Grazing and Plant Growth Interactions in a Semiarid Festuca pallescens Grassland (Patagonia)

Abstract

The effects of grazing on relative (g l g-l l d-t) and absolute (g am-* l d-l) abo veground net primary productivity and senescence of coiron blanco (Festucapaflescens(St. Yves) Parodi) were investigated in northwestern Patagonia. Aboveground net primary productivity under ungrazed and grazed conditions was estimated by means of a simulation model fitted to biomass data. Relative and absolute aboveground net primary productivity was greater for grazed than ungrazed plants during the early growing season while the inverse occurred during the late growing season (reproductive period). Grazing reduced the relative and absolute aboveground senescence during the late growing season. Relative and absolute effective primary productivity, expressed as the difference between the corresponding primary productivity and senescence, were greater for grazed than ungrazed plants. This increase is in accordance with the grazing optimization hypothesis. However, the increase was not of sufficient magnitude to compensate for losses of green biomass caused by grazing. Results imply that current grazing intensities in this region reduce aboveground productivity of coiron blanco. It follows that a reduction in stocking rates would be necessary to increase aboveground primary productivity.