Effects of Sitka black-tailed deer (Odocoileus hemionus sitkensis) on understory in old growth forest on Haida Gwaii (Queen Charlotte Islands), British Columbia
Date
2001
Authors
Engelstoft, Christian
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Abstract
In the early 1900's, the Game Commission of British Columbia introduced Sitka Black tailed Deer (Odocoileus hemionus sitkensis) to the offshore archipelago of Haida Gwaii (Queen Charlotte Islands). In the absence of predators and competing herbivores, the deer population erupted. Deer remain abundant throughout Haida Gwaii today, and concerns have been raised about the effects of deer browsing on the regeneration of Western Redcedar (Thuja plicata), which is important commercially and culturally on Haida Gwaii. There is also concern about the effects of deer on endemic plants and other components of the old-growth forest ecosystems. The objective of this study was to determine how varying levels of deer density affected understory composition and biomass in old-growth forests on Haida Gwaii. I used accumulated pellet groups to estimate relative deer density. I used regression equations to estimate aboveground, prebrowsed and utilized biomass of plant species considered to be important forage for deer, including: Red Huckleberry (Vaccinium parvifolium), Blueberry (V. ovalifolium/alaskaense), False Azalea (Menziesia ferruginea), Deer Fern (Blechnum spicant) and Western Redcedar (Models had fits (r2) ranging between 0.66 and 0.98). The study area extended from Ian Lake on Graham Island to the south end of Louise Island. All 110 sampling sites (1 ha) were located in old-growth stands. At each site, I counted pellet groups on three strip transects (1.66 m wide and 100 m long), estimated the shrub biomass on fifteen 4 m2 plots and herbaceous biomass on thirty 1 m2 plots. The estimated aboveground biomass of the selected forage species, varied from O to 2137 kg/ha, with a median of 82 kg/ha and a mean of 258 kg/ha (SD = 388). The estimated available biomass ranged from O to 93 kg/ha, with a median of 3 kg/ha and a mean of 7 kg/ha (SD = 14).
The estimated utilized biomass ranged from O to 22 kg/ha, with a median of 3 kg/ha and a mean of 4 kg/ha (SD = 4.4). The range of forage species was low, but their occurrence varied widely. I encountered Vaccinium spp. (V. parvifolium, V. ovalifolium and V. alaskaense), Salal (Gaultheria shallon), False Azalea, and Salmonberry (Rubus spectabilis) in 99%, 35%, 20% and 4% of the sites, respectively. Western Redcedar saplings were found in only one sampling site and seedlings in 34% of the sites, which corroborates concerns about the effect of deer on its regeneration. Red Huckleberry was the most common species measured, and contributed 86% of the aboveground biomass and 38 % of the available biomass. Sala! (Gaultheria shallon), which was not measured in a comparable way to other shrubs, also contributed considerably to the aboveground biomass. Density of pellet groups (PG) varied from O to 1840 PG/ha, with a median of 300 PG/ha and a mean of 402 PG/ha (SD = 362). I found no correlations between relative deer density and aboveground biomass or available biomass, and a very weak, positive correlation between relative deer density and utilized biomass. I did not find the expected I correlation between overstory canopy closure and aboveground biomass, which suggests that deer browsing is overwhelming other controlling environmental factors. Using a regression quantile method, I found that the upper limit of relative deer density was negatively correlated with aboveground biomass at 75%, 90% and 95% regression quartiles. The upper limit of relative deer density was positively correlated with utilized biomass at 75%, 90%, and 95% regression quartiles. However, the upper limit of relative deer density was not correlated with available biomass. Relative deer density, aboveground biomass, available biomass, and utilized biomass did not differ between stands considered as important winter range and other stands. These same parameters did not vary with distance from settlements either. However, on Moresby Island, relative deer density and aboveground biomass estimates were lower in remote areas compared to easily accessed areas. Deer have dramatically reduced the understory vegetation and deer carrying capacity in old-growth forests over much of Haida Gwaii. Today, relatively few deer are able to keep the sparse understory from recovering - a situation that is unlikely to change in the absence of density-independent mortality (e.g. weather). In addition to altering the character of the understory, deer will also have profound affects on the overstory by eliminating recruitment of Western Redcedar. Effects on other flora and fauna that historically occupied Haida Gwaii are perhaps less studied, but no less important.