Temperature responses of plant gas exchange from leaves to ecosystems

Warming air temperatures are increasing plant mortality globally, yet plant gas exchange responses to changing temperature are poorly understood. As average air temperatures rise and heat waves become more frequent globally, improving our ability to predict climate change is essential for anticipating global change. Currently, climate models assume a coupled system between carbon and water; above some optimal temperature for plants, carbon assimilation declines to zero and as the stomata close, water loss from the canopy ceases. However, recent evidence suggests that carbon assimilation and water loss may decouple at high temperatures such that water continues to evaporate from the leaf surfaces, even when carbon gain is declining. This dissertation studies the relationship between carbon gain and water loss in three ways. First, I study the methods of measuring carbon assimilation and stomatal conductance. Currently, many researchers use methods of branch excision to make measurements of leaf gas exchange more attainable. I found that excision decreases both carbon assimilation and stomatal conductance in deciduous trees, increases both in coniferous trees, and increases branch water potential in all individuals. I suggest that researchers should first compare excised and intact measurement values for their study species and develop a correction factor to allow continued use of the excision method. Second, I study the leaf-level relationship between carbon and water exchange via assimilation-temperature response curves along an elevational climate gradient. I found evidence of consistent decoupling, where water loss continues to climb while carbon gain drops towards zero, above optimal temperatures and no systematic trend in thermal traits along the gradient. Finally, I studied the decoupling phenomenon at the ecosystem level using eddy covariance data during the 2021 North American heat wave. I found strong decoupling of carbon and water at high temperatures and conclude that climate models are missing this important process of water loss at high temperatures. Overall, I conclude that we need to re-evaluate our use of the branch excision method of measuring plant gas exchange, and we need to incorporate this increase in water loss from leaves at high temperatures into climate models.