Impact of unburned remnant sagebrush versus outplants on post-fire landscape rehabilitation

Abstract

Nearly half of the vast sagebrush steppe in the western United states has lost many or nearly all native plant species, largely due to the interaction of invasive species and increased wildfire. Re-establishing sagebrush, a keystone component of these ecosystems, has become a management focus in recent decades using aerial broadcast seeding or limited plantings. One promising avenue for improving the planning and assessment of post-fire seedings involves the spatial patchiness of burn patterns and in the recovery of sagebrush after fire. Unburned remnant or post-fire planted islands (or patches) of sagebrush could be valuable seed sources for species recovery in surrounding burned areas. Information on how much spatial expansion of unburned remnant patches is expected over time could help in the planning of post-fire treatments. However, previous research has indicated that sagebrush seeds do not disperse far, which would imply that unburned or created patches do not contribute much to sagebrush reestablishment effects. Our objective was to determine whether remnant/unburned sagebrush patches contribute to sagebrush recovery in the surrounding burned areas. We quantified seed rain and seedling establishment in relation to patches of sagebrush that were either unburned remnant or had been planted in the first year or so after wildfire. We conducted a seed trapping experiment across 6 different wildfires during two winters to determine seed transport distances. We paired this with a seedling recruitment study on the Soda wildfire where we mapped distances between remnants and seedlings. We found that although a few seeds did travel much farther than previously recorded (maximum of 26 m), seed dispersal was highly variable across sites and patches, and only a small portion of seeds dispersed farther than a few meters from sagebrush patches. Our seedling recruitment assessment confirmed a limited contribution of remnants to seedling recruitment. Specifically, a microsite was only marginally more likely to have a sagebrush seedling even if there was >50 neighbors within a 40 m radius. There were no differences in the quantity of seeds dispersed from remnant versus actively managed patches. Overall, we found that isolated sagebrush patches are unlikely to significantly contribute to landscape regeneration of sagebrush on large fires and that aerial seeding is likely needed to overcome seed limitations. We did detect substantial variation in site-level sagebrush seed production among years, including one site that did not produce any seed in one year. Variability in seed production in space and time appeared to be a potentially more important variable potentially affecting sagebrush seed availability than dispersal distances and is a topic that merits more investigation.