Fuel treatments in shrublands experiencing pinyon and juniper expansion result in trade-offs between desired vegetation and increased fire behavior

Authors: WILLIAMS, CLAIRE - Oregon State University; ELLSWORTH, LISA - Oregon State University; STRAND, EVA - University Of Idaho; REEVES, MATT - Us Forest Service (FS); SHAFF, SCOTT - Us Geological Survey (USGS); SHORT, KAREN - Us Forest Service (FS); CHAMBERS, JEANNE - Us Forest Service (FS); Newingham, Beth; TORTORELLI, CLAIRE - University Of California, Davis

Submitted to: Fire Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/14/2023
Publication Date: 8/7/2023
Citation: Williams, C., Ellsworth, L.M., Strand, E.K., Reeves, M., Shaff, S.E., Short, K.C., Chambers, J.C., Newingham, B.A., Tortorelli, C. 2023. Fuel treatments in shrublands experiencing pinyon and juniper expansion result in trade-offs between desired vegetation and increased fire behavior. Fire Ecology. 19. Article 46. https://doi.org/10.1186/s42408-023-00201-7.
DOI: https://doi.org/10.1186/s42408-023-00201-7

Interpretive Summary: Increases in pinyon and juniper trees in sagebrush communities in the Intermountain West has increased canopy fuels and reduced surface shrub and herbaceous fuels. This woodland expansion often results in longer fire return intervals with potential for high severity crown fire. Fuel treatments are used to restore shrub-bunchgrass communities, prevent continued tree infilling and growth, improve forage quality and quantity, and/or reduce wildfire risk. Treatments may present a trade-off; they restore shrub and herbaceous cover and decrease the risk of canopy fire but increase surface fuel load and surface fire potential. We measured the accumulation of surface and canopy fuels over ten years from ten sites across the Sagebrush Steppe Treatment Evaluation Project network, which were treated with prescribed fire or mechanical tree reduction. While prescribed fire and mechanical treatments restored understory vegetation and prevented continued conifer infilling and growth, these fuel treatments also increased surface fire behavior. Thus, managers should consider the tradeoffs between desired future vegetation and wildfire risk after fuel treatments.

Technical Abstract: Background: Pinyon (Pinus spp.) and juniper (Juniperus spp.) trees are regionally native but increasing in density and distribution into shrubland communities across the western United States. These trees were historically limited by wildfire but currently often outcompete native sagebrush (Artemisia spp.) communities, resulting in new or increased canopy fuels and reduced surface shrub and herbaceous fuels. Woodland expansion often results in longer fire return intervals with potential for high severity crown fire. Fuel treatments are commonly used to restore shrub-bunchgrass communities, prevent continued tree infilling and growth, improve forage quality and quantity, and/or reduce wildfire risk. Treatments may present a trade-off; they restore shrub and herbaceous cover and decrease the risk of canopy fire but increase surface fuel load and surface fire potential. We measured the accumulation of surface and canopy fuels over ten years from ten sites across the Intermountain West in the Sagebrush Steppe Treatment Evaluation Project network, which were treated with prescribed fire or mechanical tree reduction. We used the field data in a fire modeling framework to estimate surface and canopy fire behavior in treated and control plots. Results: Increased herbaceous surface fuel following prescribed fire treatments increased the modeled rate of surface fire spread (ROS) 21-fold and nearly tripled the flame length (FL) by year ten post-treatment. In mechanical treatments, ROS increased 15-fold, FL 3.8-fold, and reaction intensity roughly doubled in year ten post-treatment compared to pretreatment and untreated controls. Treatment effects were most pronounced at 97th percentile windspeeds, with modeled ROS up to 82 m min-1 in mechanical and 106 m min-1 in prescribed fire treatments by ten years post-treatment compared to 5 m min-1 in untreated controls. Both fuel treatments resulted in greater likelihood of crown fire initiation due to increased surface fuels, however, crown fire transmissivity risk was eliminated by both fuel treatments. Conclusions: While prescribed fire and mechanical treatments in shrublands experiencing tree expansion restored understory vegetation and prevented continued conifer infilling and growth, these fuel treatments also increased surface fire behavior. Thus, managers should consider the tradeoffs between desired future vegetation and wildfire risk after fuel treatments.