Questioning the cause of calamity: using remotely sensed data to assess successive fire events

Bureau of Land Management policy regarding wildfire events on public rangelands dictates that burned areas are closed to livestock grazing until the vegetation in the burned area has reestablished itself. Ranchers and their supporters contend that extended duration of such grazing closures increases the likelihood of subsequent fire events during the grazing rest period. The ranchers attribute this effect to an over-accumulation of vegetation during the grazing rest period. With the goal of testing the claim made by ranchers, this project utilized fire history records, grazing allotment data, and remote sensing vegetation indices to identify and analyze potential rest period fires between 2000 and 2016 in and around the Nevada counties of Humboldt and Elko. GIS proximity tools were used to identify initial and subsequent fires on BLM grazing allotments which met the spatial and temporal requirements of a rest period fire. The four most likely candidates for rest period fires were selected for further examination as case studies. Scaled NDVI was used as an estimator of vegetation cover and change between selected initial and subsequent fires. Precipitation and land cover data were incorporated to provide further context. Three of the four fire perimeters showed increased vegetation cover when compared to similar nearby unburned sites during the second spring after the initial fires. This pattern suggests that increased fuel loads before the secondary fire may have been present. Evidence of cheatgrass and anthropogenic fire activity in the case study area suggest more complex explanations. Ways to improve monitoring and post-fire recovery through better record keeping, more complex sensors for satellite imagery, and targeted grazing research are discussed.

美国土地管理局（Bureau of Land Management, BLM）针对公共牧场野火事件制定的政策规定，火烧迹地需禁止牲畜放牧，直至迹地内植被完成自然恢复。牧场主及其支持者主张，此类放牧封禁的持续时长越长，封禁期内发生后续野火的概率就越高；他们将该效应归因于封禁期内植被的过度累积。为验证牧场主的这一论断，本项目依托火灾历史记录、放牧分区数据与遥感植被指数，识别并分析了2000年至2016年间内华达州洪堡县与埃尔科县及其周边区域的潜在封禁期野火。研究借助地理信息系统（Geographic Information System, GIS）邻近分析工具，筛选出符合封禁期野火时空条件的BLM放牧分区上的初始火灾与后续火灾案例，最终选取4个最具代表性的封禁期野火作为案例研究对象。本研究采用缩放后归一化差值植被指数（Scaled NDVI）作为植被覆盖度及其动态变化的估算指标，用于分析所选初始火灾与后续火灾间的植被差异；同时引入降水与土地覆盖数据以补充研究背景。对比初始火灾后第二个春季的植被状况，4处火边界中有3处与附近未受火烧的对照地块相比，植被覆盖度出现提升，这一结果暗示二次火灾发生前可能存在燃料载荷增加的情况。但案例研究区域内旱雀麦（cheatgrass）与人为野火活动的相关证据，则指向更为复杂的作用机制。本研究最后讨论了通过完善记录留存、升级卫星影像传感器以及开展针对性放牧研究，来优化野火监测与灾后恢复工作的可行路径。