Severe Drought Constrains Seedling and Sapling Growth in a Puerto Rican Tropical Rainforest

Global climate change is expected to cause a significant increase in the frequency and severity of extreme climatic events such as droughts and floods. Nevertheless, the potential impacts of these events are poorly understood for tropical forest ecosystems. For Puerto Rico, 2015 was the 6th driest year on record with below average precipitation from April through September, with peak drought conditions occurring in July. Associated reductions in soil moisture persisted for several months after rain resumed. Given that water availability is known to be an important factor regulating the success of tropical seedlings, we evaluated the effects of this drought on the mortality, growth and species composition of woody understory vegetation in a wet tropical forest in Puerto Rico. Seedlings and saplings were monitored within six 12m2 plots, which are part of a field warming experiment (Tropical Responses to Altered Climate Experiment [TRACE]) designed to warm understory plants and soils by 4°C above ambient temperatures. For this study, all plots were considered replicates since measurements were made during the pre-treatment phase of the experiment. The first census was conducted during the drought (May-June 2015), and the individuals were reassessed in November 2015 and June 2016. Comparisons between the two time periods, drought (Jun2015-Nov2015) and post-drought (Nov2015-Jun2016), revealed significant differences for overall growth rates, which were lower during the drought period, but no differences in mortality, abundance, diversity or species composition. Further analyses were conducted for the most dominant species to elucidate their particular responses to drought and if these responses were related to functional traits. Our results suggest that tropical forest seedlings and saplings may limit their growth during drought conditions, and then quickly recover when conditions return to normal. This relatively rapid recovery suggests that Puerto Rican rainforest seedlings may possess a greater resistance to drought than what has been documented for other tropical regions. Future research in these plots will assess the role of experimental warming on the success, growth, and resilience of tropical forest seedlings in the context of a changing climate.