Rapid responses to drought in a rare California annual (San Francisco collinsia, Collinsia multicolor)

As climate change continues and the frequency and intensity of droughts become more prevalent in some regions, plant populations are facing greater ecological pressures. The objective of this study was to observe the response of a rare plant species to an extreme drought event associated with climate change. To study this response, we collected seeds from three populations of Collinsia multicolor (San Francisco collinsia, Plantaginaceae) found in central California both before and after the state's historic 2012-2016 drought. We conducted a greenhouse study to examine contemporary evolution between the collection years, and included a drought treatment to study plasticity. We measured three traits that indicate life history, morphological, and physiological responses to drought, including flowering time, stomatal density, and chlorophyll fluorescence. In the two coastal populations, where interannual moisture variation is greatest, we observed evolution only in stomatal density, while we observed plasticity in all measured traits. In contrast, the driest inland population showed no response to the drought or to the watering treatments, which is consistent with other studies that have found less response to drought in pre-adapted populations. Overall, our results suggest that plasticity is favored in variable environments. However, they also highlight that the pace of evolution may be insufficient to respond to current environmental change.