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Asparagus affected by drought. Credit: William Anderegg
Diversity is the strength, even among the forests. In an article published in NatureResearchers led by William Anderegg, a biologist at the University of Utah, report that forests with trees that use a wide variety of water-related traits suffer less from drought. The results, which build on previous work on the resilience of individual tree species based on water features, lead to new directions on forest resilience and inform forest managers working on forest reconstruction. after logging or fires.
Surprisingly, says Anderegg, the hydraulic diversity of a forest is the predominant predictor of its ability to manage a drought.
"We expected hydraulic features to be important," he says, "but we were surprised that other features that the scientific community focused on were not very explanatory or predictable."
Missing the forest for trees
Anderegg is a seasoned researcher on the impacts of drought on trees, paying particular attention to the time needed for forests to recover from drought. With others in his field, he also examined the impact of hydraulic features on the chances of survival of different tree species in times of drought. Hydraulic characteristics are related to how a tree moves water in the body and the amount of water stress it can experience before the system begins to degrade.
But droughts, when they hit, do not go after individual trees – they affect entire ecosystems.
"What's different in this study is that she's now looking at the whole forest," said Anderegg.
Droughts can not touch that
Anderegg and colleagues, including collaborators from Stanford University, Princeton University, and the University of California at Davis, compiled data from 40 forest sites around the world. . The sites are equipped with instruments called flow towers that measure the carbon, water and energy flows of a forest. They are also equipped with environmental sensors, including soil moisture sensors, to get a picture of the amount of water coming in and out of the site.
They combined this data with known tree species at each site and the known hydraulic characteristics associated with these species. Non-hydraulic features would be items such as wood density or leaf area divided by leaf mass. But the hydraulic features include the hydraulic safety margin, the difference between the amount of movement of water that the tree allows during dry conditions and the absolute minimum amount of water – the point where the Hydraulic starts to stop.
Asparagus affected by drought. Credit: William Anderegg
Forests with a greater diversity of water features in their tree species showed less decline in forest function (as measured by water and energy fluxes and soil moisture) than less diverse forests. Satellite data from temperate forests around the world have confirmed their findings – droughts do not have the same effect on hydraulically diverse forests as on others.
"The species present and the hydraulic features that they seem to have the most importance for predicting drought resilience at the scale of the ecosystem," says Anderegg.
So what does a forest with a hydraulic diversity look like? First, consider the opposite – an ecosystem with only one type of tree. Image, for example, a Christmas tree farm. Each tree is exactly the same species. Diversity is not inferior to that.
But a diverse forest, says Anderegg, "will have many types of trees: conifers and angiosperms, drought tolerant and intolerant woods, and perhaps different depths of rooting." The diversity of water sources will be difficult. measure directly. "
The team sees several avenues for the future to pursue this research.
"We want to understand the detailed physiology of this resilience," says Anderegg. "What are the specific traits of different species or populations that give you resilience to the future climate?"
"Supercharged fire time"
The researchers did not specifically address the link between hydraulic features, drought and fire conditions, but a summer in the western United States caught fire and the resulting smoke spread across the country asks the question.
"More diversity in a landscape will help a forest to be more fire resistant," says Anderegg. The same underlying drought conditions, such as early snowmelt and warm summer temperatures, for example, are also the basis of hazardous fire seasons.
"It dries the fuel in the park," says Anderegg, "and creates a supercharged fire time."
So what can forest managers do to improve diversity and resilience? Opportunities can result from trauma to the ecosystem such as logging or forest fires. "After we have connected a forest or a fire passes through," says Anderegg, "we sometimes think of planting a single species, we should think of the best mixtures of several species for resilience."
Explore more:
Study identifies features of trees that may contribute to vulnerability to drought
More information:
William R. L. Anderegg et al, Forest Water Diversity Regulates Ecosystem Resilience During Drought, Nature (2018). DOI: 10.1038 / s41586-018-0539-7
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