Satellite Mapping of Forest Disturbances Can Help When Field Efforts Are Restricted
A large number of tornadoes struck the Southeast in the spring of 2020. Ordinarily, aerial surveillance and field crews would assess forest damage after windstorms and other disturbances, but the COVID-19 pandemic has limited those operations.
USDA Forest Service scientists are showing that, with recent technological advances, disturbance impacts can nonetheless be rapidly mapped at surprisingly high resolution and shared with eastern forest managers.
“After a tornado passes through a forest, managers need a way to assess impacts as soon as possible. Detailed disturbance maps are crucial, because they can direct forest managers to heavily impacted areas quickly,” says Steve Norman, a research ecologist with the Southern Research Station. “These maps also give managers insights into the dynamics of their forests for longer-term monitoring and planning.”
COVID-19 restrictions have increased the need for reliable alternatives to aircraft and ground-based assessments. Remote sensing from satellite imagery is helping to meet this need.
While satellite imagery has been successfully used for forest disturbance monitoring for decades, past technology has usually been limited by coarse resolution, infrequent satellite flyovers, high costs, or the need for specialized analysts.
Researchers at the Eastern Forest Environmental Threat Assessment Center (EFETAC) developed a workflow using newly available satellites and cloud computing to reveal likely forest impacts across large areas at high resolution.
The result has been the High-Resolution Forest Mapping, or HiForm, project – a cooperative effort between EFETAC scientists and the National Environmental Monitoring and Analysis Center (NEMAC) at the University of North Carolina, Asheville.
HiForm maps show changes in a vegetation-sensitive measure called the Normalized Difference Vegetation Index, or NDVI. The team calculates seasonal NDVI change at 10-meter resolution from the European Space Agency’s Sentinel-2 satellite sensors, which produce new imagery every several days for any location.
These changes reveal not only where disturbances have occurred, but also the likely impacts to different types of vegetation.
Mapping at high resolution and at frequent intervals is important in the eastern US, because forests are composed of a large variety of tree species and ages that respond to disturbances in different ways.
Strong tornadoes often uproot trees and damage canopies. This structural damage is relatively easy to gauge in upland pine forests, but defoliation without structural damage is common in bottomland hardwood forests. This new monitoring approach filters impacts by vegetation type to improve reliability. The team reexamines impact areas later in the season to understand longer-lasting, structural damage.
The maps have been well received by state and federal forest personnel across the South, and especially after recent tornadoes. “It’s mind-boggling what you guys are enabling us to accomplish now from the safety of our offices,” says Brian Mitchell, GIS Program Director for the Mississippi Forestry Commission. “We shared your HiForm product with field personnel who found a near perfect match with their more limited observations of tornado damage.”
HiForm can be explored on the website and available for free, upon request for download. The researcher team continues to experiment with new ways to communicate likely impacts and provide usable tools to those who need them in a timely fashion.
For more information, email Steve Norman at steve.norman@usda.gov or Bill Christie at william.m.christie@usda.gov.