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.
Highlights
Dry tropical forests (SDTF sensu Pennington et al 2009) have unique floras, characterized by small deciduous species, which are often succulent and thorny (Murphy and Lugo 1986, Banda-R et al 2016) with high wood densities (Chave et al 2009)
We posed the question: how does chronic anthropogenic disturbance, rainfall and soil fertility influence the functional composition of trees and shrubs in an area of Caatinga? To answer this question, we evaluated species turnover and intraspecific variability along these gradients, considering traits related to water-use strategy, competition, colonization and large-herbivore defense
With the exception of leaf area, which was not correlated to intraspecific trait variation (r = 0.13; P = 0.28; figure 4(e))
Summary
Dry tropical forests (SDTF sensu Pennington et al 2009) have unique floras, characterized by small deciduous species, which are often succulent and thorny (Murphy and Lugo 1986, Banda-R et al 2016) with high wood densities (Chave et al 2009). Water availability (Borchert 1994, Hulshof et al 2013) and soil fertility (Buzzard et al 2016) are two major features of SDTF that can influence the functional composition of woody plant assemblages. Considering the functional composition of plant assemblages, grazing, for instance, can influence life-form, decreasing plant height (Dıaz et al 2007) and promoting plants with tough leaves (Dıaz et al 2001). Different types of CAD together with abiotic variables, such as light, soil nutrients, and rainfall, are able to operate synergistically to restructure plant assemblages (Albuquerque 1999, Carmona et al 2012, Ribeiro et al 2015, Ribeiro-Neto et al 2016, Rito et al 2017a)
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