Mapping Disturbances to Protect the Future of Our Forests

History and function of the HiForm tool

hiform-map-tornado-2020
This HiForm map shows where tornadoes in April 2020 caused structural damage in Covington County, Mississippi. The NDVI gradient could also indicate leaf stripping in hardwood tree species. USFS image.

Our forests are changing rapidly, and with this comes the need to both understand and track how and where this change is happening. Monitoring forest disturbances is critical for effective decision making, yet our ability to do so was largely insufficient until recently. Researchers can now track a significant amount of these changes with new technologies like HiForm.

“You can’t manage a forest well without knowledge that comes from monitoring,” explains USDA Forest Service scientist Steve Norman. “One can manage forests without monitoring, but if you did, you wouldn’t know if you were achieving your objectives. Forests are long-term assets with diverse values that extend beyond any manager’s lifetime. Monitoring is how we know if we’re on course.” Norman is a research ecologist and co-developer of the HiForm tool that efficiently maps high resolution forest change.

Forest managers’ fundamental need for monitoring led to the development of two remote sensing tools that Norman has been involved with — ForWarn II and HiForm — which use satellite imagery to capture forest disturbances like hurricanes, wildfire, and insect defoliations soon after they occur.

“A little over a decade ago, I came into remote sensing to support my research unit’s ForWarn project,” says Norman. With his background in vegetation dynamics in the East and West, he helped “ground the team in ways that made our contributions more ecologically relevant and practical.”

The development of the HiForm tool (High-resolution forest mapping) arose as a sister project of ForWarn from the Eastern Forest Environmental Threat Assessment Center.

“While ForWarn relies on twice-daily satellite imagery that gives us reliable cloud-free views, the imagery is really coarse spatially,” explains Norman. “Each grid cell is 240 meters across, or about 14 acres, and that means that a lot of crucial details are lost.”

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This HiForm map shows Lymantria dispar moth defoliation patterns east of the Allegheny Reservoir in Pennsylvania. USFS image.

To fill those gaps, HiForm relies on detailed imagery at a resolution of 10 meters. “Until recently it was difficult to efficiently analyze this extra volume of data, but advances in cloud computing changed that in a profound way,” adds Norman.

In collaboration with his colleague Bill Christie and scientists at the University of North Carolina Asheville, Norman developed a user-friendly script with Google Earth Engine to map changes in the Normalized Difference Vegetation Index (NDVI), a simple indicator that is commonly used in remote sensing for tracking vegetation vigor.

Ongoing projects using HiForm include monitoring and assessing damage resulting from severe weather, wildland fire, and insects and diseases. In June 2021, Norman and his colleagues used the tool to map gypsy moth defoliation across nine states in the Northeast. “As remote sensing gets better, we’ve seen its integration with traditional aerial sketch mapping of defoliation events in ways that improve the quality of the final product,” adds Norman.

A key component of HiForm is how it represents change with its carefully crafted color palette that makes maps more readable and useful. “We’ve tried to guide interpretation while retaining flexibility,” says Norman. “Interpretation can be arduous, and we’ve worked to make that easier.”

The challenge in creating maps using HiForm is the need to use the same color scheme “across diverse forest types, disturbances, and at different times of the year” – making the correct selection of color combinations critical. “I see our maps as art, and I sometimes feel like an artist when they have gotten the colors right in a painting. We hope this makes our work more inviting,” adds Norman.

As extreme weather events such as wildfires and hurricanes continue to increase in strength over the coming years as a result of climate change, the need for remote sensing tools such as HiForm and ForWarn will become even more critical. “The remote sensing community has made incredible progress in recent years, and we’re prepared to help meet some of the challenges that lie ahead,” explains Norman.

Explore the HiForm and ForWarn tools.

For more information, email Steve Norman at steve.norman@usda.gov.

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