Fire regimes and functional traits: Adaptation and population resilience in obligate reseeding shrubs
Wildfires are important processes in Mediterranean ecosystems, and plants living in Mediterranean regions are well-adapted to withstand or embrace these disturbances, by means of traits such as thick bark, ability to resprout from underground structures, or seeds that are triggered to open or be dispersed following fire. However, different phenotypes are advantageous under different fire regimes, and there exists considerable variation in the prevalence or strength of expression of fire-adaptive traits (e.g. serotiny) between populations in regions with different fire regimes. When fire regimes shift due to human-caused ignitions, land management, or climate change, a population that was once well-adapted may be less resilient to fire. The historical strength of fire as an evolutionary pressure as well as the standing variation in fire-related traits in a population may serve as indicators of adaptive capacity of species in fire-prone systems. In this project, I am investigating the relationship between fire-adaptive traits of obligate reseeding Ceanothus species and the historical and modern fire regimes that they have experienced in order to understand the historical role of fire as a selective pressure and its potential to drive adaptation into the future. I focus on fire triggered germination, flammability, and seedling growth rates as fire-related traits, using germination experiments, field observations, and a common garden experiment.
Preliminary results focus on seed and germination traits. Experiments on seed germination in response to heat and fire cues on show that Ceanothus populations in regions with a higher historical fire frequency germinate slightly more readily with a fire cue, while populations in wetter regions with fewer fires respond more strongly to a hot water cue. Dry heat was a consistent germination trigger regardless of fire history. Control treatments showed little germination. Ongoing research seeks to expand these results to a greater number of populations, including island and coastal populations that are more likely to demonstrate a broader gradient in fire history. Work in progress will also examine trait variation within populations and look at a wider suite of fire-related traits. Variation in traits within and across populations can provide clues as to how precisely and how quickly plants have adapted to past and current fire regimes and suggest adaptive capacity of Mediterranean shrublands in the face of future change.