Here Today or Here to Stay?

ForWarn monitors seasonal duration of forest disturbance impacts

A traditional weekly ForWarn map image (left) shows the magnitude of damage from the 2015 gypsy moth outbreak in Pennsylvania based on percent changes in vegetation greenness. A 12-week Seasonal Duration map (right) shows duration of the disturbance based on the number of weekly monitoring periods in which a loss of vegetation greenness exceeded 3% during the 2015 growing season. Both maps compare vegetation greenness to that of the previous year.
A traditional weekly ForWarn map image (left) shows the magnitude of damage from the 2015 gypsy moth outbreak in Pennsylvania based on percent changes in vegetation greenness. A 12-week Seasonal Duration map (right) shows duration of the disturbance based on the number of weekly monitoring periods in which a loss of vegetation greenness exceeded 3% during the 2015 growing season. Both maps compare vegetation greenness to that of the previous year.

Some disturbances come and go, leaving forests no worse for the wear.

Hailstorms, insect defoliations, and light prescribed fires, for example, commonly occur early in the growing season, but, because of the timing and nature of these disturbances, trees and other vegetation may quickly regrow leaves after the damage is done. In such cases, even the most extreme damage diminishes by mid-summer.

Other times, disturbance damage causes longer lasting effects on forests when dieback or mortality results.

When viewed from above with coarse resolution remote sensing, such as with satellite imagery or aerial surveys, canopy impacts can appear similar whether disturbance damage is fleeting or more enduring. How can managers gauge the true impacts of forest disturbance on forest growth and productivity?

U.S. Forest Service Eastern Forest Environmental Threat Assessment Center researchers behind the satellite-based ForWarn tool have developed new Seasonal Duration map products that distinguish short-lived disturbances from lasting disturbances.

While traditional ForWarn maps are generated each week year-round to show how much vegetation greenness may have changed compared to expected conditions for a given location and day of year, seasonal disturbance impacts and recovery cannot be judged by one snapshot in space and time.

Seasonal Duration maps are generated every six weeks during the growing season and can complement weekly ForWarn maps to provide a more complete picture of the extent of the growing season impacted by disturbance.

ForWarn‘s Seasonal Duration products record the count of weekly monitoring periods that fell below a three percent drop in vegetation greenness compared to the prior year,” explains Bill Hargrove, Eastern Threat Center research ecologist and lead ForWarn researcher. “These maps provide a simple way to identify areas that have experienced long lasting change that are easily overlooked due to the low magnitude of the disturbance or persistent cloud cover during the growing season.”

Though the Seasonal Duration maps are a new innovation developed this year, they are now available for each growing season going back to 2006. ForWarn researchers plan to produce Seasonal Duration maps three times during each growing season, a frequency that can assist users with annual reporting activities.

The Seasonal Duration map products have recently been particularly useful for tracking the duration of gypsy moth damage in the northeastern Unites States — disturbance that has captured the attention of land managers, homeowners, outdoor enthusiasts, and plenty of media. Efforts to assess the impacts of the 2015 gypsy moth defoliation event spanning parts of Pennsylvania, New Jersey, and New York and the 2016 event that affected Massachusetts, Connecticut, and nearly the entire state of Rhode Island are summarized on the ForWarn website.

“Based on the lasting duration of declines in canopy greenness across a large part of the state, 2016’s gypsy moth defoliation event in southern New England was particularly severe,” says Eastern Threat Center research ecologist and ForWarn researcher Steve Norman.

The next ForWarn Seasonal Duration map will be available shortly after the last day of summer (September 21) as the 2016 growing season winds down across much of the eastern United States, and researchers look forward to the insight these new map products can provide in the future.

“For some critical measures, such as annual growth or productivity, the magnitude of change in vegetation greenness for a given day or period may not be particularly telling. Changes in forest health are best contextualized by a multi-period or seasonal perspective. ForWarn’s calculation of seasonal duration of disturbance impacts can provide that context and serve as a unique new measure for forest monitoring,” says Norman.

For more information, email Steve Norman at stevenorman@fs.fed.us.

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