Repeated fire altered succession and increased fire behavior in basin big sagebrush–native perennial grasslands

Abstract

The structure and composition of sagebrush‐dominated ecosystems have been altered by changes in fire regimes, land use, invasive species, and climate change. This often decreases resilience to disturbance and degrades critical habitat for species of conservation concern. Basin big sagebrush (Artemisia tridentata ssp. tridentata) ecosystems, in particular, are greatly reduced in distribution as land has been converted to agriculture and other land uses. The fire regime, relative proportions of shrub and grassland patches, and the effects of repeated burns in this ecosystem are poorly understood. We quantified postfire patterns of vegetation accumulation and modeled potential fire behavior on sites that were burned and first measured in the late 1980s at John Day Fossil Beds National Monument, Oregon, USA. The area partially reburned 11 yr after the initial fire, allowing a comparison of one vs. two fires. Repeated burns shifted composition from shrub‐dominated to prolonged native herbaceous dominance. Fifteen years following one fire, the native‐dominated herbaceous component was 44% and live shrubs were 39% of total aboveground biomass. Aboveground biomass of twice‐burned sites (2xB; burned 26 and 15 yr prior) was 71% herbaceous and 12% shrub. Twenty‐six years after fire, total aboveground biomass was 113–209% of preburn levels, suggesting a fire‐return interval of 15–25 yr. Frequency and density of Pseudoroegneria spicata and Festuca idahoensis were not modified by fire history, but Poa secunda was reduced by repeated fire, occurring in 84% of plots burned 26 yr prior, 72% of plots burned 15 yr prior, and 49% in 2xB plots. Nonnative annual Bromus tectorum occurred at a frequency of 74%, but at low density with no differences due to fire history. Altered vegetation structure modified fire behavior, with modeled rates of fire spread in 2xB sites double that of once‐burned sites. This suggests that these systems likely were historically composed of a mosaic of shrub and grassland. However, contemporary increases in fire frequency will likely create positive feedbacks of more intense fire behavior and prolonged periods of early‐successional vegetation in basin big sagebrush communities.

Post-Fire Vegetation Response in a Repeatedly Burned Low-Elevation Sagebrush Steppe Protected Area Provides Insights About Resilience and Invasion Resistance

“…Homogenization across ecological sites within protected areas from repeated and widespread disturbances is particularly significant because it further reduces site resilience, including spatial resilience (sensu Chambers et al, 2019), and site invasion resistance as fewer contiguous stands of perennial vegetation remain after each successive disturbance event. We observed a substantial increase in the two invasive annual grasses over time following fire, and no apparent response to fire by the two native perennial bunchgrasses, confirming the anticipated fire resiliency of all four of these species summarized by Miller et al (2013) and previously demonstrated for the perennial bunchgrasses within our study area by Reed-Dustin et al (2016) and Ellsworth et al (2020). However, the two annuals also increased with burn severity, most notably medusahead.…”

“…This diagram highlights three fundamental states with progressively steep threshold transitions into ecologically-novel conditions dominated by invasive annual grasses, from state 1 to state 2 and from state 2 to state 3. Note that other phases within these states dominated by big sagebrush and western juniper are possible (NPS, 2012;Chambers et al, 2014c;Ellsworth et al, 2020) but were not observed during our study due to past wildfires.…”

“…A second reason for low variation in grass cover among ecological sites over time in our study area was the shared attributes of soil temperature and moisture regime across all sites and only subtle differences in soil texture, historical and degraded plant community composition, and other site attributes described by NRCS for the ecological sites in the study area. Similarly, a third reason for low variation among ecological sites was the homogenizing effect (sensu Foster et al, 2013) of the 2011 fire, with likely cumulative influences from past fires and grazing (Chambers and Wisdom, 2009;Davies et al, 2012;Bernards and Morris, 2016;Mahood and Balch, 2019;Wood et al, 2019;Ellsworth et al, 2020). This homogenization has occurred throughout the study area across ecological sites, as community composition of all ecological sites converged toward an annual grass state.…”

“…Like all protected areas, those in low-elevation sagebrush steppe are fixed in geographic space even as the ecological gradient space within which they occur (e.g., temperature and precipitation, fire regime) shifts over time toward novel conditions with less native biodiversity and lower conservation value. For example, National Park Service sites containing sagebrush steppe across the Columbia Basin and Snake River Plain have all experienced substantial fire-induced shifts toward novel annual grasslands in recent decades (Bangert and Huntly, 2010;Ellsworth and Kauffman, 2010;Powell et al, 2013;Rodhouse et al, 2014;Reed-Dustin et al, 2016;Esposito et al, 2019;Ellsworth et al, 2020;Nicolli et al, 2020), yet each of these parks are also charged with preserving native steppe as established by the 1916 National Park Service Organic Act. In order to meet this charge, these parks and others faced with the same kinds of challenges to protected-area relevancy need to develop granular, place-based ecological insights that are useful for park-scale preservation and restoration planning and contextualized within the resilience and resistance framework.…”

“…The inferences supported by our statistical survey design and our models provide insights about resilience and resistance that will support a more strategic approach to park steppe conservation than what has been pursued previously, specifically revealing the importance of topography in our study area as a strong mediator of resilience and resistance which can provide a conservation foothold. More generally, our approach demonstrates the importance of using empirical data and both conceptual and statistical models in FIGURE 2 | A simplified state-transition diagram for the aridic low-elevation sagebrush steppe ecological sites within the Clarno unit of the John Day Fossil Beds National Monument, modified from Natural Resources Conservation Service ecological site descriptions (NPS, 2012), Chambers et al (2014c), and Ellsworth et al (2020). This diagram highlights three fundamental states with progressively steep threshold transitions into ecologically-novel conditions dominated by invasive annual grasses, from state 1 to state 2 and from state 2 to state 3.…”