Which functional processes control the short-term effect of grazing on netprimary production in grasslands?

Grazing has traditionally been viewed as detrimental to plant growth, but i t has been proposed that under certain conditions, grazing may lead to comp ensatory or overcompensatory growth. However, comprehensive information on the relative role of the main functional processes controlling the response of net primary production (NPP) to grazing is still lacking. In this study , a modelling approach was used to quantify the relative importance of key functional processes in the response of annual canopy NPP to grazing for a West African humid grassland. The PEPSEE-grass model, which represents radi ation absorption, NPP, water balance and carbon allocation, was used to com pute total and aboveground NPP in response to grazing pressure. Representat ions of grazing and mineral nitrogen input to the canopy were simplified to focus on the vegetation processes implemented and their relative importanc e. Simulations were performed using a constant or resource-driven root/shoo t allocation coefficient, and dependence or independence of conversion effi ciency of absorbed light into dry matter on nitrogen availability. There we re three main results. Firstly, the response of NPP to grazing intensity em erged as a complex result of both positive and negative, and direct and ind irect effects of biomass removal on light absorption efficiency, soil water availability, grass nitrogen status and productivity, and root/shoot alloc ation pattern. Secondly, overcompensation was observed for aboveground NPP when assuming a nitrogen-dependent conversion efficiency and a resource-dri ven root/shoot allocation. Thirdly, the response of NPP to grazing was main ly controlled by the effect of plant nitrogen status on conversion efficien cy and by the root/shoot allocation pattern, while the effects of improved water status and reduced light absorption were secondary.