Widespread legacy effects on net primary productivity across western US drylands.

Despite the water-limited nature of dryland ecosystems, interannual variability in precipitation (PPT) fails to explain a significant fraction of interannual variability in net primary productivity (NPP). One hypothesis states that these weak temporal NPP-PPT associations arise from the lagged effects of previous-year conditions, denoted as "legacy effects," which may amplify or constrain NPP in subsequent years. Although evidence suggests the existence of legacy effects in many ecosystem types, their generality in drylands remains unclear. We used long-term (35-year) remotely sensed estimates of NPP, climate, and a vegetation structure across the western United States to quantify the sign, magnitude, and drivers of legacy effects, defined as the lagged effects of previous-year weather and NPP anomalies on current-year NPP. Legacy effects exert a widespread effect on interannual variability in NPP across drylands spanning annual and perennial grasslands to hot and cold deserts. Previous-year NPP anomalies were the strongest predictor of current-year NPP anomalies, both across the entire time series and during specific extreme-to-average year transitions. The association between previous- and current-year NPP anomalies was consistently positive, indicating that a productive previous year will tend to result in a productive current year, and vice versa, even after accounting for the effect of current-year PPT. The strength of legacy effects increased slightly with increasing mean annual PPT and decreased slightly with an increase in the average fraction of herbaceous NPP. We conclude that legacy effects consistently effect current-year NPP in drylands and that consideration of these effects can improve predictions of temporal variation in dryland NPP.