Emulating natural disturbances in managed forests has been suggested as a potential solution to maintain habitat conditions similar to those observed in old-growth forests. We examined the gap attributes and disturbance history of old-growth Picea abies-dominated stands in the northern boreal vegetation zone of the Pallas-Yllästunturi National Park in northwestern Finland to evaluate the influence of gaps on forest dynamics and the temporal patterns of gap creation. Six stands located at two sites were sampled along 400-m-long linear transects so that all intersected gaps were measured and dated. The average proportion of the forest area in the gaps was 43.1% ± 7.5%. An average gap size was estimated to be 221 m² ± 198 m², whereas the median gap size was 170.2 m². While only 20% of the gaps were smaller than 100 m², nearly 85% of them were smaller than 300 m². Gap creation was constant with no distinct peaks from 1965 to 2005. Thus, forest dynamics were driven by continuous small-scale disturbances and were characterized by quasi-equilibrium structure. However, the results of the growth release analysis indicated that more severe disturbance(s) may have occurred almost two centuries ago. Emulating this type of forest dynamics would imply selective or group harvesting of trees as the predominant methods, but larger-scale, more intensive cuttings could also be carried out periodically.

An adequate understanding of an old-growth forest structure and its variability is needed for forest conservation and restoration. We studied the local scale structural variability of boreal old-growth forests on a Hylocomium—Myrtillus site type in the Pallas-Yllästunturi National Park in northern Finland. Principal component analysis (PCA) revealed that the main directions of structural and compositional variability were related to variables describing the total volume of coarse woody debris (CWD), volumes of living and dead Pinus sylvestris and volume of deciduous CWD. An average, living tree volume was 141.5 m³ ha^-1, varying among the 252 sample plots from 15.3 to 442.0 m³ ha^-1. The mean CWD volume was 30.0 m³ ha^-1 (0–99.3 m³ ha^-1). Downed and standing CWD comprised on average 64.6% and 35.4% of the total CWD volume, respectively. In general, the diameter distributions of living, standing and downed dead trees followed a negative exponential shape. Our results demonstrate high variability in structure and composition of old-growth forests on a northern boreal mesic Hylocomium—Myrtillus site type. This natural variability should be taken into account in forest restoration and management at landscape level.

The current heavy browsing pressure by moose (Alces alces) affects and even prevents the regeneration of deciduous species in Fennoscandia. We studied the current establishment status of Populus tremula and other deciduous species in managed regeneration stands in a moose winter-foraging area in southern Finland. The regeneration of P. tremula was more abundant in height classes 40–100 cm. There was a lag in stem density for height class 20 cm and there were no P. tremula saplings taller than two meters. The majority of living P. tremula regeneration was healthy, despite the fact that many stems had been browsed repeatedly, particularly in stands with greater sapling densities. The majority of Sorbus aucuparia and Salix spp. stems were also healthy despite recurrent browsing in all stands, and their density decreased sharply for saplings taller than 100 cm. Betula pendula and Betula pubescens were the most abundant of the studied species. They had less bites per stem and a very high proportion of them were healthy. Due to the high moose population density in the study area, the successful recruitment of mature stems of P. tremula, Salix spp., and S. aucuparia in the landscape is uncertain. The situation of Betula spp. is less problematic, due to its greater density.

We used sequential surface photography and photo-derived data to evaluate tree mortality in a windthrow area in eastern Estonia, where a storm occurred in 2001. The study is based on photographs taken from the edge of three completely destroyed areas with total canopy destruction in which wind-felled spruce trees (Picea abies) were left after disturbance. In total, 137 spruce trees were observed over a five-year period. We used a transition matrix to examine tree mortality dynamics and patterns. At the end of the five-year period, only 25% of the spruce trees survived in areas surrounding the windthrow. The mortality was highest in the second year after disturbance and the probability of a tree falling was surprisingly high over the entire study period. According to local observations, Ips typographus caused most of the tree deaths, but the co-influences of other factors were also important as there was a large proportion of falling trees in the area.

