Climate-driven changes in the American flora

Kenneth J. Feeley and Catherine H. Bravo-Avila, Department of Biology, University of Miami, Miami, FL, Belen Fadrique, University of Leeds, Leeds, United Kingdom, Timothy Perez, Department of Botany, University of British Columbia, Vancouver, BC, Canada, Daniel F. Zuleta, ForestGEO and NGEE–Tropics, Smithsonian National Museum of Natural History, Washington, DC

Background/Question/Methods
Climate change is altering the distributions of plant species worldwide. Specifically, rising temperatures can lead to reduced fitness, and hence abundances, of species in the hotter portions of their ranges and increased fitness/abundance in the colder portions of their ranges. In some cases, these differential changes in abundances may lead to range shifts, contractions, or expansions as species die back from the areas that become “too hot” and/or invade areas that were previously “too cold”. Changes in other climate variables, such as precipitation and water availability, may likewise cause changes in species’ range limits as well as abundance distributions within species’ ranges. Collectively, these distributional shifts should manifest as widespread and directional changes in the composition of plant communities over time. We used millions of georeferenced and dated observation records of thousands of plant species collected from over the past four decades (1970 – 2011) to test for changes in the composition of ecoregion floras covering most of North, Central and South America.

Results/Conclusions
Most American ecoregions (68%, binomial P < 0.0001) exhibited positive thermophilization over the past four-decades (indicating increasing relative abundances of more-thermophilic or heat-loving species). The mean thermophilization rate (TR) across all ecoregions was 0.017oC yr−1 and was significantly greater than zero (95% CI = 0.011–0.023oC yr−1, P < 0.0001). Likewise, more than half of American ecoregions (60%, binomial P = 0.0045) had positive mesophilization rates (MR, indicating increasing relative abundance of more-mesophilic or drought-tolerant species), but the mean MR across ecoregions was not significantly different from zero (mean = 0.81 mm yr−1; 95% CI = −0.55–2.17 mm yr−1). The observed rates of compositional change (TR and MR) varied greatly between ecoregions and biomes. The fastest TRs occurred in ecoregions with intermediate mean annual temperatures (MAT) and the slowest TRs were in the lowland tropics. For over half of ecoregions (60%; binomial P = 0.0029), TRs have been slower than concurrent warming (mean MATch – TR = 0.007oC yr−1; 95% CI = 0.0018–0.013oC yr−1). Consequently, many communities may be falling out of equilibrium with climate as temperatures continue to rise. Despite the many complexities and limitations inherent in such large-scale studies, our results show clear patterns of compositional change in most American ecoregion floras that are consistent with expectations based on global warming. These changes in plant community composition may in turn have important implications for the persistence of species, diversity patterns, and ecosystem functioning.