Abstract
AbstractInvasion and dominance of exotic grasses and increased fire frequency threaten native ecosystems worldwide. In the Great Basin region of the western United States, woody and herbaceous fuel treatments are implemented to decrease the effects of wildfire and increase sagebrush (Artemisia spp.) ecosystem resilience to disturbance and resistance to exotic annual grasses. High cover of the exotic annual cheatgrass (Bromus tectorum) after treatments increases fine fuels, which in turn increases the risk of passing over a biotic threshold to a state of increased wildfire frequency and conversion to cheatgrass dominance. Sagebrush ecosystem resilience to wildfire and resistance to cheatgrass depend on climatic conditions and abundance of perennial herbaceous species that compete with cheatgrass. In this study, we used longer‐term data to evaluate the relationships among soil climate conditions, perennial herbaceous cover, and cheatgrass cover following fuel management treatments across the environmental gradients that characterize sagebrush ecosystems in the Great Basin. We examined the effects of woody and herbaceous fuel treatments on soil temperature, soil water availability (13–30 and 50 cm depths), and native and exotic plant cover on six sagebrush sites lacking piñon (Pinus spp.) or juniper (Juniperus spp.) tree expansion and 11 sagebrush sites with tree expansion. Both prescribed fire and mechanical treatments increased soil water availability on woodland sites and perennial herbaceous cover on some woodland and sagebrush sites. Prescribed fire also slightly increased soil temperatures and especially increased cheatgrass cover compared to no treatment and mechanical treatments on most sites. Non‐metric dimensional scaling ordination and decision tree partition analysis indicated that sites with warmer late springs and warmer and wetter falls had higher cover of cheatgrass. Sites with wetter winters and early springs (March–April) had higher cover of perennial herbs. Our findings suggest that site resistance to cheatgrass after fire and fuel control treatments decreases with a warmer and drier climate. This emphasizes the need for management actions to maintain and enhance perennial herb cover, such as implementing appropriate grazing management, and revegetating sites that have low abundance of perennial herbs in conjunction with fuel control treatments.
Highlights
Invasions by exotic grasses and the grass/fire cycles that they initiate represent increasing threats to native ecosystems around the globe (D’Antonio and Vitousek 1992, Rossiter et al 2003, Brooks et al 2004, Balch et al 2013, Germino et al 2016)
Vegetation management treatments are widely used to reduce woody fuels, increase fire-tolerant perennial herbaceous species, and control fine fuels resulting from exotic annual grasses (Miller et al 2011, 2013, 2017)
Plant community patterns indicated by non-metric dimensional scaling (NMS) axes Ordination results indicated that annual and perennial herb cover represented major gradients from low to high cover across our sites and treatments
Summary
Invasions by exotic grasses and the grass/fire cycles that they initiate represent increasing threats to native ecosystems around the globe (D’Antonio and Vitousek 1992, Rossiter et al 2003, Brooks et al 2004, Balch et al 2013, Germino et al 2016). Across much of the biome, factors such as improper grazing, fire exclusion, and climate change have caused increases in woody fuels characterized by fire-intolerant species, such as sagebrush, pin~on pine, and juniper (Burkhardt and Tisdale 1976, Miller and Wigand 1994, Miller et al 2008, Romme et al 2009, Davies et al 2011, Summers and Roundy 2018). These changes have resulted in decreases in fine fuels characterized by perennial grasses and forbs (Miller et al 2013). Vegetation management treatments are widely used to reduce woody fuels, increase fire-tolerant perennial herbaceous species, and control fine fuels resulting from exotic annual grasses (Miller et al 2011, 2013, 2017)
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