Mountain forests are among the most important ecosystems in Europe as they support numerous ecological, hydrological, climatic, social, and economic functions. They are unique relatively natural ecosystems consisting of long-lived species in an otherwise densely populated human landscape. Despite this, centuries of intensive forest management in many of these forests have eclipsed evidence of natural processes, especially the role of disturbances in long-term forest dynamics. Recent trends of land abandonment and establishment of protected forests have coincided with a growing interest in managing forests in more natural states. At the same time, the importance of past disturbances highlighted in an emerging body of literature, and recent increasing disturbances due to climate change are challenging long-held views of dynamics in these ecosystems. Here, we synthesize aspects of this Special Issue on the ecology of mountain forest ecosystems in Europe in the context of broader discussions in the field, to present a new perspective on these ecosystems and their natural disturbance regimes. Most mountain forests in Europe, for which long-term data are available, show a strong and long-term effect of not only human land use but also of natural disturbances that vary by orders of magnitude in size and Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts frequency. Although these disturbances may kill many trees, the forests themselves have not been threatened. The relative importance of natural disturbances, land use, and climate change for ecosystem dynamics varies across space and time. Across the continent, changing climate and land use are altering forest cover, forest structure, tree demography, and natural disturbances, including fires, insect outbreaks, avalanches, and wind disturbances. Projected continued increases in forest area and biomass along with continued warming are likely to further promote forest disturbances. Episodic disturbances may foster ecosystem adaptation to the effects of ongoing and future climatic change. Increasing disturbances, along with trends of less intense land use, will promote further increases in coarse woody debris, with cascading positive effects on biodiversity, edaphic conditions, biogeochemical cycles, and increased heterogeneity across a range of spatial scales. Together, this may translate to disturbance-mediated resilience of forest landscapes and increased biodiversity, as long as climate and disturbance regimes remain within the tolerance of relevant species. Understanding ecological variability, even imperfectly, is integral to anticipating vulnerabilities and promoting ecological resilience, especially under growing uncertainty. Allowing some forests to be shaped by natural processes may be congruent with multiple goals of forest management, even in densely settled and developed countries.

Question: Have past windstorm events influenced the structure and composition of mountain forests in the Tatra Mountains? Can severe and infrequent wind disturbances lead to dynamic coexistence of two tree species with different ecological requirements? Location: Subalpine mixed spruce-larch forest at 1200-1300 m a.s.l. in the Slovakian Tatra Mountains. A forested site affected by catastrophic largescale windthrow on 19 November 2004. Methods: Sixty-seven spruce and 30 larch crosssections from the oldest cohorts were collected in a regular pattern in a 100-ha plot. Tree-ring series were analysed to reconstruct growth releases associated with past windthrows. A boundary-line release criterion was applied to detect disturbance year. Spatial patterns of release signals were statistically detected with Mantel's test. We compared reconstructed years of disturbance events with historical records. Results: Releases in both species showed three main pulses. More than 85% showed major or moderate releases in 1865-1879, 48% in 1915-1924, and 25% in 1940-1949. All of these disturbance events affected the whole 100-ha area. Releases were spatially patterned in the first disturbances, but distributed randomly in the last. Releases co-occurred in time with enhanced production of compression wood, suggesting disturbances were of wind origin. Reconstructed dates of windthrows were confirmed using historical data on storms. Conclusions: At least three windthrows of major and moderate severity took place in the last 150 years on southern slopes of the Tatra Mountains. This disturbance regime may contribute to coexistence of spruce and larch through differences in vulnerability and response to heavy windstorms.

This study examined the effects of burrow digging and habitation by the European badger (Meles meles) and the red fox (Vulpes vulpes) on soil properties and the plant community. The vegetation of control plots located in a similar but undisturbed habitat was compared with that of 18 burrow plots established at badger setts (N = 9) and fox dens (N = 9) in a lowland forest area in Poland. Soil physicochemical properties at different disturbance levels (mounds, intermounds and reference areas) were also investigated. The animals altered nutrient availability in the burrow plots considerably by excavating material from deep soil horizons that were less acidic and higher in K, Ca, Mg and available P but poorer in C and N. The effect was stronger for the badger, probably because it displaced larger amounts of material and disturbed wider areas. The activity of the two carnivores induced similar changes in plant communities. They increased herbaceous species richness and caused a shift in the herbaceous species composition: versus the control plots, the burrow plots contained more fugitive species (short-living plants typical for disturbed environments), among which ruderal forbs, including nutrient-demanding species, dominated. The carnivores also increased the species richness of fleshyfruited shrubs and trees. The primary reason for this was probably not burrowing but endozoochorous seed dispersal. Overall, the results indicate that the badger setts and fox dens differ significantly from the forest matrix in terms of soil and vegetation parameters, and that they contribute to habitat heterogeneity and biological diversity.

The structure of natural subalpine spruce forest in the Zadna´Pol'ana massif of the Western Carpathians was analysed. We focused on the variability of different aspects of stand structure, tree decay and regeneration processes in altitudinal gradient. We used systematic sampling, covering an area of 2 km 2 , to detect even subtle changes in stand structure within one forest type over a range of less than 200 m in elevation. Mean stand density was 290 trees (>7 cm DBH) per hectare, average basal area was 41 m 2 ha À1 , and the volume accumulation in living trees amounted to 500 m 3 /ha À1 . Stand volume decreased by more than 50% between 1,260 and 1,434 m a.s.l. This means for an increase of altitude of 100 m that stand volume decreased by nearly 200 m 3 . Neither stand density nor basal area was related to elevation. Maximum tree height was strongly correlated to elevation, and it decreased on average by 6 m for each 100 m increment of altitude. No significant changes in the maximum spruce diameter were recorded in relation to the elevation gradient. Spatial distribution of trees was biased toward regularity at lower altitudes. Tree clustering increased with increasing altitude. The stock of coarse woody debris (CWD) decreased slightly along the altitudinal gradient, but changes were not significant. Density of spruce saplings and their number growing on CWD significantly increased across the elevation gradient. Despite the fact that the analysed forest tract was relatively large, highly variable in respect to environmental factors, and that stand volume, spatial structure, and tree height displayed strong variability along the elevation gradient, the diameter structure of stands and regeneration measures were uniform. Our results suggest that the recruitment of new trees in the Zadna´Pol'ana subalpine spruce forest is not temporally continuous even at a scale of several square kilometres.