To burn or not to burn: Comparing reintroducing fire with cutting an encroaching conifer for conservation of an imperiled shrub‐steppe

Abstract

Woody vegetation has increased on rangelands worldwide for the past 100–200 years, often because of reduced fire frequency. However, there is a general aversion to reintroducing fire, and therefore, fire surrogates are often used in its place to reverse woody plant encroachment. Determining the conservation effectiveness of reintroducing fire compared with fire surrogates over different time scales is needed to improve conservation efforts. We evaluated the conservation effectiveness of reintroducing fire with a fire surrogate (cutting) applied over the last ~30 years to control juniper (Juniperus occidentalis Hook.) encroachment on 77 sagebrush‐steppe sites. Critical to conservation of this imperiled ecosystem is to limit juniper, not encourage exotic annual grasses, and promote sagebrush dominance of the overstory. Reintroducing fire was more effective than cutting at reducing juniper abundance and extending the period of time that juniper was not dominating the plant community. Sagebrush was reduced more with burning than cutting. Sagebrush, however, was predicted to be a substantial component of the overstory longer in burned than cut areas because of more effective juniper control. Variation in exotic annual grass cover was explained by environmental variables and perennial grass abundance, but not treatment, with annual grasses being problematic on hotter and drier sites with less perennial grass. This suggests that ecological memory varies along an environmental gradient. Reintroducing fire was more effective than cutting at conserving sagebrush‐steppe encroached by juniper over extended time frames; however, cutting was more effective for short‐term conservation. This suggests fire and fire surrogates both have critical roles in conservation of imperiled ecosystems.

Understory Vegetation Change Following Woodland Reduction Varies by Plant Community Type and Seeding Status: A Region-Wide Assessment of Ecological Benefits and Risks

“…Mastication is also known to promote annual grass establishment, more so than perennial grasses ([ 98 , 132 , 141 , 185 , 186 , 187 ], but see [ 188 ]) because the production and distribution of mulch favors annual grass growth by reducing soil temperature, increasing soil moisture, and elevating inorganic nitrogen supply to plants [ 39 , 73 , 132 , 189 , 190 ]. These results indicate that annual grasses will likely proliferate in the short-term even when perennial grasses increase following P–J reduction [ 116 , 142 , 187 ]. However, because perennial grasses are known to effectively suppress annual grasses (i.e., [ 110 , 175 , 191 ]), the expectation is that steady increases in perennial grass cover will diminish this threat over time with proper posttreatment management [ 67 , 118 , 167 , 191 , 192 ].…”

“…However, because perennial grasses are known to effectively suppress annual grasses (i.e., [ 110 , 175 , 191 ]), the expectation is that steady increases in perennial grass cover will diminish this threat over time with proper posttreatment management [ 67 , 118 , 167 , 191 , 192 ]. Consequently, extra effort during the posttreatment period (i.e., [ 5 , 167 , 192 ]) will be essential to enhance the capacity of understory herbaceous vegetation to recover and mitigate the risk of stimulating an annual grass-fire cycle (i.e., [ 39 , 116 , 142 , 193 ]). In addition, because livestock grazing is a key factor in the expansion of P–J through its direct influence on both perennial grass and shrub cover, it may be necessary to adjust management plans to reduce the speed of P–J recovery on treated sites [ 5 , 194 , 195 ].…”

“…The paramount influence of pretreatment vegetation on posttreatment understory recovery may in fact muddle the interpretation of restoration outcomes in woodland ecosystems because the choice of treatment type is usually based on pragmatic and/or workable features of treatment applications, which can confound a clear interpretation of the influences of treatment type and pretreatment vegetation on posttreatment responses. For example, in the absence of fire to naturally regulate woodland encroachment [ 12 , 19 , 116 ], numerous mechanical treatments (i.e., chaining, mastication; e.g., shredding and dispersing mulch, and cutting) have been developed to function as fire surrogates for fuel reduction, watershed improvement, and to restore understory vegetation components [ 67 , 76 , 90 , 117 , 118 , 119 ]. However, the suitability of each treatment depends on pretreatment vegetation and ecological site characteristics [ 5 , 32 ], including the amount of understory herbaceous and shrub species and the severity of P–J encroachment [ 117 , 120 ].…”

Long-Term Effectiveness of Tree Removal to Re-Establish Sagebrush Steppe Vegetation and Associated Spatial Patterns in Surface Conditions and Soil Hydrologic Properties

“…Various methods of tree removal are commonly implemented to re-establish the sagebrush steppe vegetation structure on woodland-encroached sites [13,14,34,[56][57][58][59][60][61]. The effectiveness of tree removal practices varies widely with site attributes (e.g., soil properties, climate), pre-treatment conditions (e.g., the encroachment phase, cheatgrass presence), treatment type and application, pre-and post-treatment weather trends, and post-treatment land use [13,31,33,[58][59][60][62][63][64][65]. Conceptual models based on recent longer-term data suggest a greater likelihood of re-establishing sagebrush steppe vegetation and associated ecological function where tree removal is applied in the early phases of tree encroachment at higher elevation, cooler and wetter sites [6,14,[31][32][33]57,66].…”

“…Phase 3 sites with limited sagebrush and perennial bunchgrass cover and ample bare ground are highly susceptible to cheatgrass invasion after tree removal, particularly at warmer and drier locations [62,63,69]. The re-establishment of sagebrush and perennial bunchgrass cover after tree removal in Phase 3 at higher elevations is often prolonged due to the limited residual cover of these plants, and competition with re-establishing tree cover may hinder long-term results [58,60]. Some of the variability in treatment effectiveness is related to the soil depth.…”