Effects of Commercial Thinning on Genetic, Plant Species and Structural Diversity in Second Growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) Stands : International Journal of Forest Genetics

2000

S. 193-202

Unselbständiges Werk

200084345

Forest Genetics 2000 : International Journal of Forest Genetics ; 7(3), S. 193-202

The impact of commercial thinning on biodiversity was studied in two Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations, Weeks Lake (WL) and Fairservice Creek (FC) located on southern Vancouver Island, British Columbia. The age of the stands at thinning was 53 and 70 years for FC and WL, respectively. Other tree species were also present in both sites due to natural regeneration. Biodiversity was evaluated before and after commercial thinning with respect to tree species composition/abundance, tree species genetic diversity assessed by allozyme analysis and stand structural diversity represented by the diameter class (5-cm) distribution. In addition, undersotrey plant species diversity was monitored in WL and FC for 4 and 5 years following thinning, respectively. Tree species composition changed in both plantations in a similar fashion as the proportion of Douglas-fir increased at the expense of all other tree species. Stand structural diversity was simplified due to the decreased number of trees in small diameter classes. These results were expected since the commercial thinning was conducted to promote the growth of Douglas-fir. Genetic diversity parameters (average number of alleles per locus, percent polymorphic loci, and expected heterozygosities) did not differ significantly before and after thinning; however, thinning resulted in a loss of 8 and 7 alleles across species for FC and WL, respectively. Most of the allelic loss occurred in the naturally regenerated species (93%). This allelic loss represents 7 and 6% of the total alleles present in FC and WL, respectively. Thus, only one allele was lost from the crop tree in the FC plantation. Understorey vegetation species richness decreased the year following commercial thinning and the consitently increased over time in both plantations. A total of 17 and 9 new species colonized FC and WL, respectively. One species was replaced in each plantation. In FC, diversity of the understorey plant community based on the Shannon diversity index (H) changed in an increasing linear trend that corresponded to the increase in species richness. On the other hand, H in WL remained stable. Species evenness (H/Hmax) did not change in WL and FC over the course of study expect for seasonal fluctuations. Rare species diversity increased over time in both plantations.