A drought simulation (roof experiment) was conducted in a 60-year-old Scots pine (Pinus sylvestris) stand in central Lithuania during 2003–2005. The stand represented a typical for Lithuania Scots pine forest of Pinetum vaccinio-myrtillosum type, growing on Arenosols. During the simulated-drought experiment, the changes in soil (humidity, pH, chemistry), ground vegetation (species abundant and composition) and tree condition (needle age, litterfall, crown defoliation) were recorded. The drought decreased soil humidity (O horizon and mineral soil down to the 100-cm depth) and increased the concentrations of some N compounds (in mineral topsoil). The cover of ground vegetation was significantly reduced, the needle age decreased, and crown defoliation increased as a result of drought.

We aimed to quantify the effects of anthropogenic disturbances on forest structure and diversity in deciduous and mixed boreo-nemoral stands, conditioned on potential confounding factors. Based on these results, we created a statistically-supported indicator list of stand “naturalness”. We surveyed 50 quantitative and qualitative characteristics of a stand, the understorey and forest floor, and several widely accepted biodiversity indicators in 171 forests in Estonia. Multi-factorial GLM and GLIM analyses showed that many forest structural characteristics were confounded by forest site-type specificity, stand age and/or biogeography. Near-natural old-growth forests had higher proportions of deciduous trees, a larger amount of coarse woody debris, a higher frequency of logs in each decay class, a denser understorey, and were more homogeneous in the horizontal pattern of each layer than mature managed forests. By improving light conditions, forest management indirectly increased herb layer richness and coverage and the proportion of graminoids. The critical easy-to-apply set of indicators to assess forest ecosystem quality includes the amount and type of dead wood, the canopy closure of a stand and the presence of specially-shaped trees, specific epiphytic indicator lichens, mosses and wood-dwelling insects, and forest site-type specific herb layer species.

We compared the diversity and composition of understorey vegetation of four successional stages (recently clearcut, young, middle-aged and old stands) in an Estonian boreonemoral coniferous forest under homogeneous soil conditions. The ordination analysis (NMDS) showed that successional age was the main driver of understorey species composition with soil pH and P content responsible for some variation as well. Species composition in old-growth stands was more similar to the vegetation of young and clear-cut stands than to the composition of mid-aged stands. Species richness in 1-m² plots was higher in recently disturbed and young stands due to the higher abundance of disturbance-related species. The differentiation diversity, characterising species turnover among plots within a stand, was higher in recently disturbed and young stands than in mid-aged and old stands. The results indicate that earlier successional stages are characterised by spatially heterogeneous and diverse vegetation, whereas older stands develop more homogeneous vegetation composition.

Tree survival, as affected by tree and stand variables, was studied using the Estonian database of permanent forest research plots. The tree survival was examined on the basis of remeasurements during the period 1995–2004, covering the most common forest types and all age groups. In this study, the influence of 35 tree and stand variables on tree survival probability was analyzed using the data of 31 097 trees from 236 research plots. For estimating individual tree survival probability, a logistic model using the logit-transformation was applied. Tree relative height had the greatest effect on tree survival. However, different factors were included into the logistic model for different development stages: tree relative height, tree relative diameter, relative basal area of larger trees and relative sparsity of a stand for young stands; tree relative height, relative basal area of larger trees and stand density for middle-aged and maturing stands; and tree relative height and stand density for mature and overmature stands. The models can be used as preliminary sub-components for elaboration of a new individual tree based growth simulator.

The understorey vascular plant cover and its relations with the overstorey tree species and site properties in young silver birch and hybrid aspen plantations were studied. Understorey vegetation was similar in both plantation types in terms of species richness, diversity, sensitivity to human impact, life-span and habitat preference. Nevertheless, in denser silver birch plantations some signs indicated a faster vegetation development overall, e.g., a higher share of shade tolerant plant species. The concentration of total N was higher in the humus layer of silver birch plantations consequently affecting the nutritional status of the understorey vegetation. The significant impact of the plantation type on the understorey vegetation was confirmed by the NMDS analysis. The hypothesis that semi-exotic hybrid aspen plantations may support the spread of alien species or may show a tendency towards smaller indigenous species richness was not confirmed. Irrespective of the overstorey tree species, a strong previous land use impact, i.e. disturbance history, on the ground vegetation was eminent.