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| ------Mean annual precipitation accounts for a large proportion of the variation in mean aboveground net primary production (ANPP) of grasslands worldwide. However, the interannual variation of production in any grassland site is only loosely correlated with precipitation. The longest record of variation in production and precipitation for a site corresponds to a shortgrass steppe in Colorado. A previous study of this record showed that precipitation accounted for 39% of the interannual variation in ANPP. In this note, we show that about one third of the unexplained variation is related to previous-year ANPP: in years preceded by wet, more productive years, ANPP per mm of precipitation was higher than in years preceded by average years; similarly, in years preceded by dry, less productive years ANPP per mm of precipitation was lower than in years preceded by average years. This finding not only increases our predictive ability, but it also changes our understanding of how ANPP responds to precipitation fluctuations. If ANPP is thought to vary according to current-year precipitation only, it will simply track annual precipitation in time. According to our model, however, ANPP fluctuations are buffered if wet, more productive years alternate with dry, less productive years, and they are amplified if wet or dry sequences of several years take place. These results support the hypothesis that inertial mechanisms regulate the temporal fluctuations of ANPP. |