Accuracy, realism and general applicability of European forest models.

Forest models are instrumental for understanding and projecting the impact of climate change on forests. A considerable number of forest models have been developed in the last decades. However, few systematic and comprehensive model comparisons have been performed in Europe that combine an evaluation of modelled carbon and water fluxes and forest structure. We evaluate 13 widely used, state-of-the-art, stand-scale forest models against field measurements of forest structure and eddy-covariance data of carbon and water fluxes over multiple decades across an environmental gradient at nine typical European forest stands. We test the models' performance in three dimensions: accuracy of local predictions (agreement of modelled and observed annual data), realism of environmental responses (agreement of modelled and observed responses of daily gross primary productivity to temperature, radiation and vapour pressure deficit) and general applicability (proportion of European tree species covered). We find that multiple models are available that excel according to our three dimensions of model performance. For the accuracy of local predictions, variables related to forest structure have lower random and systematic errors than annual carbon and water flux variables. Moreover, the multi-model ensemble mean provided overall more realistic daily productivity responses to environmental drivers across all sites than any single individual model. The general applicability of the models is high, as almost all models are currently able to cover Europe's common tree species. We show that forest models complement each other in their response to environmental drivers and that there are several cases in which individual models outperform the model ensemble. Our framework provides a first step to capturing essential differences between forest models that go beyond the most commonly used accuracy of predictions. Overall, this study provides a point of reference for future model work aimed at predicting climate impacts and supporting climate mitigation and adaptation measures in forests.

Growth resistance and resilience of mixed silver fir and Norway spruce forests in central Europe: Contrasting responses to mild and severe droughts.

Extreme droughts are expected to increase in frequency and severity in many regions of the world, threatening multiple ecosystem services provided by forests. Effective strategies to adapt forests to such droughts require comprehensive information on the effects and importance of the factors influencing forest resistance and resilience. We used a unique combination of inventory and dendrochronological data from a long-term (>30 years) silvicultural experiment in mixed silver fir and Norway spruce mountain forests along a temperature and precipitation gradient in southwestern Germany. We aimed at examining the mechanisms and forest stand characteristics underpinning the resistance and resilience to past mild and severe droughts. We found that (i) fir benefited from mild droughts and showed higher resistance (i.e., lower growth loss during drought) and resilience (i.e., faster return to pre-drought growth levels) than spruce to all droughts; (ii) species identity determined mild drought responses while species interactions and management-related factors strongly influenced the responses to severe droughts; (iii) intraspecific and interspecific interactions had contrasting effects on the two species, with spruce being less resistant to severe droughts when exposed to interaction with fir and beech; (iv) higher values of residual stand basal area following thinning were associated with lower resistance and resilience to severe droughts; and (v) larger trees were resilient to mild drought events but highly vulnerable to severe droughts. Our study provides an analytical approach for examining the effects of different factors on individual tree- and stand-level drought response. The forests investigated here were to a certain extent resilient to mild droughts, and even benefited from such conditions, but were strongly affected by severe droughts. Lastly, negative effects of severe droughts can be reduced through modifying species composition, tree size distribution and stand density in mixed silver fir-Norway spruce forests.

Climate change and the provision of biodiversity in public temperate forests - A mechanism design approach for the implementation of biodiversity conservation policies.

The provision of forest biodiversity remains a major challenge in the management of forest resources. Biodiversity is mostly considered a public good and the fact that societal benefits from biodiversity are private information, hinders its supply at adequate levels. Here we investigate how the government, as a forest owner, may increase the biodiversity supply in publicly-owned forests. We employ a mechanism design approach to find the biodiversity provision choices, which take into account agents' strategic behavior and values towards biodiversity. We applied our framework to a forest landscape in Southwestern Germany, using forest birds as biodiversity indicators and evaluating the impacts of climate change on forest dynamics and on the costs of biodiversity provision. Our results show that climate change has important implications to the opportunity cost of biodiversity and the provision levels (ranging from 10 to 12.5% increase of the bird indicator abundance). In general, biodiversity valuations needed to surpass the opportunity cost by more than 18% to cope with the private information held by the agents. Moreover, higher costs under more intense climate change (e.g. Representative Concentration Pathway 8.5) reduced the attainable bird abundance increase from 12.5 to 10%. We conclude that mechanism design may provide key information for planning conservation policies and identify conditions for a successful implementation of biodiversity-oriented forest management.

