In early summer 2016 we established 12 plots with one P. abies or L. decidua tree in the centre. Six of these plots (3 P. abies and 3 L. decidua) were then assigned to limited top-soil water availability caused by rain exclusion by roofing the forest floor (“rain exclusion” treatment), while the remaining plots served as controls in the absence of any top soil water manipulation (“control” treatment).Sap flow density (Q_s) through the trunks of the selected study trees was monitored with thermal dissipation sensors throughout the wet growing season of 2017 and the dry growing season of 2018 . During in the second year (wet 2017) rain exclusion caused daily mean normalized Q_sof P. abies to decline, although statistically not significant (p =0.71) on average by 16% below the level of control trees. Compared to evergreen P. abies, withholding precipitation in wet 2017 had no effect of normalized Qs ofdeciduous L. decidua in the rain exclusion plots and normalized Qs was at average 7% above the corresponding value of control trees. In dry 2018 by contrast withholding precipitation had no effect on normalized Q_s of P. abies and L. decidua trees in rain exclusion plots and normalized Q_s was at average 3 (P. abies) and 1%, (L. decidua) above the values of the corresponding control trees.

Observations and experiences by practitioners in North and South Tyrol show that the proportion of unstocked areas in Alpine protection forests is increasing. Results of the Austrian Forest Inventory (ÖWI) support these observations (1992/96: 13.2 %; 2007/09: 17.0 %).

Therefore, the effects of reduced forest cover on water turnover and runoff in protection forests is investigated. Within the framework of the Interreg Project BLÖSSEN (the project is co-financed by the EU and ERDF, through the Interreg Programme V-A “Italien-Österreich” 2014-2020; Forestry Service of Tyrol in cooperation with Forestry Service of the Province of Bolzano) two test sites in North Tyrol (Schallerbach, Istalanzbach in the Paznaun valley) respectively South Tyrol (Kapron and Tanaserberg) have been chosen. Extensive fieldwork was started in summer 2018 comprising forest and soil surveys, rain simulation experiments on 50 m² plots representing dominant land cover units (intensity of 100 mm/h) as well as mapping of surface runoff, surface roughness and channel characteristics. Soil moisture measuring stations are monitoring the seasonal soil water content at forest and non-forest sites.

Seasonal or annual water balances of different land cover units within the test sites are calculated with the model GEOtop, in addition precipitation/runoff modelling for different land cover units and scenarios is done with the model ZEMOKOST. First analyses of the rain simulations show higher surface runoff on intensively used non-forest areas, predominantly higher runoff on areas with sparse ground vegetation compared to areas with high groundcover under forest cover or areas covered by dwarf shrub heath. First evaluations of soil moisture data show higher soil moisture content in non-forest soils. This indicates that forest covered soils have a higher storage capacity during precipitation events, with less deep seepage.

Final results of the Interreg-Project BLÖSSEN will be available in spring 2020.

Effects of delayed reforestation in protection forests on runoff formation

In temperate mountains, overwintering buds of trees are often less cold hardy than adjoining stem tissues or evergreen leaves. Yet, scarce data are available about freezing resistance (FR) of buds and the underlying functional frost survival strategy.

The freezing pattern in and around of vegetative buds of 37 temperate woody species was studied by infrared-video-thermography and cryo-microscopy, and their FR was assessed.

Only in 4 species, after controlled ice nucleation in the stem (-1,6±0,9°C) ice was observed to immediately invade the bud. These buds tolerated extracellular ice and were the most freezing resistant (-61,8°C). In all other species, buds remained supercooled and free of ice, despite a frozen stem. A structural ice barrier prevents ice penetration. Extraorgan ice masses grew in the stem and scales but in 50% exclusively between supercooled bud leaves. Buds of 14 species supercooled temporary and ice nucleated spontaneously at -20,5±4,6°C. This freezing was intracellular as it matched the bud killing temperature (-22,8°C). Temporarily supercooled buds had the lowest FR. Buds of 19 species remained persistently supercooled down to below the killing temperature without indication for the cause of damage. Although having a moderate mean midwinter FR of -31,6°C, some species within this group attained a FR similar to ice tolerant buds. This variability could indicate mechanistic differences.

Our study represents the first comprehensive overview of frost survival mechanisms of vegetative buds of temperate trees. The majority of buds survived in a supercooled state. In 50% of species, extraorgan ice masses harmlessly grew between supercooled bud leaves which was aided by secretion of lipophilic substances. Strikingly, despite exposure to the same environmental demand, bud FR varied intra-specifically (-17,0 to -90,0°C) leaving different leeways to frost damage. Particularly, species, whose buds are killed after temporary supercooling, have a generally lower maximum FR, which limits their geographic distribution.

For about ten years, I have been interested in the influence of frost constraints on plant physiology and in species adaptative responses. My approach combines different scales and disciplines (physics, bioclimatology, physiology, ecology and modeling) and integrates the underlying physiological processes (e.g. hydraulic conductivity in the vascular tissues, frost acclimation of living tissues and frost avoidance via dormancy and phenology), ultimately leading to better predict frost risks in overwintering plants. Mountain ecosystems are strongly limited by frost constraint at high and, even, lower elevation, but climate change is likely to jeopardize the current distribution. Actually, three components interact in the definition of frost risks: hazard (temperature), exposure (organ and freezing temperature) and vulnerability (temperature of resistance). Will climate change only release frost pressure by decreasing the hazard, or, more likely, will we assist to complex interactions including non-linear relationships, threshold effects and feedback loops? To which extent current species distribution will be affected by frost in the future is not easily predictable because

(i) the vulnerability to frost acclimation is under environmental (photoperiod and temperature) and physiological control (synthesis of cryoprotective compounds), limiting acclimation in autumn and increasing deacclimation in spring

(ii) the exposure to frost is mainly controlled by snow cover height and duration (insulating perennial parts from low temperature, freeze-thaw cycles and ice-induced dehydration).

To predict and decrease frost risks under climate change, I develop generic and process-based approaches in perennial species to predict frost vulnerability, exposure and damages. It should help to select adapted ideotypes with respect to future environmental conditions, including interacting constraints (eg. prolonged warm and/or dry period followed by sudden freezing).

