UAV remote sensing of plant-soil-nutrient dynamics in burned rangeland

Abstract:

Rangelands cover 70% of the world’s land surface, and provide critical ecosystem services of primary production, soil carbon storage, and nutrient cycling. Rangelands are very susceptible to wildfire and post-fire soil erosion. Prescribed burning is also a commonly used rangeland management tool. Worldwide, an estimated 423 Mha of grass and shrublands are burned each year. The effects of fire on ecosystem services in rangelands are governed by very fine-scale spatial patterning of soil carbon, nutrients, and plant species at the centimeter-to-meter scales, a phenomenon known as “islands of fertility”. While remote sensing would appear an optimal tool for monitoring fire effects in rangelands, such fine-scale dynamics can’t be detected with most satellite and manned airborne platforms. Remote sensing from unmanned aerial vehicles (UAVs) provides an alternative option for detecting fine-scale soil nutrient and plant species changes in rangelands. We demonstrate that the fusion of UAV multispectral and structure-from-motion photogrammetry accurately classifies plant functional types and bare soil cover with an overall accuracy of 95% in a desert grassland degraded by shrub encroachment and disturbed by fire at the Sevilleta National Wildlife Refuge in New Mexico USA. We further demonstrate that UAV hyperspectral and LiDAR fusion greatly improve upon these results by accurately classifying 9 different plant species and soil fertility microsite types with an overall accuracy of 87%. Among them, creosote bush and black grama, the most important native species in the desert rangeland, have the highest producer’s accuracies at 98% and 94%, respectively. The integration of LiDAR-derived plant height differences was critical in these improvements. Finally, we use synthesis of the UAV datasets with ground-based LiDAR surveys and lab characterization of soils to estimate that the burned rangeland potentially lost 1,474 kg/ha of C and 113 kg/ha of N owing to soil erosion processes during the first year after a prescribed fire. However, during the second-year post-fire, grass patches and plant-interspaces functioned as net sinks for sediment and nutrients and gained approximately 175 kg/ha C and 14 kg/ha N, combined. While fire and subsequent erosion can degrade some rangelands, we show that post-fire plant-soil-nutrient dynamics might provide a competitive advantage to grasses in rangelands degraded by shrub encroachment. These novel UAV and ground-based LiDAR remote sensing approaches are thus an important contribution towards more accurate accounting of the effects of fires on soil erosion, carbon, and nutrients in rangelands.