Prefire vegetation structure of high severity wildfires in nonherbaceous-dominated rangelands in the western United States

Authors: LI, ZHENG - Texas A&M University; Angerer, Jay; WU, X. BEN - Texas A&M University

Submitted to: Earth's Future
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2022
Publication Date: 9/19/2022
Citation: Li, Z., Angerer, J.P., Wu, X. 2022. Prefire vegetation structure of high severity wildfires in nonherbaceous-dominated rangelands in the western United States. Earth's Future. 10(10). Article e2021EF002624. https://doi.org/10.1029/2021EF002624.
DOI: https://doi.org/10.1029/2021EF002624

Interpretive Summary: In the last 30 years, the land area burned by large wildfires has increased in the western United States, with many of these fires being classified as high severity. These high severity fires can impact trees, shrubs, and grasses, which in turn, can affect water quality, carbon storage, the atmosphere. In addition, high severity fires can affect human safety and infrastructure. In order to better understand the vegetation conditions that resulted in historical high severity fires, a study was conducted using historical remote sensing data that identified high severity fires (Monitoring Trends in Burn Severity), as well as a new product derived from remote sensing data that identifies the amount of plant cover on rangelands over time (Rangeland Analysis Platform Cover 2.0). Using these datasets, statistics were used to identify groups of fires having similar rangeland cover characteristics. Nine groups were identified. Five of these groups accounted for 76% of the total land area having high severity wildfires. These 5 groups were characterized as having high tree and shrub cover (approximately 50%) prior to the fires. Another two of these 9 groups, accounted for 13% of the land area with high severity wildfires, and had high cover of perennial forbs and grasses (40%) and moderately high tree and shrub cover (25%) prior to the wildfires. Management strategies, such as prescribed burning, that could decrease tree and shrub cover, reduce standing dead vegetation, and break up continuous fuel loads could be effective in reducing the risk and extent of high severity wildfires on rangelands in the western US.

Technical Abstract: In the last three decades, the increase of large wildfires has led to the rise in rangeland area burned across the western United States. High severity rangeland wildfires have enormous negative impacts on human safety and infrastructure, as well as long-term consequences on ecosystem structure, function and services, across the vast rangeland areas in the western US. Understanding of the patterns of prefire vegetation structure where high severity wildfires occurred can help identify vegetation conditions that are conducive to high severity burns and inform policies and rangeland management to reduce the impact of high severity wildfires. This study explored prefire vegetation structure of all high severity wildfires in western United States rangelands from 1985 to 2018 recorded by the Monitoring Trends in Burn Severity (MTBS) project, based on the fractional cover data for plant functional types identified from the Rangeland Analysis Platform (RAP). These high severity rangeland wildfires were first clustered using Expectation Maximization (EM) algorithms based on the prefire per-pixel rangeland fractional cover, which included annual forbs and grasses (AFGC), perennial forbs and grasses (PFGC), bare ground (BG), shrubs (SHR), trees (TREE) and litter (LTR). Patterns of prefire rangeland cover composition and the major vegetation types where high burn severity wildfires occurred for these clusters were then assessed. Nine clusters were identified and dominant patterns in the prefire vegetation structure of high severity rangeland wildfires emerged. A group of five clusters, accounting for 76% of the areas with high severity wildfires, were characterized by high (~50%) woody cover in prefire vegetation structure, which when accompanied by abundant fine fuel, high structural connectivity, and/or highly flammable materials, could facilitate the ignition of the woody crowns leading to high severity wildfires. Another group of two clusters, accounting for 13% of the areas with high severity wildfires, were characterized by high cover of perennial forbs and grasses (~ 40%) and moderately high woody cover (~25%). Within the same ecosystems, significant differences in prefire vegetation structure existed between areas with high severity burns and the areas with low or moderate severity burns, with the former having higher woody cover and lower bare ground cover than the latter in all nine clusters. Management strategies (such as prescribed burning) that can decrease woody cover, reduce standing dead vegetation and fuel laddering, and lower the continuity of fuels could be effective in reducing the risk and extent of high severity wildfires on rangelands in the western US.