COS 98-3 - Amazonian trees show limited acclimation to elevated temperatures in futuristically hot forests

Abstract: Amazonian trees will not be able to escape rising global temperatures through either species migrations or adaptation. It is therefore imperative to understand the mechanisms through which Amazonian trees may be able to acclimate to rising temperatures. In this study, we leveraged a unique and extreme natural thermal gradient in forests along the geothermally heated Boiling River in Peru’s central Amazon to assess the ability of several tropical tree species to acclimate to elevated air temperatures. Maximum air temperatures at the Boiling River are hotter than any other known rainforest and are analogous to predicted future conditions. Specifically, our objectives were to (1) assess the acclimation of leaf thermoregulatory traits in six species and determine whether such acclimation was sufficient to offset increased growth temperatures; (2) assess the acclimation of photosynthetic thermal tolerances to elevated leaf temperatures; and (3) determine if acclimation of leaf temperatures and thermal tolerances was sufficient to maintain constant thermal safety margins (TSMs) and apparent plant heath across different growth temperatures. We first characterized the thermal gradients in the Boiling River Forest with a network of soil and air temperature loggers deployed over one year and interpolated using Landsat 9 thermal imagery. Then, for 10-15 individuals of each of the six focal tree species, we measured a suite of 10 leaf thermoregulatory traits and estimated leaf temperatures with an energy balance model (n = 5 leaves per tree), measured the thermal tolerance of each tree, calculated TSMs as the difference between thermal tolerance and leaf temperature, and estimated plant health as the remotely sensed normalized difference vegetation index (NDVI at 3-m resolution) at each tree’s location. We found that some thermoregulatory traits acclimated to increased temperatures in a few of the focal species and that two species increased their thermal tolerances when growing under increased temperatures. Overall acclimation was insufficient to maintain constant TSMs in any of our study species, and leaf temperatures can become dangerously high in the futuristically hot portions of the thermal gradient. Perhaps because of these high leaf temperatures, four species had lower NDVI (our metric of plant health) at hotter temperatures. Our results support previous hypotheses and findings that Amazonian tree species will be increasingly vulnerable to thermal stress as temperatures rise and that future conditions may be unsuitable for many species.