Regional patterns of aboveground net primary production (ANPP) are well correlated with spatial precipitation gradients. However, in arid ecosystems, mean annual precipitation accounts for only 20-40% of annual ANPP variability in any particular site, mainly due to observed legacies in the response of vegetation growth to changes in precipitation. A better understanding of the mechanisms responsible for the legacies is needed to understand the controls of the global carbon cycle, and to predict changes in aboveground net primary productivity with changes in climate. Our hypothesis predicts that legacies in ANPP are a result of preceding precipitation (PPT) that constrains the density of tillers in these perennial communities. We tested the specific hypotheses that previous-year precipitation constrains production by changes in meristem density or by changes in the frequency of active axillary buds per tiller. We conducted an experiment with five levels of water input (-80% reduced PPT, -50% reduced, ambient PPT, +50% increased, and 80% increased PPT) in the Northern Chihuahuan desert (NM, USA), counted the number of Bouteloa eriopoda tillers in fixed quadrants of 0.125 m2 within our experimental plots, and estimated the number of active axillary buds per tiller from harvested crowns under a dissecting microscope using a staining technique.
After two years of manipulations, tiller density increased with increasing PPT; it was 50% higher in irrigated treatments than in drought plots. More importantly, the number of active axillary buds per tiller doubled with increased PPT treatment, which produced a fourfold increase in the density of active buds per unit of area. We found that the effect of PPT on active buds per tiller was greater than the effect on tillers density. We concluded that the number of meristems may constrain ANPP response to an increase in water availability.