Will forest dynamics continue to accelerate throughout the 21st century in the Northern Alps?

Observational evidence suggests that forests in the Northern Alps are changing at an increasing rate as a consequence of climate change. Yet, it remains unclear whether the acceleration of forest change will continue in the future, or whether downregulating feedbacks will eventually decouple forest dynamics from climate change. Here we studied future forest dynamics at Berchtesgaden National Park, Germany by means of a process-based forest landscape model, simulating an ensemble of 22 climate projections until the end of the 21st century. Our objectives were (i) to assess whether the observed acceleration of forest dynamics will continue in the future, (ii) to analyze how uncertainty in future climate translates to variation in future forest disturbance, structure, and composition, and (iii) to determine the main drivers of future forest dynamics. We found that forest dynamics continue to accelerate in the coming decades, with a trend towards denser, structurally more complex and more species rich forests. However, changes in forest structure leveled off in the second half of the 21st century regardless of climate scenario. In contrast, climate scenarios caused trajectories of tree species change to diverge in the second half of the 21st century, with stabilization under RCP 2.6 and RCP 4.5 scenarios and accelerated loss of conifers under RCP 8.5. Disturbance projections were 3 to 20 times more variable than future climate, whereas projected future forest structure and composition varied considerably less than climate. Indirect effects of climate change via alterations of the disturbance regime had a stronger impact on future forest dynamics than direct effects. Our findings suggest that dampening feedbacks within forest dynamics will decelerate forest change in the second half of the 21st century. However, warming beyond the levels projected under RCP 4.5 might profoundly alter future forest disturbance and composition, challenging conservation efforts and ecosystem service supply.

Do bark beetle outbreaks amplify or dampen future bark beetle disturbances in Central Europe?

Bark beetle outbreaks have intensified in many forests around the globe in recent years. Yet, the legacy of these disturbances for future forest development remains unclear. Bark beetle disturbances are expected to increase further because of climate change. Consequently, feedbacks within the disturbance regime are of growing interest, for example, whether bark beetle outbreaks are amplifying future bark beetle activity (through the initiation of an even-aged cohort of trees) or dampening it (through increased structural and compositional diversity).We studied bark beetle-vegetation-climate interactions in the Bavarian Forest National Park (Germany), an area characterised by unprecedented bark beetle activity in the recent past. We simulated the effect of future bark beetle outbreaks on forest structure and composition and analysed how disturbance-mediated forest dynamics influence future bark beetle activity under different scenarios of climate change. We used process-based simulation modelling in combination with machine learning to disentangle the long-term interactions between vegetation, climate and bark beetles at the landscape scale.Disturbances by the European spruce bark beetle were strongly amplified by climate change, increasing between 59% and 221% compared to reference climate. Bark beetle outbreaks reduced the dominance of Norway spruce (Picea abies (L.) Karst.) on the landscape, increasing compositional diversity. Disturbances decreased structural diversity within stands (α diversity) and increased structural diversity between stands (ß diversity). Overall, disturbance-mediated changes in forest structure and composition dampened future disturbance activity (a reduction of up to -67%), but were not able to fully compensate for the amplifying effect of climate change. Synthesis. Our findings indicate that the recent disturbance episode at the Bavarian Forest National Park was caused by a convergence of highly susceptible forest structures with climatic conditions favourable for bark beetle outbreaks. While future climate is increasingly conducive to massive outbreaks, the emerging landscape structure is less and less likely to support them. This study improves our understanding of the long-term legacies of ongoing bark beetle disturbances in Central Europe. It indicates that increased diversity provides an important dampening feedback, and suggests that preventing disturbances or homogenizing post-disturbance forests could elevate the future susceptibility to large-scale bark beetle outbreaks.

The influence of climate change and canopy disturbances on landslide susceptibility in headwater catchments.

