Drought intensity and duration interact to magnify losses in primary productivity.

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e. ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme. Editor's summary: Climate change is bringing more severe and longer-lasting droughts to many locations around the world. Some ecosystems have shown resilience to increasing drought, but this could change as droughts become more severe. Ohlert et al. assessed drought effects with a rainfall-exclusion experiment distributed across 74 grasslands and shrublands on six continents. On average, primary productivity declined by 29% in moderate-drought years, but effects did not compound over multiyear droughts. Extreme droughts (those falling outside the historic range of rainfall variability) caused much higher productivity declines and lower recovery, especially in drier locations and those with lower plant diversity. This study adds to growing evidence that future climate extremes will likely affect ecosystems in new and concerning ways. —Bianca Lopez [ABSTRACT FROM AUTHOR]