Research published in Nature last week finds that “hydraulically diverse” forests are particularly resilient in the face of drought, which could help inform strategies for restoring forests after they’ve been degraded by wildfires or logging.
University of Utah biologist William Anderegg led a team of researchers who compiled data from 40 temperate and boreal forest sites around the globe that were equipped with instruments to measure the carbon, water, and energy flowing in and out of the forests in order to examine the impacts of droughts. The team combined that data with information on the tree species present at each study site and the hydraulic traits of those species to determine that forests with trees that possess a highly diverse set of traits related to water use are impacted less severely by drought.
Hydraulic traits are essentially the mechanisms by which a tree moves water through itself, which in turn help determine the degree of drought stress a tree can withstand before its water-transport system starts to shut down. These characteristics, the researchers write in the study, were found to be “the predominant significant predictors” of drought response across all of the forest sites they studied.
“By contrast, standard leaf and wood traits, such as specific leaf area and wood density, had little explanatory power,” the researchers add. “Our results demonstrate that diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and is likely to have an important role in future ecosystem-atmosphere feedback effects in a changing climate.”
Anderegg and team were not necessarily expecting to discover that the chief determining factor in how well a forest tolerates drought was its hydraulic diversity. “The species present and the hydraulic traits they have seem most important for predicting resilience to drought at an ecosystem scale,” he said in a statement. “We expected that hydraulic traits should matter, but we were surprised that other traits that a lot of the scientific community have focused on weren’t very explanatory or predictive at all.”
A diverse forest, according to Anderegg, “will have many different types of trees — conifer and angiosperm, drought tolerant and intolerant wood, and maybe different rooting depths. It’s going to involve some diversity in water source.”
Exchanges of water, carbon and energy between the land surface and atmosphere “strongly influence” the climate, the researchers note in the study. These exchanges are typically dominated by plants in most ecosystems, meaning that the physiological responses of plants under water stress can have a powerful influence on this exchange.
“Anthropogenic climate change is expected to intensify the hydrological cycle globally, leading to more frequent and more severe droughts in many regions,” Anderegg and team write. “Therefore, understanding the drivers of land-atmosphere feedback effects during drought and simulating them in Earth system models is critical for robust future projections and assessment of climate change impacts.”
The research team plans to continue along this line of research in order to more fully understand the detailed physiology behind drought resilience, Anderegg said. “What are the specific traits, either of different species or different populations, that give you resilience to future climate?”
In the meantime, Anderegg suggested that there are steps forest managers can take to improve the diversity and drought resilience of forests, especially following a traumatic disturbance of the ecosystem such as logging or wildfire.
“After we log a forest or a fire comes through, we sometimes think about planting a single species,” he said. “We should be thinking about the best mixes of multiple species for resilience.”
• Anderegg, W. R., Konings, A. G., Trugman, A. T., Yu, K., Bowling, D. R., Gabbitas, R., … & Zenes, N. (2018). Hydraulic diversity of forests regulates ecosystem resilience during drought. Nature, 1. doi:10.1038/s41586-018-0539-7
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