The interacting effect of climate change and herbivory can trigger large-scale transformations of European temperate forests.

In many regions of Europe, large wild herbivores alter forest community composition through their foraging preferences, hinder the forest's natural adaptive responses to climate change, and reduce ecosystem resilience. We investigated a widespread European forest type, a mixed forest dominated by Picea abies, which has recently experienced an unprecedented level of disturbance across the continent. Using the forest landscape model iLand, we investigated the combined effect of climate change and herbivory on forest structure, composition, and carbon and identified conditions leading to ecosystem transitions on a 300-year timescale. Eight climate change scenarios, driven by Representative Concentration Pathways 4.5 and 8.5, combined with three levels of regeneration browsing, were tested. We found that the persistence of the current level of browsing pressure impedes adaptive changes in community composition and sustains the presence of the vulnerable yet less palatable P. abies. These development trajectories were tortuous, characterized by a high disturbance intensity. On the contrary, reduced herbivory initiated a transformation towards the naturally dominant broadleaved species that was associated with an increased forest carbon and a considerably reduced disturbance. The conditions of RCP4.5 combined with high and moderate browsing levels preserved the forest within its reference range of variability, defining the actual boundaries of resilience. The remaining combinations of browsing and climate change led to ecosystem transitions. Under RCP4.5 with browsing effects excluded, the new equilibrium conditions were achieved within 120 years, whereas the stabilization was delayed by 50-100 years under RCP8.5 with higher browsing intensities. We conclude that forests dominated by P. abies are prone to transitions driven by climate change. However, reducing herbivory can set the forest on a stable and predictable trajectory, whereas sustaining the current browsing levels can lead to heightened disturbance activity, extended transition times, and high variability in the target conditions.

Tree Biomass and Leaf Area Allometric Relations for Betula pendula Roth Based on Samplings in the Western Carpathians.

Biomass allometric relations are necessary for precise estimations of biomass forest stocks, as well as for the quantification of carbon sequestered by forest cover. Therefore, we attempted to create allometric models of total biomass in young silver birch (Betula pendula Roth) trees and their main components, i.e., leaves, branches, stem under bark, bark, and roots. The models were based on data from 180 sample trees with ages up to 15 years originating from natural regeneration at eight sites in the Western Carpathians (Slovakia). Sample trees represented individuals with stem base diameters (diameter D0) from about 4.0 to 113.0 mm and tree heights between 0.4 to 10.7 m. Each tree component was dried to constant mass and weighed. Moreover, subsamples of leaves (15 pieces of each tree) were scanned, dried, and weighed. Thus, we also obtained data for deriving a model expressing total leaf area at the tree level. The allometric models were in the form of regression relations using diameter D0 or tree height as predictors. The models, for instance, showed that while the total tree biomass of birches with a D0 of 50 mm (and a tree height of 4.06 m) was about 1653 g, the total tree biomass of those with a D0 of 100 mm (tree height 6.79 m) reached as much as 8501 g. Modeled total leaf areas for the trees with the above-mentioned dimensions were 2.37 m2 and 8.54 m2, respectively. The results prove that diameter D0 was a better predictor than tree height for both models of tree component biomass and total leaf area. Furthermore, we found that the contribution of individual tree components to total biomass changed with tree size. Specifically, while shares of leaves and roots decreased, those of all other components, especially stems with bark, increased. The derived allometric relations may be implemented for the calculation of biomass stock in birch-dominant or birch-admixed stands in the Western Carpathians or in other European regions, especially where no species- and region-specific models are available.

Spatial configuration matters when removing windfelled trees to manage bark beetle disturbances in Central European forest landscapes.

Windfelled Norway spruce (Picea abies) trees play a crucial role in triggering large-scale outbreaks of the European spruce bark beetle Ips typographus. Outbreak management therefore strives to remove windfelled trees to reduce the risk of outbreaks, a measure referred to as sanitation logging (SL). Although this practice has been traditionally applied, its efficiency in preventing outbreaks remains poorly understood. We used the landscape simulation model iLand to investigate the effects of different spatial configurations and intensities of SL of windfelled trees on the subsequent disturbance by bark beetles. We studied differences between SL applied evenly across the landscape, focused on the vicinity of roads (scenario of limited logging resources) and concentrated in a contiguous block (scenario of spatially diversified management objectives). We focused on a 16 050 ha forest landscape in Central Europe. The removal of >80% of all windfelled trees is required to substantially reduce bark beetle disturbances. Focusing SL on the vicinity of roads created a "fire break effect" on bark beetle spread, and was moderately efficient in reducing landscape-scale bark beetle disturbance. Block treatments substantially reduced outbreaks in treated areas. Leaving parts of the landscape untreated (e.g., conservation areas) had no significant amplifying effect on outbreaks in managed areas. Climate change increased bark beetle disturbances and reduced the effect of SL. Our results suggest that past outbreak management methods will not be sufficient to counteract climate-mediated increases in bark beetle disturbance.

Post-disturbance recovery of forest carbon in a temperate forest landscape under climate change.

Disturbances alter composition, structure, and functioning of forest ecosystems, and their legacies persist for decades to centuries. We investigated how temperate forest landscapes may recover their carbon (C) after severe wind and bark beetle disturbance, while being exposed to climate change. We used the forest landscape and disturbance model iLand to quantify (i) the recovery times of the total ecosystem C, (ii) the effect of climate change on C recovery, and (iii) the differential factors contributing to C recovery. We reconstructed a recent disturbance episode (2008-2016) based on Landsat satellite imagery, which affected 39% of the forest area in the 16,000 ha study landscape. We subsequently simulated forest recovery under a continuation of business-asusual management until 2100. Our results indicated that the recovery of the pre-disturbance C stocks (C payback time) was reached 17 years after the end of the disturbance episode. The C stocks of a theoretical undisturbed development trajectory were reached 30 years after the disturbance episode (C sequestration parity). Drier and warmer climates delayed simulated C recovery. Without the fertilizing effect of CO2, C payback times were delayed by 5-9 years, while C parity was not reached within the 21st century. Recovery was accelerated by an enhanced C uptake compared to undisturbed conditions (disturbance legacy sink effect) that persisted for 35 years after the disturbance episode. Future climate could have negative impacts on forest recovery and thus further amplify climate change through C loss from ecosystems, but the effect is strongly contingent on the magnitude and persistence of alleviating CO2 effects. Our modelling study highlights the need to consider both negative and positive effects of disturbance (i.e., C loss immediately after an event vs. enhanced C uptake of the recovering forest) in order to obtain a comprehensive understanding of disturbance effects on the forest C cycle.