Research updates

Mo et al. combined ground-sourced and satellite-derived approaches to evaluate the scale of global forest carbon potential outside agricultural and urban lands. The study indicates that the total potential for additional forest carbon storage is 226 Gt. 61% of this can be achieved by protecting existing forests and allowing them to regrow to maturity. The remaining 39% can be achieved by reconnecting fragmented landscapes through community-driven ecosystem restoration and management.

In the first continental-scale analysis of post-disturbance forest recovery in Europe, Senf and Seidl used satellite data from 1986–2018 across 35 countries. Their analyses show that 69% of Europe's forests recover from disturbances within 30 years, but 14% have low or critical resilience. With climate change potentially increasing natural disturbances, enhancing forest resilience should be a priority for management, by reducing the intensity of harvest and planting trees after disturbances.

Throughout history, wood resources have been essential for human welfare. The volume of growing stock (GS) is considered one of the most important forest attributes monitored by National Forest Inventories (NFIs) to inform policy decisions and forest management planning. Gschwantner et al. provide an up-to-date review focusing on large-area GS monitoring, from the history of European NFIs to current methods, as well as an outlook under changing climate and forest-based bioeconomy objectives.

Carbon accounting in the land sector needs a reference level to calculate past carbon losses and potential gains. The carbon carrying capacity of primary forests is an ecologically-based reference, estimating mitigation potential achievable through implementing forest protection and restoration measures. Analysing tree inventory data from 7,982 sites across Europe, Keith et al. found that protecting and restoring forests could yield 309 megatons of CO₂ equivalents annually, exceeding current sinks and aligning with the Green Deal 2030 target.

The annual wood harvest in the EU was 1.1 m³ per year per capita in 2021. Rougieux et al. assess future management options and their impacts on the forest carbon sink. Their scenarios indicate that an intensified use of non-merchantable wood resources would reduce the area affected by direct management activities, and the forest carbon sink would almost meet the EU LULUCF target for forest land. Thereafter, however, the forest carbon sink will deteriorate rapidly due to increased wood consumption. Only a reduction in wood consumption appears to be compatible with the forest carbon budget which ensures meeting of the EU targets until 2050, while increase in harvesting intensity and wood utilisation could undermine those targets.

For German forest greenhouse gases reporting, deadwood carbon stock is calculated using volume, deadwood density, and carbon concentration for each decay class. Herrmann et al. developed a method to harmonize data collected from literature to supplement and improve the German data. Their meta-analysis on Central European tree species found the IPCC default value of 50% might under- and overestimate the real carbon concentration of spruce, pine and beech, depending on the decay class, by about 4% at the maximum.

Dalmonech et al. used a state-of-the-art biogeochemical forest growth model to simulate the effect of different management practices on productivity and carbon storage in European forests under 20 climate change scenarios. The study findings suggest that the business-as-usual forest management practices may be a close-to-optimum scheme for maintaining both the carbon uptake and woody stocks in forests, even under a changing climate. The results indicate that there is little further leeway to increase the carbon sink capacity in the forests without sacrificing the existing carbon storage. Instead, the authors emphasize that it is today crucial for EU countries to preserve forests’ functionalities under the pressure of the rapidly changing climate conditions, in order to maintain the climate mitigation potential and the supply of wood products and many ecological goods and services. The study highlights the limited potential for simultaneously increasing both net primary productivity (NPP) and potential carbon woody stocks (pCWS) through intensified management practices.

Stritih et al. used spaceborne lidar data to identify forest structure patterns across the European Alps, covering over 10.5 million ha. They identified two alternative states: tall, closed-canopy forests (76%) and short, open-canopy forests (24%). Within 35 years after disturbance, 72% of forests recovered to a closed-canopy state, except in submediterranean forests where recovery is slower. As climate warming increases disturbances and causes thermophilization of vegetation, transitions to open-canopy conditions could become more likely in future. This could pose a challenge for forest management, as open-canopy forests have lower capacities for providing important ecosystem services.

Tree crown defoliation is an important parameter in monitoring forests and climate disturbances, eg within the pan-European ICP Forests programme. Defoliation is defined as the loss of needles or leaves compared to a reference tree and serves as an unspecific indicator of tree health and vitality. Bussotti et al. highlight the need to connect defoliation levels with the physiological functioning of trees, as it integrates various intrinsic and extrinsic factors. The authors propose a set of physiological indicators for application in forest monitoring programs, including water relations, photosynthesis and carbon metabolism, growth, and mineral nutrients of leaves. They emphasize the importance of integrating these physiological measurements with traditional visual assessments to improve the prediction of tree mortality and forest decline under changing climatic conditions.

Unmanaged land areas are not included in current national reports on greenhouse gas emissions for the Paris Agreement. Nabuurs et al. argue that CO₂ fluxes from all forest land need to be recorded to help track progress towards global climate targets and fill the knowledge gap. They propose a gradual 4-step transition process to start in 2-3 years’ time, including adequate financial support for developing countries to improve their monitoring systems.

Remote sensing technology is a powerful tool for monitoring different stages of pest disturbance in a timely manner. Luo et al. evaluate remote sensing platforms, such as ground instruments to monitor needles, unmanned aerial vehicles for stand evaluations, manned aircraft and satellites for larger scales, as well as sensor technology (eg LiDAR, radar) and detection models. Precisely identifying host tree species or differentiating between wood-boring pests causing similar damage is challenging. 

To develop ecologically sustainable forest management practices, it is important to understand the management impacts on forest-dwelling organisms. Tinya et al. study metadata from 28 experimental field studies on the effects of forestry treatments on multi-taxa biodiversity. Based on the results they pose 8 research questions for management- and ecology-oriented studies which could be upscaled to the European level, and set out 11 knowledge gaps which require additional field experiments.

In the transition towards a sustainable economy, forests and their ecosystem services play a vital role. Schulz et al. propose a conceptual framework to describe and classify the potentially competing demands on forests and forest management, and the conflicts that may result. They suggest differentiating between goal conflicts at the policy formulation level, and trade-off situations in forest management and planning. They further distinguish between conflicts and trade-offs that occur “in” or “with” the forest. 

Accurate assessments of above-ground biomass carbon stocks are needed to quantify the climate mitigation benefits of e.g. forest restoration. Calders et al. use 3D laser measurements across the full range of tree size and shape in a typical UK temperate forest to assess tree size-to-mass allometric models, used since the 1960s for biomass calculation. They find 1.77 times more biomass than expected, due to bias towards small trees in the original models, and more abundant large trees from changes in forest management. 

To meet carbon neutrality goals, the EU27 net carbon sink from forests should increase to −450 Mt CO_2eq yr⁻¹ by 2050. Pilli et al. use a meta-modelling approach to show that if current management practices are continued, the EU27 + UK forest carbon sink would decrease to c. −250 Mt CO_2eq yr⁻¹ in 2050 and −80 Mt CO_2eq yr⁻¹ by 2100. However, climate change adds a considerable uncertainty, potentially nearly doubling or halving the sink associated with management. 

Van der Woude et al. investigate the impact of the 2022 summer drought in central and SE Europe on carbon exchange between European forests and the atmosphere, using ground-and space-based monitoring platforms. They find a reduction of net biospheric carbon uptake, despite partial compensation by a warm autumn. Comparison to the 2018 drought suggests this is no longer an exceptional situation, and important to factor into Europe’s plans for net-zero greenhouse gas emissions that rely on carbon uptake by forests.

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