Resolving the Dust Bowl paradox of grassland responses to extreme drought

During the Dust Bowl drought, central US grasslands responded unexpectedly
to a decade of hot, dry conditions. Grass species adapted to high
temperatures with higher water use efficiency (C4 grasses) decreased while
those preferring cooler climates (C3 grasses) increased. We reproduced
this surprising response by experimentally imposing extreme drought in two
native grasslands. Analysis of historical climate records revealed that
during extreme drought years, the proportion of annual precipitation that
occurs during cooler months increases. This previously unidentified shift
in seasonal precipitation patterns during extreme drought years provides a
mechanism for C3 grasses to increase despite overall hot, dry conditions.
Thus, alterations in precipitation seasonality may be as important as
reduced precipitation amount when forecasting ecosystem responses to
extreme drought.