The effects of individual tree species on species diversity in a tropical dry forest change throughout ontogeny

Abstract

Understanding how diversity is maintained in species‐rich communities, such as tropical forests, remains a challenge in ecology. Recent work suggests that the controversy between competing theories could be better resolved by considering the spatial scale at which different processes rule community assembly. Here we use individual species–area relationships (ISAR) to evaluate the spatial organization of tree diversity around individuals of different species in a completely‐mapped tropical dry forest in south Ecuador. We test two hypotheses. First, stressful environmental conditions promote facilitative interactions that will generate spatial signals of accumulation of diversity around individual trees – contrary to what has been reported in humid tropical forests. Second, spatial signals will shift through ontogeny. As, as larger, older trees generate new microsite conditions that affect the recruitment of younger, smaller trees. We compute ISAR functions for adult trees, for young trees and a new crossed‐ISAR function measuring the accumulation of diversity of young trees around the old trees. We compare observed ISARs to the expectations of inhomogeneous Poisson (i.e. null) models controlling for the effects of environmental variation and habitat association on tree distribution. Although the prevalent response among adult trees was not different from null expectations, which means that the organization of diversity in this size class could be explained by environmental heterogeneity alone, most species accumulated more diversity than expected over short spatial scales in the small stem size class. Only two species accumulated significant diversity in the crossed‐ISARs. Our study indicates the role of facilitation in the organization of plant diversity in this dry forest, but that facilitation is limited to some key species acting on early life stages and accumulating diversity around them. Our results demonstrate the benefit of considering different life‐stages and crossed analyses to disentangle the processes affecting community assembly in tropical dry forests.

Effects of biotic interactions on tropical tree performance depend on abiotic conditions

“…For example, in a tropical dry forest in Ecuador, Espinosa et al. () found evidence of facilitation but that site has a more pronounced dry season and receives substantially less precipitation than the driest site in our study (ca. 650 mm/yr vs. ca.…”

“…It is also possible that the range of the gradient studied here is not wide enough to observe positive interactions in the dry site. For example, in a tropical dry forest in Ecuador, Espinosa et al (2016) found evidence of facilitation but that site has a more pronounced dry season and receives substantially less precipitation than the driest site in our study (ca. 650 mm/yr vs. ca.…”

Pond drying cues and their effects on growth and metamorphosis in a fast developing amphibian

“…), one of the last remnants of lowland tropical dry forests in the world and one of the few remaining habitats for the Pacific horned frog (Ortiz et al, 2013). The annual precipitation average is 676 mm, but with large variations between years (between 65 and 2647 mm, Huaquillas meteorological station, 45 years records), and distributed almost exclusively during the four months of the wet season (January-April; 515 mm on average), while the dry season has at least 4 months with less than 10 mm monthly precipitation (Espinosa et al, 2016). Because of the extended dry season, there are no permanent water habitats, even longer-lasting ponds drying up in a few weeks.…”

Is the spatial pattern of a tree population in a seasonally dry tropical climate explained by density‐dependent mortality?

“…), local dispersal events (Hubbell ; Hardy & Sonké ), facilitation (Espinosa et al . ), topography and the availability of microsites with favourable conditions for survival up to maturity (Hay et al . ; Costa & Santos ).…”

“…In addition, spatial repulsion between adults and juveniles may be a response to environmental heterogeneity (Espinosa et al . ), since different stages have different environmental requirements. However, despite the fact that the detection of DDM can be masked by the influence of habitat heterogeneity, DDM is still the predominant regulator of the spatial structure of most tropical trees, especially of abundant species (Zhu et al .…”