Rain Forest Fragmentation and the Proliferation of Successional Trees

Abstract

Abstract. The effects of habitat fragmentation on diverse tropical tree communities are poorly understood. Over a 20-year period we monitored the density of 52 tree species in nine predominantly successional genera (Annona, Bellucia, Cecropia, Croton, Goupia, Jacaranda, Miconia, Pourouma, Vismia) in fragmented and continuous Amazonian forests. We also evaluated the relative importance of soil, topographic, forest dynamic, and landscape variables in explaining the abundance and species composition of successional trees. Data were collected within 66 permanent 1-ha plots within a large (ϳ1000 km 2 ) experimental landscape, with forest fragments ranging from 1 to 100 ha in area.Prior to forest fragmentation, successional trees were uncommon, typically comprising 2-3% of all trees (Ն10 cm diameter at breast height [1.3 m above the ground surface]) in each plot. Following fragmentation, the density and basal area of successional trees increased rapidly. By 13-17 years after fragmentation, successional trees had tripled in abundance in fragment and edge plots and constituted more than a quarter of all trees in some plots. Fragment age had strong, positive effects on the density and basal area of successional trees, with no indication of a plateau in these variables, suggesting that successional species could become even more abundant in fragments over time.Nonetheless, the 52 species differed greatly in their responses to fragmentation and forest edges. Some disturbance-favoring pioneers (e.g., Cecropia sciadophylla, Vismia guianensis, V. amazonica, V. bemerguii, Miconia cf. crassinervia) increased by Ͼ1000% in density on edge plots, whereas over a third (19 of 52) of all species remained constant or declined in numbers. Species responses to fragmentation were effectively predicted by their median growth rate in nearby intact forest, suggesting that faster-growing species have a strong advantage in forest fragments.An ordination analysis revealed three main gradients in successional-species composition across our study area. Species gradients were most strongly influenced by the standlevel rate of tree mortality on each plot and by the number of nearby forest edges. Species composition also varied significantly among different cattle ranches, which differed in their surrounding matrices and disturbance histories. These same variables were also the best predictors of total successional-tree abundance and species richness. Successional-tree assemblages in fragment interior plots (Ͼ150 m from edge), which are subjected to fragment area effects but not edge effects, did not differ significantly from those in intact forest, indicating that area effects per se had little influence on successional trees. Soils and topography also had little discernable effect on these species.Collectively, our results indicate that successional-tree species proliferate rapidly in fragmented Amazonian forests, largely as a result of chronically elevated tree mortality near forest edges and possibly an increased seed rain from successional p...

Habitat fragmentation and the future structure of tree assemblages in a fragmented Atlantic forest landscape

“…In particular (1) population declines and local extinctions in human-modified landscapes predominantly affect particular functional groups (XXXX), (2) edge-affected habitats tend to support distorted tree species assemblages in both taxonomic and functional terms (Laurance et al 2006a;Laurance et al 2006b;Tabarelli et al 2008), (3) these distortions can be readily observed in seedling assemblages (Santo-Silva et al 2013); (3) small forest fragments are becoming increasingly dominated by pioneer species (cf. Pütz et al 2011;Tabarelli et al 2008), (4) some native pioneer species are proliferating at multiple spatial scales , particularly Byrsonima sericea, Bowdichia virgilioides, Cupania oblongifolia and Cupania racemosa.…”

“…For example, the collapse of the old-growth tree flora and the proliferation of pioneers, particularly short-lived species, have been well documented in Amazonian forest landscapes dominated by both cattle pastures and young secondary forests (Laurance et al 2006b) and open water (Benchimol and Peres 2015). Based on the patterns found here, we also expect that recently fragmented Amazonian forest landscapes will eventually experience a proliferation of long-lived, canopy pioneers, including hardwooded species.…”

“…Impoverished tree assemblages are dominated by some pioneer species, which tend to proliferate from local to regional scales (Lôbo et al 2011;Tabarelli et al 2010a). Proliferation of pioneer and light-demanding species (successional species, sensu Laurance et al 2006b) parallels population collapses and extirpations of species in several ecological groups belonging to the old-growth flora, such as large-seeded ) and emergent species , and those with specialized floral biology (Girão et al 2007;Lopes et al 2009). Collectively, these shifts have been interpreted as a successional trajectory triggered by habitat fragmentation transforming edge-affected habitats into early-successional systems .…”

Efeito de borda na diversidade, composição e estrutura da comunidade arbórea em uma floresta estacional decidual no Brasil Central

“…The processes that rule edge effects in forest fragments are well understood and reasonably consistent throughout the studies carried out in temperate (WHITNEY;RUNKLE, 1981;PALIK;MURPHY, 1990;MATLACK, 1994;HANSSON, 2000) and tropical rain forests (LOVEJOY et al, 1983;WILSON;CROME, 1989;WILLIAMS-LINERA, 1990a;LAURANCE, 1991;LAURANCE et al, 1998;RODRIGUES, 1998;WILLIAMS-LINERA et al, 1998;LAURANCE et al, 2006). However, edge effect trends found elsewhere may not be directly applicable to tropical dry forests.…”

“…Even in the Amazon rain forest, where edge effects were considered formerly as one of the most extensive impacts for biodiversity conservation of forest fragments (LAURANCE, 2000;LAURANCE et al, 2006). PHILLIPS et al (2006) reported the absence of edge effects fragments in rain forests from the Southwestern Peruvian Amazon.…”