Editing IPCC:AR6/SRCCL/Chapter-3 (section)

==== 3.7.3.4 United States of America ====

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Sagebrush ecosystems have declined from 25 Mha to 13 Mha since the late 1800s (Miller et al. 2011 <sup>[[#fn:r1644|1644]]</sup> ). A major cause is the introduction of non-native cheatgrass ( ''Bromus tectorum'' ), which is the most prolific invasive plant in the USA. Cheatgrass infests more than 10 Mha in the Great Basin and is expanding every year (Balch et al. 2013 <sup>[[#fn:r1645|1645]]</sup> ). It provides a fine-textured fuel that increases the intensity, frequency and spatial extent of fire (Balch et al. 2013). Historically, wildfire frequency was 60 to 110 years in Wyoming big sagebrush communities and has increased to five years following the introduction of cheatgrass (Balch et al. 2013 <sup>[[#fn:r1646|1646]]</sup> ; Pilliod et al. 2017 <sup>[[#fn:r1648|1648]]</sup> ).

The conversion of the sagebrush steppe biome to annual grassland with higher fire frequencies has severely impacted livestock producers, as grazing is not possible for a minimum of two years after fire. Furthermore, cheatgrass and wildfires reduce critical habitat for wildlife and negatively impact species richness and abundance – for example, the greater sage-grouse ( ''Centocercus urophasianus'' ) and pygmy rabbit ( ''Brachylagus idahoensis'' ) which are on the verge of being listed for federal protection (Crawford et al. 2004 <sup>[[#fn:r1649|1649]]</sup> ; Larrucea and Brussard 2008 <sup>[[#fn:r1650|1650]]</sup> ; Lockyer et al. 2015 <sup>[[#fn:r1651|1651]]</sup> ).

Attempts to reduce cheatgrass impacts through reseeding of both native and adapted introduced species have occurred for more than 60 years (Hull and Stewart 1949 <sup>[[#fn:r1652|1652]]</sup> ) with little success. Following fire, cheatgrass becomes dominant and recovery of native shrubs and grasses is improbable, particularly in relatively low-elevation sites with minimal annual precipitation (less than 200 mm yr <sup>–1</sup> ) (Davies et al. 2012 <sup>[[#fn:r1653|1653]]</sup> ; Taylor et al. 2014 <sup>[[#fn:r1654|1654]]</sup> ). Current rehabilitation efforts emphasise the use of native and non-native perennial grasses, forbs and shrubs (Bureau of Land Management 2005 <sup>[[#fn:r1655|1655]]</sup> ). Recent literature suggests that these treatments are not consistently effective at displacing cheatgrass populations or re-establishing sage-grouse habitat, with success varying with elevation and precipitation (Arkle et al. 2014 <sup>[[#fn:r1656|1656]]</sup> ; Knutson et al. 2014 <sup>[[#fn:r1657|1657]]</sup> ). Proper post-fire grazing rest, season-of-use, stocking rates, and subsequent management are essential to restore resilient sagebrush ecosystems before they cross a threshold and become an annual grassland (Chambers et al. 2014 <sup>[[#fn:r1658|1658]]</sup> ; Miller et al. 2011 <sup>[[#fn:r1659|1659]]</sup> ; Pellant et al. 2004 <sup>[[#fn:r1660|1660]]</sup> ). Biological soil crust protection may be an effective measure to reduce cheatgrass germination, as biocrust disturbance has been shown to be a key factor promoting germination of non-native grasses (Hernandez and Sandquist 2011). Projections of increasing temperature (Abatzoglou and Kolden 2011 <sup>[[#fn:r1662|1662]]</sup> ), and observed reductions in and earlier melting of snowpack in the Great Basin region (Harpold and Brooks 2018 <sup>[[#fn:r1663|1663]]</sup> ; Mote et al. 2005 <sup>[[#fn:r1664|1664]]</sup> ) suggest that there is a need to understand current and past climatic variability as this will drive wildfire variability and invasions of annual grasses.

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