Can we reconstruct browsing history and how far back? Lessons from Vaccinium parvifolium Smith in Rees

Abstract

We assessed the impact of browsing by black-tailed deer ( Odocoileus hemionus sitkensis) on a common long-lived shrub (the red huckleberry, Vaccinium parvifolium) on Haida Gwaii (British Columbia, Canada). We studied how deer impact can be used as a marker of deer abundance and fluctuation and a means to reconstruct the recent history of deer browsing over a significant section of the archipelago. We compared islands with and without deer to understand processes involved in these changes. We compared shrub features such as number of stems and regenerating sprouts, age and height of stems and stem age-structures between deer-free and deer-affected islands and analysed their spatial and temporal variation. Deer, by browsing regenerating sprouts, stopped stem replacement. On deer-affected islands the number of stems per individual shrub was 2–4 times lower than on deer-free islands. The number of regenerating sprouts was 8–15 times higher. Stems were, on average, 2–3 times older. There was no variation in these characteristics among deer-free islands. They varied both spatially and temporally among deer-affected islands revealing spatial and temporal variation in deer impact. Deer impact has been prevalent for at least 40–50 years before this study in all sites with deer but one. In the latter, the most distant from the point of introduction, severe impact seemed to date to less than 10 years before this study. On Reef Island, Ramsay Island and Burnaby Island, deer impact was prevalent 10–20 years earlier than on Louise and Haswell Islands, although the two latter were much closer and more easily accessible from the point of introduction. Using independent information, we interpreted this pattern as the result of differences in climate and habitat rather than of a delay in colonisation. Effects of isolation on dispersal, pattern of land use or access to alpine summer range are all likely to affect delay between colonisation and severe impact.