Interaction between vegetation and lateral nutrient fluxes in a forested watershed. A model study

Abstract

In terrestrial forest systems, ecology is strongly influenced by hydrological and nutrient fluxes, but the ecosystem itself can control these fluxes as well. A model study was done on a watershed with a temperate forest to investigate if upslope vegetation can control downslope vegetation through the control of water and nutrient fluxes. The ecohydrological ‘WALNUTS’ (water, nutrients & succession) model was based on three existing models: Water and Tracer Balance (WTB) model (Rebel et al., 2007), Multiple Element Limitation (MEL) ecosystem model (Rastetter et al., 2001) and Brook90 hydrological model (Federer, 2002). The WALNUTS model was parameterized for watershed 3 of Hubbard Brook (NH, USA). A model experiment was done to assess the effects of upslope vegetation on nutrient fluxes and the effects of these fluxes on downslope vegetation at a time scale of several decades. Result showed that nutrient fluxes downslope could be limited if the upslope ecosystem had limited N output. This should not be attributed to vegetation alone, but also to soil microbial processes and other controls on nutrient transport. Biomass accumulation rates of downslope vegetation were affected by nutrient fluxes; these rates were highest in areas with much inflow of nutrients. Furthermore, a buffering effect was noticed: if upslope areas would export very little nutrients, mid-slope areas would react with increased mineralization, therefore allowing downslope ecosystems to develop at the same rate as ecosystems that received more nutrients from the upslope system. However, as the results on nutrient transport were not realistic (due to high net mineralization and no retention), the extent of these interactions could not be quantified in a realistic way. Furthermore, the results of the simulations can not be confirmed by observations. Further research is necessary to gain understanding of the interaction between vegetation and nutrient fluxes