LB 23-201 - Botany at the Boiling River: Changes in the diversity and composition of tropical trees across a thermal gradient

Global warming is an immediate threat to tropical forests. However, our understanding of how warming may affect tropical tree diversity and composition is incomplete. This is primarily due to the challenge of decoupling temperature from other environmental variables in natural systems and the limitations of warming experiments. Here, we attempt to address this gap in knowledge by introducing the Boiling River, a unique geothermal feature in the central Peruvian Amazon and highlight its use as a large-scale, natural warming experiment. Hot water inputs at the Boiling River heat the surrounding air and, in turn, warm the riparian forest. We first characterize the natural thermal gradient at the Boiling River by collecting a year of temperature data and show that there is a steep increase in air temperature surrounding the river over a relatively small distance. We then leverage this thermal gradient to study the effects of increased temperature on tree diversity and composition. To do this, we install a series of tree inventory plots along the gradient and measure and identify all woody stems above 2cm diameter at breast height. We also measure a suite of abiotic variables, including soil properties, to control for potential confounding effects along the thermal gradient. Finally, we test the effect of air temperature on tree diversity and composition through various statistical tests. Our analyses show a decrease in tree diversity due to temperature and clear compositional changes along the thermal gradient. We also find evidence for the thermophilization of tree communities due to increases in temperature. Our results show that warming may lead to significant reductions in tropical tree diversity and that increasing temperature may be contributing to observed compositional changes already underway in the Amazon. Finally, warming in the Amazon may cause a directional shift in species composition, with species associated with warmer forests increasing in abundance while those associated with cooler forests may decrease. This study highlights the potential of using the Boiling River as a natural warming experiment and finds evidence that increases in temperature may reduce the diversity and change the composition of tropical tree communities.