 550 tree species to examine taxonomic and functional composition at 72 sites across strong gradients of soil phosphorus (P) and rainfall in central Panama.

    We determined whether functional and taxonomic composition was nested or turned over completely and whether community mean traits and species composition were more strongly driven by P or moisture.

    Turnover characterized the functional composition of tree communities. Leaf traits responded to both gradients, with species having larger and thinner leaves in drier and more fertile sites than in wetter and less fertile sites. These leaf trait–moisture relationships contradict predictions based on drought responses and suggest a greater role for differences in light availability than in moisture. Shifts in wood density and maximum height were weaker than for leaf traits with taller species dominating wet sites and low wood density species dominating P‐rich sites.

    Turnover characterized the taxonomic composition of tree communities. Geographic distances explained a larger fraction of variation for taxonomic composition than for functional composition, and community mean traits were more strongly driven by P than moisture.

    Synthesis. Our results offer weak support for the tolerance hypothesis for tree communities in central Panama. Instead, we observe functional and taxonomic turnover reflecting trade‐offs and conflicting species' responses to multiple abiotic factors including moisture, soil phosphorus and potentially other correlated variables (e.g. light).

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  7. Abstract

    Patterns of seed dispersal and seed mortality influence the spatial structure of plant communities and the local coexistence of competing species. Most seeds are dispersed in proximity to the parent tree, where mortality is also expected to be the highest, because of competition with siblings or the attraction of natural enemies. Whereas distance‐dependent mortality in the seed‐to‐seedling transition was often observed in tropical forests, few studies have attempted to estimate the shape of the survival‐distance curves, which determines whether the peak of seedling establishment occurs away from the parent tree (Janzen–Connell pattern) or if the peak attenuates but remains at the parent location (Hubbell pattern). In this study, we inferred the probability density of seed dispersal and two stages of seedling establishment (new recruits, and seedlings 20 cm or taller) with distance for 24 tree species present in the 50‐ha Forest Dynamics Plot of Barro Colorado Island, Panama. Using data from seed traps, seedling survey quadrats, and tree‐census records spanning the 1988–2014 period, we fit hierarchical Bayesian models including parameters for tree fecundity, the shape of the dispersal kernel, and overdispersion of seed or seedling counts. We combined predictions from multiple dispersal kernels to obtain more robust inferences. We find that Hubbell patterns are the most common and Janzen–Connell patterns are very rare among those species; that distance‐dependent mortality may be stronger in the seed stage, in the early recruit stage, or comparable in both; and that species with larger seeds experience less overall mortality and less distance‐dependent mortality. Finally, we describe how this modeling approach could be extended at a community scale to include less abundant species.

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