Plant competition and the growth of western hemlock in the Eve River Watershed
Date
2018-06-21
Authors
Quesnel, Harold
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Abstract
In the Eve River watershed on northeastern Vancouver Island, chlorotic and stunted stands of naturally regenerated western hemlock (Tsuga heterophylla (Raf.) Sarg.) occur on several hundred hectares of clearcuts that have been burned. Similar stands have been observed in adjacent watersheds. Unburned parts of the same clearcuts have nonchlorotic and vigorously growing western hemlock. In this study, microsite, soil, tree, and vegetation data were collected from two sites with adjacent burned and unburned areas. All stands in the area were characterized by low concentrations of foliar N, P, K, S, and Cu. Stem volume was not significantly correlated with any site variable on the burned or unburned area of either site. Although not significant, the largest correlations with stem volume on the burned areas involved slope, seedling density, and cover of decaying wood. For the unburned areas, the largest correlations with stem volume involved seedling density and depth of forest floor. Because only two sites were sampled, this approach was inconclusive for assessing the factors which control growth of western hemlock in this watershed; sampling of additional sites is needed.
Growth responses of western hemlock on two burned sites were monitored for two years following herbicide application to herbs and shrubs, conifer removal, and these two treatments combined. For these treatments, plots of 2m radius were centered on individual western hemlock target trees. On average, the trees on these sites were 2m tall and 8 years old. One site had greater tree density, and the other had greater cover of herb and shrub species. A trenching treatment was included for a limited number of plots on the first site. The response to treatments was affected by initial site characteristics, leader breakage, herbicide damage, and substrate characteristics. Significantly greater growth in diameter and stem volume of target trees occurred during both growing seasons after conifer removal. Responses to herbicide application were similar. These responses were more pronounced after the second growing season. Results for trenching were not significantly different from the combined treatment.
Foliage of western hemlock had significantly greater concentrations of N, P, and K after conifer removal and of N, K, and Cu after herbicide application. Significant positive correlations were found between stem volume growth after both growing seasons and increases in foliar concentrations after the first growing season; this was true for N, K, and S on site E55 and for N, S, and Cu on site E44. Moisture deficits may have limited growth during both growing seasons.
This study indicates that after slashburning the growth of western hemlock is reduced by competition from herb and shrub species as well as from conifers.
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Keywords
Slashburning, Western hemlock, British Columbia, Eve River Watershed