Stumping Deer: Are Old Growth Stumps Creating Refugia for Understory Plants on Haida Gwaii?

Abstract

Sitka black-tailed deer (Odocoileus hemionus sitchensis) were introduced to Haida Gwaii, British Columbia in the late 19th century and are having a dramatic impact on the regeneration of understory vegetation. Although never formally tested, past investigations have reported that in parts of Haida Gwaii understory vegetation is restricted to old growth stumps, suggesting they are providing refugia from browsing deer. Additionally, little is known about which plant species utilize stumps and whether these communities' changes during stand development. The objectives of this study is to (1) determine if old growth stumps are providing refugia for understory plants and (2) determine if stand age influences the understory plant community. Species density(# of species/m2) and percent cover of vegetation present on stumps above (>1.5 m) and below (<1.5 m) the deer browse line as well on the forest floor was assessed across sitka spruce (Picea sitchensis) dominated stands of different ages: 10 years after clear cut harvesting, 30 years after clear cut harvesting, 60 years after clear cut harvesting and old growth (>250 years). Species density and percent cover was found to be greatest on the forest floor in the 10 and 30 year old stands, indicating that stumps are providing relatively poor refugia in younger stands. The only indication that stumps provided protection for understory plants was found in the 60 and >250 year old stands. In these stands percent cover was significantly greater on stumps above the browse line than on stumps below the browse line or the forest floor. Possible reasons for why stumps in younger stands are not providing refugia for vegetation include substrate preference and microclimate. Many plant species prefer to germinate and establish on soil rather than woody material. Furthermore, stumps extending beyond the browse line likely experience more extreme high and low temperatures limiting plant establishment and growth. However, these factors likely diminish as the stand ages. Most plant species appeared not to have a clear association with a single stand age class. The exceptions to this include Gaultheria shallon which appears to prefer 30 year old stands and graminoids which showed the greatest species richness on the forest floor of the 10 year old stands. Overall, the results from this study suggest that the understory vegetation of older stands will benefit the most from stump refugia. Species with strong preferences for soil substrate will benefit the least.