Climatic determinants of plant phenology in vernal pool habitats

Vernal pool plants are small, colorful, and specialized to both desiccated and inundated conditions that distinguish the ephemeral wetlands in which they grow. These species germinate rapidly in response to the first rain and grow quickly to take advantage of available water supplies. The floral phenology of vernal pool plant species is little understood despite being a crucial developmental stage for producing seeds and determining population growth rates. The current study focuses on two vernal pool plants, Limnanthes douglasii ssp. rosea (meadowfoam), a vernal pool specialist, and Trifolium variegatum (whitetip clover), a generalist vernal pool associate, and characterizes their phenology in response to interannual climate variation. We recorded phenology and climate data over seven years during a period of highly variable precipitation and temperature patterns, which serve as a robust dataset for quantifying the relationship of floral phenology with various climatic factors. We found that warmer and drier environmental conditions occurring during early growth periods were strongly associated with advanced floral phenology later in the life cycle for both species. Over the seven-year dataset, which was increasingly warm and dry, phenology advanced by 4.7 days per year for meadowfoam and 5.6 days per year for whitetip clover, respectively. The floral duration of the habitat specialist was influenced by microtopographic features of vernal pools, whereas no such patterns were observed for the habitat generalist. Finally, warmer and drier conditions were associated with reduced occupancy rates of both focal species within vernal pools. To our knowledge, this is the first study quantifying the relationship between vernal pool floral phenology and climate, offering insights into how phenology may shift in response to modern climate change.