How “thirsty” trees may make forests more vulnerable to climate change

A new study suggests that increased maple populations may leave forests in western North Carolina more vulnerable to extreme weather conditions like flooding and drought.  

The southern Appalachian Mountains feature large, intact forests with frequent precipitation. This kind of area would not typically be a place to look for the effects of climate change, but the emergence of more “thirsty” trees like maples shifts that dynamic. Maples are an example of “diffuse-porous” trees, which require more water to grow than “ring-porous” trees like oaks.

“Due to climate change, we have seen increasing dry periods across the world and regionally. We’re having more periods where it’s just not raining,” said Katherine Martin, Center for Geospatial Analytics Faculty Fellow and co-author of a paper on the study. “When you have more of these trees that need more water, it means that when it does rain, less of that water ends up in streams and the effects of the drought are magnified.”

Using a suite of data tools, Martin and her team modeled these effects in future scenarios including both low and heavy rainfall. In both dry and wet future climate projections, the model predicted that forests with more diffuse-porous trees would have more water loss and less streamflow, as well as a decreased ability to sequester carbon through photosynthesis.

Previous models did not account for the different water needs of various tree species, Martin said. This led to a potential underestimation of the threat posed by climate change in areas with increasing diffuse-porous tree populations.

“To make these predictions, we take the projected precipitation and subtract the amount of water that we think the trees will use, which lets us estimate how much water will be in these mountain streams. What we’re seeing is that there will be less,” Martin said. “You could have done that with the old model, but you wouldn’t have had details about how different species intersect with climate change to create a larger vulnerability.”

Naturally, trees like maples have been primarily located immediately adjacent to streams or in steep, moist coves in the southern Blue Ridge Mountain due to their vulnerability to fire, which other trees like oaks are more resistant to. However, fire suppression efforts in the region combined with a long-running wet period led to an explosion in diffuse-porous trees across the landscape, which have now begun reaching maturity. This highlights the importance of considering the interaction between tree species and climate when planning land-use and conservation efforts, Martin said.

“With climate change predicted to intensify, it will become more and more important for us to consider how these different tree species will respond,” she said. “This modeling framework could provide a tool to examine these effects globally, and better inform forest managers on how to create more resilient ecosystems in the future.”

The paper, “Interactions Between Climate and Species Drive Future Forest Carbon and Water Balances,” is published in Ecohydrology. This work was supported by the Center for Geospatial Analytics at NC State, the Nature Conservancy NatureNet, the USDA National Institute of Food and Agriculture (2017-67019-26544) and the Nature Conservancy, University of Minnesota. The lead author of the paper is Katie A. McQuillan of NC State. Co-authors include A. Christopher Oishi, USDA Forest Service; Zachary J. Robbins, Los Alamos National Laboratory and NC State’s Robert Scheller.

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Note to editors: The study abstract follows.

Authors: Katie A. McQuillan, Robert Scheller and Katherine L. Martin, North Carolina State University; A. Christopher Oishi, USDA Forest Service; Zachary J. Robbins, Los Alamos National Laboratory

Published: Dec. 4, 2024

DOI: 10.1002/eco.2748

Abstract: Global change is altering forest carbon and water balances; however, the extent to which tree species shape ecosystem-scale responses to climate, particularly in biodiverse forests, remains unclear. To address this, we simulated the effects of an envelope of future climate conditions on watershed carbon and water balances and quantified the contributions of tree species based on their xylem anatomy. We accomplished this by incorporating species-level transpiration calculations into a landscape-scale ecosystem process model. Our revised model linked the effects of forest succession, species composition, and climate change on water and carbon. Calibration of forest water fluxes using sap flux measurements and catchment water balances captured variability in species transpiration and interannual ET in biodiverse, humid temperate forest catchments in the southern Blue Ridge Mountains, USA. Across wet and dry future climate projections, ET increased, and streamflow and net carbon uptake decreased, particularly under a scenario of increasing drought. Despite accounting for just 30% of current biomass, diffuse-porous tree species were the main driver of carbon and water flux responses now and in the future, thus intensifying the increase in ET and decline in streamflow. As diffuse-porous biomass continues to increase, these forests will be increasingly sensitive to drought, amplifying losses of carbon sequestration and freshwater delivery.