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

Abstract

Grazing has traditionally been viewed as detrimental to plant growth, but it has been proposed that under certain conditions, grazing may lead to compensatory 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 radiation absorption, NPP, water balance and carbon allocation, was used to compute total and aboveground NPP in response to grazing pressure. Representations of grazing and mineral nitrogen input to the canopy were simplified to focus on the vegetation processes implemented and their relative importance. Simulations were performed using a constant or resource-driven root/shoot allocation coefficient, and dependence or independence of conversion efficiency of absorbed light into dry matter on nitrogen availability. There were three main results. Firstly, the response of NPP to grazing intensity emerged as a complex result of both positive and negative, and direct and indirect effects of biomass removal on light absorption efficiency, soil water availability, grass nitrogen status and productivity, and root/shoot allocation pattern. Secondly, overcompensation was observed for aboveground NPP when assuming a nitrogen-dependent conversion efficiency and a resource-driven root/shoot allocation. Thirdly, the response of NPP to grazing was mainly controlled by the effect of plant nitrogen status on conversion efficiency and by the root/shoot allocation pattern, while the effects of improved water status and reduced light absorption were secondary.

Forage accumulation in brachiaria grass under continuous grazing with single or variable height during the seasons of the year

“…The lower LSR on pasture managed with lower height (15 cm) in the winter compared with the pasture maintained at 25 cm height can be attributed to more intensive grazing management in the first condition, which provided greater removal of not synthetically active tissues and, indeed, it may have increased the photosynthetic rate of the pasture (Leriche et al, 2001). Thus, the lower pasture in the winter probably had lower negative energy balance, which explains their lower LSR.…”

Effects of grazing exclusion on carbon sequestration and plant diversity in grasslands of China—A meta-analysis

“…For carbon sequestration, some studies found that grazing exclusion facilitated vegetation recovery, increased plant productivity and thus enhance soil carbon stocks in degraded grasslands (Biondini et al, 1998;McIntosh and Allen, 1998;Li et al, 2008;Mekuria et al, 2007;Mekuria and Veldkamp, 2012;Xiong et al, 2014). Others, however, reported no change (McNaughton et al, 1998;Pucheta et al, 2004) or even decline of carbon stocks in grasslands (Schuman et al, 1999;Leriche et al, 2001;Frank et al, 2002;Wienhold et al, 2001). Controversial results have also been found on a regional and global scale.…”

Effects of aboveground grazing on coupling among nitrifier activity, abundance and community structure

“…Negative effects can also be observed in nutrient-poor systems or under high grazing pressure (Bardgett and Wardle, 2003). Because N is the limiting factor for primary production in many ecosystems (Vitousek and Howarth, 1991), grazing-induced enhancement of soil N dynamics is a key mechanism through which herbivores can influence plant productivity in such a way as to alleviate nutritional deficiencies (McNaughton et al, 1997;Leriche et al, 2001). In particular, the regulation of nitrification has often been regarded as the key for an efficient nitrogen cycling in this context, as in contrast to most other steps in nitrogen turnover only a limited number of microbes is able to convert ammonia into nitrate.…”