Meristem density constrains primary production variability along a precipitation gradient in the Patagonian steppe, Argentina

L. G. REICHMANN; O. E. SALA

Memphis, Tennessee, USA

Congreso; 91st Annual Meeting of the Ecological Society of America; 2006

Aboveground net primary production (ANPP) and its variability have been topics of interest to ecologists for a long time. In arid ecosystems, ANPP increases linearly along spatial precipitation gradients, but ANPP variability is largest in sites with intermediate mean annual precipitation (MAP). Our hypothesis is that both annual precipitation and meristem density (MD) may constrain ANPP differentially along an MAP gradient. To evaluate the role of biotic constraints on ANPP, we studied MD and ANPP of dominant species along a precipitation gradient in the southern Patagonian steppe. Within the 80 km transect, we chose three sites with 170, 250 and 290 mm MAP, with MAP increasing to the west. The assemblage of dominant species changed among sites, while plant life-form composition (shrubs and grasses) did not. We measured meristem number and ANPP of dominant species at 10 independent points within each site using a harvesting technique. MD was calculated multiplying meristem number, plant size and plant density, which was estimated using the closest individual method at 25 random points in each site. In a stepwise multiple regression analysis, we found that MD and MAP effects combined explained 73% and 62% of the ANPP variation for shrubs and grasses respectively, whereas MAP alone only explained 20% and 8% of the ANPP variability. Our results suggest that when biotic components are taken into account we can make a better prediction of ANPP than MAP alone. Abiotic controls may better explain the ANPP short-term response while biotic controls may act at a long-term scale, imposing constrains on the ecosystem response to climate variability.