By Ellen H. Brisendine
| Thanks to
satellite imagery and the patience of a group of dedicated scientists,
there may be a tool available to landowners who have dealt with wildfire
that can help them develop strategies to assist and monitor recovery. Diana Doan-Crider, an adjunct faculty member at the Texas A&M Department of Ecosystem Science and Management, collaborated with her peers at Texas A&M AgriLife Research, Jay Angerer and Ed Rhodes, to test the accuracy of “differenced Normalized Burn Ratio,” or dNBR, maps. The team hiked through some of the roughest West Texas terrain that had been burned in the wildfires of 2011 and 2012 — the Rock House and Tejano Canyon fires in the Davis Mountains, and the Livermore Ranch and Spring Mountain fires. Together, these fires burned more than 340,000 acres, eliminating critical forage that ranchers were hoping would last them through a historic drought. Due to high fuel buildup from an earlier hurricane, dry weather, and high spring winds, these fires proved to be catastrophic to many landowners and their livestock. The team wanted to develop models that would help predict the recovery of key wildlife foods and livestock forage after wildfire, and dNBR maps seemed to provide a baseline. When Doan-Crider showed one of the preliminary dNBR maps to a local West Texas rancher whose ranch had burned, he said, “I feel like you just showed me an x-ray of what really happened to my land.” It was then that she realized that these maps might be useful as an outreach tool to help landowners cope with such a life- and landscape-changing event. Doan-Crider said, “The initial shock of seeing your property converted into a blackened landscape would fool anyone, including myself, into believing that all hope is lost, especially in the middle of such a serious drought. However, burn severity maps can help landowners develop a plan of attack for recovery.” Burn severity Burn severity is an index that measures fire’s impact on the soil and vegetation. Wildfire often burns unevenly across a landscape, and can have varying levels of “severity” throughout a burned area. Accordingly, post-fire management needs to be adapted. Some patches may require more time and effort, and different approaches, to recover. The team’s objective was to compare before-and-after satellite images of the burned areas to determine 4 levels of burn severity: severe, moderate, low, and unburned. They used a special index to address variables like scorching, soil color, tree and vegetation mortality, and the replacement of previously existing vegetation communities by invasive species to “validate” what the satellite images reflected. Validation is necessary for areas that have not been previously mapped to determine how close the satellite imagery information is relative to the actual ground conditions. Once validation has been sufficiently conducted for a particular area, then the maps can be adjusted accordingly and become fairly reliable without as much fieldwork. “The use of Landsat satellite-derived burn severity maps for wildfires on large landscapes is common in the western U.S.,” Doan-Crider wrote in her paper on the topic, particularly on public lands where large areas are difficult to access. “This information can then be used to target areas in need of attention for erosion control or restoration, and can also be used as a layer for other GIS-based (geographic information system) research and restoration monitoring efforts.” Doan-Crider said the burn severity products are produced by USDA Forest Service Remote Sensing Applications Center, Salt Lake City. “Their products are commonly used by most federal fire-management agencies, but land conservation agencies such as the NRCS (USDA Natural Resources Conservation Service) have just recently started considering the usefulness of these products for post-fire assessment and recovery efforts on private lands.” Maps compare before- and after-fire vegetative cover  Doan-Crider briefly explained the process. “Satellite imagery picks up visible and infrared light bands of soil and vegetation.” By comparing before- and after-fire satellite images, the dNBR maps “show you the areas of the greatest change based on the difference between those values. If you have green vegetation before the fire and then you have no green vegetation after the fire, then that’s going to show up as severely burned (we picked the color red as the key for that category on our maps).” Areas of lower burn severity will show up with less of a change (we used lighter colors such as light blue and light green for the map key).” Typically, dNBR maps are field validated after the first growing season. However, Doan-Crider and colleagues had to wait for a year and a half until it rained. Even then, the maps were 83 percent accurate, which is considered to be very good. The dNBR maps might be most useful to large-acreage landowners. Doan-Crider said, “When you’re dealing with sections of land, and everything appears to be burned, it’s really difficult to evaluate the overall impacts to the landscape. For example, you could easily have a large patch of severely burned soil and vegetation right in the center of an area that was only slightly burned. Those spots can be difficult to see with the naked eye, may be vulnerable to erosion or to the spread of invasive species.” This targeted management will help landowners and land management agencies make sound decisions when it comes to recovery. For example, identifying areas of moderate-to-high burn severity may help managers identify sites vulnerable to erosion, and that might pose threats to fresh water sources important to cattle and wildlife. For post-fire restoration efforts, the maps can help managers make better decisions about reseeding, given that the introduction of seeds on moderate to severely burned soils too soon after a fire may result in more problems than benefits. “In many cases,” Doan-Crider said, “it might be best to allow soils and their microbial communities some time to recover. You might be surprised, but grasses and other vegetation are very well adapted to recovering on their own after fire over time, if the conditions are right. Sometimes they need some help, but many times they don’t.” In addition, many agencies under pressure to “do something” report high rates of failure for reseeding right after a fire, particularly for moderate to severely burned soils. This can be expensive and possibly bring in unwanted invasive species that can spread to other areas. The dNBR maps, in the hands of trained resource management professionals, can help landowners weigh all of those factors and make prudent decisions. Doan-Crider said that other uses for the maps should also be evaluated. “There is no reason these maps can’t be used to determine new fencing boundaries for grazing deferment, or even for determining the level of disaster funding received by individuals who have experienced different levels of burn severity. Some areas may require more time to recover, and landowners may not be able to graze those areas anytime soon. If models can be developed to correlate burn severity with recovery time and conditions, maybe funding can be calibrated accordingly,” Doan-Crider explained. In her report to the NRCS, she wrote, “The potential for these maps to be used for landowner outreach after wildfires has been largely unexplored and warrants further attention. While burn severity maps are critical in identifying areas that need immediate attention because of erosion or danger to sensitive resources, we believe they offer several other benefits for agency Extension and outreach.” Agencies in Texas, such as the Texas A&M Forest Service and Texas Parks and Wildlife Department, have received some training in the application of these maps. Doan-Crider is working with NRCS to help make dNBR maps available to landowners; however, this is a relatively new technology for the private sector and there remains a learning curve to distribute and apply the information. For more information about burn severity maps, you can contact Dr. Diana Doan-Crider at d-crider@tamu.edu. -TC |
No comments:
Post a Comment