Updating beliefs and combining evidence in adaptive forest management under climate change: a case study of Norway spruce (Picea abies L. Karst) in the Black Forest, Germany.

We study climate uncertainty and how managers' beliefs about climate change develop and influence their decisions. We develop an approach for updating knowledge and beliefs based on the observation of forest and climate variables and illustrate its application for the adaptive management of an even-aged Norway spruce (Picea abies L. Karst) forest in the Black Forest, Germany. We simulated forest development under a range of climate change scenarios and forest management alternatives. Our analysis used Bayesian updating and Dempster's rule of combination to simulate how observations of climate and forest variables may influence a decision maker's beliefs about climate development and thereby management decisions. While forest managers may be inclined to rely on observed forest variables to infer climate change and impacts, we found that observation of climate state, e.g. temperature or precipitation is superior for updating beliefs and supporting decision-making. However, with little conflict among information sources, the strongest evidence would be offered by a combination of at least two informative variables, e.g., temperature and precipitation. The success of adaptive forest management depends on when managers switch to forward-looking management schemes. Thus, robust climate adaptation policies may depend crucially on a better understanding of what factors influence managers' belief in climate change.

High economic costs of reduced carbon sinks and declining biome stability in Central American forests.

Tropical forests represent important supporting pillars for society, supplying global ecosystem services (ES), e.g., as carbon sinks for climate regulation and as crucial habitats for unique biodiversity. However, climate change impacts including implications for the economic value of these services have been rarely explored before. Here, we derive monetary estimates for the effect of climate change on climate regulation and habitat services for the forests of Central America. Our results projected ES declines in 24-62% of the study region with associated economic costs of $51-314 billion/year until 2100. These declines particularly affected montane and dry forests and had strong economic implications for Central America's lower-middle income countries (losses of up to 335% gross domestic product). In addition, economic losses were mostly higher for habitat services than for climate regulation. This highlights the need to expand the focus from mere maximization of CO2 sequestration and avoid false incentives from carbon markets.

Sharp decline in future productivity of tropical reforestation above 29°C mean annual temperature.

Tropical reforestation is among the most powerful tools for carbon sequestration. Yet, climate change impacts on productivity are often not accounted for when estimating its mitigation potential. Using the process-based forest growth model 3-PGmix, we analyzed future productivity of tropical reforestation in Central America. Around 29°C mean annual temperature, productivity sharply and consistently declined (-11% per 1°C of warming) across all tropical lowland climate zones and five tree species spanning a wide range of ecological characteristics. Under a high-emission scenario (SSP3-7.0), productivity of dry tropical reforestation nearly halved and tropical moist and rain forest sites showed moderate losses around 10% by the end of the century. Under SSP2-4.5, tropical moist and rain forest sites were resilient and tropical dry forest sites showed moderate losses (-17%). Increased vapor pressure deficit, caused by increasing temperatures, was the main driver of growth decline. Thus, to continue following high-emission pathways could reduce the effectiveness of reforestation as climate action tool.

Climate change may induce connectivity loss and mountaintop extinction in Central American forests.

The tropical forests of Central America serve a pivotal role as biodiversity hotspots and provide ecosystem services securing human livelihood. However, climate change is expected to affect the species composition of forest ecosystems, lead to forest type transitions and trigger irrecoverable losses of habitat and biodiversity. Here, we investigate potential impacts of climate change on the environmental suitability of main plant functional types (PFTs) across Central America. Using a large database of occurrence records and physiological data, we classify tree species into trait-based groups and project their suitability under three representative concentration pathways (RCPs 2.6, 4.5 and 8.5) with an ensemble of state-of-the-art correlative modelling methods. Our results forecast transitions from wet towards generalist or dry forest PFTs for large parts of the study region. Moreover, suitable area for wet-adapted PFTs is projected to latitudinally diverge and lose connectivity, while expected upslope shifts of montane species point to high risks of mountaintop extinction. These findings underline the urgent need to safeguard the connectivity of habitats through biological corridors and extend protected areas in the identified transition hotspots.

