To burn or not to burn: Comparing reintroducing fire with cutting an encroaching conifer for conservation of an imperiled shrub‐steppe
Seventy-seven sagebrush steppe sites encroached by western juniper (Juniperus occidentalis) in the northern Great Basin and Columbia Basin.
Description of Site
Elevation – 894 to 1,996 m (2,950-6,560 ft); Ppt – xeric (variable); Soils – variable and occupying all slope positions; Plant Communities – mountain big sagebrush (Artemisia tridentata spp. vaseyana) ranging from warm-cool to cool with Thurber’s needlegrass (Achnatherum thurberianum), western needlegrass (A. occidentale), bluebunch wheatgrass (Pseudoroegneria spicata), and Idaho fescue (Festuca idahoensis).
Treatments – 77 burn and cut areas, most in Phase II (mid successional tree encroachment) at time of treatment; Period – 3 to 33 years post-treatment with space (treatment location) substituted for time since treatment; Measurements – cover and density of western juniper, big sagebrush (Artemisia tridentata), other shrubs, and large perennial grasses (excluding Sandberg bluegrass [Poa secunda]); cover of annual grasses.
Juniper increased at a faster rate in the cut compared than burn treatment; cover 0.4 and 7.8% and density 141 and 587/ha in burn and cut treatments, respectively, at 30 years after treatment. Perennial grass cover and density were not different between treatments nor were there differences in exotic grass abundance. Authors reported: “Exotic annual grass cover was influenced by northness, elevation, large perennial grass density, and their interactions.”
The differences in tree cover and density between treatments reinforces the importance of follow-up when using mechanical treatments. This study also supports other studies concluding lower resistance to exotic annual grasses on hotter and drier sites compared to cooler sites (Bates & Davies, 2017; Chambers, Bradley, et al., 2014; Chambers, Miller, et al., 2014; Roundy, Miller, et al., 2014).
However, authors finding no difference in exotic annual grass abundance between treatments does appear to contradict many other studies that report larger increases in burned than mechanically treated plots. This may simply be a result of longevity of the studies, with the early post-fire years having a larger annual grass response, but persistence primarily influenced by the key components mentioned above in addition to post-treatment disturbance that influences resilience and resistance. Three short-term factors that differentiate burns from cutting treatments favoring cheatgrass are: 1) warmer soil surface temperatures, 2) increased available N, and 3) often a decline in perennial grass cover (reduced competition) in the first one or two post-fire years.