Forests have an important regulating function on water runoff and the occurrence of shallow landslides. Their structure and composition directly influence the risk of hydrogeomorphic processes, like floods with high sediment transport or debris flows. Climate change is substantially altering forest ecosystems, and for Central Europe an increase in natural disturbances from wind and insect outbreaks is expected for the future. How such changes impact the regulating function of forest ecosystems remains unclear. By combining methods from forestry, hydrology and geotechnical engineering we investigated possible effects of changing climate and disturbance regimes on shallow landslides. We simulated forest landscapes in two headwater catchments in the Eastern Alps of Austria under four different future climate scenarios over 200 years. Our results indicate that climate-mediated changes in forest dynamics can substantially alter the protective function of forest ecosystems. Climate change generally increased landslide risk in our simulations. Only when future warming coincided with drying landslide risk decreased relative to historic conditions. In depth analyses showed that an important driver of future landslide risk was the simulated vegetation composition. Trajectories away from flat rooting Norway spruce (Picea abies (L.) Karst.) forests currently dominating the system towards an increasing proportion of tree species with heart and taproot systems, increased root cohesion and reduced the soil volume mobilized in landslides. Natural disturbances generally reduced landslide risk in our simulations, with the positive effect of accelerated tree species change and increasing root cohesion outweighing a potential negative effect of disturbances on the water cycle. We conclude that while the efficacy of green infrastructure such as protective forests could be substantially reduced by climate change, such systems also have a strong inherent ability to adapt to changing conditions. Forest management should foster this adaptive capacity to strengthen the protective function of forests also under changing environmental conditions.

What drives the future supply of regulating ecosystem services in a mountain forest landscape?

Forest ecosystems provide a wide variety of ecosystem services to society. In harsh mountain environments, the regulating services of forests are of particular importance. Managing mountain forests for regulating services is a cost- and labor intensive endeavor. Yet, also unmanaged forests regulate the environment. In the context of evidence-based decision making it is thus important to scrutinize if current management recommendations improve the supply of regulating ecosystem services over unmanaged development trajectories. A further issue complicating decision making in the context of regulating ecosystem services is their high sensitivity to climate change. Climate-mediated increases in natural disturbances, for instance, could strongly reduce the supply of regulating services from forests in the future. Given the profound environmental changes expected for the coming decades it remains unclear whether forest management will still be able to significantly control the future trajectories of mountain forest development, or whether the management effect will be superseded by a much stronger climate and disturbance effect. Here, our objectives were (i) to quantify the future regulating service supply from a 6456 ha landscape in the Stubai valley in Tyrol, Austria, and (ii) to assess the relative importance of management, climate, and natural disturbances on the future supply of regulating ecosystem services. We focused our analysis on climate regulation, water regulation, and erosion regulation, and used the landscape simulation model iLand to quantify their development under different climate scenarios and management strategies. Our results show that unmanaged forests are efficient in providing regulating ecosystem services. Both climate regulation and erosion regulation were higher in unmanaged systems compared to managed systems, while water regulation was slightly enhanced by management. Overall, direct effects of climate change had a stronger influence on the future supply of regulating services than management and natural disturbances. The ability of management to control ecosystem service supply decreased sharply with the severity of future climate change. This finding highlights that forest management could be severely stymied in the future if climate change continues to proceed at its current rate. An improved quantitative understanding of the drivers of future ecosystem service supply is needed to more effectively combine targeted management efforts and natural ecosystem dynamics towards sustaining the benefits society derives from forests in a rapidly changing world.

Trade-offs between temporal stability and level of forest ecosystem services provisioning under climate change.

The ability of forests to continuously provide ecosystem services (ES) is threatened by rapid changes in climate and disturbance regimes. Consequently, these changes present a considerable challenge for forest managers. Management of forests often focuses on maximizing the level of ES provisioning over extended time frames (i.e., rotation periods of more than 100 yr). However, temporal stability is also crucial for many ES, for example, in the context of a steady provisioning of resources to the industry, or the protection of human infrastructure against natural hazards. How temporal stability and the level of ES provisioning are related is of increasing interest, particularly since changing climate and disturbance regimes amplify temporal variability in forest ecosystems. In this simulation study, we investigated whether forest management can simultaneously achieve high levels and temporal stability of ES provisioning. Specifically, we quantified (1) trade-offs between ES stability and level of ES provisioning, and (2) the effect of tree species diversity on ES stability. Simulating a wide range of future climate scenarios and management strategies, we found a negative relationship between temporal stability and level of ES provisioning for timber production, carbon cycling, and site protection in a landscape in the Austrian Alps. Tree species diversity had a predominantly positive effect on ES stability. We conclude that attempts to maximize the level of ES provisioning may increase its temporal variability, and thus threaten the continuity of ES supply. Consequently, considerations of stability need to be more explicitly included in forest management planning under increasingly variable future conditions.

