Facilitation mediates species presence beyond their environmental optimum

Species distributions are driven by abiotic conditions that filter species with specific traits and physiological tolerances and match them with their suitable environment. Plant-plant interactions can constrict (through competition) or loosen (through facilitation) the strength of these environmental fillers, which in turn inhibit or enhance establishment and recruitment of plant species at a finer spatial scale. Although competition is often the focus of community assembly processes that further impede the entry of plant species into a site, facilitation is also important for potentially loosening environmental filters (especially climatic filters such as temperature and precipitation), ultimately enhancing species occurrence beyond their physiological optimum. We used multiple data sets from the arid site of Rambla del Saltador Valley to test the hypothesis that facilitation by a nurse-plant promotes the presence of herbaceous, beneficiary species beyond their environmental optimum relative to open sites. Furthermore, we propose that the median elevation and community composition of herbaceous species expands with the age of the nurse-plant, and we tested this hypothesis by examining 105 beneficiary species under 50 nurse-plant shrubs varying in age from 6 to 48 years old. We found nurse-plants both facilitate herbaceous species occurrence beyond their median elevation and support more diverse and a distinctly different composition of species in contrast to open sites. Specifically, herbaceous species that originate from a median elevation more than 600 to 700 m above the site only existed beneath nurse-plants, and below this median elevation, half the species only occurred below the nurse-plant. Moreover, the richness and elevation provenance of the herbaceous species increased with increasing nurse-plant age. Our results highlight the importance of facilitation for alleviating physiological strain (in support of the strain hypothesis) and mediating regional species distributions, which has implications for understanding species movements and community assembly at larger-scales under hotter and drier climates

Restinga forests of the Brazilian coast: richness and abundance of tree species on different soils

The aim of this study was to determine changes in composition, abundance and richness of species along a forest gradient with varying soils and flood regimes. The forests are located on the left bank of the lower Jucu River, in Jacarenema Natural Municipal Park, Espírito Santo. A survey of shrub/tree species was done in 80 plots, 5x25 m, equally distributed among the forests studied. We included in the sampling all individuals with >3.2 cm diameter at breast height (1.30 m). Soil samples were collected from the surface layer (0-10 cm) in each plot for chemical and physical analysis. The results indicate that a significant pedological gradient occurs, which is influenced by varying seasonal groundwater levels. Restinga forest formations showed significant differences in species richness, except for Non-flooded Forest and Non-flooded Forest Transition. The Canonical Correlation Analysis (CCA) showed that some species are distributed along the gradient under the combined influence of drainage, nutrient concentration and physical characteristics of the soil. Regarding the variables tested, flooding seems to be a more limiting factor for the establishment of plant species in Restinga forests than basic soil fertility attributes.O objetivo deste estudo foi o de determinar as mudanças na composição, abundância e riqueza de espécies ao longo de um gradiente de floresta com variações de solo e inundações. As florestas localizam-se na margem esquerda do Rio Jucu, no Parque Natural Municipal de Jacarenema, Espírito Santo. O levantamento das espécies arbustivo/arbóreas foi realizado em 80 parcelas de 5x25m, distribuídas equitativamente entre as florestas estudadas. Foram incluídos na amostragem todos os indivíduos com >3,2 cm diâmetro à altura do solo (1,30 m). Foram coletadas amostras de solo superficial (0-10 cm) em cada parcela para análise química e física. Os resultados mostraram a existência de gradiente pedológico significativo, que é influenciado pelas variações sazonais do lençol freático. As formações florestais da Restinga apresentaram diferenças significativas na riqueza de espécies, com exceção da Floresta Não Inundável em relação a Floresta Não Inundável de Transição. A Análise de Correlação Canônica (CCA) mostrou que algumas espécies apresentam sua distribuição ao longo do gradiente sob influência da drenagem, das concentrações de nutrientes e das características físicas dos solos. Entre as variáveis testadas, a inundação parece ser o fator mais limitante para o estabelecimento de espécies vegetais nas florestas de Restinga do que os atributos básicos de fertilidade do solo