Approximately 70% of total annual rainfall in lowland California falls in the winter months (Dec- Feb), but prolonged dry and warm spells (“winter droughts”) are not uncommon. Winter rainfall is crucial to the germination, survival, and early growth of annual species. At our principal study site in the Inner North Coast Range, a directional decline in winter rainfall from 2000-2016 appeared to drive a decline in grassland species richness across all functional groups. Within the most numerous functional group, native annual forbs, the selective loss of drought-intolerant species during this period was indicated by a decline in the community mean of specific leaf area (SLA).
Beginning in December 2015, we are using wintertime watering and rainout shelters to test the mechanisms behind and potential reversibility of this climate-driven loss of diversity.
Following the first full year of treatments, in spring 2016, watered plots compared with controls showed (1) lower seedling mortality, (2) higher mean SLA of native annual forbs, and (3) higher total and native annual forb cover. These results suggest that winter drought affects diversity via actual demographic change -- higher mortality, lower growth and seed production -- as opposed to via long-term seed dormancy. Thus, we predict that grassland diversity will show only partial recovery in wetter years such as 2016-2017. Since greater aridity and more frequent droughts are forecast in California, this apparent lack of resilience to prolonged drying in native plant communities is cause for concern.