Harnessing landscape heterogeneity for managing future disturbance risks in forest ecosystems.

In order to prevent irreversible impacts of climate change on the biosphere it is imperative to phase out the use of fossil fuels. Consequently, the provisioning of renewable resources such as timber and biomass from forests is an ecosystem service of increasing importance. However, risk factors such as changing disturbance regimes are challenging the continuous provisioning of ecosystem services, and are thus a key concern in forest management. We here used simulation modeling to study different risk management strategies in the context of timber production under changing climate and disturbance regimes, focusing on a 8127 ha forest landscape in the Northern Front Range of the Alps in Austria. We show that under a continuation of historical management, disturbances from wind and bark beetles increase by +39.5% on average over 200 years in response to future climate change. Promoting mixed forests and climate-adapted tree species as well as increasing management intensity effectively reduced future disturbance risk. Analyzing the spatial patterns of disturbance on the landscape, we found a highly uneven distribution of risk among stands (Gini coefficients up to 0.466), but also a spatially variable effectiveness of silvicultural risk reduction measures. This spatial variability in the contribution to and control of risk can be used to inform disturbance management: Stands which have a high leverage on overall risk and for which risks can effectively be reduced (24.4% of the stands in our simulations) should be a priority for risk mitigation measures. In contrast, management should embrace natural disturbances for their beneficial effects on biodiversity in areas which neither contribute strongly to landscape-scale risk nor respond positively to risk mitigation measures (16.9% of stands). We here illustrate how spatial heterogeneity in forest landscapes can be harnessed to address both positive and negative effects of changing natural disturbance regimes in ecosystem management.

Assessing the resilience of Norway spruce forests through a model-based reanalysis of thinning trials.

As a result of a rapidly changing climate the resilience of forests is an increasingly important property for ecosystem management. Recent efforts have improved the theoretical understanding of resilience, yet its operational quantification remains challenging. Furthermore, there is growing awareness that resilience is not only a means to addressing the consequences of climate change but is also affected by it, necessitating a better understanding of the climate sensitivity of resilience. Quantifying current and future resilience is thus an important step towards mainstreaming resilience thinking into ecosystem management. Here, we present a novel approach for quantifying forest resilience from thinning trials, and assess the climate sensitivity of resilience using process-based ecosystem modeling. We reinterpret the wide range of removal intensities and frequencies in thinning trials as an experimental gradient of perturbation, and estimate resilience as the recovery rate after perturbation. Our specific objectives were (i) to determine how resilience varies with stand and site conditions, (ii) to assess the climate sensitivity of resilience across a range of potential future climate scenarios, and (iii) to evaluate the robustness of resilience estimates to different focal indicators and assessment methodologies. We analyzed three long-term thinning trials in Norway spruce (Picea abies (L.) Karst.) forests across an elevation gradient in Austria, evaluating and applying the individual-based process model iLand. The resilience of Norway spruce was highest at the montane site, and decreased at lower elevations. Resilience also decreased with increasing stand age and basal area. The effects of climate change were strongly context-dependent: At the montane site, where precipitation levels were ample even under climate change, warming increased resilience in all scenarios. At lower elevations, however, rising temperatures decreased resilience, particularly at precipitation levels below 750-800 mm. Our results were largely robust to different focal variables and resilience definitions. Based on our findings management can improve the capacity to recover from partial disturbances by avoiding overmature and overstocked conditions. At increasingly water limited sites a strongly decreasing resilience of Norway spruce will require a shift towards tree species better adapted to the expected future conditions.

