Temporal trends in the protective capacity of burnt beech forests (Fagus sylvatica L.) against rockfall

Beech (Fagus sylvatica L.) forests covering relief-rich terrain often provide direct protection from rockfall for humans and their property. However, the efficacy in protecting against such hazards may abruptly and substantially change after disturbances such as fires, windthrows, avalanches and insect outbreaks. To date, there is little known about the mid-term evolution of the protective capacity in fire-injured beech stands. We selected 34 beech stands in the Southern European Alps that had burnt in different intensity fires over the last 40 years. We inventoried all living and dead trees in each stand and subsequently applied the rockfall model Rockfor.net to assess the protective capacity of fire-injured forests against falling rocks with volumes of 0.05, 0.2, and 1 m3. We tested forested slopes with mean gradients of 27°, 30°, and 35° and lengths of 75 and 150 m. Burnt beech forests hit by low-severity fires have nearly the same protective capacity as unburnt forests, because only thin fire-injured trees die while intermediate-sized and large-diameter trees mostly survive. However, the protective capacity of moderate- to high-severity burns is significantly reduced, especially between 10 and 30 years after the fire. In those cases, silvicultural or technical measures may be necessary. Besides the installation of rockfall nets or dams, small-scale felling of dying trees and the placement of stems at an oblique angle to the slope can mitigate the reduction in protection provided by the forest

Shallow landslide disposition in burnt European beech (Fagus sylvatica L.) forests

Tree roots contribute significantly to soil strength on hillslopes. In the case of wildfires, this effect may abruptly vanish and be lacking for a considerable period of time depending on the resistance and resilience of the forest. Despite its importance, quantitative data on the impact and dynamics of wildfires on slope stabilization is still lacking. We use the study case of the Fagus sylvatica L. to quantify the medium-term evolution of root reinforcement and its effect on slope stability in fire-injured forests. In the study, we upscale root reinforcement using field data for the calibration of the Root Bundle Model and detailed information on forest structure in 244 plots, and calculate the spatio-temporal dynamics of forest protective capacity using a three-dimensional probabilistic slope stability model (slideforNET) for different site types. In unburnt and low-burn forests, the protective capacity was found to remain constant over time. Forests hit by moderate burns continue to provide adequate protection for shallow (depth < 0.5 m) and cohesive soils only, whereas in the case of high severity fires, the protective capacity vanishes for 15 years and an increased shallow landslide probability remains for at least 40 years. These conditions call for appropriate sylvicultural post-fire measures

Spatial patterns and broad-scale weather cues of beech mast seeding in Europe.

Mast seeding is a crucial population process in many tree species, but its spatio-temporal patterns and drivers at the continental scale remain unknown . Using a large dataset (8000 masting observations across Europe for years 1950-2014) we analysed the spatial pattern of masting across the entire geographical range of European beech, how it is influenced by precipitation, temperature and drought, and the temporal and spatial stability of masting-weather correlations. Beech masting exhibited a general distance-dependent synchronicity and a pattern structured in three broad geographical groups consistent with continental climate regimes. Spearman's correlations and logistic regression revealed a general pattern of beech masting correlating negatively with temperature in the summer 2 yr before masting, and positively with summer temperature 1 yr before masting (i.e. 2T model). The temperature difference between the two previous summers (DeltaT model) was also a good predictor. Moving correlation analysis applied to the longest eight chronologies (74-114 yr) revealed stable correlations between temperature and masting, confirming consistency in weather cues across space and time. These results confirm widespread dependency of masting on temperature and lend robustness to the attempts to reconstruct and predict mast years using temperature data

The synchronicity of masting and intermediate severity fire effects favors beech recruitment

The fire ecology of European beech (Fagus sylvatica L.) is poorly understood. We analyzed beech recruitment after a mast year in recently burnt and unburnt stands to answer to the questions: (i) Does post-fire mast seed production and recruitment in beech depend on fire severity, and (ii) which are the processes by which fire and the environment affect beech seed production, germination and seedling emergence and establishment in the first year after masting? We selected three beech stands in the Southwestern Alps, burnt in either the winter of 2012 or 2013 but before the 2013 beech mast year. In the summer of 2013, at each stand, we established 30 sampling plots stratified by fire severity based on the percent basal area loss of beech (low; intermediate; high). Another 10 plots per stand were assigned to a control (unburnt) group. In the spring of 2014, we counted cupules, seeds, germinated seeds, and emergent seedlings (i.e., rooted in mineral soil) in four squares (0.4 × 0.4 m) at each plot. In the summer of 2014, at each plot, we measured stand characteristics (i.e., a circular area of 12-m in a planar radius) and counted established seedlings in 12 squares (1 × 1 m). Control stands had 448 ± 38 cupules m−2 and 489 ± 44 seeds m−2 with a germination rate of 11%. In comparison to the control, production of cupules and seeds was significantly lower only under high fire severity (−75% and −63%, respectively). At intermediate and low severity sites, cupule and seed production were similar to unburnt sites, while seed germination and seedling emergence were higher. At intermediate severity sites established seedlings (86,000 ± 10,574 seedlings ha−1) were significantly more frequent than the control. Generalized linear and additive models demonstrated that intermediate disturbance of litter and canopy cover favored beech regeneration. Mixed severity fires are an important ecological factor for the natural regeneration of beech. Such insights in beech disturbance ecology can help improve silviculture and post-fire restoration of Alpine forests. The synergy between fire and masting raises new questions concerning the role of fire in temperate beech forests

What drives European beech (Fagus sylvatica L.) mortality after forest fires of varying severity?

