Forests and a future of drought: Area exposed to severe drought projected to triple by 2070
By 2070, drought is likely to affect three times the forest area as it has in recent decades. This is based on projections from a new USDA Forest Service study that shows where in the conterminous U.S. drought exposure is set to increase and which forest types could be most affected.
The study, led by Jennifer Costanza, combined the Standardized Precipitation-Evapotranspiration Index (SPEI) with 10 different climate models. They used this to compare drought in a recent period, 1991–2020, to projected drought for a mid-century period, 2041–2070.
Among eastern forest types, those in the oak-hickory group have the greatest potential for increased exposure to prolonged drought in the mid-century period.
According to Costanza, “Other studies have looked at drought regionally, particularly in the West, but this was the first to look at the entire conterminous U.S. It is also the first to focus on forest land by linking the SPEI climate data with forest types from Forest Inventory and Analysis (FIA) plots.
There were some startling results. For example, 11% of forested land was exposed to a three-year drought in the recent period. For the mid-century period, that area is projected to increase to 38%. That’s a 238% increase in forested land area that will undergo a three-year drought based on the average of all 10 climate models. The models do not agree on exact numbers, but all 10 forecast greater exposure to prolonged drought in the mid-century period.
Much of this increased drought exposure will occur in the West, especially the Southwest and Great Basin. Pinyon-juniper forests, already affected by drought, are projected to see the greatest increases in drought exposure.
Some forest types show a broad range of projected drought exposures. The aspen-birch forest type group shows high exposure for most of its range, but a low exposure in the Northeast. On the other hand, forest types restricted to coastal regions of the Pacific Northwest show a low exposure to drought in the future.
In the East, several models show a substantial increase in drought exposure. Among eastern forest types, those in the oak-hickory group show the greatest potential for increased drought exposure. Oaks have traditionally been considered drought-tolerant, but this resistance may not be as effective over longer periods and as the atmosphere warms and dries.
But from research comes hope. “There is mounting evidence that forest management can help mitigate the effects of drought,” says Costanza. “In some forests a lower basal area may reduce mortality, and in others, it’s known that older trees may be more sensitive to prolonged drought.”
Managing forests for more open stands by thinning, planting at lower densities, and frequent prescribed burning may be an effective strategy for some Southeastern forests, according to a 2019 Forest Service report on drought and management strategies for it.
However, to find the most effective strategies for each region and forest type, it is important to know which are likely to undergo the most change. This study gives researchers and managers a way to find those regions and forest types and will help in their efforts to adapt forests to a changing climate.
To support the most recent Resource Planning Act Assessment chapter on Disturbances to Forests and Rangelands, Costanza and the team have provided a more thorough look at drought exposure for the next 50 years. “This project was a starting point linking FIA data to future drought trends,” says Costanza.
Moving forward, Costanza and the team plan to use these drought models to look at specific impacts on the nation’s forests for the mid-century period. This is important because the increased drought exposure combined with other effects of a drying or warming climate such as wildfire and insect activity could have devastating effects. Among other things, the changing forests could have reduced biodiversity and carbon storage as well as a reduced capacity to provide clean water.
For more information, email Jennifer Costanza at jennifer.costanza@usda.gov.