Monitoring of freezing dynamics in trees: an insight into complexity

During winter, trees have to cope with harsh conditions, including extreme freeze-thaw stress. The present study focused on ice nucleation and propagation, related water shifts and xylem cavitation as well as cell damage, and was based on in situ monitoring of xylem (thermocouples) and surface temperatures (infrared imaging), ultrasonic emissions and dendrometer analysis. Field experiments during late winter on Picea abies growing at the alpine timberline revealed three distinct freezing patterns: (i) from the top of the tree towards the base, (ii) from thin branches towards the main stem’s top and base, and (iii) from the base towards the top. Infrared imaging showed freezing within branches from their base towards distal parts. Such complex freezing causes dynamic and heterogeneous patterns in water potential and probably in cavitation. This study highlights the interaction between environmental conditions upon freezing and thawing and demonstrates the enormous complexity of freezing processes in trees. Diameter shrinkage, which indicated water fluxes within the stem, and acoustic emission analysis, which indicated cavitation events near the ice front upon freezing, were both related to minimum temperature, and upon thawing, related to vapor pressure deficit and soil temperature. These complex patterns, emphasizing the common mechanisms between frost and drought stress, shed new lights on understanding winter tree physiology.

Keywords:

cavitation, dendrometer, freezing stress, ice propagation, infrared imaging, non-destructive monitoring, ultrasonic acoustic emissions, xylem embolism.

Climate change will affect forest damages caused by pests (1) directly due to more favourable climatic conditions for pathogen survival and reproduction, and (2) indirectly by altering tree fitness and defence responses. Knowledge of physiological responses of mountain forest trees to pathogen attack is limited, and it is unclear how abiotic stresses may influence tree susceptibility to biotic stresses and vice versa.

We study a pathosystem which comprises the economically most important European conifer Norway spruce (Picea abies) and the rust fungus Chrysomyxa rhododendri. This biotrophic pathogen attacks the current-year needles of the host and causes distinct yellowing and defoliation during summer. Consequently, reduced growth or even dieback of diseased trees causes economic losses and rejuvenation problems in high alpine forests. Research approaches focus on rare specimens with enhanced resistance, which lack symptoms even in years with high spore pressure. By applying ecophysiological, biochemical and genetic analyses, defence and resistance mechanisms of spruce are analysed as well as their interrelation with climatic factors and abiotic stresses. Investigations showed, for example, that fungal spore dispersal in spring is favoured by higher temperatures, leading to higher infection intensities. Water limitation during summer and higher nutrient deposition can affect vulnerability to infection by altering the concentration of secondary phenolic metabolites in the needles.

Better knowledge can help to develop sustainable forest management strategies by optimising selected plant material, genetic breeding approaches, and appropriate monitoring, which may be adopted also to other pathosystems. Resistant plant material represents a highly promising approach to counter economic damages caused by pests and to maintain the protective function of high elevation forests.

 Analysis of rust resistance in Norway spruce: New tools and strategies for sustainable afforestation in alpine regions

Climate change will affect forest damages caused by pests directly due to more favourable climatic conditions for pathogen survival and reproduction, but also indirectly by altering tree fitness and defence responses. Norway spruce trees in subalpine forests of the European Alps are increasingly attacked by the needle rust Chrysomyxa rhododendri. This biotrophic parasite undergoes a complex life cycle with a host shift between rhododendrons (Rhododendron spp.) and Norway spruce (Picea abies). It causes yellowing and defoliation of the current-year needles in summer and, in consequence, reduced growth or even dieback of infected trees. The susceptibility to infection varies between individual trees, and rarely specimens with enhanced resistance can be found. Based on this observations, we aim at investigating the underlying resistance mechanisms and to develop new tools to obtain and use resistant plant material for sustainable afforestations.

Research approaches include (i) the development of new image analysis methods to quantify Chrysomyxa infections and tree resistance based on unmanned aerial vehicle (UAV) gained images, (ii) the quantification of benefits of resistant trees in terms of better growth performance and survival, (iii) detailed analysis of the biochemical defence response of Norway spruce, and (iv) the identification of gene expression patterns and allelic variants involved in tree resistance.

Project outcomes help to better understand the genetic and biochemical mechanisms underlying resistance against C. rhododendri. Based on this knowledge, genetic markers and diagnosis tools are developed to select and reproduce resistant plant material, which is able to defend pathogen attack but also adapted to the extreme alpine environment. Results may be crucial to develop future strategies for sustainable management of mountain ecosystems, especially in the face of the looming challenges predicted by climate change.

Alpine treelines are expected to shift as a result of climatic warming. For the potential treeline elevation, which is determined by a lack of warmth, this expectation is relatively straightforward: additional warmth should result in an upward shift. In contrast, for treeline ecotones realised in mountain landscapes the rate, timing and even direction of elevational shifts are much harder to predict. These will depend on local climatic constraints, species traits, biotic interactions, and disturbance regimes, among other things. Ecological processes such as seed dispersal and seedling establishment, tree growth and dieback, modified by neighbour interactions, play pivotal roles here.

Treeline ecotones, i.e. transition zones between forest and alpine vegetation, can have different types of spatial pattern, including closed, diffuse, or island-forming arrangements of trees and abrupt or gradual changes in tree height. Assuming that such patterns reflect the underlying processes controlling treeline dynamics, we are using spatial models to understand pattern-process relationships and to predict treeline responses to climatic change. This pattern-oriented approach should allow a generalised prediction about climate-change responses of different spatial treeline types. In the workshop it will be interesting to discuss the experience of mountain-forest researchers from around the world with respect to observed spatial patterns, probably underlying causes and observed and expected shifts due to climate or other environmental changes.

