Date of Award

2019

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Forest Science (PhD)

Administrative Home Department

College of Forest Resources and Environmental Science

Advisor 1

Molly Cavaleri

Committee Member 1

Andrew Burton

Committee Member 2

Robert Froese

Committee Member 3

Sarah Green

Abstract

Tropical forests cycle one third of Earth’s carbon, yet we are still unsure how tropical vegetation will respond to climate warming. Tropical biomes experience a smaller temperature margin compared to other systems, possibly making them less capable of thermal adjustments. In addition, thermal responses of vegetation have been identified as one of the areas of greatest uncertainty for global carbon models. This dissertation works to quantify tropical forest photosynthetic responses to temperature as well as assessing physiological thermal acclimation of four tropical species. In Chapter, 2 we conducted a meta-analysis to investigate global tropical photosynthetic responses to temperature. We presented algorithms that quantify how instantaneous temperature responses vary for different climate regimes within the tropics. We found that mean annual temperature was the single variable that best predicted most temperature response variables. Stepwise regression showed that including light in net photosynthetic models improved predictive power but, overall, we need better representations of tropical responses to different growth types and conditions. We implemented two in situ warming experiments in a Puerto Rican rainforest to assess physiological thermal acclimation. One experiment was implemented in the understory (Chapter 3) and one in the canopy (Chapter 4). Our understory warming experiment found evidence for net photosynthetic acclimation; however, acclimation did not systematically occur across both warming studies. Some species showed evidence of acclimation of the optimum temperature for photosynthesis (Topt) or both Topt and the photosynthetic rate; while, neither of our canopy species photosynthetically acclimated. Contrary to common hypotheses surrounding plant respiration, only one of the four species showed evidence of respiratory acclimation. Our understory vegetation temperature responses were more strongly controlled by soil moisture than temperature itself. Specifically, the photosynthetic rate declined as soils dried, a response that coincided with stomatal conductance. Surprisingly, Topt decreased with increasing height for our canopy species, and this response was likely, in part, due to higher thermal sensitivity of stomatal conductance in the mid and upper canopies. Additionally, our canopy species were found to be operating right at or above their Topt. The results of this dissertation better quantify tropical physiological responses to temperature, as well as assesses the potential of tropical plants to physiologically acclimate.

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