The forest structure and ecosystem quality in conditions of anthropogenic disturbance along productivity gradient

Abstract

Forest structure and diversity have generally been well studied, but the effects of anthropogenic disturbance on structural differences of hemi-boreal old-growth and managed mature stands have not been quantified along a wide range of the productivity gradient. This study compared 39 quantitative or qualitative characteristics and various widely accepted biodiversity indicators of a stand, its understorey and the forest floor in old-growth and conventionally managed mature stands of dry alvar and fresh boreo-nemoral forests. This study has to produce a statistically supported set of indicators for evaluating of forest “naturalness”. We examined 154 stands over a wide biogeographical range in Estonia. Many forest structural characteristics, including stand basal area and composition, amount of dead wood, and coverage of understorey layers (shrub, herb and moss layers) followed productivity and/or biogeographical gradients. Old-growth forests had higher proportions of deciduous trees, a larger amount of coarse woody debris (CWD, consisting of logs and snags), a higher frequency of logs in each decay class, a denser understorey and herb layer, and a more heterogeneous pattern of herb layer diversity than mature managed forests. We found a correlation between live trees greater than 40 cm in diameter and forest age. The effects of forest management on the forest floor are both direct and indirect. Anthropogenic disturbance directly decreased shrub and herb layer coverage. Indirectly forest management increased herb layer coverage, proportion of graminoids, and species richness of the herb layer by improving light conditions as stand canopy was opened. The sets of indicators for assessing ecosystem quality in each forest site-type have overlapping features, such as the amount of dead wood, canopy closure of the stand and the presence of woodpeckers and specific lichens. Several other old-growth or disturbance indicators are forest-type specific, related to the composition of shrub and herb layer vegetation. Results suggest that stands managed with an extended rotation can be compositionally very similar to old-growth forests. Stands differ quantitatively in structural features, but these differences are confounded by other environmental factors, such as soil productivity, forest age or biogeography. Mature and old-growth forests still enclose features created by the mid-term (decades) and long-term (centuries) history of forest management practices.