The mountain treeline in the northern Andes is a complex transition zone between mountain forests (subpáramo) and open páramo grasslands, and is considered a key environment for monitoring the effects of global climate change. We analyzed the vegetation structure along two altitudinal transects with contrasting orientation in a forest-paramo transition zone (including the upper limit of continuous forests, small forest islands and in open páramo) located in the Sierra Nevada National Park (Mérida, Venezuela). Our results indicate clear changes in dominant tree species with both altitude and aspect. We found a clear change in species composition and abundance along the altitudinal gradient. In continuous forests bellow 3300 m cloud forest and subpáramo species were dominant, while they were completely absent in forest islands and open páramos at higher altitudes. In the forest islands and páramo-forest borders the dominant woody species were Diplostephium venezuelense and Cybianthus marginatus, considered as pioneers, given their resistance to the high radiation levels characteristic of the páramo (ej. Diplostephium venezuelense). Even though D. venezuelense was absent in the páramos above 3500 m it was the dominant species of the sparse tree layer of the open páramos at lower altitudes. This suggests it could act as pioneer species in scenarios of altitudinal advance of the treeline induced by climate change.

An increase in the severity and frequency of drought events across many party of Europe is predicted by climate projections. However, there is a lack in understanding about how trees in regions with previously ample water supply respond to sustained and recurrent drought periods. At a recently established subalpine LTER-forest site (Stubai Valley, Austria, south-facing slope at 2000 m) we performed a replicated rain exclusion experiment, excluding a large percentage of the precipitation during three consecutive growing seasons. We continuously monitored radial growth and sap flux dynamics under ambient and drought conditions for the two dominant tree species Norway spruce (Picea abies) and European larch (Larix decidua). Drought significantly reduced radial growth of both species during the first years of the treatment, but drought effects became larger for spruce under recurrent drought. Furthermore, we observed that growing periods become shorter with recurring drought. Sap flow rates and their threshold responses to environmental drivers were less affected by drought, but tended to decrease under drought during periods of high evaporative demand, effects being more pronounced for spruce than for larch. Contrary to our expectation, saplings did not show any stronger drought responses of sap flow than adult trees. Stem conductivity was measured on mature spruce trees using in situ tomography at the end of the drought treatment in 2018, demonstrating that drought had significantly reduced conductivity across all sections of the trunk. This indicates the potential for an increased vulnerability of spruce to dry periods extending into the late season. Overall, our findings indicate that prolonged and recurrent drought periods can significantly reduce tree growth at the treeline, affecting spruce more strongly than larch. However, while reducing hydraulic conductivity, extended drought periods were found to have only minor effects on water use at the alpine tree line.

With climate change causing amplified warming at high altitudes and latitudes, improved growing conditions may trigger a rapid advancement of tree-lines. These changes may be significant even under a moderate warming scenario, as an increased global average temperature of 2°C has been estimated to increase the arctic forest extent by up to 55%. Such an advancement can have devastating consequences to adjacent Arctic and mountainous ecosystems due to their inherently restricted geographic range, and has been predicted to cause a reduction in tundra extent of up to 40%. These predicted dramatic shifts in vegetation are caused by a combination of the above-average warming anticipated in the arctic and mountain regions along with the high temperature-sensitivity of the associated vegetation. These changes have already started to occuring at large scale, with almost 70% of tree-line sites indicating either an altitudinal or latitudinal shift. It is important to better understand the drivers of these cchanges in order to predict future movements of tree-lines. Furthermore, changes in vegetation composition have shown to influence carbon storage of ecosystem, with the presence of vascular plants not only thretening alpine vegetation but also resulting in climatic feedback mechanisms.

The influence of tree-line migration on the worlds carbon budgets

I would like to discuss work that is currently in progress on how presence of vascular plants on previous tree-less alpine vegetation will influence the overall carbon budgets of the area. Three main hypothesis exist on how tree-line movements will influence carbon dynamics: 

1.Increased above ground biomass will lead to greater carbon sequestration and storage, with more litter increasing both soil and above-ground carbon storage 2.Shrub expansion will cause a net release of carbon, as shrubs increases the rate of respiration and causes more rapid soil carbon turnover 3.These changes will balance themselves out, leading to no change in the overall carbon storage of the ecosystem I would like to take the opportunity to discuss my findings in this topic with other conference participants to better understand the climatic feedback mechanisms associated with tree-line movements.

It is well established that tree growth at high elevations and within dry inner Alpine valleys is mainly limited by climate conditions, i.e. low temperature at treeline and water availability at xeric sites. Climate change is expected to influence tree growth especially at these boundaries of tree existence. Therefore, we developed several tree ring series of co-occurring conifers covering the last 100 yr along alpine treeline ecotones (1960-2250 m asl) and at drought-prone inner Alpine sites (750 m asl) to evaluate whether long-term growth trends (basal area increments, BAI) are affected by climate warming and climate extremes. Unexpectedly, results revealed that recent increase in air temperature is not consistently reflected in radial tree growth along the alpine treeline ecotone. At the treeline and the krummholz-limit BAI distinctly increased at all sites and in all tree species, whereas missing adequate growth response at timberline is most likely caused by local site or stand peculiarities, e.g. stand density or grazing intensity. At the drought-exposed site recent trend in BAI strikingly differed among co-occurring species. Dominating Scots pine stands showed constant BAI under dry-mesic conditions, whereas at xeric sites recurring drought periods in spring initiated a stepwise growth decline until tree death occurred. To conclude, although a warming-induced increase in density of seedlings in the forest-alpine tundra ecotone is common, lack of suitable microsites and competition with dwarf shrubs and grass vegetation markedly influence successful tree establishment and growth above treeline causing a time lag in its upslope advance. On the other hand, with progressive climate warming and increase in frequency and duration of drought periods, xeric sites within dry inner Alpine valleys might gradually become treeless, if already suffering Scots pine stands are not replaced by more drought tolerant tree species like oak.

Climate-driven changes of the environment can affect trees of mountain and alpine ecosystems in various ways. Increases in frequency, severity and duration of drought events can push trees closer to their eco-physiological limits by affecting tree water relations and, consequently, productivity. Climate change is also associated with an enhanced risk of wildfires, which affects even relatively humid regions such as the Austrian Central Alps. However, knowledge of the resilience of alpine tree species to fires is scarce and potential long-term heat effects on their physiology are not well understood.

Based on a conceptual framework, which considers plant carbon and water relations, we analyzed how fire injuries resulting from heat transfer can trigger short- and long-term impairments in tree physiology and functionality. Crucial aspects and knowledge gaps were identified and respective experimental studies conducted: in heat experiments on forest tree species, the insulation capacity of bark, the heat protection of buds, and heat transfer processes to meristematic tissues during fires were analyzed. Furthermore, fire-effects on xylem functionality and increment growth were studied.

