Changes in tropical forests in response to climate change may have global consequences due to their exceptionally high levels of biodiversity and major role in the global carbon cycle. In this study, we document the effects of experimental warming on the abundance and composition of a tropical forest floor herbaceous plant community (ferns, graminoids, forbs, and non-climbing herbaceous vines) in the Luquillo Experimental Forest, Puerto Rico. This study was conducted within TRACE plots (Tropical Responses to Altered Climate Experiment) which uses infrared heaters under free-air, open-field conditions, to warm understory vegetation and soils 4°C above that of ambient control plots. Hurricanes Irma and Maria damaged the heating infrastructure in the second year, therefore our three-year study included one pre-treatment year, one year of warming, and one year of hurricane recovery with no warming. We measured percent leaf cover of individual herbaceous species, fern population dynamics, and species richness and diversity within three warmed and three control plots.
Results/Conclusions
Results showed that one year of experimental warming did not significantly affect the cover of individual herbaceous species, fern population dynamics (density, mortality, size, and fertility of Blechnum occidentale ), species richness, or species diversity. The hurricanes did, however, have significant effects. Herbaceous cover increased more than threefold, from 20% to 70%, and bare cover decreased from 70% to 6% (pre- to post-hurricane). Non-metric dimensional scaling also revealed that the largest shift in herbaceous species composition occurred post-hurricane. The negligible effects of warming may have been due to the short nature of the warming treatment, or an understory that is somewhat resistant to higher temperatures. This finding is consistent with experiments showing that shaded conditions can buffer the effects of experimental warming in temperate forests. Our results indicate that temperature increases will not be the most consequential component of climate change for these understory herbaceous communities over the short term. Climate extremes that are also predicted to increase with climate change, such as hurricanes, droughts, and extreme heat waves, may cause more abrupt changes in tropical forest structure, function, and composition than longer-term sustained warming.