Disturbance regimes and "thermophilization" of understory plant communities

Background/Question/Methods

The composition of plant communities can change over time to favor species from warmer climates, in response to increases in regional temperature. This “thermophilization” can be slowed by increases in forest cover, which can have a strong cooling effect on understory microclimate. We explore the corollary of this prediction, and ask whether increases in disturbance severity contribute to increased local thermophilization of understory plant communities. We sampled understory forest vegetation at twelve forested sites in California, where each site had a gradient of disturbance severity due to forest thinning, low-severity wildfire, and high-severity wildfire. To measure thermophilization, we assigned species to one of two biogeographic affinities in western North America: North-temperate species, which diversified during the mild, wet Eocene, and Southern-xeric or “thermophilic” species, which diversified during the late Tertiary and Quaternary during a period of regional drying. We compared the relative abundance of these two groups, and the distribution of individual species’ functional traits, among different disturbance severities and across a precipitation gradient. This allowed us to investigate the importance of disturbance and associated higher climatic water deficit in filtering the regional species pool into local plant communities.

Results/Conclusions

We found that the ratio of North-temperate to thermophilic species decreased as forest disturbance severity increased. This pattern held across the strong precipitation gradient in this study, however the degree of thermophilization following disturbance was greater in forests with higher precipitation and thus greater pre-disturbance canopy cover. Variation in climatic water deficit appears to drive thermophilization: among high-severity wildfire sites with extremely low canopy cover, there was still a trend towards more thermophilic species at low precipitation sites. Furthermore, despite the strong negative correlation between leaf carbon-nitrogen ratios (C:N) and specific leaf area (SLA) at the species-level, high-severity wildfire stands had lower plot-level C:N without increased SLA, suggesting that water deficit is constraining SLA in these highly disturbed stands. Collectively, these results indicate that small-scale decreases in forest canopy cover due to different disturbance regimes leads to strong, predictable shifts in understory plant communities. Thermophilization of understory plant communities can be attributed to increased understory temperature and climatic water deficit, and can occur rapidly in response to disturbance without any accompanying change in regional climate.