A better understanding of post-fire eco-physiological processes will help to estimate vulnerability of alpine trees during fires and long-term responses to fire injuries. Species-specific data on fire- and post-fire effects will enable improved risk assessment and forest management strategies in areas of high fire risk.

Heat resistance of trees: Bark insulation is crucial in a more fire-prone future 

Andreas Bär¹, Stefan Mayr¹

¹ Department of Botany, University of Innsbruck, Sternwartestraße 15, 6020 Innsbruck, Austria

 Climate-driven increases in frequencies and intensities of wild fires will affect even relatively humid regions such as the Austrian Central Alps, where higher temperatures in combination with snow-poor winters, lightning activity and more pronounced dry periods favour an increased occurrence of forest fires. During moderate fires, the resistance of trees is mainly determined by the insulation capability of their bark, which protects the cambium from lethal temperatures. However, little is known about fire resistance and bark heat insulation of tree species in alpine ecosystems.

Bark insulation capability of 10 forest tree species was analyzed in the laboratory by measuring temperature gradients along heat-exposed bark samples. Obtained heat transfer parameters, lethal exposure times and bark traits were then linked and analyzed with multivariate statistics. Our results revealed an overall strong relationship between bark thickness and cambial temperature responses. Depending on species, also traits like bark density, bark-to-phloem ratio and moisture content considerably contributed to the bark insulation capability.

Exact knowledge of the underlying properties determining species-specific bark insulation will help to estimate resistance of alpine trees during fires and enable improved risk assessment and forest management strategies in areas of high fire risk.

The conservation of cultural landscapes and traditional forest practices are key ways to achieve sustainable rural development. The dynamics of the landscape is the result of human action over several centuries. In this paper, we study the development and changes that have occurred, the environmental, socioeconomic and historical changes. Our objective is to propose a set of actions for innovation in landscape conservation and the recovery of the traditional uses.

For that, we have studied the resilience and changes of the cultural landscapes in the mountains of eastern Galicia. The study focused on the progression of natural broadleaf forests, intensively exploited since ancient times.

These ecosystems were transformed into agricultural land, exploited for the naval, metallurgical and railway industries, joined the Church’s possessions, suffered forest fires and were replaced by fast-growing species, mainly coniferous and, nowadays, Eucalyptus nitens Shining Gum. All these actions have led to a reduction in the area occupied by them. Now, broadleaf forests cover small and generally sloping sites, remaining where soil characteristics often prevent other use, however, their natural regeneration is very limited by human activities. In these areas, there is a significantly modified landscape but with a slow transformation, and where the conservation of biodiversity, hunting and cultural or environmental tourism have great importance.

Its current situation raises the problem of its socioeconomic reconversion. From a positive point of view, the area covered by these forests has recently increased, and there is a greater awareness of the importance of their conservation given the recognition as habitats of interest of the European Union.

Management of forest stands of Quercus robur L. in mountainous areas of Galicia (NW Spain) according to the environmental factors

The objective of our work was to study the management of the forest stands of the oak species that lives in a larger area of Galicia (northwest of Spain), Quercus robur L. To achieve our objective, we have started with the characterization of the occupied forest site by natural stands of that species. Given that some parameters have a greater meaning in relation to the environmental descriptors than others, a discriminant plot analysis was carried out in order to identify the parameters with greater descriptive weight, using the Two Way Indicative Species Analysis program (TWINSPAN). This analysis is based on the estimation of 25 ecological parameters (ie, topographic, climatic and edaphic) through the sampling of 39 plots of Quercus robur. The results show that the distribution of oaks is mainly related to climate and topography, and to a lesser extent with soil factors. Therefore, it is necessary to find silvicultural treatment alternatives to obtain a more profitable economic production instead of the ones obtained now with the traditional methods and the inadequate forestry practices used (cutting of the best trees and pollarding of others for firewood). Some alternative methods could be the following: i) conversion to high forest, ii) maintenance of the coppice forest in areas of certain quality and homogeneity where firewood is still used, iii) silvopastoral improvement in areas of grazing importance, and iv) restoration of degraded stands through reforestation with other native broadleaf species.

Keywords: Ecological factors, Northwester Spain, TWINSPAN, Oak species

Human-shaped treelines are common in mountain landscapes across Eurasia including long-term managed Mediterranean high-elevation areas. The centuries-long history of transhumant land use of alpine pastures significantly depressed the upper treelines. The natural establishment of trees near alpine treelines was widespread during the 20th century in Europe after a progressive decline of traditional agro-pastoral practices at high elevation, combined with the absence of relevant geo-morphological constraints and climate warming. Differences are mainly due to local scale influences, as forest line advancement is strongly related to dispersal characteristics of the current treeline species such as production of viable seeds, seed dispersal kernels and seedlings competition with ground vegetation. Our studies of recolonization on abandoned high-elevation grasslands in the central Apennines (Italy) showed that Pinus nigra is expanding upwards in formerly treeless grasslands. The process is evident on steep rocky slopes and partially controlled by the distance from pine plantations, acting as seed sources. Recruitment of tree cohorts above the treeline is likely to occur only if climatic and edaphic conditions are favorable and if land use changes are suitable for their establishment and range expansion. Similar processes occurred with other pine species naturally occurring in southern European mountain chains, appearing either as a high-altitude tree densification process or as an initial treeline upshift. The increase of these young tree cohorts within the treeline ecotones can induce changes in major biogeochemical cycles, soil properties, and mountain eco-hydrological processes, leading to a significant long-term plant diversity decline in species-rich grasslands. Forest densification could improve slope protection against landslides and snow avalanches but also produce more fuel, and together with climate warming, increase the risk of mountain wildfires. More accurate predictions are necessary including better estimation of demographic and dispersal parameters in similar low-density expanding tree populations. 

