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The driving forces of tree mortality following wind disturbances of mountain mixed European temperate forests belongs among issues not comprehensively resolved. Hence, we aimed to elucidate the key factors of tree resistance to historical severe disturbance events in the Boubínský Primeval Forest, one of the oldest forest reserves in the Czech Republic. By using spatially explicit tree census, dendrochronological and soil data, we study spatial and temporal patterns of past disturbances and mathematically compared selected characteristics of neighboring trees that were killed by a severe storm in 2017 and those that remained undisturbed. The tendency of trees toward falling was primarily driven edaphically, limiting severe events non-randomly to previously disturbed sites occupied by hydromorphic soils and promoting the existence of two spatially-separated disturbance regimes. While disturbed trees usually recruited in gaps and experienced only one severe release event, surviving trees characteristically regenerated under the canopy and were repeatedly released. Despite the fact that disturbed trees tended to reach both lower ages and dimensions than survivors, they experienced significantly higher growth rates. Our study indicates that slow growth with several suppression periods emerged as the most effective tree strategy for withstanding severe windstorms, dying of senescence in overaged life stage. Despite the selective impact of the Herwart storm on conifer population, we did not find any difference in species sensitivity for most characteristics studied. We conclude that the presence of such ancient, high-density wood trees contributes significantly to the resistance of an entire stand to severe storms.

Unlike many studies on the stand structure of European beech-dominated natural forests we explicitly examined the spatial variability of six general stand characteristics: density, basal area and volume of living trees, volume of deadwood, total volume and the proportion of deadwood in the total volume. We asked whether and how these stand characteristics are spatially organized and autocorrelated, and how their spatial autocorrelation varies among particular characteristics, study sites and observation scales. The study was conducted at three forest stands dominated by Fagus sylvatica L. and co-dominated by Picea abies (L.) Karsten and Abies alba Mill., which represent the few sizable remnants of beech-dominated natural forests in central Europe. Vector stem position maps of the three sites were examined by the computer-simulated placement of differently sized square sample plots (10 x 10; 20 x 20; 30 x 30; 50 x 50 and 70 x 70 m). The six general stand characteristics were calculated for every simulated sample plot. Experimental semivariograms were calculated for all sampling plot (grain) sizes, model semivariograms were only fitted for 20 x 20 m plots. The spatial variability of stand characteristics significantly changes with the scale of observation. At the finest grain the spatial autocorrelation is mostly quite low and usually very nearly approximate the nugget model. However, autocorrelation increases with increasing sampling plot size. A peak in the first lag of the semivariograms was observed only at the finest grains, documenting the competition of large trees, whereas a recurring pattern of patches with similar stand characteristics was identified at larger observation scales. Nested structures formed by the high nugget and relatively less distinct but still apparent sill/range and fluctuation signal were detected in most of the model semivariograms, indicating different sources of variability operating at multiple scales. Moreover, significant differences among particular stand variables were demonstrated. The relative nugget varied from 61% (stand density) to 96% (stand volume) at the 20 x 20 m sampling plots; the autocorrelation ranges varied from more than 320 m to 64 m, respectively. The irregular periodic patchiness found (usually 400-1100 m² in size) may serve as a model for close-to-nature forestry, which emulates the spatial structures of natural forests. The level of positive spatial autocorrelation acknowledged for stand density should be reflected in an effective sampling design. For better estimates of the variance in this variable, sampling in a regular grid or stratified sampling is suggested. [ABSTRACT FROM AUTHOR]

Abstract: Interactions between pit-mound dynamics and tree species populations were studied in a natural mountain (fir)-spruce-beech forest. Pit-mounds are special habitats with unique erosion-sedimentation and microclimatic conditions, which continually influence the trees growing there. Our assumption was that these factors would impact on the competitive potential of the trees and that the interaction between the pit-mound dynamics and the tree layer would not be static, but would change depending on the ages of both the trees and the pit-mounds. Over an area of 74.2ha that was repeatedly studied in 1975, 1997, and 2008 in terms of the tree layer structure (about 23,000 trunks), pit-mound evaluation was performed on a regular network of 354 circular plots with a 23m diameter (1733 pit-mounds in total). Dendrochronological cores were drilled in 1986 samples in order to establish an age structure of the tree layer. Using tree-census, dendrochronological, and mathematical methods, direct or indirect dating of the pit-mounds was performed. The actual occurrence of trees on the pit-mounds was compared with a null model corresponding to random occurrence for various age categories of the trees/pit-mounds. The number of trees decreased smoothly with age in the respective classes. The dominant species was Fagus sylvatica, which like Picea abies and Abies alba reached an age of >450years. A multi-peak pit-mound age structure suggested the occurrence of stronger episodic disturbance events in the past. Mounds covered 8.5% and pits 3.7% of the studied area (121pieces/ha) and the average pit-mound size was 9.92m2. F. sylvatica and other marginally represented trees (A. alba, Sorbus aucuparia, Acer pseudoplatanus, Ulmus glabra) significantly preferred mounds over other microsites (χ 2 =147.37, p <0.001; resp. χ 2 =14.73, p =0.005). The preference to mounds by marginally represented tree species decreased with the age of the trees, whereas the affinity of F. sylvatica increased with age. Also, older individuals of P. abies were more frequently found on mounds compared to the null model, although the overall presence of P. abies on mounds was significantly deficient (χ 2 =11.21, p =0.024). These results suggest that the competitive potential of the trees on mounds decreases with age only in trees of initial succession stages. Mounds older than 101years were most favourable to natural regeneration. [Copyright &y& Elsevier]

