Relações solo-vegetação em áreas sob processo de desertificação no Estado de Pernambuco.

Abstract

Land degradation in semiarid regions is a result of natural processes that can be induced by man, through the inadequate use of natural resources, resulting in the degradation of vegetation, soil and water resources. Aiming to relate vegetation and soil degradation processes in the Agreste and Sertão region of Pernambuco State, soil and vegetation were characterized in twelve plots, representative of preserved, moderately degraded and degraded environments in the municipalities of Jatauba and Floresta. To study the vegetation, plants were stratified in three layers or height classes: 3 – plants with more than 3.0m of height, were sampled in the total plot area(200m²); 2 – plants with height between 0.51-3.0m, were sampled in an area of 100m2; 1 – plants with height equal or less than 0.50m, were sampled in 18 miniplots of 25 x 50cm. Soils were morphologically characterized in each plot and samples were collected by horizons for physical, chemical and mineralogical analysisIn Jatauba County, the vegetation of the 2nd stratum showed a great decrease in absolute density as a result of soil degradation intensity. Species with greatest relative densities were Neoglaziovia variegata (caroá) and Cordia leucocephala(moleque-duro) in the preserved area; Bromelia laciniosa (macambira),Aspidosperma pyrifolium (pereiro) and Caesalpinia pyramidalis (catingueira), in the moderately degraded area; and C. pyramidalis and Sida galheirensis (malva branca),in the degraded area. Soil properties (Planosols) best correlated with the preserved vegetation were the thickness of A + E horizons and the amounts of organic carbon. The occurrence of soil crusting and erosion, as well as, the high percentages ofexchangeable sodium were observed with greater intensity in the degraded sites, particularly related to sparse vegetation. Associations between vegetation and soil variable groups, obtained through canonic correlation analysis, indicate that the presence of higher plants and great soil covering by the shrub stratum gives protection to soil surface, reducing the formation and development of soil surface crusting. On the other hand the vegetation cover seems to require a greater nitrogen uptake for plant nutrition or organic matter decomposition, resulting in a decrease of total nitrogen.In Floresta County the vegetation showed a great decrease in absolute density with soil degradation intensity, in relation to all studied strata. C. pyramidalis was the dominant specie in the 3rd vegetation stratum in all environments. Species with the greatest relative densities were: C. leucocephala and Croton mucronifolius, in the preserved site; Malvastrum coromandelianum, A. pyrifolium and Melochia tomentosa (capa bode), in the moderately degraded site; and M. coromandelianum and C. pyramidalis, in the degraded site. Soil properties (Luvisols) best correlated with the degraded areas was: shallow profile, small thickness or removal of the surface horizon, and soil erosion intensity, as well as, high levels of exchangeable sodium. The decrease in the amounts of organic carbon and total nitrogen were also related to the smaller vegetation cover. The canonic correlation analysis in Floresta region showed a straight relationship between the density of woody plants (>0.5m of height) and the amounts of total-N and organic carbon. This is probably a result of the greater amount of plant residues incorporated by the dense vegetation. Greater amounts of clay in the B horizon also positively influenced the density of woodyplants, due to increase in water holding capacity. The smaller soil density values in the surface horizon of the preserved sites are, probably, a result of the soil protection given by the plants and also influenced by the greater organic matter contents in this horizon.