Effects of grazing pattern and nitrogen availability on primary productivity.

SEMMARTIN, M.; MARTÍN OESTERHELD

OECOLOGIA

SPRINGER

Año: 2001 vol. 126 p. 225 - 230

0029-8549

Abstract A major part of the impact of grazing on pri-mary productivity results from the joint action of tissue removal and nutrient return to the soil via dung and urine. Grazing, however, is not uniformly distributed in space: grazed grasslands show a matrix of grazed and ungrazed patches, which in turn, may or may not be af-fected by faecal or urine deposition. This paper investi-gates the effects of grazing spatial pattern and nitrogen availability on primary productivity. We propose that grazed plants located at the edge of a grazed patch are more shaded by their taller ungrazed neighbours than plants at the center. Since the border effect is less im-portant as patch size increases, the effects of grazing will be more positive, or less negative, when grazing pattern is coarse-grained than when it is fine-grained. We also propose that nitrogen availability will affect this response to grazing through its effects on the inten- sity of competition for light and on the amount of com- pensatory growth. We performed a field experiment in a grassland community of the Flooding Pampa, Argen- tina, in which we compared the productivity of undefo- liated controls and defoliated patches of different size, with and without nitrogen application. Defoliation re- duced primary productivity and this effect was greater in the smallest, fertilized patches. Productivity was highest at patches of intermediate and large sizes.