Abstract: Current knowledge of the within-site variability of major stand structural features in beech dominated natural forests is limited. Numerous studies have used just several small plots for characterizations of natural stands, but this may lead to generalizations based on unreliable results. This study shows how major stand structural features vary at the local scale, and how suitable sampling may reflect this variability. Stem position maps of three natural forests in the Czech Republic (Zofin 71ha, Salajka 19ha and Zakova hora 17ha) were used. Each vector stem position map representing all live and dead trees with DBH≥10cm was intensively analyzed using square sample plots of different sizes (10×10m; 20×20m; 30×30m; 50×50m; 100×100m; 140×140m and 200×200m). Basic statistics (mean, standard deviation, coefficient of variation, min., and max.) were calculated for every plot size and each of six major stand features: density, basal area and volume of living trees, volume of course woody debris, total volume and proportion of course woody debris in total volume. As expected, the local relative variability of major stand structural features decreased with increasing plot size. For particular stand features, the observed trend was expressed by significant regression models (y = a × x b ) with high coefficients of determination across all study sites. The results further show that even hectare-to-hectare values are still highly variable; e.g. the volume stock of living trees can range from 474 to 1049m3/ha within one study site. Hence, single samples of 1ha can be poorly representative. We also calculated the minimal number of plots and total sampled area required for estimations of major stand features to within 20% (±10%) of the mean with 95% confidence for particular plot sizes. It appears that plot sizes between 0.01 and 0.09ha would be the most efficient for sampling the above-mentioned variables in European beech-dominated natural forests. [Copyright &y& Elsevier]

• Canopy trees altered forest floor by litter and deadwood production and uprooting. • Deep beech litter accumulations prevented regeneration of spruce, but not of beech. • Deadwood and uprooting mounds were the best microsites for spruce regeneration. • Soil wetness became more limiting for beech regeneration in larger size classes. • Canopy trees of both species showed the ability to support their own regeneration. Natural regeneration of European beech (Fagus sylvatica) and Norway spruce (Picea abies) plays a crucial role in the future of many European mountain forests. It is affected by various soil and stand-related factors whose relative importance, especially in mixed stands, is still not known. In this study, we assessed the importance of stand composition, soil wetness, disturbances and different microsites and seedbeds for regeneration of beech and spruce in a mixed old-growth mountain forest. We also focused on how the effects of these factors change as regeneration gets older. We sampled all regeneration in 563 plots from different microsite types (deadwood, intact soil, treethrow pits and mounds), distinguishing three seedbeds (mosses, beech litter, bare substrate) for seedlings. We used soil survey and tree census data with generalized linear mixed models and variance partitioning to identify the main factors driving tree regeneration and their relative importance. Although beech was slightly less abundant in the canopy than spruce, it strongly outnumbered spruce in regeneration. Beech regeneration showed an affinity for beech litter-rich microsites and drier soils, while spruce was more common on deadwood and moister soils and its response to the seedbed was microsite-specific. The regeneration of both species was positively related to the proportion of their own species in the canopy, but more so in seedlings than in older regeneration cohorts, where soil wetness was more important. The overall pattern of tree regeneration thus resulted from a complex interplay between site conditions and their alterations by current and former generations of canopy trees through the creation of new microsites (deadwood, uprooting mounds) or litter production. Where beech regeneration is not suppressed by excess soil wetness, it is much more successful than spruce due to its shade tolerance and ability to be established in the beech litter that dominates the forest floor. On the other hand, spruce regeneration is mostly restricted to elevated microsites with lower litter accumulation, such as deadwood and treethrow mounds. Our results indicate that both species exhibit an ability to modify their environment in favor of their own regeneration, but under current conditions, beech is more successful than spruce and can be expected to increase its dominance in the future. [ABSTRACT FROM AUTHOR]