Evaluating the suitability of management strategies of pure Norway spruce forests in the Black Forest area of southwest Germany for adaptation to or mitigation of climate change.

The study deals with the problem of evaluating management strategies for pure stands of Norway spruce (Picea abies Karst) to balance adaptation to and mitigation of climate change, taking into account multiple objectives of a forest owner. A simulation and optimization approach was used to evaluate the management of a 1000 ha model Age-Class forest, representing the age-class distribution of an area of 66,000 ha of pure Norway spruce forests in the Black Forest region of Southwest Germany. Eight silvicultural scenarios comprising five forest conversion schemes which were interpreted as "adaptation" strategies which aims at increasing the proportion of Beech, that is expected to better cope with climate change than the existing Norway spruce, and three conventional strategies including a "Do-nothing" alternative classified as "mitigation", trying to keep rather higher levels of growing stock of spruce, were simulated using the empirical growth simulator BWINPro-S. A linear programming approach was adapted to simultaneously maximize the net present values of carbon sequestration and timber production subject to the two constraints of wood even flow and partial protection of the oldest (nature protection). The optimized plan, with the global utility of 11,687 /ha in forty years, allocated a combination of silvicultural scenarios to the entire forest area. Overall, strategies classified as "mitigation" were favored, while strategies falling into the "adaptation"-category were limited to the youngest age-classes in the optimal solution. Carbon sequestration of the "Do-nothing" alternative was between 1.72 and 1.85 million tons higher than the other alternatives for the entire forest area while the differences between the adaptation and mitigation approaches were approximately 133,000 tons. Sensitivity analysis showed that a carbon price of 21 /t is the threshold at which carbon sequestration is promoted, while an interest rate of above 2% would decrease the amount of carbon.

Retention as an integrated biodiversity conservation approach for continuous-cover forestry in Europe.

Retention forestry implies that biological legacies like dead and living trees are deliberately selected and retained beyond harvesting cycles to benefit biodiversity and ecosystem functioning. This model has been applied for several decades in even-aged, clearcutting (CC) systems but less so in uneven-aged, continuous-cover forestry (CCF). We provide an overview of retention in CCF in temperate regions of Europe, currently largely focused on habitat trees and dead wood. The relevance of current meta-analyses and many other studies on retention in CC is limited since they emphasize larger patches in open surroundings. Therefore, we reflect here on the ecological foundations and socio-economic frameworks of retention approaches in CCF, and highlight several areas with development potential for the future. Conclusions from this perspective paper, based on both research and current practice on several continents, although highlighting Europe, are also relevant to other temperate regions of the world using continuous-cover forest management approaches.

Evaluating the effectiveness of retention forestry to enhance biodiversity in production forests of Central Europe using an interdisciplinary, multi-scale approach.

Retention forestry, which retains a portion of the original stand at the time of harvesting to maintain continuity of structural and compositional diversity, has been originally developed to mitigate the impacts of clear-cutting. Retention of habitat trees and deadwood has since become common practice also in continuous-cover forests of Central Europe. While the use of retention in these forests is plausible, the evidence base for its application is lacking, trade-offs have not been quantified, it is not clear what support it receives from forest owners and other stakeholders and how it is best integrated into forest management practices. The Research Training Group ConFoBi (Conservation of Forest Biodiversity in Multiple-use Landscapes of Central Europe) focusses on the effectiveness of retention forestry, combining ecological studies on forest biodiversity with social and economic studies of biodiversity conservation across multiple spatial scales. The aim of ConFoBi is to assess whether and how structural retention measures are appropriate for the conservation of forest biodiversity in uneven-aged and selectively harvested continuous-cover forests of temperate Europe. The study design is based on a pool of 135 plots (1 ha) distributed along gradients of forest connectivity and structure. The main objectives are (a) to investigate the effects of structural elements and landscape context on multiple taxa, including different trophic and functional groups, to evaluate the effectiveness of retention practices for biodiversity conservation; (b) to analyze how forest biodiversity conservation is perceived and practiced, and what costs and benefits it creates; and (c) to identify how biodiversity conservation can be effectively integrated in multi-functional forest management. ConFoBi will quantify retention levels required across the landscape, as well as the socio-economic prerequisites for their implementation by forest owners and managers. ConFoBi's research results will provide an evidence base for integrating biodiversity conservation into forest management in temperate forests.

