ROLE OF VEGETATION COVER IN THE BIOGEOCHEMICAL BALANCES OF SMALL AFFORESTED CATCHMENT IN SOUTHEASTERN SPAIN

The budgets of major dissolved nutrients have been determined in a small micaschist catchment (0.34 km2) covered with Mediterranean shrubs and afforested pines at 1560 m altitude in the Filabres range (southeast Spain). The climate is semiarid with 395 mm annual rainfall and 12-degrees-C annual mean temperature. During the hydrological year 1988-1989, major element concentrations in open field precipitation, throughfall, stemflow and runoff were recorded as well as the element fluxes of the main solutes. The total rainfall recorded during the sampling year was 650 mm of which 86% was lost through evapotranspiration and 14% through runoff. Mean annual rainfall interception is estimated as 15% and net rainfall reaching the soil is divided between 90% throughfall and 10% stemflow. Two methods were applied to compute bulk precipitation inputs to the ionic balances: (I) bulk open field deposition was estimated from data provided by open field polyethylene collectors (bulk open field precipitation inputs); (II) net throughfall and stemflow were corrected for canopy leaching. The results show that the total deposition calculated using balance (II) is higher than that calculated by (I); the effect of canopy surfaces in scavenging atmospheric dry fallouts and gases is identified in a more efficient way than by simple polyethylene collectors. When comparing both methods of computing balances in several environments, using published data, the authors conclude that the plant canopy effect is larger in sermiarid areas with dusty atmospheres and in industrial regions with higher concentrations of sulphur. These conclusions suggest that the retention of nutrients within the ecosystem may be more widespread than was previously thought. This calls for more research on the assessment of biomass removal and the dynamics of intermediate pools (precipitates). Consequently, it throws doubts on most of the figures available for catchment weathering rates; these were often estimated by difference between bulk open field precipitation inputs measured with polyethylene collectors and runoff outputs.