Succession of Ephemeral Secondary Forests and Their Limited Role for the Conservation of Floristic Diversity in a Human-Modified Tropical Landscape

<div><p>Both local- and landscape-scale processes drive succession of secondary forests in human-modified tropical landscapes. Nonetheless, until recently successional changes in composition and diversity have been predominantly studied at the patch level. Here, we used a unique dataset with 45 randomly selected sites across a mixed-use tropical landscape in central Panama to study forest succession simultaneously on local and landscape scales and across both life stages (seedling, sapling, juvenile and adult trees) and life forms (shrubs, trees, lianas, and palms). To understand the potential of these secondary forests to conserve tree species diversity, we also evaluated the diversity of species that can persist as viable metapopulations in a dynamic patchwork of short-lived successional forests, using different assumptions about the average relative size at reproductive maturity. We found a deterministic shift in the diversity and composition of the local plant communities as well as the metacommunity, driven by variation in the rate at which species recruited into and disappeared from the secondary forests across the landscape. Our results indicate that dispersal limitation and the successional niche operate simultaneously and shape successional dynamics of the metacommunity of these early secondary forests. A high diversity of plant species across the metacommunity of early secondary forests shows a potential for restoration of diverse forests through natural succession, when trees and fragments of older forests are maintained in the agricultural matrix and land is abandoned or set aside for a long period of time. On the other hand, during the first 32 years the number of species with mature-sized individuals was a relatively small and strongly biased sub-sample of the total species pool. This implies that ephemeral secondary forests have a limited role in the long-term conservation of tree species diversity in human-modified tropical landscapes.</p></div

Increasing dissimilarity between seedling and the initial assemblages of trees.

<p>Mean of pair-wise dissimilarities between the seedling assemblages of SBA classes 1, 2 and 3 with the assemblages of plants ≥ 1 cm DBH of SBA class 1, using the Chao Jaccard Abundance Estimator. Error bars indicate ± 95% confidence limits. Calculations included only species that can potentially grow to a diameter of at least 5 cm.</p

Diversity in relationship with relative size and forest age.

<p>A) Number of tree and shrub species per age class (lianas and palms not included!), when all individuals are taken into account (blue dots), when only individuals above the relative size threshold (RST) of 10% (green dots) or of 30% (darker orange dots) are counted and when only species are counted with individuals ≥ RST_30% in more than one plot (lighter orange dots). Per age class, species were counted for the pooled data of 15 SFD plots (3 ha). Dotted lines are for illustrative purposes. B-C) Number of species per maximum-size class (blue) and the subsamples of species with individuals above relative size thresholds of 10% (green) and 30% (orange). D-E) Number of stems per maximum size class. Graphs show data for the 2–7 y age class (B, D) and the 18–32 y age class (C, E).</p

Regression models and coefficients of determination for the relationships between indices of community structure (response variables) and age since abandonment, stand basal area (SBA), or light (predictor variables).

<p>⁽²⁾ calculated with presence/absence data; M  =  Model. P  =  power model; L  =  Linear model; E  =  Exponential model. <i>ns</i>  =  non-significant relationship (p>0.05), in all other cases significance is p ≤ 0.01.⁽¹⁾ Calculated with abundance weighted data; </p

Changes in diversity with successional stand development on local and landscape scale.

<p>Indices of species diversity were calculated for four different groups of woody plants, and for individual sites (white dots and regression lines) and the metacommunity (diamonds and triangles). A-D) ⁰<i>D</i>  =  species density. E-H) ¹<i>D</i>  =  the exponential of Shannon entropy. I-L), ²<i>D</i>  =  the inverse Simpson concentration. For all three diversity measures, units are in number of species. Metacommunity diversity was calculated for the pooled data of randomized samples of 12 SFD sites. Colors indicate SBA classes as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082433#pone-0082433-g001" target="_blank">figure 1</a> and error bars give the 95% confidence limits. The lines connecting the symbols are for illustrative purposes only. Sample area per plot and per SBA class is indicated above the graphs. Equation type and regression statistics are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082433#pone-0082433-t002" target="_blank">Table 2</a>.</p

Compositional change with successional stand development.

<p>Variation in species composition was assessed for four different groups of woody plants by calculating the relative positions of sites along the axes of a non-metric multidimensional scaling (NMDS) ordination, based on the Jaccard abundance-based dissimilarity index. See methods for definition of plant groups and sample areas. Equation type and regression statistics are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0082433#pone-0082433-t002" target="_blank">Table 2</a>.</p

Functional trait data

Functional trait dat

Species abundances

Contains the number of individuals in old-growth and secondary forest for each species at each site

Site locations

Latitude and longitude of sites used in this study

Above-ground biomass of Neotropical secondary forests database

This database is the product of the 2ndFOR collaborative research network on secondary forests. The database contains aboveground biomass data (in Mg/ha) for 1334 secondary forest plots differing in time since abandonment. The plots belong to different chonosequence studies in the Neotropics. For a description of the database, see Poorter et al. 2016. Biomass resilience of Neotropical secondary forests. Nature doi:10.1038/nature16512