Response of water and nutrient fluxes to improvement fellings in a tropical montane forest in Ecuador

Management of natural forests might be one option to reduce the high deforestation rate in Ecuador. We therefore evaluated the response of water and nutrient cycles in a natural tropical montane forest to improvement fellings with the aim of favoring economically valuable target trees which will later be harvested with additional ecosystem impacts not considered here.The study was conducted at ca. 1900–2200 m above sea level in the south Ecuadorian Andes on the east-exposed slope of the east cordillera. In June 2004, one of two paired ca. 10-ha large catchments was thinned by felling 10.2% of the initial basal area (dbh ≥ 10 cm) on 30% of the catchment. The stems remained in situ. We measured ecosystem fluxes from rainfall via throughfall and stemflow to soil solution (litter leachate, soil solution at 15 and 30 cm depth) and stream flow between May 2004 and May 2005. After the fellings, soil solutions were extracted from the gaps created by the felled trees and the forest next to the gaps. We determined aboveground water fluxes by direct measurement and soil water fluxes with a budget approach. In the solutions, we measured concentrations of NH4+-N, NO3−-N, total dissolved N, PO43−-P, total dissolved P, Ca, Mg, K, Na, and Cl−. Fluxes were calculated as volume-weighted mean (vwm) concentrations times water fluxes. Dry deposition was estimated using Cl− as inert tracer.The fellings increased concentrations of N, K, Ca, and Mg in the organic layer of the resulting gaps compared with the forest next to the gaps (vwm concentrations of N: 6.4 mg l−1 in the forest next to the gap/8.7 mg l−1 in the gaps, K: 9.8/11, Mg: 1.8/3.0, Ca: 3.4/5.8). Lower nutrient concentrations and fluxes in the mineral soil of the gaps than in forest next to the gaps suggested that these nutrients were taken up by ground vegetation and target trees. The paired modified and undisturbed catchments had similar water and nutrient budgets. The fellings did not have a significant impact on the water and nutrient budget at the catchment scale.