Chronic human disturbance affects plant trait distribution in a seasonally dry tropical forest

Abstract

The effects of human disturbance on biodiversity can be mediated by environmental conditions, such as water availability, climate and nutrients. In general, disturbed, dry or nutrient-depleted soils areas tend to have lower taxonomic diversity. However, little is known about how these environmental conditions affect functional composition and intraspecific variability in tropical dry forests. We studied a seasonally dry tropical forest (SDTF) under chronic anthropogenic disturbance (CAD) along rainfall and soil nutrient gradients to understand how these factors influence the taxonomic and functional composition. Specifically we evaluated two aspects of CAD, wood extraction and livestock pressure (goat and cattle grazing), along soil fertility and rainfall gradients on shrub and tree traits, considering species turnover and intraspecific variability. In addition, we also tested how the traits of eight populations of the most frequent species are affected by wood extraction, livestock pressure, rainfall and soil fertility. In general, although CAD and environmental gradients affected each trait of the most widespread species differently, the most abundant species also had a greater variation of traits. Considering species turnover, wood extraction is associated with species with a smaller leaf area and lower investment in leaf mass, probably due to the indirect effects of this disturbance type on the vegetation, i.e. the removal of branches and woody debris clears the vegetation, favouring species that minimize water loss. Livestock pressure, on the other hand, affected intraspecific variation: the herbivory caused by goats and cattle promoted individuals which invest more in wood density and leaf mass. In this case, the change of functional composition observed is a direct effect of the disturbance, such as the decrease of palatable plant abundance by goat and cattle herbivory. In synthesis, CAD, rainfall and soil fertility can affect trait distribution at community and species levels, which can have significant implications for the ecosystem functioning of SDTF under increasing levels of disturbance, climate change and soil nutrient depletion.

Functional organization of woody plant assemblages along precipitation and human disturbance gradients in a seasonally dry tropical forest

“…Our results reinforce the notion that both CAD and variation in precipitation operate as species assembly and community‐level organizing forces in SDTFs (Rito, Arroyo‐Rodríguez, et al, ), even if not synergistically. Yet, in the Caatinga dry forest they influence the spatial distribution and values of some traits within populations (intraspecific variation) and between species (interspecific variation), with predictable effects on patterns of community‐level trait diversity (Sfair et al, ). In accordance with previous studies (Lôbo, Leão, Melo, Santos, & Tabarelli, ; Ribeiro‐Neto et al, ; Rito, Tabarelli, & Leal, ), increased human pressure tended to cause community‐level biotic homogenization, which in most cases results from the proliferation of a few disturbance‐adapted species—that is, winners (sensu Tabarelli, Peres, & Melo, ).…”

“…Accordingly, the traits we found to be responsive to CAD were those related to both light capture and water conservation strategies (i.e., leaf area and specific leaf area; Westoby et al, ; Wright et al, ; Wright et al, ). On the other hand, precipitation has been recognized as a key driver of community‐level patterns in SDTFs (Allen et al, ; Díaz & Cabido, ; Le Bagousse‐Pinguet et al, ; Maestre et al, ), including in Caatinga dry forests, particularly through environmental filtering acting on species and trait distribution (Rito, Arroyo‐Rodríguez, et al, ; Sfair et al, ). Previous studies have in fact proposed that changes in precipitation may be more important than CAD (Rito, Arroyo‐Rodríguez, et al, ).…”

“…While the focus on trait differences among species is motivated by the aim of generalized predictability (McGill et al, ), recent evidence points to a substantial contribution from plant intraspecific trait variability (ITV) in determining patterns of trait variation within and across communities (Albert et al, ; Siefert et al, ). Although the effects of within‐species trait variation on community dynamics and ecosystem functioning are being increasingly investigated (de Bello et al, ; Kichenin, Wardle, Peltzer, Morse, & Freschet, ; Sfair, Bello, França, Baldauf, & Tabarelli, ; Violle et al, ), the understanding of the contexts in which plant ITV is relevant, and how such variation relates to measurable environmental and disturbance gradients to determine community functional patterns, still remains poorly understood (Shipley et al, ).…”

“…Moreover, slash‐and‐burn agriculture and the exploitation of forest products have converted most of the Caatinga old growth forest into a mosaic of forest patches exposed to varying levels of chronic disturbance (Ribeiro et al, ; da Silva et al, ; Sobrinho et al, ). Recent studies in the Caatinga have documented the influence of CAD and changes in precipitation levels as drivers of plant community organization, including taxonomic (Ribeiro et al, ; Ribeiro‐Neto, Arnan, Tabarelli, & Leal, ; Rito, Arroyo‐Rodríguez, et al, ; Siqueira et al, ), phylogenetic (Ribeiro et al, ), and functional patterns (Ribeiro et al, ; Sfair et al, ). Overall, these findings suggest environmental filtering and intraspecific trait variation as key drivers of the spatial distribution of plants along environmental and disturbance gradients.…”

“…Specifically, we expected changing precipitation conditions to reduce cross‐community trait overlap due to species sorting, while disturbance should drive niche differentiation and species coexistence at the local scale. We examine variation in resource‐use traits (e.g., wood density, specific leaf area) that have been widely recognized to confer the ability for plants to cope with reduced water availability and disturbance (O’Brien et al, ; Reich, ; Sfair et al, ). To reveal potential mechanisms driving coexistence patterns within and across communities, we decompose the effects of precipitation and CAD on community functional diversity into species turnover and intraspecific variability components.…”