Influences of elk browse on aspen stand structure and landbirds at the National Elk Refuge, Jackson Hole, Wyoming
Edwards, Jennifer Lindsay
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Aspen stands in the Rocky Mountains are hot spots of biodiversity meeting the habitat needs of many species, including wintering ungulates. Ungulates have the potential to alter habitat composition and structure via browsing, especially near areas of concentrated use associated with winter feedgrounds. The National Elk Refuge, established in Jackson, Wyoming in 1912, has provided supplemental winter feed for elk during all but nine winters since its establishment. I conducted the current study to assess aspen stand structure, estimate the likelihood of aspen recruitment given current browse levels, and examine if existing bird communities are reflective of aspen stand structure. I sampled 27 aspen stands at varying distances from feedgrounds during June 2014. I quantified aspen structure using five height class categories along 10 x 40 m belt transects. Twenty trees in height range of elk browse (i.e., 50-150 cm) were selected and measured within each belt to determine the Live-Dead Index, a quantification of browse intensity. I conducted two rounds of bird point counts to assess bird community composition in relation to aspen stand structure. My results indicated current browse levels were largely precluding aspen recruitment. LD Index values indicated 89% of 27 stands sampled in the National Elk Refuge were experiencing high browse intensity and aspen < or = 150 cm were being browsed back to ground level. A relationship between distance to feedgrounds and LD Index was not present, possibly resulting from intensive browsing throughout the refuge. In addition, bird communities were responding to the altered aspen stand structure. For example, Yellow Warblers were strong indicators of stands that had experienced recent aspen recruitment (i.e. greater mid-story canopy and structure). White-breasted Nuthatches were an indicator of a bird community group associated with larger aspen stands. Based on the current lack of recruitment on the refuge, large exclosures surrounding targeted aspen stands are recommended to protect juvenile aspen until they grow out of the browse zone.