Tree recruitment failure in old‐growth forest patches across human‐modified rainforests

Abstract

Land‐use change threatens biodiversity in tropical landscapes, but its impact on rainforest regeneration remains poorly known. In fact, the landscape‐scale patterns driving the diversity of regenerating plants within forest fragments have been rarely explored, and we are uncertain whether such drivers vary across regions with different land‐use change patterns. We assessed the effect of landscape composition (forest cover and matrix openness) and configuration (forest patch density) on species diversity of sapling assemblages (trees ≥30 cm height and <1 cm diameter) in old‐growth forest fragments from three Mexican rainforest regions with different disturbance levels (n = 20 landscapes per region). We separately assessed old‐growth forest specialists (OGS) and forest generalist (FG) species to test the hypotheses that: (a) OGS species show recruitment limitation (‘loser’ species), and can therefore be negatively impacted by landscape changes, especially by forest loss and matrix openness in more deforested regions; and (b) FG species can regenerate and even proliferate in more disturbed landscapes (‘winner’ species). We recorded ~24,000 plants from 415 species. Landscape composition showed stronger effects than landscape configuration. The diversity of OGS species generally decreased in more deforested landscapes dominated by open matrices, and FG species followed the opposite response, especially in the regions with high‐to‐intermediate degree of disturbance. Overall, forest fragmentation (patch density) showed weak or no effects on species diversity, especially after controlling for forest cover effects (i.e. fragmentation per se). In contrast to the fragmentation threshold hypothesis, the effect of fragmentation was independent of the regional context. Moreover, FG species were affected by landscape attributes operating at larger scales than OGS species. Synthesis. Our findings support our hypotheses, and suggest that forest loss and matrix openness, not fragmentation per se, can cause the recruitment failure of tree assemblages in highly deforested rainforests. This can be related to source and dispersal limitation in more deforested landscapes with treeless matrices. Therefore, to promote the regenerative potential (resilience) of forest patches in human‐modified tropical landscapes, conservation programs should focus on preventing forest loss (even the smallest forest patches) and improving matrix quality with treed elements, particularly in highly deforested tropical regions.

Landscape structure shapes the diversity of tree seedlings at multiple spatial scales in a fragmented tropical rainforest

“…This is the first study to assess the effects of landscape structure on the α-and β-diversity of the animal-dispersed tree seedling community at multiple spatial scales (but see [30]). As expected, we found that landscape composition affected α-diversity whereas β-diversity was influenced by landscape configuration.…”

“…As result, the regenerating forests are dominated by generalist and abiotically-dispersed tree species, supports a lower density of saplings, show reduced recruitment of animal-dispersed species, and their composition and structure resemble to secondary forests [24][25][26]. Although a growing body of conservation strategies have emerged in order to preserve plant diversity in the tropics [12,27,28], none of them refer to seedling communities in HMTLs, since the effects of the types and amount of land covers in the landscape (landscape composition), and their spatial arrangement (landscape configuration) on the α-and β-diversity are poorly understood (but see [29,30]).…”

“…Evidence suggests that seedling α-diversity is affected by landscape composition, while βdiversity is more influenced by landscape configuration. Recent studies have shown that reduction of α-diversity in seed and sapling assemblages by forest loss and dominance of open matrices (landscape composition) is associated with limitations in local seed sources, seed dispersal and sapling establishment of animal-dispersed species [10,30,31]. Additionally, seedling α-diversity may also be influenced by seed sources from nearby secondary forests [31][32][33].…”

“…Additionally, seedling α-diversity may also be influenced by seed sources from nearby secondary forests [31][32][33]. Finally, the effects of landscape configuration on α-diversity were negligible [30]. On the contrary, seedling β-diversity within patches may be linked to forest fragmentation and edge effects (landscape configuration).…”

“…In these approaches, it is necessary the use of a wide range of scales, as assessments performed using very few scales within narrow ranges overlook the scale of effect [41,45]. To our knowledge, studies that have employed multi-scale analysis on plant communities in the tropics are scant [30,47], and none of them have assessed the effect of landscape structure on seedling communities in HMTLs (but see [30]). In this study, we employed a multi-scale approach to assess the contribution of landscape composition and configuration on tree seedling α-and β-diversity within rainforest patches.…”