Recent increases in drought frequency cause observed multi-year drought legacies in the tree rings of semi-arid forests

Recent observations on the full recovery of forests after extreme droughts have shown multi-year legacies, particularly in semi-arid, conifer-dominated ecosystems. Such legacies are usually attributed to ecophysiological memory. However, the potential for successive droughts could impose overlapping recovery times and can also contribute to these legacies. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios and ring-width increments in Pinus ponderosa at 13 montane sites in the semi-arid Southwestern U.S. The spatial hydroclimate gradient and seasonal dynamics were well-predicted by the correlation between δ¹³C and atmospheric vapor pressure deficit (VPD). Using seasonal VPD records together with the tree response to VPD, we show that the probability of extreme drought has substantially increased up to 70% in this region during the past two decades.

Multi-year legacies lasting 2-5 years were observed at most sites when the recent increase in drought frequency was retained. However, when the increase in drought frequency, and its effect on δ¹³C, was detrended, some sites exhibited shorter legacies, but when considered across all sites and for the entire time series, no significant legacies were observed. The absence of legacies, when studied within the context of a long-term shift in the climate system, contrasts with the multi-year legacies reported in past studies, which were most frequently assessed for single drought events and isolated from recent climate trends. Our study revealed that a previously unrecognized contribution to observed multi-year legacies is related to time-dependent shifts in the climate system itself, an exogenous factor that must be considered along with physiological memory.