资源环境科技发展态势分析平台

Mortality is an important process to include when modelling forest stand development. Previous attempts to model mortality after partial cutting mostly focused on overturning and stem breakage. However, in old irregular and unmanaged stands, many processes may be acting at the same time, portions of the stands being at different developmental stages. In order to better understand the mortality processes in such stands, mortality after 5 years was analyzed in old-growth black spruce (Picea mariana) and balsam fir (Abies balsamea) stands subjected to a wide range of harvesting intensities. The study comprises three sites where five treatments were applied at two scales. Mixed logistic regressions explaining total mortality and its distribution across mortality types were developed. Results show that the effects of partial cutting in these stands are complex. Mortality in uncut stands was rather high and many trees died standing. Stem breakage was of minor importance, regardless of treatment. Mortality increased with DBH and harvesting intensity and was higher for balsam fir in comparison with black spruce. Mortality was influenced not only by the locally applied treatment (2500 m(2)) but also by the treatment applied over a larger area (10-20 ha). Critical wind speeds derived form an adaptation of the ForestGales model were not successful in explaining total mortality or mortality type, although critical wind speeds were generally lower for dead trees in comparison with living trees. The increased mortality was directly related to an increase in the relative importance of uprooting. The type of mortality was more difficult to predict than total mortality and was related to individual tree variables. Results show an influence of crown width, with overturning being more prevalent for trees with wider crowns. Our study indicates that, until there is a better understanding of the factors leading to trees that die standing, it will continue to be difficult to predict mortality in these types of stand.