Why do the trees grov where they grow.

Submitted by editor on 24 March 2015.Get the paper!

The southwestern Ecuador holds one of the last remnants of seasonally tropical dry forest of the Tumbesian region, a place where amazing biodiversity, endemism and adaptation to extreme conditions can be found. However, this forest face high anthropogenic pressures such as deforestation and land use changes, and more research is needed to fully understand the ecological function of these ecosystems in order to find strategies that help us to preserve its biodiversity.

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Despite the extreme conditions of dry forest, much heterogeneity occurs at small scales, affecting the spatial structure of plants. Plant species have developed a high variety of fruit and seed adaptations to disperse and perpetuate successfully. This seed dispersal processes are an important issue to understand the structure of plant communities.

 

 

Our study “Does spatial heterogeneity blur the signature of dispersal syndromes on spatial patterns of woody species?” look at  determining if there is a signal of dispersal syndromes on the spatial structure of plants. Moreover, we assess how these signals could be altered by environmental heterogeneity. Our working hypothesis is that the realized spatial pattern of a plant species is the result of a two-step process where dispersal syndrome sets the initial template and environmental heterogeneity potentially blurs the original pattern of seed dispersal, generating the realized pattern of adult trees.

We established a 9-ha (300 x 300 m) permanent plot in a well conserved area within the Arenillas Ecological Reserve, in southwestern Ecuador. All trees and shrubs exceeding 5 cm diameter at breast height were mapped, measured, and identified to species. We collected diaspores for all species in the area and classified them into three major dispersal syndromes according to the dispersal agent: animal, wind or explosive/unassisted.

 

One way to analyze spatial relationships in woody plants is by means of spatial point processes, which allow us to model the spatial patterns of plants and getting the ecological processes underlying. To define the spatial pattern of each species we used four models representing different processes affecting the spatial pattern: randomness, environmental heterogeneity, limited dispersal and joint effects of limited dispersal and environmental heterogeneity. We selected the model that best fitted the spatial pattern of each species and we analyzed the relationship with the dispersal syndrome.

 

Our results showed that environmental heterogeneity, and not only dispersal limitation, affected species spatial patterns in the studied seasonally tropical dry forest. Thus, ignoring habitat heterogeneity could bias the analysis of relationships between dispersal syndrome and species realized spatial patterns.

We analyzed the cluster size of species with different dispersal syndrome and found a trend from animal-dispersed species, which showed the largest cluster sizes, followed by wind dispersed species and explosive/unassisted species. However, there was a large variation even among species with the same dispersal syndrome. We proposed some explanations based on morphology of seeds and fruits, and the behavior of dispersers. For example, in animal-dispersed plants, for which we expected larger cluster sizes, some species such as in Achatocarpus pubescens and Erythroxylum glaucum, showed small clusters. This result might be explained by the increased recruitment around some trees or other objects employed as perches by dispersal agents (perch effects) (Herrera et al. 1994, Schupp and Fuentes 1995, Webb and Peart 2001). In unassisted plants, for which we expected the smallest cluster sizes, some species such as Mimosa acantholoba and Pithecellobium excelsum, showed large clusters. This pattern might be a consequence of secondary dispersal events.

In wind-dispersed plant species, dispersal distances depend on the rate of fall, given by the interaction between propagule morphology, which affects aerodynamics, and the structure of the patch (which affects air currents) (Schupp and Fuentes 1995, Muller-Landau et al. 2008). For these species we expected large clusters similarly to animal-dispersed plants. However, the two Tabebuia spp. showed small cluster sizes. We proposed that this pattern might be explained because they fruited early in the rainy season (A. Jara-Guerrero, pers. obs.) when wind circulation is lower and wet atmospheric conditions drastically reduce seed uplifting (Wright et al. 2008).

We believe that this work enlightens the academic debate about the prevalence of different mechanisms in the formation of realized assemblages in tropical dry forests.

 

References

Herrera, C.M. et al. 1994. Recruitment of a mast-fruiting, bird-dispersed tree – bridging frugivore activity and seedling establishment. Ecol. Monogr. 64: 315–344.

Muller-Landau, H. C. et al. 2008. Interspecific variation in primary seed dispersal in a tropical forest. – J. Ecol. 96: 653-667.

Schupp, E. and Fuentes, M. 1995. Spatial patterns of seed dispersal and the unification of plant population ecology. - Écocience 2: 267-275.

Webb, C. and Peart, D. 2001. High seed dispersal rates in faunally intact tropical rain forest: theoretical and conservation implications. – Ecol. Lett. 4: 491-499.

Wright, S. J. et al. 2008. Understanding strategies for seed dispersal by wind under contrasting atmospheric conditions. - Proc. Natl. Acad. Sci. USA 105: 19084-19089.

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