Kleber Andrade da Silva

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.

INTERAÇÕES ENTRE OS COMPONENTES HERBACEO E ARBUSTIVO-ARBÓREO EM UMA REGIÃO SEMIÁRIDA NO NORDESTE DO BRASIL: COMPETIÇÃO OU FACILITAÇÃO?

Under conditions of high stress, interactions between species can be positive. Islands of perennial vegetation can improve the conditions of the understory and facilitate the establishment of herbaceous plants. The hypothesis of this study is that islands of perennial vegetation in an area of caatinga harbor, a greater richness, diversity and density of herbaceous plants, and that individuals reach a greater height and diameter than in open spaces. The study was conducted in Petrolândia, Pernambuco, Brazil. Twenty-seven plots were installed in the center of the islands, 38 at the edge of the islands (in a total of 38 islands) and 35 in the open spaces. A total of 51 species were recorded in the center and 55 on the edge of the islands and 48 in the open spaces. The mean richness of the open spaces was lower than on the islands. The diversity was greater in the center of the island and became less on the edge of the island and in the open spaces. The mean density was lower in the open spaces than on the islands. The mean density at the edge of the islands was greater than in the center of the islands. There was no difference in mean diameter of herbaceous plants. The mean height of the individuals was higher in the center of the islands. The herbaceous community growing on the islands exhibited higher richness, diversity, density and height than in open spaces. Thus, islands of perennial vegetation facilitate the establishment of herbaceous species.

VARIAÇÃO TEMPORAL NA PRECIPITAÇÃO CAUSA MUDANÇA NA COMPOSIÇÃO FLORÍSTICA, RIQUEZA DE ESPÉCIES E A DENSIDADE DE SEMENTES DO BANCO DO SOLO?

Introdução As variações anuais e sazonais que ocorrem na precipitação em função do tempo podem causar mudanças na dinâmica do banco de sementes das florestas secas (ARAÚJO et al., 2013; SANTOS et al., 2013). Entre os atributos mais afetados da comunidade vegetal estão a riqueza de espécies e a densidade de sementes (TRABA et al., 2006 ; SILVA et al., 2013) que em algumas áreas são maiores em anos mais úmidos (FACELLI et al., 2005) em outras áreas são maiores em períodos secos (ONAINDIA e AMEZAGA, 2000). Além disso, as precipitações de anos anteriores também podem ter um maior poder de explicação sobre os atributos do banco de sementes do que as chuvas do ano corrente (SILVA et al., 2013). Portanto, os atributos do banco de sementes do presente pode ser um reflexo da influência da precipitação do passado sobre a dinâmica da comunidade vegetal. Na Região Nordeste do Brasil, o tipo predominante de floresta seca é denominado de caatinga, na qual ocorrem baixos totais pluviométricos anuais, irregularidade de distribuição das chuvas (SANTOS et al., 2013; SILVA et al., 2013) que a torna um bom exemplo para verificar o efeito da precipitação do presente e passado. Então, admitindo que a dinâmica do banco de sementes do solo das florestas tropicais secas seja regida pela heterogeneidade temporal, este estudo tem por hipótese que: 1) As variações na precipitação, entre estações climáticas e entre anos consecutivos, causam mudanças na composição florística e determinam o número de sementes e de espécies do banco de sementes. Para testar esta hipótese este estudo se propõe a responder a seguinte pergunta: (1) Os atributos do banco de sementes do solo (Composição florística, riqueza de espécies e a densidade de sementes) são afetados pela variação temporal na precipitação?

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.