Agent-based modeling of the effects of forest dynamics, selective logging, and fragment size on epiphyte communities

Abstract

Forest canopies play a crucial role in structuring communities of vascular epiphytes by providing substrate for colonization, by locally varying microclimate, and by causing epiphyte mortality due to branch or tree fall. However, as field studies in the three-dimensional habitat of epiphytes are generally challenging, our understanding of how forest structure and dynamics influence the structure and dynamics of epiphyte communities is scarce.Mechanistic models can improve our understanding of epiphyte community dynamics. We present such a model that couples dispersal, growth, and mortality of individual epiphytes with substrate dynamics, obtained from a three-dimensional functional-structural forest model, allowing the study of forest-epiphyte interactions. After validating the epiphyte model with independent field data, we performed several theoretical simulation experiments to assess how (a) differences in natural forest dynamics, (b) selective logging, and (c) forest fragmentation could influence the long-term dynamics of epiphyte communities.The proportion of arboreal substrate occupied by epiphytes (i.e., saturation level) was tightly linked with forest dynamics and increased with decreasing forest turnover rates. While species richness was, in general, negatively correlated with forest turnover rates, low species numbers in forests with very-low-turnover rates were due to competitive exclusion when epiphyte communities became saturated. Logging had a negative impact on epiphyte communities, potentially leading to a near-complete extirpation of epiphytes when the simulated target diameters fell below a threshold. Fragment size had no effect on epiphyte abundance and saturation level but correlated positively with species numbers.Synthesis: The presented model is a first step toward studying the dynamic forest-epiphyte interactions in an agent-based modeling framework. Our study suggests forest dynamics as key factor in controlling epiphyte communities. Thus, both natural and human-induced changes in forest dynamics, for example, increased mortality rates or the loss of large trees, pose challenges for epiphyte conservation.

Keywords: canopy dynamics; community assembly; demography; epiphyte assemblages; epiphyte interactions; forest; individual‐based model; long‐term dynamics; vascular epiphytes.

FIGURE 1

Flowchart of the coupled forest–epiphyte model. Based on dynamic three‐dimensional input data from a functional–structural forest model (Petter et al., 2020), a microhabitat matrix characterizing the epiphytic habitat at each time step is generated. To this end, the simulated spatial distribution of leaf area, branches, and trunks for each annual time step (left panel) is used to calculate the light distribution, total substrate area and relative annual change of substrate area for each 1 m³‐voxel in the microhabitat matrix (middle panel), which ultimately influences the initialization and all three submodels of the epiphyte model (right panel)

FIGURE 2

Simulated long‐term dynamics of vascular epiphyte communities. Five replicates of a typical Neotropical lowland forest stand (see Appendix S3: Figure S1 for forest attributes) were used as input data for the epiphyte model. On each of these forest replicates, the development of epiphyte communities, which initially consisted of 100 individuals of 100 species, was simulated over 600 years. Ten different initial species sets were simulated on each forest replicate and means (bold lines) and standard deviations (shaded areas) of abundance (a) and species richness (b) are shown

FIGURE 3

Rank abundance distributions and vertical distributions (a‐b), size‐distributions (c‐e), and vertical stratifications (f‐h) of simulated epiphyte communities in comparison with data from Panama and Ecuador. (a) Relative abundances of species sorted by their abundance rank in descending order in one representative model run at several time steps in comparison with empirical data from rainforests in Panama and Ecuador. (b) Simulated vertical distribution of epiphytes in comparison with empirical data from Panama and Ecuador. The simulated vertical distribution ± standard deviation was calculated based on the pooled model results of the years 300–600 in one representative model run and resulted from an upward shift in abundances from the initial distribution (see Appendix S3: Figure S6). (c‐e) Simulated size distribution was calculated based on the pooled model results of the years 300–600 in one representative model run. Please note that due to limited availability, different proxies for plant size are plotted. (f‐h) Vertical stratification is represented as height distribution for each species, arranged by mean height. The simulated stratification after 300 years is shown for one representative model run

FIGURE 4

Saturation level, abundance, and species richness of simulated epiphyte communities in forests differing in dynamics, logging regimes, and fragment size. Each panel shows the averaged temporal development of epiphyte communities over 600 years: (a‐c) Forests differing in their natural dynamics (Appendix S3: Figure S1), (d‐f) forests differing in their logging intensity (Appendix S3: Figure S2), and (g‐i) forests differing in their fragment size (Appendix S3: Figure S3). For each of these forest scenarios, five replicates were simulated and used as input data for the epiphyte model. Ten different species sets of vascular epiphytes were separately simulated for each forest replicate. Thus, for each forest scenarios, a total of 50 epiphyte simulations were conducted, and mean values (bold lines) and standard deviations (shaded areas) are shown here. Note that the communities in the first 100–200 years are still re‐assembling from the evenly distributed initial conditions (100 individuals per species) due to typically slow dynamics. Therefore, differences among scenarios are more apparent after 100–200 years