Alexandre F. Souza

Community assembly in harsh environments: the prevalence of ecological drift in the heath vegetation of South America

Metacommunity dynamics is marked by a gradient ranging from pure ecological determinism to pure stochasticity. This gradient encompasses compositional turnover that is governed by ecological drift, selection and dispersal. Here we estimate the influences of selection, dispersal limitation acting in concert with drift, drift acting alone and homogenizing dispersal on the structure of tropical restinga heath vegetation growing under stressful conditions in north-eastern South America. We hypothesize that if abiotic heterogeneity is strong enough, it could select distinct sets of colonizing species from neighbor ecosystems, with stress sensitive species occupying refuges created by abiotic heterogeneity and stress tolerators dominating the more exposed areas. In this case selection would occur at both biogeographical and local scales. Under its prevalence, we expect selection to have a major signature in the woody plant community structure in a phylogenetic null model. Alternatively, if abiotic heterogeneity...

Detecting ecological groups from traits: a classification of subtropical tree species based on ecological strategies

We evaluated the frequently assumed but rarely tested hypothesis that in tropical and subtropical forests species form discontinuities along gradients in trait variation, which can be detected to classify species into ecologically meaningful and statistically defined groups. We also tested the hypothesis that the dominant conifer Araucaria angustifolia have a contrasting trait syndrome from the pool of angiosperms. Data were collected in subtropical mixed conifer-hardwood forests in southern Brazil. Eleven trait variables (relative growth rates, growth rates under favourable conditions, annual mortality rates, seed length, seed dispersal mode, wood density, maximum height, crown depth, stem slenderness, specific leaf area, and leaf blade length) were measured for 66 large tree species. The conifer Araucaria angustifolia differed significantly in most traits and vital rates from the angiosperm distributions. The Simple Structure Index criterion was used to identify the optimum number of non-hierarchical k-means groups. This index was largest for a solution with five groups. Non-hierarchical groups were more strongly related with the resource capture and height gradients than groups formed by hierarchical clustering. We propose the recognition of seven ecological species groups in the studied forests depending on growth rates, leaf size, wood density, height, stem slenderness, dispersal, and crown depth: Conifers, Palms, Pioneers, Large-seeded Pioneers, Wind-dispersed Large trees, Large shade-tolerant trees, and Small Shade-tolerant trees. The classification produced was coherent with the ecological strategies present in the community and represents a subdivision of Westoby’s leaf-height-seed plant strategy scheme.

Determinants of variation in heath vegetation structure on coastal dune fields in northeastern South America

Despite its implications for carbon storage, animal conservation, and plant regeneration, the variation in the structure of heath vegetation in South America is still poorly studied. In this study, we aimed at examining the edaphic and topographic determinants of this variation along 85 plots (5xa0×xa05xa0m) randomly distributed in a restinga heath vegetation occurring on coastal dune fields in northeastern Brazil. We carried out a PCA analysis to reduce eleven vegetation descriptors into a small number of structural gradients, which were then assessed by a stepwise standard least-squares multiple regression to reveal the effects of the abiotic environment on structure. The three following hypotheses were tested: (1) both soils and topography are important to explain variation in vegetation structure at local scale; (2) herbaceous plants, cactus, and woody plants show differential responses to soil and topographic variations; and (3) soil acidity and salinity are more important determinants of herbaceous cover than woody plant variation. PCA analysis revealed three major structural gradients related to biomass, herbaceous cover, and leaning plants, respectively. These gradients were only related to calcium and nitrogen contents, which partially supports our first hypothesis. Our results also suggest that different groups of plants have different responses to abiotic gradients that are exposed. The effect of the soil acidity and salinity did not appear to present an immediate strong influence on the herbaceous community. It seems that a reduced number of edaphic factors promote the variation in vegetation structure in the restinga heath vegetation.

Aridity drives plant biogeographical sub regions in the Caatinga, the largest tropical dry forest and woodland block in South America

