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1. Fruit traits of pioneer trees structure seed dispersal across distances on tropical deforested landscapes: Implications for restoration

   https://doi.org/10.1111/1365-2664.13697  

   Camargo, Paulo H. S. A. ; Pizo, Marco A. ; Brancalion, Pedro H. S. ; Carlo, Tomás A. ; Hampe, ed., Arndt ( July 2020 , Journal of Applied Ecology)

   Pioneer trees with fleshy fruits are typically planted in restoration projects to attract frugivores as a mean to increase dispersal and accelerate forest regeneration. However, differences in fruit traits of pioneer trees can potentially influence dispersal and their restoration outcomes.

   Here we investigated the effects of bird and plant traits, and distance to forest fragments, on the seed rain using a tree‐planting experiment replicated in 12 deforested sites in Brazil. Factors were fruit traits of pioneer trees (wind‐dispersed, bird‐dispersed with lipids or with carbohydrates and controls) and distance (10, 50, 300 m) from forest fragments.

   We found that density and richness of birds and seeds decreased exponentially with distance from fragments, yet these effects were minor compared to the effects of fruit traits on the structure of the seed rain.

   Overall, plots with fleshy fruited pioneers attracted much greater bird activity and seed dispersal than plots with wind‐dispersal pioneers and the controls. For instance, plots with carbohydrate‐rich fruits received more than twice the average species richness and density of birds and seeds of plots with lipid‐rich pioneer trees, surpassing wind‐dispersed pioneers by more than 80%, and controls by over 90%. Furthermore, the fruit trait treatments resulted in morphological shifts in the average traits of visiting birds. Significant differences in bill gape and flight capacities (wing‐loading) were associated with the differences in the seed rain associated with each treatment.

   Synthesis and applications. Understanding how trait‐matching processes mediating mutualistic seed dispersal by frugivores interact with distance‐dependent dispersal limitation on deforested tropical landscapes is critical for improving forest restoration efforts. This is especially relevant in the context of applied nucleation. As shown here, avian seed dispersal can thus be manipulated in restoration projects in order to increase connectivity and speed up forest recovery and the provision of the multiple ecosystem services that follow forest succession.

    

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2. Intraspecific variation in seed dispersal of a Neotropical tree and its relationship to fruit and tree traits

   https://doi.org/10.1002/ece3.1905  

   Augspurger, Carol K. ; Franson, Susan E. ; Cushman, Katherine C. ; Muller‐Landau, Helene C. ( January 2016 , Ecology and Evolution)

   The distribution of wind‐dispersed seeds around a parent tree depends on diaspore and tree traits, as well as wind conditions and surrounding vegetation. This study of a neotropical canopy tree,Platypodium elegans,explored the extent to which parental variation in diaspore and tree traits explained (1) rate of diaspore descent in still air, (2) distributions of diaspores dispersed from a 40‐m tower in the forest, and (3) natural diaspore distributions around the parent tree. The geometric mean rate of descent in still air among 20 parents was highly correlated with geometric mean wing loading^1/2(r = 0.84). However, diaspore traits and rate of descent predicted less variation in dispersal distance from the tower, although descent rate⁻¹consistently correlated with dispersal distance. Measured seed shadows, particularly their distribution edges, differed significantly among six parents (DBHrange 62–181 cm) and were best fit by six separate anisotropic dispersal kernels and surveyed fecundities. Measured rate of descent and tree traits, combined in a mechanistic seed dispersal model, did not significantly explain variation among parents in natural seed dispersal distances, perhaps due to the limited power to detect effects with only six trees. Seedling and sapling distributions were at a greater mean distance from the parents than seed distributions; saplings were heavily concentrated at far distances. Variation among parents in the distribution tails so critical for recruitment could not be explained by measured diaspore or tree traits with this sample size, and may be determined more by wind patterns and the timing of abscission in relation to wind conditions. Studies of wind dispersal need to devote greater field efforts at recording the “rare” dispersal events that contribute to far dispersal distances, following their consequences, and in understanding the mechanisms that generate them.

    

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3. Crossing the threshold: Invasive grasses inhibit forest restoration on Hawaiian islands

   https://doi.org/10.1002/eap.2841  

   Rehm, Evan M. ; D'Antonio, Carla ; Yelenik, Stephanie ( April 2023 , Ecological Applications)

   Forest removal for livestock grazing is a striking example of human‐caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration. We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawaiʻi Island, USA. Since the 1980s, over 400,000Acacia koa(koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody‐plant recruitment. Between this koa/grass matrix are remnant nativeMetrosideros polymorpha(ʻōhiʻa) trees beneath which native woody plants naturally recruit. We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and ʻōhiʻa) with a range of native woody understory basal area (BA). We found a positive relationship between total seed rain (but not bird‐dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant ʻōhiʻa (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources.

    

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4. Seed‐to‐seedling transitions exhibit distance‐dependent mortality but no strong spacing effects in a Neotropical forest

   https://doi.org/10.1002/ecy.2926  

   Marchand, Philippe ; Comita, Liza S. ; Wright, S. Joseph ; Condit, Richard ; Hubbell, Stephen P. ; Beckman, Noelle G. ( December 2019 , Ecology)

   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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5. Do seed dispersal strategies reflect adaptation to environmental variability?

   https://doi.org/10.1111/1365-2435.14336  

   Van Den Elzen, Courtney L. ; Sigman, Nathan ; Emery, Nancy C. ( April 2023 , Functional Ecology)

   Dispersal is one of the primary mechanisms by which organisms adapt to spatial and temporal variation in the environment. Theory predicts that increasing spatiotemporal variation drives selection for offspring dispersal away from their natal habitat and one another. However, due to inherent difficulties in measuring dispersal in plant systems, there are few empirical tests of the extent to which this hypothesis can explain variation in seed dispersal strategies.

   In this study, we characterized and compared the dispersal patterns of three closely related plant species that segregate across gradients in spatiotemporal variation in seasonal wetlands.

   We tracked individual seeds as they dispersed in their natural habitats to measure seed dispersal distance (the distance travelled from the maternal plant) and inter‐seed spread (distances between dispersed seeds) and to identify the plant traits causing within‐species variation in seed dispersal. We also evaluated the seed traits causing within‐species variation in seed flight distance and terminal velocity in a wind tunnel and a drop tube, respectively.

   We found that average seed dispersal distance was lowest in the species that occupies the most spatiotemporally variable habitat, contradicting our predictions; however, inter‐seed spread was lowest in the species from the least variable habitat, which aligned with our expectations.

   The maternal plant and seed traits explaining intraspecific variation in seed dispersal varied among species as well as the method used to measure dispersal potential. Two traits had non‐intuitive effects on dispersal, including pappus size, which reduced seed flight distance in two of the focal taxa.

   Overall, our results indicate that the differences we detected in seed dispersal among three closely related plant taxa can be only partially explained by current patterns of environmental variability in their respective habitats and that the traits driving within‐species variation in seed dispersal can evolve rapidly and change with the environmental context in which they are measured.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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