Controls of grass and shrub aboveground production in the Patagonian steppe

We studied the temporal dynamics of aboveground net primary production (ANPP) and senescence in a semiarid steppe representative of the Occidental District of Patagonia. We focused on the association of ANPP and senescence with climatic fluctuations at annual and seasonal scales. We estimated annual ANPP based on biomass harvests of grasses and shrubs (the dominant functional groups) at the peak of the growing season (January). The ANPP and senescence of grasses alone were also estimated for several periods within each year. The mean annual ANPP of the studied community for 10 years was 56 g.m(-2).yr(-1), and the coefficient of variation was 26%. Grass and shrub mean annual ANPP had similar levels. Annual shrub production was associated with cumulative precipitation for the whole year, whereas annual grass production was not associated with precipitation. The average seasonal dynamics of grass ANPP showed its peak during spring, and there was no grass production during late summer and early fall. The peak green biomass of grass was achieved in early summer. Average senescence rates were more constant throughout the year, except for winter, when senescence was almost interrupted. Winter grass ANPP was positively associated with fall temperature, spring grass,ANPP was positively related to winter precipitation, and summer grass ANPP was positively related to spring precipitation. Grass senescence during spring was negatively related to precipitation in the same season. Our results highlight the importance of separating functional types in productivity studies. The differential effect of precipitation on grass and shrub ANPP masked the association when these functional groups were pooled. ANPP association with climatic variables in the Occidental District of the Patagonian steppe shifted from precipitation alone to precipitation and temperature when the temporal scale of analysis changed from annual to seasonal.