Wildfire is an important component of Mediterranean ecosystems, and plants living in Mediterranean regions are well-adapted to withstand or embrace disturbances. 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 between populations in regions with different fire regimes. When fire regimes shift due to human-caused ignitions or climate change, a population that was once well-adapted may be less resilient to fire. 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 in the face of future change. In this project, we investigate the relationship between fire-adaptive traits of obligate reseeding Ceanothus species (C. cuneatus and C. megacarpus) and the historical and modern fire regimes that they have experienced. Traits studied include fire triggered germination, flammability, and seedling growth rates as fire-related traits, using germination experiments, field observations, and a common garden experiment. Study areas included 12 populations of C. cuneatus from coastal, coast range, and inland parts of California and 8 populations of C. megacarpus from the Southern California mainland and the Channel Islands.
Results/Conclusions
We found that variation in germination and growth traits weakly reflect historical and recent fire regimes, with the biggest differences observed between island and mainland C. megacarpus, where historical fire intervals show the biggest contrast. Fire-proxy enhanced seed germination was strongest for populations of both species with high historical fire frequencies, especially for mainland C. megacarpus (p<.001 when compared to island populations). Germination in the absence of proxy fire was strongest for C. cuneatus populations in regions with low historical fire frequencies (p<.01) and island C. megacarpus, but the latter was not significant. Common garden seedling growth rates did not show any differences that reflected historical fire patterns. We conclude that fire patterns can shape fire-adaptive traits in shrubs on a population level, but some traits reflect these patterns more strongly than others. In addition, traits reflect long-term regional fire patterns more so than 20th century fire data, suggesting that it takes many generations for adaptation to take place and that shrubs may not adapt quickly to changes in fire frequency. However, considerable within population variation may allow population resilience as fire regimes change.