Tropical wet forest root specific respiration responses to experimental warming and hurricane disturbance

Rob Tunison, Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI, Molly A. Cavaleri, School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI, Sasha Reed, Southwest Biological Science Center, U.S. Geological Survey, Moab, UT and Tana E. Wood, International Institute of Tropical Forestry, USDA Forest Service, Rio Piedras, PR

Background/Question/Methods
Tropical forests exchange more carbon with the atmosphere than any other biome, but little is known about how carbon fluxes will change in tropical forests with projected climate warming. Root respiration acclimation allows plants to conserve carbon resources under higher temperatures and decreases carbon loss to the atmosphere. While experimental warming in temperate forests has resulted in down-regulation of root respiration, recent studies show a lack of respiration acclimation of tropical tree roots. We investigated root respiration acclimation to in situ experimental warming in a tropical wet forest. This study was conducted at the Tropical Responses to Altered Climate Experiment (TRACE) project located in a tropical wet forest in Luquillo, Puerto Rico. TRACE has six 4m diameter plots containing 3 ambient temperature plots and 3 plots warmed (+4 °C above ambient) with infrared heaters. Two major hurricanes created large disturbances at TRACE in 2017, a year after experimental warming initiation, providing a unique opportunity to measure the legacy effects of warming on root respiration in early secondary succession. Data were collected for root specific respiration, root morphological traits, and soil nutrients for multiple bulk soil and root ingrowth core campaigns sampled from March 2017 to October 2020.
Results/Conclusions
Root specific respiration did not acclimate to experimental warming in the TRACE plots for either basal respiration rate at 25 °C (R25) or temperature sensitivity (Q10, multiplicative rate change per 10°C). However, we did see a small increase in R25 after the hurricanes, followed by a decrease nearly a year later. These differences in R25 could be characterized by the rapid change in community composition following the hurricanes. Q10, which had a mean value of 1.74 across all measurements, showed no change across either treatment or time. R25 increased with increasing root tissue nitrogen concentration for ingrowth cores, but not fresh cores, indicating basal respiration rates were influenced by enzymatic activity associated with new root growth. With no root respiration acclimation to warmer conditions, we conclude that roots were not able to conserve carbon under higher temperature scenarios. Changes in basal root respiration rates following the hurricanes suggest that areas with high disturbance frequency may have more variable root respiration. These results lend rare insight into how tropical forest plant carbon cycling will respond to the interacting effects of warming and disturbance and can be used to parameterize models that describe root respiration contributions to tropical soil carbon efflux.