Cutting history, forest structure and biodiversity in boreal forests
The human impact on boreal forests started thousands of years ago and has intensified during the last approximately 500 years through growing populations and technological development, culminating with the introduction of the stand replacing forestry. As a result of the human exploitation the natural dynamics of the forest has been altered and this has lead to a decrease in important forest structural components as for instance old trees and dead wood. The lack of such forest structure elements in managed forests has been proposed as one of the main reasons for the observed differences in biodiversity between managed and old-growth forests. Selective cutting has been suggested as an alternative to clear-cutting in order to maintain structural elements important to biodiversity in the forest landscape. This thesis has two main aims. First, to examine the link between historic selective cuttings and recent biodiversity. Second, to study the relationship between species richness of epiphytic lichens and host tree age and size. Historic selective cutting and forest structure were studied using historical documents and dendroecological methods. Past and present forest structures were analysed in relation to epiphytic lichens, wood-inhabiting fungi and field vegetation in eight study plots (0.2 ha) in Siljan, south-eastern Norway. The relationship between tree age and size and epiphytic lichen species number growing on stems and branches of 157 trees of Picea abies was studied in the same area. Epiphytic lichen biomass of 200 branches at different heights in the tree canopy was quantified in a mountainous Picea abies forest in Kittilbu, south-eastern Norway. The results of the research are summarised in the following (Paper I-IV): I) Reconstructions of historical selective cuttings dated the last cutting to 1903-1910.The forest landscape of the same period consisted of large forest stands of multi-aged and multi-layered forest with old trees present in all forest stand categories. The volumes of living trees were comparable to the volumes of forest in low productivity sites in southern Norway today. II) The present forest structure in the study plots showed typical characteristics of old-growth forest, but the previous selective cutting could still be traced. There were positive correlations between trees left after cutting and tree age, large trees and amounts of dead wood at present. A productivity gradient was also influencing the forest structure. Epiphytic lichen species number correlated positively with tree age both at past and at present. The number of wood-living fungi species correlated positively with the amounts of dead wood at present and number of large trees left after cutting. Number of stumps or estimates of cut volumes did not prove useful in quantifying cutting impact. III) The number of epiphytic lichen species growing on tree stems increased with increasing tree age and tree size. To lichens growing on branches the amount of branches was the most important measured habitat variable. IV) Lichen biomass correlated positively with branch size. The branches were sampled at two different heights in the tree canopy, 2-3 m and 4-6 m above ground. The foliose lichens decreased with increasing height within the canopy, whereas the alectoroid lichen biomass increased.
Publisert i 2009
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