Pine recolonization dynamics in Mediterranean human-disturbed treeline ecotones

Alessandro Vitali, Matteo Garbarino, J. Julio Camarero, Francesco Malandra, Elvin Toromani, Velibor Spalevic, Milić Čurović, Carlo Urbinati

In this study we compared the encroachment patterns of four pine species across anthropogenic treelines in Southern Europe. Using a synchronic approach, we studied structure and recent spatio-temporal patterns of pine recruitment at upper treeline ecotones in Albania, Italy, Montenegro and Spain. Within altitudinal transects we mapped and sampled 964 living individuals of Pinus heldreichii, Pinus peuce, Pinus sylvestris and Pinus uncinata growing above the current forest line. We measured their basal diameter, total height,and counted the number of seed cones. We differentiated seedlings (height < 0.5 m) from saplings (0.5m≤height < 2 m) and trees (height≥2 m). From individuals with basal stem diameter>4 cm we extracted one increment core for cambial age determination and tree-ring width measurements. On smaller specimens, we estimated the age by counting annual internodes (terminal bud scars) along the whole stem. We compared the ground cover around each pine, applied point pattern analyses, modelled the probability of seed cone production and estimated the average distance of seed dispersal. The four pine species exhibited heterogeneous density values (87-1552 N° ha−1). The overall averaged means ranged 2–7 cm for basal diameter, 54–106 cm for total height and 9–20 years for cambial age, suggesting a recent encroachment process. None of these structural variables decreased with increasing relative altitude and distribution patterns exhibited a few higher density spots but not cohort spatial structure. Ground cover differed between species and more significantly between size classes. Grass was the most frequent type at all sites except for P. sylvestris where shrubs prevailed (> 50%). Further differences appeared when discriminated by height thresholds, with larger share of saplings and trees neighboring shrubs and rocks. Basal area increments increased from 1990 and stabilized in recent years at all species except for P. peuce. Height and basal diameter predicted cones production better than cambial age. P. heldreichii and P. peuce dispersed seeds at longer distances than P. uncinata and P. sylvestris, suggesting different potential for further encroaching. Pine recruitment above the forest lines is quite synchronic at all sites (last 30 years), but in some cases it appeared as a high altitude tree densification process, whereas in others as a starting treeline advance.

Climate change (CC) will considerably alter environmental conditions like temperature and precipitation patterns affecting Central European tree species, especially in alpine regions. To reduce CC-related risks for forests, the planting of different tree species and provenances better adapted to the expected climate has been suggested. However, the amount of seeds produced by temperate tree species varies between years and depends on numerous factors including site and weather conditions during and preceding the year of seed maturation. This study aims to identify the most important climate factors affecting variable seed production in order to estimate the need for forest plantations of key tree species in Austria under different adaptation strategies. Historical data on mast seeding and seed harvest from 1960 to 2016 were collected and analyzed for the currently main tree species. The main climatic conditions and regimes affecting their reproductive features were determined and the occurrence of mast fruiting in each species clustered and mapped at the district level to allow the application of predictive models for all Austrian regions. The results show that the climatic variables affecting masting vary between species and geographical areas and that elevation likewise influences seed production. A heterogeneous distribution of mast fruiting clusters was found for all key tree species. Forest seed producers and forest nurseries are key stakeholders for the implementation of adaptation measures to reliably meet forest seed & seedling demand. Results of this study can support the development of a transnational CC adaptation management strategy in Austria and Central Europe.

In mountains, the protective effect of forests against natural hazards is a particularly important ecosystem service. The quantification of this service is key for an efficient management. This, however, is a challenging task, especially over longer periods since uncertainties in the protective function of forests are high. The protective effect of forests is often not constant in time but varies due to natural dynamics and disturbances. Understanding and quantifying the temporal evolution of the protective effect of forests is crucial for a realistic valuation of this ecosystem service and becomes increasingly challenging in the face of a changing climate. A combination of forest dynamic modelling of future forest development under climate change with process-based hazard modelling would allow for a comprehensive analysis of future trends in the protection function of forests. This study aims at a quantification of the temporal evolution of the rockfall protection service of forests in the face of climate change for the Vaud Alps (Switzerland). Changes in the protective effect of the forest against rockfall will be assessed using a risk-based approach considering vegetation dynamics driven by climate change and altering disturbance regimes. The disturbing effect caused by the rockfall process itself and other natural disturbances, such as fire and wind, will be integrated in the forest landscape model TreeMig. Future forest development will then be simulated and a possible change in the protective function of forests against rockfall will be evaluated using a risk-based approach. This study will enhance the quantitative knowledge on changes in the protection function of forests against rockfall and provide an important basis for efficient natural hazard and forest management. This is of high relevance for a region such as the Vaud Alps, where the protection from natural hazards is a very important ecosystem service provided by the forest.

Windthrow create large amount of surface roughness such as lying stems, root plates and stumps. These roughness elements in windthrown areas provide a considerable protective function against avalanches during the first years after a disturbance. The importance of surface roughness is not only recognized for the potential avalanche release, but it may also contribute to deceleration of small to medium sized avalanches through snow detrainment. However, if an extreme amount of snow covers the surface roughness elements, a windthrow area may become prone to avalanche release. Snow accumulation may produce terrain smoothing, which is an important factor in avalanche formation. The effective heights in windthrown areas decrease with time, as the fallen trees decompose. However, windthrow create also favourable conditions for tree regeneration. Information on wood decay in combination with tree regeneration were evaluated using data from long term post-disturbance observation in the study area of Disentis (Switzerland, windthrow Vivian 1990), in order to model effective protection capacity against natural hazards. To assess the effect of snow on surface roughness in windthrown areas, we quantify terrain smoothing using the vector ruggedness measure and the snow heights. Elevation models from summer and winter terrain in the windthrown area close to Bergün (Switzerland, windthrow Vaia 2018) were produced from repetitive drone flights. Based on these data series, we can quantify how surface roughness is smoothed depending on the snow height. Increasing snow height leads to decreasing surface roughness, which can produce local release areas. We expect that with continuous increase of snow height, these release areas expand in size. Good knowledge of effective heights and maximum snow heights is a key in evaluation of an avalanche risk in windthrown areas.

Healthy, dense forests growing in avalanche terrain reduce the likelihood of slab avalanche release by inhibiting the formation of continuous snow layers and weaknesses in the snowpack. Driven by climate change, trends towards more frequent and severe bark beetle disturbances may reduce the protective capacity of mountain forests against avalanches.