Predicting the timing and amount of tree mortality after a forest fire is of paramount importance for post-fire management decisions, such as salvage logging or reforestation. Such knowledge is particularly needed in mountainous regions where forest stands often serve as protection against natural hazards (e.g., snow avalanches, rockfalls, landslides). In this paper, we focus on the drivers and timing of mortality in fire-injured beech trees (Fagus sylvatica L.) in mountain regions. We studied beech forests in the southwestern European Alps, which burned between 1970 and 2012. The results show that beech trees, which lack fire-resistance traits, experience increased mortality within the first two decades post-fire with a timing and amount strongly related to the burn severity. Beech mortality is fast and ubiquitous in high severity sites, whereas small- (DBH <12 cm) and intermediate-diameter (DBH 12–36 cm) trees face a higher risk to die in moderate-severity sites. Large-diameter trees mostly survive, representing a crucial ecological legacy for beech regeneration. Mortality remains low and at a level similar to unburnt beech forests for low burn severity sites. Beech trees diameter, the presence of fungal infestation and elevation are the most significant drivers of mortality. The risk of beech to die increases toward higher elevation and is higher for small-diameter than for large-diameter trees. In case of secondary fungi infestation beech faces generally a higher risk to die. Interestingly, fungi that initiate post-fire tree mortality differ from fungi occurring after mechanical injury. From a management point of view, the insights about the controls of post-fire mortality provided by this study should help in planning post-fire silvicultural measures in montane beech forests

How future-proof is Sweet chestnut (Castanea sativa) in a global change context?

In the debate about tree species suited to cope with the ongoing global changes, Sweet chestnut (Castanea sativa Mill.) is frequently discussed as a potentially future-proof tree species for Central Europe. This is mainly due to its current southern distribution range, which climatically corresponds to what is expected for Central Europe in the near future. Present chestnut forests are, however, the result of historic long-term intensive management, which has extended and partially obscured the species' natural ecological range and autecology. Therefore, knowledge about the competitiveness and suitability of chestnut for future conditions is still poor. Here, we analysed data of the Swiss National Forest Inventory (NFI) for Southern Switzerland to compare the survival probability of chestnut to other co-occurring tree species and detect drivers of chestnut mortality under natural dynamics including the Asian Chestnut Gall Wasp (Dryocosmus kuriphilus) using survival models, such as the Kaplan-Meier estimator and the Cox-Proportional Hazards model. Despite the potential longevity of cultivated chestnuts, under natural conditions the survival curves rank the chetsnut among short-lived and light-demanding tree species, especially for individuals smaller than 50 cm diameter at breast height. Moreover, the survival models confirm the pioneer-like characteristics of the chestnut tree such as its low shade tolerance and poor competitiveness in comparison to late-successional species. The mortality probability of chestnut increases when growing under dry conditions and under grazing pressure from goats, while fire disturbances have a positive effect on survival. Severe damage due to repeated D. kuriphilus attacks represents an additional significant factor of increased chestnut mortality risk. Our results suggest that chestnut is not per se a future-proof tree species. Rather, forest managers should be aware that chestnut might not be the best option to face expected rises in summer temperatures and dry periods on xeric site conditions, whereas the species can be considered on good site conditions when regular silvicultural management is provided

Canopy Disturbances Catalyse Tree Species Shifts in Swiss Forests

Widely observed inertia of forest communities contrasts with climate change projections that suggest dramatic alterations of forest composition for the coming decades. Disturbances might be a key process to catalyse changes in tree species composition under environmental change by creating opportunities for ‘new’ species to establish. To test this assumption, we compared two assessments (1993–1995, 2009–2017) from the Swiss National Forest Inventory to evaluate which forests were opened by natural canopy disturbance (that is, wind, insect outbreaks, fire and drought) and if these disturbances altered tree species composition both in terms of species-specific basal area and recruitment densities. Natural disturbances affected 14% of the Swiss forests within 25 years, with wind and insect outbreaks being the most frequent (75%) and fire and drought being rare (< 1.5%). Disturbances led to a shift from conifer to broadleaf tree species at low elevation, in particular in dense Picea abies stands, but no change was observed at higher elevations. The composition of undisturbed sites persisted during the same period. Our results demonstrate that undisturbed forests widely resist changes in tree species composition as an effect of direct ingrowth by stand-forming species. Disturbance events seem necessary to create opportunities for climatically ‘better suited and site-adapted’ species to (re-)establish and therefore potentially catalyse tree species turnover under environmental changes. We detected a reduction of tree species that were—centuries ago—cultivated outside their primary natural range, in particular P. abies, or depended on traditional management practices (Pinus sylvestris, Castanea sativa), which may inform us on how the projected increase in disturbance frequency and severity might filter tree species composition and hereby alter forest structure.ISSN:1432-9840ISSN:1435-062