The influence of microhabitat on the population dynamics of four herbaceous species in a semiarid area of northeastern Brazil.

Variation in annual rainfall is considered the most important factor influencing population dynamics in dry environments. However, different factors may control population dynamics in different microhabitats. This study recognizes that microhabitat variation may attenuate the influence of climatic seasonality on the population dynamics of herbaceous species in dry forest (Caatinga) areas of Brazil. We evaluated the influence of three microhabitats (flat, rocky and riparian) on the population dynamics of four herbaceous species (Delilia biflora, Commelina obliqua, Phaseolus peduncularis and Euphorbia heterophylla) in a Caatinga (dry forest) fragment at the Experimental Station of the Agronomic Research Institute of Pernambuco in Brazil, over a period of three years. D. biflora, C. obliqua and P. peduncularis were found in all microhabitats, but they were present at low densities in the riparian microhabitat. There was no record of E. heterophylla in the riparian microhabitat. Population size, mortality rates and natality rates varied over time in each microhabitat. This study indicates that different establishment conditions influenced the population size and occurrence of the four species, and it confirms that microhabitat can attenuate the effect of drought stress on mortality during the dry season, but the strength of this attenuator role may vary with time and species.

The influence of microhabitat on the population dynamics of four herbaceous species in a semiarid area of northeastern Brazil / A influência dos microhabitats sobre a dinâmica de populações de quatro espécies herbáceas em uma área semiárida no nordeste do Brasil

Abstract Variation in annual rainfall is considered the most important factor influencing population dynamics in dry environments. However, different factors may control population dynamics in different microhabitats. This study recognizes that microhabitat variation may attenuate the influence of climatic seasonality on the population dynamics of herbaceous species in dry forest (Caatinga) areas of Brazil. We evaluated the influence of three microhabitats (flat, rocky and riparian) on the population dynamics of four herbaceous species (Delilia biflora, Commelina obliqua, Phaseolus peduncularis and Euphorbia heterophylla) in a Caatinga (dry forest) fragment at the Experimental Station of the Agronomic Research Institute of Pernambuco in Brazil, over a period of three years. D. biflora, C. obliqua and P. peduncularis were found in all microhabitats, but they were present at low densities in the riparian microhabitat. There was no record of E. heterophylla in the riparian microhabitat. Population size, mortality rates and natality rates varied over time in each microhabitat. This study indicates that different establishment conditions influenced the population size and occurrence of the four species, and it confirms that microhabitat can attenuate the effect of drought stress on mortality during the dry season, but the strength of this attenuator role may vary with time and species.

Resumo A variação anual na precipitação é considerada o fator mais importante que influencia a dinâmica populacional em ambientes secos. No entanto, diferentes fatores podem controlar a dinâmica populacional em diferentes microhabitats. A hipótese deste estudo é que as variações de microhabitats podem atenuar a influência da sazonalidade climática sobre a dinâmica populacional de espécies herbáceas em uma áreas floresta seca (Caatinga) no Brasil. Então, objetivou-se avaliar a influência de três microhabitats (plano, rochoso e ciliar) sobre a dinâmica das populações de quatro espécies herbáceas (Delilia biflora, Commelina obliqua, Phaseolus peduncularis e Euphorbia heterophylla) em um fragmento de Caatinga da Estação Experimental do Instituto de Pesquisas Agronômicas de Pernambuco, no Brasil, durante um período de três anos. D. biflora, C. obliqua e P. peduncularis foram encontrados em todos os microhabitats, mas elas estavam presentes em baixas densidades no microhabitat ciliar. Não houve registro de E. heterophylla no microhabitat ciliar. O tamanho das populações, as taxas de mortalidade e de natalidade variaram ao longo do tempo em cada microhabitat. Este estudo indica que diferentes as condições de estabelecimento influenciaram o tamanho das populações e a ocorrência das quatro espécies e confirma que microhabitat pode atenuar o efeito do estresse hídrico sobre a mortalidade durante a estação seca, mas a força desse papel atenuador pode variar com o tempo e a espécie considerada.

Does proximity to a mature forest contribute to the seed rain and recovery of an abandoned agriculture area in a semiarid climate?

Proximity to forests contributes to the recolonisation of anthropogenic-disturbed areas through seed input. We evaluated the role of proximity to a mature forest in the recolonisation of an agricultural area that has been abandoned for 18 years and is currently a young forest. Seed rain was monitored at fixed distances from the mature forest. The type of surface recolonisation (germination versus resprouting) and the reproductive season were measured in both forests. The majority of plants recolonising the young forest originated from seed germination. Proximity to the mature forest contributed to the seed rain in the young forest; however, 18 years has not provided sufficient time for the recolonisation of 80 species present in the mature forest. Some species shared between forests differed in their fruiting season and seed dispersal. The seed rain had a total species richness of 56, a total density of 2270 seeds·m(-2)·year(−1) and predominance of self- and wind dispersal. A significant reduction in seed rain with increasing distance from the mature forest was observed. The young forest contained 35 species not observed in the mature forest, and the floristic similarity between the two forests was 0.5, indicating that the two forests are floristically distinct.

Spatial variation in the structure and composition of the herbaceous community in a semiarid region of northeastern Brazil.

In the arid and semiarid environments of the world, microhabitats serve as models for the structure of vegetation communities. The goal of this study was to identify differences in the structures of the herbaceous communities growing on a crystalline substrate and those growing on a sedimentary substrate in a semiarid region of northeastern Brazil. One hundred 1 × 1 m plots were established in each area for quantitative sampling, with 69 species recorded in the crystalline area and 76 in the sedimentary area. The average plant density was higher in the sedimentary area, and average diameters and heights were greater in the crystalline area. The families and species with a high Importance Value Index (IVI) and a high Mixed Ecological Value Index (MEVI) differed between the areas. Of the species with high densities, only four were found in both areas. Shannon-Weiner diversity index values in the crystalline (2.96 nats/ind.(-1)) and sedimentary (2.89 nats/ind.(-1)) areas were similar. Evenness values on both substrates were also similar (0.72 and 0.71 in the crystalline and sedimentary areas, respectively). This study shows that variations in plant establishment conditions between crystalline and sedimentary areas in a semiarid region of northeastern Brazil should be considered as structure-modeling factors for the herbaceous community.