Diversification of forest management regimes secures tree microhabitats and bird abundance under climate change.

The loss of biodiversity in temperate forests due to combined effect of climate change and forest management poses a major threat to the functioning of these ecosystems in the future. Climate change is expected to modify ecological processes and amplify disturbances, compromising the provisioning of multiple ecosystem services. Here we investigate the impacts of climate change and forest management on the abundance of tree microhabitats and forest birds as biodiversity proxies, using an integrated modelling approach. To perform our analysis, we calibrated tree microhabitat and bird abundance in a forest landscape in Southwestern Germany, and coupled them with a climate sensitive forest growth model. Our results show generally positive impacts of climate warming and higher harvesting intensity on bird abundance, with up to 30% increase. Conversely, climate change and wood removals above 5% of the standing volume led to a loss of tree microhabitats. A diversified set of management regimes with different harvesting intensities applied in a landscape scale was required to balance this trade-off. For example, to maximize the expected bird abundance (up to 11%) and to avoid tree microhabitat abundance loss of >20% necessitates setting aside 10.2% of the forest area aside and application of harvesting intensities < 10.4% of the standing volume. We conclude that promoting forest structural complexity by diversifying management regimes across the landscape will be key to maintain forest biodiversity in temperate forests under climate change.

Forest carbon allocation modelling under climate change.

Carbon allocation plays a key role in ecosystem dynamics and plant adaptation to changing environmental conditions. Hence, proper description of this process in vegetation models is crucial for the simulations of the impact of climate change on carbon cycling in forests. Here we review how carbon allocation modelling is currently implemented in 31 contrasting models to identify the main gaps compared with our theoretical and empirical understanding of carbon allocation. A hybrid approach based on combining several principles and/or types of carbon allocation modelling prevailed in the examined models, while physiologically more sophisticated approaches were used less often than empirical ones. The analysis revealed that, although the number of carbon allocation studies over the past 10 years has substantially increased, some background processes are still insufficiently understood and some issues in models are frequently poorly represented, oversimplified or even omitted. Hence, current challenges for carbon allocation modelling in forest ecosystems are (i) to overcome remaining limits in process understanding, particularly regarding the impact of disturbances on carbon allocation, accumulation and utilization of nonstructural carbohydrates, and carbon use by symbionts, and (ii) to implement existing knowledge of carbon allocation into defence, regeneration and improved resource uptake in order to better account for changing environmental conditions.

Multiple uncertainties require a change of conservation practices for saproxylic beetles in managed temperate forests.

In Europe, intensive forest management has severely compromised the habitat of forest insects, especially saproxylic beetles, due to the removal of deadwood and veteran trees. The loss of insect diversity may disrupt ecosystem functioning and affect the provision of important ecosystem goods and services in the future. Here we propose a novel approach for the implementation of conservation policies, by optimally allocating forest reserves and deadwood islands under multiple sources of uncertainty and minimizing economic risk. We use the saproxylic beetle Lucanus cervus as umbrella species, requiring that deadwood islands were spaced within its dispersal capacity. We show that current management and conservation practices are increasingly inefficient under changing environmental conditions and that the consideration of uncertainty requires a major expansion of conservation areas. Moreover, our results indicate that a strong diversification of management regimes, with a focus on selection forest systems, is required to reduce economic risk of forest management. We conclude that the integration of uncertainty into conservation planning may reduce the trade-off between production and conservation objectives in forest landscapes and is key to increase the efficiency of forest management in the future.