The impact of future forest dynamics on climate: interactive effects of changing vegetation and disturbance regimes.

Currently, the temperate forest biome cools the earth's climate and dampens anthropogenic climate change. However, climate change will substantially alter forest dynamics in the future, affecting the climate regulation function of forests. Increasing natural disturbances can reduce carbon uptake and evaporative cooling, but at the same time increase the albedo of a landscape. Simultaneous changes in vegetation composition can mitigate disturbance impacts, but also influence climate regulation directly (e.g., via albedo changes). As a result of a number of interactive drivers (changes in climate, vegetation, and disturbance) and their simultaneous effects on climate-relevant processes (carbon exchange, albedo, latent heat flux) the future climate regulation function of forests remains highly uncertain. Here we address these complex interactions to assess the effect of future forest dynamics on the climate system. Our specific objectives were (1) to investigate the long-term interactions between changing vegetation composition and disturbance regimes under climate change, (2) to quantify the response of climate regulation to changes in forest dynamics, and (3) to identify the main drivers of the future influence of forests on the climate system. We investigated these issues using the individual-based forest landscape and disturbance model (iLand). Simulations were run over 200 yr for Kalkalpen National Park (Austria), assuming different future climate projections, and incorporating dynamically responding wind and bark beetle disturbances. To consistently assess the net effect on climate the simulated responses of carbon exchange, albedo, and latent heat flux were expressed as contributions to radiative forcing. We found that climate change increased disturbances (+27.7% over 200 yr) and specifically bark beetle activity during the 21st century. However, negative feedbacks from a simultaneously changing tree species composition (+28.0% broadleaved species) decreased disturbance activity in the long run (-10.1%), mainly by reducing the host trees available for bark beetles. Climate change and the resulting future forest dynamics significantly reduced the climate regulation function of the landscape, increasing radiative forcing by up to +10.2% on average over 200 yr. Overall, radiative forcing was most strongly driven by carbon exchange. We conclude that future changes in forest dynamics can cause amplifying climate feedbacks from temperate forest ecosystems.

Disturbance legacies increase the resilience of forest ecosystem structure, composition, and functioning.

Disturbances are key drivers of forest ecosystem dynamics, and forests are well adapted to their natural disturbance regimes. However, as a result of climate change, disturbance frequency is expected to increase in the future in many regions. It is not yet clear how such changes might affect forest ecosystems, and which mechanisms contribute to (current and future) disturbance resilience. We studied a 6364-ha landscape in the western Cascades of Oregon, USA, to investigate how patches of remnant old-growth trees (as one important class of biological legacies) affect the resilience of forest ecosystems to disturbance. Using the spatially explicit, individual-based, forest landscape model iLand, we analyzed the effect of three different levels of remnant patches (0%, 12%, and 24% of the landscape) on 500-year recovery trajectories after a large, high-severity wildfire. In addition, we evaluated how three different levels of fire frequency modulate the effects of initial legacies. We found that remnant live trees enhanced the recovery of total ecosystem carbon (TEC) stocks after disturbance, increased structural complexity of forest canopies, and facilitated the recolonization of late-seral species (LSS). Legacy effects were most persistent for indicators of species composition (still significant 500 years after disturbance), while TEC (i.e., a measure of ecosystem functioning) was least affected, with no significant differences among legacy scenarios after 236 years. Compounding disturbances were found to dampen legacy effects on all indicators, and higher initial legacy levels resulted in elevated fire severity in the second half of the study period. Overall, disturbance frequency had a stronger effect on ecosystem properties than the initial level of remnant old-growth trees. A doubling of the historically observed fire frequency to a mean fire return interval of 131 years reduced TEC by 10.5% and lowered the presence of LSS on the landscape by 18.1% on average, demonstrating that an increase in disturbance frequency (a potential climate change effect) may considerably alter the structure, composition, and functioning of forest landscapes. Our results indicate that live tree legacies are an important component of disturbance resilience, underlining the potential of retention forestry to address challenges in ecosystem management.