COS 222-6 - Limited thermal acclimation of photosynthesis and respiration in Puerto Rican tropical rainforest plants exposed to long-term experimental warming and hurricane disturbance

Abstract: Climate change projections over the next 80 years predict increased global temperatures and more frequent and intense hurricane disturbances. How plants respond to these changes could have huge implications for how carbon is cycled between ecosystems and the atmosphere. Thermal acclimation, or the ability of individuals to change metabolic processes to work more efficiently at different temperatures, could help plants perform better at higher temperatures or mitigate the effects of temperature damage. While thermal acclimation has been observed in temperate systems, it could be minimal to non-existent in tropical systems which experience narrower temperature ranges than temperate systems. We performed our work at the Tropical Responses to Altered Climate Experiment (TRACE) in eastern Puerto Rico, a long-term in situ experimental warming project where a tropical forest understory is warmed +4 °C above ambient temperature using infrared heaters. The work presented here was collected after two major hurricanes caused a large disturbance at the study site. We conducted temperature response curves on root respiration, leaf photosynthesis and respiration, and leaf thermotolerance for experimentally warmed and ambient temperature plants. We assessed the acclimation potential of these physiological processes to experimental warming and tracked their changes during post-hurricane recovery.

Tropical root respiration did not acclimate through changes in root specific respiration rates, but ecosystem root respiration declined under warming due to lower root biomass. Hurricane disturbance increased variability in root respiration, but the effects are likely short-term. We observed photosynthetic thermal acclimation through wider thermal niche breadth, but at the cost of overall lower photosynthesis for warmed plants. Additionally, photosynthesis was highest after the hurricanes and decreased as the canopy closed. The temperature optimum for photosynthesis, which did not show signs of acclimation, was near the temperature of warmed plots. Leaf respiration was not affected by experimental warming, but decreased with canopy closure after the hurricanes. Leaf thermotolerance did not acclimate to warming and was lowest when the canopy was open and increased as the canopy closed. Together, these results suggest that there is a brief boost to understory carbon assimilation after hurricane disturbance, independent of warming effects, but at the potential cost of higher respiration. Scaling these responses to an ecosystem or global scale is dependent on overall biomass of the system. Our future goals are to estimate the biomass at our site and model the carbon flux responses to experimental warming using the ELM-FATES model.