An understanding of the long-term vegetation structure, patterns of fuel succession, and potential for reburn in sagebrush-dominated ecosystems is important for managing the landscape at a temporal scale that is appropriate for the ecological interactions in these systems. Our overarching research objective was to fill existing knowledge gaps about long term fire effects by 1) remeasuring a suite of long-term post-fire studies, 2) quantifying fuels accumulation in a chronoseries of time-since-fire plots, and 3) measuring the impact of repeated burns on fuel composition, structure, and reburn potential. Finally, this research project provided data on two previously understudied sagebrush ecosystem types, basin big sagebrush (Artemisia tridentata spp. tridentata) and low sagebrush (Artemisia arbuscula).

Seventeen years following fire (17YPF) Wyoming big sagebrush communities at Hart Mountain National Antelope Refuge (HMNAR) were dominated by native herbaceous vegetation, with 8% cover of broad-leaved forbs and bunchgrasses in the understory, compared to 4% cover in unburned controls. Seventeen years after fires, shrub cover was 0.4-4% in burned plots compared to 13-24% in unburned controls. In 17 years, overstory fuels only recovered to 13% of pre-fire levels and understory fuels only reached 25% of pre-fire levels. This resulted in potential fire behavior that was far lower in burned plots than in unburned controls, with rates of spread under the highest fire danger modeled scenario ranging from 0.4-2 m min-1 in burn plots and 3-6 m min-1 in controls (P<0.01).

Similar resilience to fire was seen in the more mesic mountain big sagebrush communities 29 years following high severity wildfire at HMNAR. Shrub cover was over 50% before fire, and declined to 10% 1YPF, then continuously increased with time since fire to 26% at 29YPF. Bunchgrass cover was fairly consistent, ranging from 29% prefire to 20% 1YPF and 28% at 29YPF. Cheatgrass cover was only 2% prefire, increased to over 30% for the first 8 years, and then declined as natives successfully outcompeted it. At 29YPF, cheatgrass cover was back down to 3%.

In basin big sagebrush sites 25-26 years post fire, total fuel recovery was variable, recovering to 7-191% of pre fire levels at Bear Creek (BEAR) and to 113-209% at John Day Fossil Beds National Monument (JODA) Repeated burns at JODA significantly altered fuels composition. Fifteen years after a single fire (15YPF), herbaceous fuels made up 44%, and shrubs were 39% of total fuels. Total fuel loads of twice-burned sites (2xB; 26 and 15 YPF) had a composition of 71% herbaceous and 12% shrub mass. Total fuel loads in 15YPF and 2xB sites ranged from 4-6 Mg ha-1 and did not differ by site (P=0.85). Potential fire behavior was not different between 26YPF and 15YPF plots, but in some cases was higher for herbaceous-dominated 2xB plots.

Collectively, these studies show higher levels of resilience to fire than is typically discussed in the sagebrush steppe, in part because the studied ecosystems were in good condition pre-fire, but also because the longer post-fire monitoring time may be more appropriate to capture patterns of succession in these ecosystems. We saw no cases of post-fire conversion to invasive dominance. In some cases, we saw previously-burned areas acting as a fuel break, likely reducing the intensity of the next fire event. Further, unburned control plots were dominated by woody vegetation and exhibited losses in herbaceous understory, possibly indicating that they are outside of their natural fire return interval. Our results illustrate that management of all ecosystem components, including natural disturbance and a mosaic of successional stages is important for persistent resilience, and that suppression of all fires in the sagebrush steppe may create long-term losses of heterogeneity in good condition sagebrush ecosystems. Additionally, this study underscores the need for additional studies looking at multiple decades of post-fire response in the sagebrush steppe.