Our aims were to quantify and map the plant sub regions of the the Caatinga, that covers 844,453 km2 and is the largest block of seasonally dry forest in South America. We performed spatial analyses of the largest dataset of woody plant distributions in this region assembled to date (of 2,666 shrub and tree species; 260 localities), compared these distributions with the current phytogeographic regionalizations, and investigated the potential environmental drivers of the floristic patterns in these sub regions. Phytogeographical regions were identified using quantitative analyses of species turnover calculated as Simpson dissimilarity index. We applied an interpolation method to map NMDS axes of compositional variation over the entire extent of the Caatinga, and then classified the compositional dissimilarity according to the number of biogeographical sub regions identified a priori using k-means analysis. We used multinomial logistic regression models to investigate the influence of contemporary climatic productivity, topographic complexity, soil characteristics, climate stability since the last glacial maximum, and the human footprint in explaining the identified sub regions. We identified nine spatially cohesive biogeographical sub regions. Current productivity, as indicated by an aridity index, was the only explanatory variable retained in the best model, explaining nearly half of the floristic variability between sub regions. The highest rates of endemism within the Caatinga were in the Core and Periphery Chapada Diamantina sub regions. Our findings suggest that the topographic complexity, soil variation, and human footprint in the Caatinga act on woody plant distributions at local scales and not as determinants of broad floristic patterns. The lack of effect of climatic stability since the last glacial maximum probably results from the fact that a single measure of climatic stability does not adequately capture the highly dynamic climatic shifts the region suffered during the Pleistocene. There was limited overlap between our results and previous Caatinga classifications.

Weak whole-plant trait coordination in a seasonally dry South American stressful environment

Abstract A core question involving both plant physiology and community ecology is whether traits from different organs are coordinated across species, beyond pairwise trait correlations. The strength of within‐community trait coordination has been hypothesized to increase along gradients of environmental harshness, due to the cost of adopting ecological strategies out of the viable niche space supported by the abiotic conditions. We evaluated the strength of trait relationship and coordination in a stressful environment using 21 leaf and stem traits of 21 deciduous and evergreen woody species from a heath vegetation growing on coastal sandy plain in northeastern South America. The study region faces marked dry season, high soil salinity and acidity, and poor nutritional conditions. Results from multiple factor analyses supported two weak and independent axes of trait coordination, which accounted for 25%–29% of the trait variance using phylogenetically independent contrasts. Trait correlations on the multiple factor analyses main axis fit well with the global plant economic spectrum, with species investing in small leaves and dense stems as opposed to species with softer stems and large leaves. The species’ positions on the main functional axis corresponded to the competitor‐stress‐tolerant side of Grimes CSR triangle of plant strategies. The weak degree of trait coordination displayed by the heath vegetation species contradicted our expectation of high trait coordination in stressful environmental habitats. The distinct biogeographic origins of the species occurring in the study region and the prevalence of a regional environmental filter coupled with local homogeneous conditions could account for prevalence of trait independence we observed.

Mean trait values and the CSR ecological strategies

This file contains the mean trait values for leaf morphology, litterfall productivity, stem morphology, stem anatomy and the CSR ecological strategies of 33 Restinga species at Parnamirim, Northeastern Brazil. Leaf and stem samplings were collected from 80 25-m² plots distributed along 16 transects of 100 m-long (five plots per transect). Whenever possible, we collected the organs from the same individuals. Traits and abbreviations: Leaf dry mass (Ldmass), leaf area (Larea), specific leaf area (SLA), leaf moisture (Lmois), stem moisture (Smois), bark (Bark), stem density (Sdens), vessel diameter (Vdiam), vessel density (Vdens), vulnerability index (Vindex), stem length (Slength), stem diameter (Sdiam), litter production (Litter), variability in litter production (Vlitter), leaf-shedding behavior (Lshed) [E=evergreen, SD= semi-deciduous, D=deciduous], and the CSR plant strategy.

Does hydroelectric reservoirs affect the structure of surrounding tree communities? A test of hypotheses in subtropical South America

Despite its advantages over other energy sources, hydropower facilities are recognized as causes of terrestrial and freshwater environmental impacts. A deeper understanding of hydropower effects on surrounding riparian forests is thus important to the development hydropower facilities of less impact on the environment as well as to define criteria to manage the vegetation in the vicinity of large hydroelectric dams. We compared tree community structure on forest slopes exposed to a hydropower reservoir created in 2005 with control ones, in the biodiversity-rich subtropical forests of southern Brazil. Field data were collected between 2009 and 2011 on six forest slopes of which three were affected by the rise in water level caused by a hydropower dam, while the other three were in nearby sites unaffected by damming. Ninety 10xa0×xa010 plots were set along six 50xa0mxa0×xa010xa0m transects. Forest structure was significantly affected by vicinity of reservoir, but this effect differed significantly between different tributaries of the Pelotas River. Rarefaction curves showed homogeneity in species richness among the studied hills. Composition differed significantly between reservoir and control areas. Variance partition results showed that pure reservoir effects explained a fraction of the variance in floristic composition comparable to environmental factors. Vicinity to reservoir was the main factor able to distinguish between communities in the studied system in a multiple regression tree. Our results highlight that hydroelectric facility building produces more extensive damage to remnant native forests than anticipated and that even forests upslope from reservoir border may suffer rapid structural and compositional degradation due to changes in environmental conditions caused by altered hydrology.