We examined the spatial variability in snow stratigraphy, i.e., the characteristic layering of the snowpack, by repeatedly measuring vertical hardness profiles with a digital snow micro penetrometer (SMP) in a spruce beetle infested Engelmann spruce forest in Utah, USA. Four study plots were selected: non-infested/green, infested > 3 years ago/gray stage, salvage-logged, and non-forested/meadow. Based on our SMP measurements and a layer matching algorithm, we quantified the spatial variability in snow stratigraphy, and tested which forest, snow and/or meteorological conditions influenced differences between our plots using linear mixed effects models.

Spatial variability in snow stratigraphy was best explained by the percentage of canopy cover (R² = 0.71, p < 0.001), and only 14% of the variance was explained by the stage within the outbreak cycle. That is, differences between green and gray stage stands did not depend on the reduction in needle mass per se. At both study plots, a more heterogeneous snow stratigraphy developed, which translates to disrupted and discontinuous snow layers and therefore reduced avalanche formation. In contrast, salvage logging that reduced the canopy cover to ~25%, led to a spatially less variable and similar snow stratigraphy as observed in the meadow. At these two study plots, a homogeneous snow stratigraphy consisting of distinct vertical and continuous slope-parallel soft and hard snow layers had formed, a condition which is generally more prone to avalanche release.

Our findings therefore emphasize advantages of leaving dead trees in place, especially in protection forests where bark beetle populations have reached epidemic levels. 

Besides increasing temperatures climate change is predicted to cause an increasing frequency and intensity of extreme events like droughts and especially in the south-eastern Alps a general reduction of precipitation. Thus, inner-alpine dry valleys, for instance the Vinschgau/Val Venosta in Italy, can serve as proxies for future conditions in larger parts of the Alps, especially when taking advantages of changing climatic characteristics along elevation gradients. Within the framework of the LTER IT25 - Val Mazia/Matschertal – Italy we are combining dendrometer and sap flow measurements with tree ring analysis to monitor short- and long-term of climate conditions on tree hydraulics and growth. We observed that European Larch (Larix decidua) was effected by droughts up to the lower subalpine belt (1700 m asl.) while radial growth at the forest line was increasing strongly with warming in the last three decades. Swiss stone pine (Pinus cembra) which is only abandoned at high elevations reacted far less. For Norway spruce (Picea abies) the water limitations led to higher continuously growth rates on North- than South-slopes. Even Scotts pine (Pinus sylvestris) and introduced black pine (Pinus nigra) which are supposed to be rather drought resistant showed clear signs of water limitation with positive growth responses on precipitation and lower growth rates at drier, low elevation sites. Overall, we observed water limitation of radial growth for all conifers up to the lower subalpine belt. Future drier condition in larger parts of the south-eastern Alps could therefore affect timber production. While we did not observe drought-related tree mortality directly at our research sites, especially black pine and Scotts pine were affected by major die-backs in neighboring areas. Only around the forest line, water limitation will play no role in the intermediate future and tree growth, although with differences between species, will benefit from warming.

In tropical forest ecosystems land use has been and is forecast to continue to be the major driver of change in biodiversity and ecosystem services. About 90% of the forest in the Atlantic Forest domain has been lost and the sustained anthropogenic pressure, climate change impacts, along with other global change drivers pose challenges for the regeneration/restoration. One characteristic montane forest type in the south-eastern Brazilian mountain ranges is mixed forest, dominated by the endemic Araucaria angustifolia. The cover of these forests has been reduced and fragmented by land conversion, and paleo evidence suggests that climate change will further reduce the potential niche of Araucaria. We studied the regeneration dynamics of Araucaria in its northernmost range by comparing aerial photographs and describing the age structure of tree populations along fragment edges. There has been no expansion over the last 60 years from forest edge into abandoned grazing land. Limitations identified included (a) seed dispersal limitation; (b) high degree of frugivory of seed crops; (c) regeneration limited to few individuals, mostly within the existing borders likely caused by (d) reduction of the numbers of a native parrot, the main disperser of the species. Furthermore, (e) any seedlings from incidentally dispersed seeds outside of the fragments are vulnerable to man-made fires. With climate warming making dry seasons drier, fires might become more frequent and more intense, preventing tree species from colonising open vegetation and killing trees at fragment edges. With temperature rises we expect the ranges of species with sufficient dispersal capacity to expand uphill; however, for Araucaria this appears unlikely, making the survival of its populations depend on conservation measures. The observed and projected status quo of the forest-open vegetation configuration is likely to have long-term consequences for ecosystems services, such as carbon sequestration and water yield regulation.

Title: Limitations to the recruitment of Araucaria angustifolia beyond forest fragment edges and its implications for forest expansion

Edge expansion and nucleation of mixed ombrophilous Atlantic montane forests have been shown to rely on nurse plant effects by Araucaria angustifolia, the dominant species of this forest type. Spatio-temporal patterns of seed dispersal, seedling establishment and survival of Araucaria beyond forest edges onto neighbouring open secondary vegetation are key factors that determine the dynamics of forest recovery.

We simulated using R and GAMA a number of small patches of Araucaria forest fragments surrounded by open secondary vegetation, to determine (a) the regeneration potential of Araucaria and its dependence on seed dispersal limitation, (b) to what extent fires could prevent the expansion of Araucaria stands; (c) the frequency and intensity of the fires to cause forest regression at fragment edges. 

We measured and mapped all regeneration since the abandonment of livestock grazing (young trees, saplings and seedlings) currently found outside of forest fragment edges to determine dispersal range. We compared the spatial structure recorded in the field, deduced from aerial photographs and of the simulated forest expansion with strict and relaxed propagule availability, dispersal and absence and presence of fire (frequency and intensity) .

We present results and interpretation, together with management recommendations.

