Graduate Thesis Or Dissertation
 

Shrub reestablishment following fire in the mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) alliance

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  • Currently, the of lack information on shrub reestablishment following fire and the wide variability in rates of recovery have lead to uncertainty in using prescribed burning as a management tool in the mountain big sagebrush (Artemisia tridentata Nutt. ssp. vaseyana (Rydb.) Beetle) alliance. This study examined the recovery of shrub density and percent canopy cover of mountain big sagebrush and other associated shrub species across 16 large fires, with between 4-49 years of recovery, in southeastern Oregon, northwestern Nevada, and northeastern California. Sagebrush recovery within the interior of these large (400 to 4000 ha each), uniform burns resulted from existing soil seed pools and in the absence of seed rain from adjacent unburned plants. We sampled 175 sites with over 31 km of line intercept and 6.3 ha of shrub density plots. On four of the fires, Badger Mountain (6 years since fire), Miller Canyon (10 yrs), Kiger (15 yrs) and Murdock (41 yrs) we further explored the chronosequence of shrub reestablishment by harvesting over 1400 mountain big sagebrush and 450 bitterbrush (Purshia tridentata (Pursh) D.C.) plants from interior locations where post-fire seed sources were limited to soil seed pools and unclaimed rodent seed caches. Shrub crowns were prepared in the lab and aged by at least two separate technicians using binocular dissecting microscopes. In our study area, the median % live canopy cover of mountain big sagebrush returned to 20-25% within 32-36 years after the fire event. Linear regression analysis showed that median % live canopy cover increased 3.429 times (3.932 to 2.990, 90% CI, p-value 0.001) with doubling of years since fire. Similarly, mean sagebrush densities increased 0.227 shrubs / m2 (0.267 to 0.188, 90% CI, p-value 0.001) with each doubling of years since fire. Years since fire explained 57% to 36% of the cover and density variation respectively and variation increased as recovery time increased. Shrub chronology suggests that where seed is limited to surviving soil seed pools, shrub reestablishment following fire occurred in three phases: Phase One) the opportunity for immediate shrub establishment from surviving soil seed pools, Phase Two) a lull in seedling establishment resulting from depleted soil seed pools, and Phase Three) the beginning of modal establishment from newly established on-site seed sources. The success or failure of soil seed pools to establish shrub densities during Phase One probably explains some of the variability in the formulas describing the rate of % shrub cover and density recovery following fire. Both regression analysis and the chronology data emphasize the importance of shrub reestablishment in the first 3-4 years following the fire (Phase One) in influencing the rate of shrub recovery. It would appear that quantifying sagebrush density and % cover after the first 3-4 years following fire will aid land managers in developing long-term, landscape level fire management plans by estimating future shrub recovery.
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