Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration

Abstract

Restoration of shrubs in arid and semi‐arid rangelands is hampered by low success rates. Planting shrub seedlings is a method used to improve success in these rangelands; however, it is expensive and labor intensive. The efficiency of shrub restoration could be improved by identifying microsites where shrub survival is greater. Bitterbrush (Purshia tridentata Pursh DC) is an important shrub to wildlife that has declined because of conifer encroachment, excessive defoliation, wildfires, and low recruitment. We investigated planting bitterbrush seedlings in western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroached shrublands after prescribed fire was used to control trees. Bitterbrush seedlings were planted in under (canopy) and between (interspace) former juniper canopies at five blocks and evaluated for three growing seasons. Bitterbrush survival was greater than 50% in the former canopy, but only 5% in the interspace microsite by the third growing season. Growth of bitterbrush was also greater in the former canopy compared with the interspace, potentially due to markedly less perennial vegetation in this microsite. Exotic annual grasses and annual forbs became prevalent in the former canopy in the second and third growing season, suggesting that soil resource availability was greater in this microsite. These results suggest that restoration success will vary by specific locations within a burned landscape and that this variability can be used to improve restoration efficiency. In this situation, bitterbrush restoration can be improved by planting seedlings in former canopy compared with interspace microsites.

Improving restoration success through microsite selection: an example with planting sagebrush seedlings after wildfire

“…The more favorable environment for seedling establishment and growth in former canopy microsites resulted in sagebrush cover exceeding 16% on average 4 years post‐fire; in contrast, interspace microsites averaged less than 3% cover. Bitterbrush growth was similarly greater in former juniper canopy than interspace microsites (Davies et al ). These results suggest that recovery of shrub cover can be enhanced by planting shrub seedlings in former shrub canopy compared to interspace microsites.…”

“…Seeded perennial grasses also had greater establishment in former sagebrush canopy microsites compared to interspace microsites post‐fire (Boyd & Davies ). Likewise, survival of planted bitterbrush seedlings was >50% and 5% in former juniper tree canopy and interspace microsites, respectively (Davies et al ). Our study reaffirms that woody vegetation creates post‐fire microsites that can be favorable to seedling establishment and survival.…”

“…Spatial variability in restoration responses can be used to improve restoration success by prioritizing efforts (Brudvig et al ; Davies et al ). Establishment of shrub seedlings differs with spatial variability across post‐fire landscapes (Davies & Bates ; Davidson et al ).…”

“…Fire is more severe under woody vegetation canopies, leading to much greater mortality of understory vegetation in these microsites (Boyd et al ). This results in less vegetation immediately post‐fire in former canopy microsites than surrounding areas, and therefore seedlings may experience less competition in these microsites (Davies et al , ; Bates & Davies ). Shrubs may preferentially establish in microsites where disturbances have reduced the grass layer (Jurena & Archer ).…”

“…Bitterbrush ( Purshia tridentata [Pursh] DC) seedlings in former western juniper ( Juniperus occidentalis ssp. occidentalis Hook) canopy locations had 10‐fold greater survival compared to interspaces (Davies et al ). Western junipers (10 to >15 m tall), however, are much larger than Wyoming big sagebrush (<1 m tall) and likely their former canopy locations influence resources and competition differently.…”