Supervisor: vanemteadur Jaan Liira
Opponent: Joachim Strengbom, assistant professor (Rootsi Põllumajandusülikool)
The biodiversity of boreal forests has been reported to be increasingly endangered. The main reasons for this are related to the intensification of silvicultural practices and the loss of old-growth forests. One way to respond is to establish conservation areas and change management practices in commercial forests to maintain or restore the elements characteristic to natural forests. In order to identify the degree of naturalness we need indicators. The purpose of current thesis is to define which structural and compositional characteristics of Estonian boreal forests are sensitive to forest management and useful as indicators. From compositional aspects we studied the effects of management on forest regeneration and field layer composition.
Forest characteristics negatively affected by management were: number of large-diameter trees and diameter classes, the proportion of spruce and deciduous trees, the amount of dead wood of various decay stages, canopy closure and the coverage of lower canopy layers. Only a fraction of forest characteristics was statistically significant for predicting the naturalness of forests. The set of significant indicators were: the amount of standing dead wood, the proportion of deciduous trees in the Myrtillus site type, the presence of specially shaped trees, signs of woodpeckers and as negative indicator the abundance of herb Melampyrum pratense. Indirect forest management mainly affected tree regeneration and field layer composition through manipulated canopy structure and composition. Light conditions represented the dominant factor complex explaining the compositional variation of the field layer in site types with low and high productivity. Soil and ground layer were the most important determinants of field layer composition in site types of intermediate productivity.
Sustainable forest management in Estonian boreal forests should focus at the stand scale on the maintenance of structural features like dead wood, old trees and deciduous tree species. To enhance the diversity of the field layer, management should be site type specific in diversifying stand structure. At the landscape scale, forest management should begin to emphasize the importance of successional phases after stand-replacing disturbances.