Blesner Creek runs near Superior National Forest in Duluth, Minnesota. Superior National Forest is one of 154 national forests in the U.S. Photo by Tony Webster from Minneapolis, Minnesota, United States, CC BY-SA 2.0, via Wikimedia Commons.By Emily Case-Buskirk, Communications Specialist
With the diverse array of plant life in forests, tracking drought can be a challenge. A new tool developed by the National Drought Mitigation Center (NDMC), the Forest Drought Response Index (ForDRI), aims to address the intricacies of drought within forest ecosystems.
ForDRI provides a way to monitor forest drought in the continental U.S. This research was funded by a three-year cooperative agreement with the U.S. Department of Agriculture (USDA) Office of the Chief Economist. ForDRI was developed in collaboration with the USDA, U.S. Forest Service (USFS), and Center for Advanced Land Management Information Technologies (CALMIT) at the University of Nebraska-Lincoln.
The composite index will be helpful to fire managers, producers, climatologists and stakeholders, said Tsegaye Tadesse, NDMC geospatial coordinator and ForDRI lead researcher.
“Even though forest drought status is a key control on the carbon cycle, very few indices exist to monitor and predict drought in forests,” he said. “ForDRI combines multiple complementary data inputs to help identify forest drought stress.”
There are 154 national forests in the U.S., according to the USFS. Many of these forests are composed of different tree species with varying levels of drought tolerance, and droughts in these areas tend to be more intermittent than the seasonal or multi-year droughts experienced in the western U.S.
Drought scientists use climate- and satellite-based data to depict drought conditions and impacts on vegetation. However, the variety within forest ecosystems makes it difficult to monitor forest drought only with climatic or satellite data.
The ForDRI map from Oct. 23, 2023, shows forest drought conditions across the continental U.S. ForDRI was developed by the National Drought Mitigation Center.
ForDRI works by combining 12 data types including satellite, climate, evaporative demand, groundwater, and soil moisture. The model uses Principal Components Analysis (PCA), which assigns a weight to each variable based on historical data and how the variables change together over time.
The ForDRI maps show drought conditions only on the forest cover areas determined by the national forest type dataset. This dataset was produced by the USFS Forest Inventory and Analysis program and the Remote Sensing Applications Center.
To evaluate ForDRI’s efficacy, NDMC selected drought years depicted by the ForDRI weekly maps since 2003 and compared them to the U.S. Drought Monitor maps. In that period, ForDRI effectively identified extreme (D3) or exceptional (D4) drought in numerous research forests.
NDMC also found significant correlations between ForDRI values and biophysical indicators, including normalized Bowen ratio data and tree ring data, for areas of intense drought. The Bowen ratio is the ratio of heat flux to moisture flux near the surface, according to the Encyclopedia of Atmospheric Sciences.
This indicates ForDRI’s ability to predict the likelihood of extreme or exceptional drought before or early in growing seasons, Tadesse said.
“A multiyear pattern of drought stress is clearly visible in ForDRI and the normalized Bowen ratio at a number of sites, and critically, in all those that reached D3 or D4,” he said. “This is an important result as it implies that serious forest drought, the kind that we are most concerned about, takes a long time to develop.”
NDMC is continuing to evaluate ForDRI and associated products by collecting feedback from experts and users to improve the model. The ForDRI model will be updated with any new inputs that would improve the maps.
View the ForDRI maps and learn more at https://fordri.unl.edu/Home.aspx.