John and his colleagues published a new study in Tree Physiology investigating whether there is substantial within-species variation in tree response to elevated concentrations of carbon dioxide eCO2. The work was led by Mike Aspinwall at the University of North Florida. Measuring photosynthesis in the greenhouse. Photo by Renee Smith. Eucalyptus camaldulensis is an extremely widely-distributed tree species in Australia. These widely distributed species are expected to contain substantial genetic variation, as genotypes are expected to be adapted to their local environmental conditions. This genetic variation may cause some trees to respond differently to eCO2 than others. Growth from small cuttings to saplings. Photos by Renee Smith. Despite the large geographic and climatic distance between the seed sources, we actually found little evidence that different genotypes will respond differently to eCO2. Most of the genotypes displayed higher rates of photosynthesis and growth under eCO2. There was some variation, and genotypes that had large increases in photosynthetic nitrogen use efficiency and large increases in root mass fraction showed the largest growth responses to eCO2.
This work implies that eCO2 is likely to positively affect the growth of trees across the wide range of this species. However we note that other aspects of climate change, notable droughts and heatwaves, are likely to dampen or eliminate this positive effect of eCO2 on tree growth.
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Our recent Global Change Biology publication demonstrating how some trees successfully cope with an extreme heat wave has received significant media attention. Of special note was the recent story in the Scientific American titled "Trees Sweat to Keep Cool". We were excited to see our work highlighted in this excellent magazine. There were some other interesting press articles as well, including stories from The Guardian, The Futurist, Phys.org, Outdoor Design, Salon, The Weather Channel, and others. Much of this coverage was very good, but some points were missed in some of these stories. (1) We never suggested that these trees need to be genetically engineered to cope with extreme heatwaves. I don't know where the genetic engineering idea crept in, as we don't mention this at any point. The primary message of our work is that these trees already possess a remarkably ability to withstand extreme heat with little detrimental effects. (2) Several stories highlighted how these trees "lost their ability to absorb carbon". We think this misses the central message. Yes, photosynthesis was reduced an average of 40% over the 4-day heatwave; this was entirely expected and is directly predictable from the temperature dependence of photosynthesis. The much more interesting point is that after the heatwave passed, the heatwave trees and the control trees continued to photosynthesize at the same rates. That is, the heatwave had no impact on the trees "ability to absorb carbon" over the long term. We find this remarkable. Heatwaves, which are several consecutive days of extremely high temperatures, are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. How do trees respond to heatwaves? John Drake, along with 19 collaborators across 9 institutions, exposed trees to an extreme experimental heatwave of four days exceeding 43 degrees Celsius (~110 degrees F). The team expected to document the process by which trees physiologically fail and begin to die. However, the trees surprised us. They were fine! You can read the paper in Global Change Biology here. The trees coped with this extreme heatwave through two mechanisms. First, they accessed deep soil water and continued to transpire, which cooled their leaves through the process of latent heat exchange. This is similar to how sweating on a hot day helps keep humans cool. This continued transpiration cooled the leaves by an average of 3 degrees C. Secondly, the trees increased the thermotolerance of their leaves. That is, after just one day, the leaves were able to tolerate a much higher temperature before physiological failure. We are not sure what biochemical process was responsible for this increased thermotolerance, but it likely involved volatile organic C emissions, the expression of heat shock proteins, and/or alterations to the lipids in the cellular membranes. We conclude that this tree species was remarkably capable of tolerating an extreme heatwave via mechanisms that have implications for future heatwave intensity and forest resilience in a warmer world. |
Drake lab
Tree ecophysiology at SUNY-ESF Archives
October 2022
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