Climate change-induced treeline shifts ultimately depend on successful recruitment that requires dispersal of viable seeds followed by successful establishment of individual propagules in novel environments. The Global Treeline Range Expansion Experiment (G-TREE) was initiated in 2013 as an international initiative aimed at disentangling environmental limitations on range expansion by conducting multi-factorial seeding experiments in Arctic and alpine treeline ecotones worldwide. At the G-TREE site in the Swiss Alps near Davos, we evaluated the effects of several abiotic and biotic drivers of early tree seedling recruitment across an alpine treeline ecotone. In two consecutive years, we sowed seeds of low- and high-elevation provenances of European larch and Norway spruce below, at, and above the current treeline into intact vegetation and into open microsites where surface vegetation was experimentally removed, as well as into plots protected from seed predators and herbivores. Seedling emergence and early establishment in treatment and in control plots were monitored over six years. Tree seedling emergence occurred at and several hundred metres above the current treeline when viable seeds and suitable microsites for germination were available. Dense understorey vegetation in open subalpine forests and dwarf-shrubs at treeline strongly limited seedling emergence. A first worldwide synthesis of data from around 15 G-TREE sites confirmed strong seed and substrate limitations on seedling emergence. Continued observations at the Swiss site for six years showed that winter mortality significantly reduced seedling survival in particular above treeline. These results demonstrate the importance of evaluating multi-year seedling survival and performance, because treeline shifts depend on long-term seedling survival. Furthermore, the continuation and expansion of the global collaborative G-TREE approach will lead to a better understanding of the importance of different recruitment drivers at alpine and Arctic treelines, and ultimately to improved predictions of global treeline dynamics in the face of climate change.

Mountain treeline forests often have a patchy distribution, for reasons related to the current and historic human land uses and climatic constraints, both of which vary with topography. In this study, we aimed to understand the topographic controls on the distribution of Polylepis treeline forests in the Andes of southern Peru. We mapped P. subsericans forests in the Cordillera Urubamba, Cusco, using high-resolution aerial images and related forest cover to topographic variables extracted from a digital terrain model (30-m resolution). The variables were selected based on their expected biological relevance for tree growth at high elevations. We constructed logistic regression models of forest cover separately for 100-m elevational belts to study potential changes in topographic control with elevation. The results show a consistent shift in topographic preference with elevation, forests at lower elevations showing a preference for topographically protected sites near water courses and forests at higher elevations being increasingly restricted to north-facing and well-drained microsites. With this elevational shift in the topographic control, our study serves as the first indication of the ability of Andean treeline forests to benefit from the topographic heterogeneity of the high-Andes, forests seeking suitable microsites in a gradient of varying landscape features and climatic conditions. This suggests that local relocations between microsites could help these forests to persist regionally in spite of changing climatic conditions, providing that dispersal and establishment are possible. To ensure an effective conservation and restauration of these forests in the future, it would be important to spatially model suitable microsites at different elevations and macroclimatic zones, taking into account the connectivity between these sites in the fragmented landscape.

Forest cover has fluctuated in the world’s mountain regions over the last hundreds of thousands of years due to natural climatic shifts. In the Andes, palaeoecological studies suggest that Polylepis forest cover has been at its maximum in warmer and wetter than modern conditions, during the transitions into, and out of, the previous interglacial, and at its minimum in drier than modern conditions, during previous interglacial periods. The current human-induced climate change is faster than any previous change and together with an intensive human land use, it is causing a strong contemporary threat for mountain forests worldwide. Forest cover has disappeared from several, currently suitable sites due to logging for firewood, grazing by introduced domestic animals and burning to pastures. Consequently, the current forest cover is highly fragmented. During the past climatic shifts, microrefugial areas, consisting of suitable microsites over time, have thought to play an important role in vegetation maintenance in the world’s mountain regions under the shifting climate. In these microrefugial areas, local relocations between suitable microsites may have helped vegetation to persist regionally in spite of changing conditions, providing that dispersal and establishment have been possible. These areas may play an important role also under the current and future climate change, and should therefore be considered as the current priority areas for conservation and restauration activities. They could be spatially modelled using topographic heterogeneity as one of the main criteria, as topographically heterogeneous areas are expected to offer microsites with varying climatic conditions in close proximity, making forest relocations possible when conditions change, as well as protected microsites with a climatic buffering. Identifying these priority areas and focusing the conservation and restoration actions on these would largely help to reach the United Nation’s 2030 Agenda for Sustainable Development, regarding especially the protection of healthy mountain ecosystems.

Owing to their environmental and socio-economic characteristics, mountain regions are key providers of ecosystem services, which guarantee human well-being not just locally, but over much larger areas. Mountain forests, in particular, are responsible for a significant share of such services, including biomass production, erosion control, carbon sequestration, etc.

The definition of policies to safeguard these services requires detailed information about supply levels across space and possibly the identification of areas characterized by the coexistence of different services. Similarly, the general public may need a visual representation of where such important functions are delivered across the landscape. All of this is possible through maps, to be produced from spatially-explicit forest data and advanced spatial modelling in a Geographic Information System.

Pan-European maps are a great way to compare different mountain regions and possibly identify areas that, although far from each other, provide similar sets of forest ecosystem services. This may encourage international cooperation and the share of views and approaches to the management of mountain regions.

At the present time, European beech (Fagus sylvatica L.) would naturally dominate most lowland forests in the parts of Central Europe where oceanic climate prevails. However, current projections suggest that beech will loose its dominance in a substantial part of the Central European lowlands until the end of the 21st century due to higher drought frequency and intensity. Contrary to the situation in the lowlands, the growth of beech shows a positive trend at higher elevations of the northern slopes of the Alps. Trees growing at high elevations profit from the rising temperatures and while they less frequently endure pronounced drought events than those growing in the lowlands. As the elevational limit of beech is most likely determined by spring frosts, it is expected to rise significantly in the near future. In fact, already today foresters observe an upwards migration of beech at some places in the Alps.

In order to gain insight into the possible reactions of existing and emerging beech stands to future climatic conditions, we measured hydraulic and anatomical features of 7 beech stands along an elevational transect above Innsbruck, spanning from 700 to 1600 m a.s.l. Our results show that elevation does influence wood anatomical and foliar traits independent from tree height but has no effect on key hydraulic traits such as embolism resistance and hydraulic conductivity. We conclude that hydraulic traits do primarily adapt to water-availability during tissue formation, which shows a strong small-scale variation due to factors not directly related to elevation. Moreover, we found support for recent findings suggesting that embolism resistance in angiosperm trees is chiefly determined by anatomical parameters other than vessel size. Finally, we observed that remaining mature trees of recently disturbed stands suffered most from the summer drought in 2018, irrespective of elevation.

