Retention forestry as a conservation measure for boreal forest ground vegetation

The boreal forest is prone to natural disturbances, especially fire. Despite this, the disturbance caused by forest management is shown to have a severe effect on the system’s biodiversity. Besides threatening rare species especially sensitive to clearfelling, forestry in Sweden and Finland is reported to negatively affect also common and functionally important plant species. A strategy that is proposed to halt these negative effects is the implementation of retention forestry. This measure is based on retaining dead and living structures during clearfelling and is meant to increase the heterogeneity of both managed stands and landscapes. Retained structures such as standing trees and forest patches are hypothesized to affect the restructuring process of the stand both by acting as lifeboats for and by promoting recolonization of forest species. In this thesis I examine the effects of retained forest patches on the dynamics of boreal forest ground vegetation in Sweden and Finland. As study systems I use an experimental set-up in East Finland as well as a normally managed landscape in Central Sweden containing both harvested stands and mature forest stands of different age. I show that retaining patches constituting as much as 17% of the stand area does not affect vegetation dynamics over a whole stand. However, if the aim of the retention patches is that they should function as lifeboats that can secure that species survive in situ over the clearcut phase, also patches as small as 0.1 ha can be effective. Such patches favour functionally important species like the dwarf-shrub Vaccinium myrtillus and the moss Hylocomium splendens. To effectively lifeboat more sensitive species however, the canopy openness and patch size is important. I.e. the red-listed orchid Goodyera repens does generally not survive in 0.1 ha patches, probably due to poor microclimatic conditions. I also show that G. repens is, probably as a consequence of dispersal limitation, strongly associated with forest older than 120 years. The normal rotation time in Swedish forestry of about 100 years is thus problematic for this species. Finally I show that retention forestry in combination with prescribed burning can promote the production of bilberries (V. myrtillus) and cowberries (V. vitis-idaea). My results demonstrate that the application of retention forestry promotes both biodiversity and provisioning of non-timber products. However, in order to be effective, both level and type of retention need to match the specific goal of the measure