Realizing Mitigation Efficiency of European Commercial Forests by Climate Smart Forestry.

European temperate and boreal forests sequester up to 12% of Europe's annual carbon emissions. Forest carbon density can be manipulated through management to maximize its climate mitigation potential, and fast-growing tree species may contribute the most to Climate Smart Forestry (CSF) compared to slow-growing hardwoods. This type of CSF takes into account not only forest resource potentials in sequestering carbon, but also the economic impact of regional forest products and discounts both variables over time. We used the process-based forest model 4 C to simulate European commercial forests' growth conditions and coupled it with an optimization algorithm to simulate the implementation of CSF for 18 European countries encompassing 68.3 million ha of forest (42.4% of total EU-28 forest area). We found a European CSF policy that could sequester 7.3-11.1 billion tons of carbon, projected to be worth 103 to 141 billion euros in the 21st century. An efficient CSF policy would allocate carbon sequestration to European countries with a lower wood price, lower labor costs, high harvest costs, or a mixture thereof to increase its economic efficiency. This policy prioritized the allocation of mitigation efforts to northern, eastern and central European countries and favored fast growing conifers Picea abies and Pinus sylvestris to broadleaves Fagus sylvatica and Quercus species.

How do people's perceptions and climatic disaster experiences influence their daily behaviors regarding adaptation to climate change? - A case study among young generations.

Adaptation is a commonly applied strategy used to address individual behavior changes, in response to climate change. However, in-depth, evidence-based investigations of the relationships among individual perceptions, climatic disaster experiences, and daily behaviors regarding adaptation to climate change remain to be conducted. We obtained survey data from 488 respondents in southwestern China, a region prone to frequent and severe droughts, to assess factors that influence adaptive behaviors and to identify their pathways. We applied Construal Level Theory (CLT) and the Theory of Planned Behavior (TPB) to differentiate between respondents' high-level abstract construals and their low-level concrete construals. We analyzed the influences of these two levels of perception, combined with drought experiences on water-saving behaviors. We developed a structural equation model to estimate the correlation coefficients of the latent and observed variables in the structural process linked to the respondents' adaptive behaviors. The results found that a concrete perception of saving water plays a more significant part than an abstract perception of climate change in prompting specific adaptive behaviors. Improving public perceptions of climate change might increase the desirability of adaptation, whereas improving perceptions of water saving might increase the feasibility of implementing adaptive measures. Experience influenced individual behaviors, but that influence was indirect through its effects on perceptions.

Are forest disturbances amplifying or canceling out climate change-induced productivity changes in European forests?

Recent studies projecting future climate change impacts on forests mainly consider either the effects of climate change on productivity or on disturbances. However, productivity and disturbances are intrinsically linked because 1) disturbances directly affect forest productivity (e.g. via a reduction in leaf area, growing stock or resource-use efficiency), and 2) disturbance susceptibility is often coupled to a certain development phase of the forest with productivity determining the time a forest is in this specific phase of susceptibility. The objective of this paper is to provide an overview of forest productivity changes in different forest regions in Europe under climate change, and partition these changes into effects induced by climate change alone and by climate change and disturbances. We present projections of climate change impacts on forest productivity from state-of-the-art forest models that dynamically simulate forest productivity and the effects of the main European disturbance agents (fire, storm, insects), driven by the same climate scenario in seven forest case studies along a large climatic gradient throughout Europe. Our study shows that, in most cases, including disturbances in the simulations exaggerate ongoing productivity declines or cancel out productivity gains in response to climate change. In fewer cases, disturbances also increase productivity or buffer climate-change induced productivity losses, e.g. because low severity fires can alleviate resource competition and increase fertilization. Even though our results cannot simply be extrapolated to other types of forests and disturbances, we argue that it is necessary to interpret climate change-induced productivity and disturbance changes jointly to capture the full range of climate change impacts on forests and to plan adaptation measures.