Danielle Melo dos Santos

Natural regeneration of the herbaceous community in a semiarid region in Northeastern Brazil

This study aimed to describe and compare the interannual changes in the diversity and population structure of herbaceous plants in an anthropogenic area that has been regenerating for 15xa0years and to identify the similarities and differences in the biological attributes of the community compared with the characteristics of a regenerating conserved area. In total, 105 plots measuring 1xa0m2 were established. In each plot, the herbaceous plants were identified, and their height and stem diameter were measured for two consecutive years. The herbaceous flora of the anthropogenic area was represented by 86 species in 70 genera and 27 families, and there were no significant differences in the average richness between years. The conserved area was represented by 71 species in 63 genera and 35 families, and there was a significant difference in the total richness between areas and between years, except when comparing the richness between the conserved area and the anthropogenic area during the second year. Considering both the anthropogenic and conserved areas, 123 herbaceous species were listed, and the similarity between areas was 60xa0%. For the anthropogenic area, the floristic similarity between years was 95xa0%, and in the fragment of the conserved area, the similarity was 74xa0%. The diversity and density were significantly different between years and between areas. Given these results, this study suggests that 15xa0years of natural regeneration for the caatinga is not sufficient to reestablish its native flora with respect to its herbaceous component.

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?