Ação do pastejo sobre a composição florística do estrato herbáceo e atributos do solo do Semi-Árido da Paraíba.

Abstract

Extensive animal raising, mainly goats, is one of the main economic activities in the semiarid region of northeast Brazil. This activity is based mainly on the native vegetation to provide food for the animals. The poor animal performance indexes indicate the need to study rangeland conditions and the effect of the animals on it. The scarcity of information on this topic gave rise to the interest to elucidate the negative effects of animal browsing and trampling on the composition of the herbaceous layer and on soil compaction. A method to establish the quantitative relations between the native or infesting flora and animal browsing and trampling consists in the study of the seed bank pool of the soil. This study had the objective to evaluate the effect of animal browsing and trampling on seed bank pool and soil characteristics from a native rangeland of the semiarid region of Paraíba. The study was carried out from May to August 2007, at Fazenda NUPEARIDO/UFCG, in Patos-PB, in a 1.5 ha deforested caatinga site in which 24 goats (19 kg of live body weight, and 120-day old) browsed during 104 days. Litter and soil samples were collected before and after animal browsing. Litter samples (10) were collected under and out of tree canopy from protected (AP) 1mx1m plots. Adjacent to each protected plots, other 10 litter samples were collected (AL), as well as 10 litter samples along the margins of a dam (AA). These samples had their seed bank pool evaluated in the UFCG Seedling Nursery facilities during 90 days, when the emerged plantules were identified according to their vegetative and/or reproductive characteristics. Plantule frequencies were tested by the c 2 (P<1%) procedure. Goat browsing and trampling decreased floristic composition and seed bank density, in general, and increased the participation of four grasses and one legume species, while other 25 species were affected negatively. Soil compaction was estimated from soil samples (0-20cm deep) collected before and after animal browsing, in a South-North and East-West orientation around trees present in the area, at 1.5 (under) and 3.0 m (out of canopy) from tree trunks. These soil samples were characterized chemically and physically. Goats preferred to stay under the tree canopy of legume trees. Litter fall and the herbaceous layer provided the highest levels of organic matter. Leucaena and juazeiro canopies together with the herbaceous layer resulted in, respectively, the highest and the lowest P and K level before and after browsing.