No abstract

<p>In the past, terrestrial ecosystems have largely functioned as carbon sinks, capturing nearly 30% of anthropogenic carbon dioxide emissions (Le Quéré et al. 2009). Forest ecosystems, which cover roughly 30% of the land surface, play a fundamental role in maintaining this sink by storing nearly half of all terrestrial carbon (Pan et al. 2011; Bonan 2008). Over large parts of Europe, these forest ecosystems are dominated by European beech. Consequently, the reaction of beech to climate extremes is central to the ability of European forests to act as carbon sinks. Disconcertingly, the projected – and indeed already observed – increase in frequency and severity of drought across Europe threatens to shift forest ecosystems from carbon sinks to carbon sources (Ciais et al. 2005). Concurrently, the incidence of late-spring frost events in Europe is on the rise. While these events are considerably more localized and do not result in the same widespread reduction of ecosystem productivity as droughts, the damage to the photosynthetic apparatus of affected trees forces the mobilization of non-structural carbohydrates (NSC) to ensure tree survival. We analyze high-resolution historical (E-OBS 0.1°) and projected (EURO-CORDEX RCP 2.6 & RCP 8.5 0.11°) climate data to identify localized changes in the frequency of sequentially occurring drought and late-spring frost events across Europe. Subsequently, we use a modified version of the standalone NSC-model SUGAR (Jones et al. 2020) to ascertain the effect of sequentially occurring climate extremes on the carbon reserves of European beech forests. Here, we identify differences in the impact of isolated extremes (either frost or drought) and sequential extremes (frost followed by drought and vice versa) on the regulation of the NSC pool. Through the integration of SUGAR with the LPJ-GUESS DGVM (Smith et al. 2014; Sitch et al. 2003) we further quantify the effect of sequentially occurring climate extremes on the productivity of beech forest ecosystems in central Europe.</p><p> </p>

<p>According to climate projections, extreme summer drought conditions as those striking Central and Southern Europe in 2022 will become more frequent under climate change. Consequently, studying forests’ response to such extreme conditions may reveal important insights on how forests will cope with anticipated climate conditions. Of particular interest are questions related to forest-type specific drought sensitivities (e.g. broadleaved vs. coniferous, Mediterranean vs. temperate) and the existence of legacy effects from previous droughts (e.g. the extreme 2018 drought, see Buras <em>et al</em> 2020).</p> <p>While many approaches exist to address these questions at local scale, satellite borne remote sensing offers the opportunity to tackle these topics at large scale. Here, the MODIS mission provides a valuable source of information due to a relatively long observational period since the year 2000 at sufficient spatial resolution (250 m x 250 m) and a high sampling frequency (daily images which are used to compute 16-day maximum value composites). In context of monitoring forests' response to environmental conditions, MODIS NDVI renders a frequently considered data source since it reflects canopy greenness and consequently mirrors – among others – early leaf coloration and senescence as direct responses of trees to extreme drought. Yet, MODIS NDVI time-series need to pass a multi-step processing chain to mask poor-quality pixels, remove remaining outliers, gap-fill, and finally apply a pixel-specific standardization to achieve relative measures of canopy greenness. The recently launched European Forest Condition (EFCM, Buras <em>et al</em> 2021) provides correspondingly processed data, which can be used to monitor forest canopy condition in Europe through space and time.</p> <p>Here, we present first insights on the impact of the 2022 drought on European forest ecosystems based on the EFCM. Preliminary results indicate the drought 2022 to supersede previous droughts with regards to the spatial extent of severely affected pixels, thus breaking the former record from the 2018 drought (Buras <em>et al</em> 2020) just four years later. Our analyses suggest that legacy effects from previous years have contributed to this development. Moreover, we found different drought sensitivities of different forest types. In combination, these factors draw a complex picture of forests climate-change resilience, which we here seek to disentangle. Corresponding knowledge will likely provide valuable empirical information to improve model-based projections of tree-species performance under anticipated climate change.</p> <p>Buras A, Rammig A and Zang C S 2020 Quantifying impacts of the 2018 drought on European ecosystems in comparison to 2003 <em>Biogeosciences</em> <strong>17</strong> 1655–72</p> <p>Buras A, Rammig A and Zang C S 2021 The European Forest Condition Monitor: Using Remotely Sensed Forest Greenness to Identify Hot Spots of Forest Decline <em>Frontiers in Plant Science</em> <strong>12</strong> 2355</p>

