Pre‐fire vegetation drives post‐fire outcomes in sagebrush ecosystems: evidence from field and remote sensing data

Abstract

Understanding the factors that influence vegetation responses to disturbance is important because vegetation is the foundation of food resources, wildlife habitat, and ecosystem properties and processes. We integrated vegetation cover data derived from field plots and remotely sensed Landsat images in two focal areas over a 37-yr period to investigate how historical changes to community composition influence contemporary responses of vegetation to fire in sagebrush ecosystems in the Great Basin. Our objectives were (1) to quantify the magnitude and direction of change in the cover of native and exotic plant functional groups in relation to their exposure to fire; (2) to relate plant community changes to their historical composition, exposure to fire, and environmental conditions; and (3) to test for consistency of trends revealed by vegetation cover data derived from field plots and Landsat images. Historical (1979Historical ( -1981 field data originated from 298 locations, Landsat-derived data and contemporary (2011-2016) field data originated from 448 locations, and an expanded set of locations were included in some analyses of Landsat-derived data. We found that areas burned by fire since the 1980s had higher annual herbaceous cover than unburned areas both historically and contemporarily. Models revealed a significant interaction between historical community composition and exposure to fire, which suggests that plots with historically high herbaceous cover were more susceptible to burning. Trends revealed by field and Landsat-derived cover data were only partially consistent, potentially due in part to methods used to predict cover values from Landsat images, and the time period over which each data set was collected. Our results suggest that burned areas historically occupied by sagebrush-dominated plant communities may have been invaded by exotic annuals prior to burning, possibly because of prior land uses, and after burning, have now transitioned to a persistent herbaceous-dominated state. This type of state transition has important consequences for forage quality, wildlife habitat, soil nutrients, and future disturbances, such as drought and wildfire.

Rapid Monitoring of the Abundance and Spread of Exotic Annual Grasses in the Western United States Using Remote Sensing and Machine Learning

“…Exotic annual grass invasions rapidly altered disturbance characteristics, as has been observed by others (e.g., Bradley et al, 2018) and documented here as a fourfold increase in wildfire occurrence probabilities when exotic annual cover was greater than 10%, which can trigger abrupt shifts in ecosystem states and create novel environments that are maintained by new sets of legacies and reinforcing feedbacks. While ecosystem trajectories varied widely after wildfires, mainly due to vegetation composition and other site conditions (Figure 5; Barker et al, 2019), we found that enhanced rates of exotic annual growth generally occurred until about 20 years after fire and then increased at a decreasing rate thereafter. Individual fires events (on average) resulted in an 5% increase in exotic annual cover over a 40-year period, underscoring the role wildfires have on perpetuating the annual grass-fire cycle.…”

Harvester ant seed removal in an invaded sagebrush ecosystem: Implications for restoration

“…Non‐native cheatgrass ( Bromus tectorum ) has steadily invaded sagebrush ecosystems for decades and, as a fire‐prone species, creates a cheatgrass–wildfire cycle. In this cycle, degraded sagebrush systems are susceptible to invading cheatgrass, which increases wildfire frequency, further facilitating cheatgrass establishment; this process often leads to human interventions in the form of extensive and expensive restoration efforts (Barker et al., 2019; Brooks et al., 2004; Chambers et al., 2014). Sagebrush restoration techniques include protecting native plant communities, preventing non‐native invasion, and restoring degraded areas (Chambers et al., 2014).…”

Application of Empirical Land-Cover Changes to Construct Climate Change Scenarios in Federally Managed Lands

“…Federal land managers must also understand the impact of allowable use (i.e., grazing policy) and budgets in the development of long-term management plans. The impacts of abrupt disturbances such as fire, industrial development, and vegetation treatments on rangeland condition have been relatively well documented [4,12]. However, spatially explicit data of projected climate impacts is often lacking, constraining the prioritization of areas for climate adaptation practices [13].…”