Alpine treelines represent an important ecological boundary, separating the boreal forest belt from the treeless alpine tundra. Disregarding local and regional constraints on tree growth, the alpine treeline appears at a relatively uniform growing season temperature on a global scale. It therefore follows that global warming will result in changes to treeline dynamics and elevation. Paulsen and Körner (2014) constructed a mathematical model (TREELIM) based solely on meteorological data, in order to predict the position of global treelines for the present climate at a continental to global scale. We have subsequently recoded TREELIM using Python in combination with ArcGIS, adapting the model to examine treeline elevation in Norway and the Swedish Scandes from a meteorological perspective, at a 1 km spatial resolution. TREELIM requires a digital elevation model (DEM) in addition to the following climate data as input: monthly values of the mean, minimum and maximum temperature, precipitation and cloudiness. Climatic datasets for these variables are gridded at 1 km resolution for the normal period 1971-2000. The model calculates length, mean temperature and other values for the growing season of the site based on the site georeference (latitude, longitude, and elevation) and the minimum temperature for a day required to belong to the growing season. The modelling hypothesis assumes that trees are able to persist if the snow-free and sufficiently moist part of the season reaches a minimum warmth and duration. Maps showing the observed location of the modern treeline were imported into ArcGIS and digitised in order to select control points on the visible treeline. Observed and modelled data were compared to evaluate where TREELIM provides an accurate prediction of the present treeline elevation in Scandinavia. Our results indicate that the treeline is below its potential climatic position in some locations due to grazing or other disturbances.

The Swiss Federal Institute for Forest, Snow and Landscape Research WSL maintains a landslide database with currently 734 comprehensively documented shallow landslides of which 305 were triggered in forests. In terms of a better understanding on the influence of forest structure on slope stability, a Multifactor-Analysis (MFA) approach was applied to 207 datasets, additionally comprising geotechnical information.

Conventional structural properties of forests (layering, development stage, mixture, canopy cover), geotechnical (friction angle, cohesion, water content, void ratio, fine content [silt, clay]) and environmental variables (altitude, inclination) were considered as well as the triggering rain storm events and the different affected geographic regions. Furthermore, the soil material of the shearing zone was determined according to USCS and categorised in three soil stability types, either particularly governed by friction, suction, or a combination of both.

These input variables account for roughly 93% of the variance on the first four MFA dimensions (47%, 22%, 14%, 10%). Results show significant and positive correlation of water content, void ratio, and fine content with the first dimension, contributing around 28, 27, and 20%, respectively. The friction angle is negatively correlated and accounts for 24%, totalising the geotechnical contribution to about 98%. Further significant correlation with the first dimension was found for altitude and the categorical variables soil stability type, specific region, rain storm event, and development stage.

Highest contribution to the second dimension resulted from cohesion (41%) and inclination (8%) both significantly positive as well as altitude (36%) and fine content (10%) both negatively correlated. Additional significant correlation was identified for void ratio, water content, soil stability type, specific region, rain storm event, canopy cover, and layering.

The Multifactor-Analysis has been proven a valuable statistical approach to evaluate combined, numerical and categorical variables and confirms the importance of forest structure as regards the protection against shallow landslides.

Frost drought is one of abiotic stresses for subalpine evergreen conifers inwinter, and induces fatal xylem dysfunction on branches. In the present study, we monitored seasonal changes in water conduction, and observed water distribution and a position of a pit membrane in an inter-tracheid pit with a cryo-SEM on one-year-old stem xylem using Abies veitchii trees grown at wind swept sites in Yatsugatake Mountains, Central Japan. In 2015 winter, percent loss of conductivity (PLC) reached ~60% in mid-February. Notably, only ~10% of tracheids were embolized in the xylem but many pit membranes were aspirated although the two adjacent tracheid lumina were filled with water. In 2017, likewise, PLC was >80% while few embolized tracheids were found and ~90% of inter-tracheid pits were aspirated until early-March. However, in later-March, the shoot water potential decreased to <−2.5 MPa and PLC wasnearly 100%. Then, ~60% of tracheids were embolized but aspirated pit membranes decreased to ~25%. The shoot water potential fully recoveredand almost all pits opene immediately after the first rain in mid-April. In contrast, PLC gradually decreased until July as the embolized tracheids refilled slowly with water in stem xylem. These results suggest that pit aspiration caused the substantial inhibition of stem water transport at less negative water potential and rapidly recovered when the drought stress was relieved. Besides, we hypothesized that the pit aspiration occurred due to water flow generated by increment in ice volume pushing the unfrozen water in xylem at a speed similar to ice propagation. Water perfusion experiment indicated the pit aspiration occurred above ~0.4 m/h of water flow which was similar to the maximum sap flux of the Abies veitchii in summer but much less than the ice propagation speed (e.g., 3 to 14 m/h in Picea abies wood, Charra-Vaskou et al. 2017).

Mountains are rich repositories of biodiversity and provide essential ecosystem services to billions of people, but these functions are under threat. However, given the sensitivity and significance of the mountain ecosystems, the lack of intensive and extensive research in some mountains of the world, especially tropical mountain ranges, is conspicuous. Transition zones between mountain forests and alpine grasslands are termed treeline ecotones. As temperature is one of the most important, but not only, limiting factors for tree growth at high elevation around the world, treelines ecotones have been known to respond to changes in long-term temperature fluctuations. However, global patterns of recent shifts and their controlling factors are yet to be identified. Our research focuses on detecting the spatial patterns of alpine treelines globally, assessed via high-resolution remote sensing and field-based validation with new and existing treeline data sets. These spatial patterns are a signature of the underlying processes controlling treeline dynamics. In combination with individual-based spatial process modelling, such spatial patterns can therefore be used to predict the most likely response to climate change. A part of the data is gathered from a network of researchers and citizen scientists. Building up a network and discussing concepts with mountain ecologists and treeline experts from around the world will be important for including more global treelines into this analysis framework.