<p>Besides offering numerous important ecosystem services, sustainably managed forests can help reduce atmospheric CO<sub>2</sub> concentrations and thus mitigate climate change. Forest-based mitigation occurs through the carbon sink in the forest itself, the carbon sink in wood products, and through substitution effects when wood products replace carbon-intensive materials and fuels.</p> <p>The relative importance of each of these three mitigation dimensions depends on a multitude of factors. First, forest type and structure, site conditions, and climate change and associated disturbances determine the amount of carbon that may be sequestered over the next decades at a given site. Second, the type and intensity of management determines the trade-off between on-site carbon sequestration and carbon storage in wood products. Third, management, wood usage patterns, and the carbon-intensity of the economy determine the amount of avoided emissions via substitution effects.</p> <p>To assess their impact on the total forest mitigation potential, we conducted a factorial modeling experiment by varying all of the aforementioned factors. Specifically, we looked at the forest type (needle-leaved vs broad-leaved) and age (young vs mature), increased and decreased harvest intensities, increased material wood usage and cascading, decarbonization rates, climate change and disturbance scenarios, and salvage logging practices after disturbance.</p> <p>Under an assumed "closer-to-nature forest management" our results show a higher mitigation potential of young forests compared to mature forests, whereas the forest type does not have a clear effect. The importance of substitution effects outweighs the importance of the forest and product carbon sink on shorter time scales. This changes towards the end of the century, assuming that substitution effects decrease because the substituted materials can be produced in a less carbon-intensive way. Increases in harvest intensity consequently are also only beneficial for climate change mitigation on these shorter time scales, though they likely have adverse effects on other ecosystem services. Our results also show that increased material usage (as opposed to energy usage) of wood can be an important lever for mitigation. Finally, changes in disturbances strongly affect the mitigation potential, though the mitigation impact of a subsequent salvaging operation heavily depends on the forest type and the product portfolio created from the salvaged wood.</p> <p>In conclusion, our results quantify the impacts and interactions of the different factors that govern forest-based mitigation, while highlighting the complexity of the topic and the importance of the considered time-scales.</p>

<p>Ecosystem disturbances such as wildfires, storms or insect outbreaks are important elements of forest dynamics. As a changing and more extreme climate is expected to lead to an increase in such disturbances in many places, they have to be considered in coupled land surface – atmosphere dynamics.</p><p>Next to releasing large pulses of carbon to the atmosphere through large-scale forest mortality, disturbances can also play an important role in catalyzing or enhancing ecosystem state shifts. In the boreal zone, results from field and landscape modeling studies indicate that disturbances drive transient or permanent shifts from needleleaf evergreen to broadleaf deciduous species. While such changes are also visible in biome-wide simulations, the role of disturbances therein remains open.</p><p>We here investigate the impact of changing disturbance regimes on the species composition of the boreal zone under climate change. We perform simulations with the Dynamic Global Vegetation Model LPJ-GUESS, which allows simulating disturbances and post-disturbance recovery through its representation of vegetation demographics and patch dynamics. We combine varying rates of stylized disturbances with different climate scenarios to create controlled simulation experiments of changing climate, changing disturbance regimes and their interactions.</p><p>Our simulations reproduce findings from previous studies and theory, with increasing disturbance rates leading to higher shares of deciduous trees in areas where they would be negligible in the absence of disturbance. We further investigate if these changes represent (1) a transient state of early-successional species that disappears again once disturbance pressure is lifted or (2) a stable reorganization of the ecosystem towards a deciduous-dominated forest.</p>