In arid ecosystems, current year precipitation explains a low proportion of the annual aboveground net primary production (ANPP). There is evidence that precipitation that occurred in previous years is responsible for the observed difference between actual and expected ANPP, a concept called legacy. Here, we study the mechanisms that generate these legacies, thus we will able to better understand the controls of the global carbon cycle, and to forecast changes in ANPP with a changing climate. Our hypothesis predicts that legacies in ANPP are a result of preceding precipitation 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 in the Northern Chihuahuan desert (NM, USA), where 132 6 m² plots were subjected to either five levels of water input (-80% reduced precipitation, -50% reduced, ambient PPT, +50% increased, and 80% increased PPT). First, we counted the number of Bouteloa eriopoda tillers in fixed quadrants of 0.125 m² within our experimental plots. Later, we harvested 2-3 crowns and estimated the number of active axillary buds per tiller 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 tiller density. We concluded that the number of meristems may constrain ANPP response to an increase in water availability.