BG3.11

Genetics, in concert with environmental factors, affect plant chemical defenses, and thereby plant susceptibility to pests, pathogens, and ultimately mortality. Plant resource allocation strategies in response to stress play important roles in balancing trade-offs and coordinating non-structural carbohydrate (NSC) investment between critical functions such as growth, storage, and chemical defense. Yet stress-induced growth-storage-defense (GSD) dynamics and their consequences for tree function in situ in the face of severe insect defoliation events are lacking. While vegetation model simulations have suggested that incorporating these dynamics will vastly impact our ability to predict outbreak “hotspots” and ultimately outbreak trajectories, we lack sufficient empirical studies describing herbivore-induced GSD relationships. Improving our predictive capabilities of tree-insect dynamics at the landscape level requires accurate quantification of plant defense dynamics in relationship to growth and storage, which can be accomplished using image spectroscopy.

To gain a more robust understanding of genetically-driven variation in NSC-chemical defense relationships—and the link to susceptibility or resilience in the face of invasive insect outbreak events—we leveraged a current Lymantria dispar outbreak occurring in an aspen (Populus tremuloides) common garden in Arlington, Wisconsin, USA comprised of 519 genotypes collected along a latitudinal gradient across Wisconsin. We measured shifts in the metabolome of targeted genotypes with known dissimilarity in phenolic glycosides and condensed tannin concentrations, the former being biologically active in defense against L. dispar. Targeted and untargeted metabolomics were used to assess shifts in leaf chemistry throughout the outbreak and whole-tree NSCs were measured concurrently. To evaluate the utility of imaging spectroscopy to quantify stress-induced chemical variation, remote sensing data were acquired concurrently using airborne and UAV-based HySpex and LiDAR sensors along with leaf-level reflectance measurements.

During the 2021 growing season, hyperspectral imagery shows distinct changes in foliar traits spatially and among genotypes over the course of the defoliation and during foliar reflush. LiDAR data illustrate discontinuous temporal patterns of defoliation during the event, likely due to spatial patterns of egg mass distribution rather than differences among genotypes. The leaf spectral dissimilarity analysis across all bands shows greater spectral variation among genotypes after defoliation than before defoliation. However, untargeted metabolomics indicates that leaf phytochemical profiles are more homogeneous following the outbreak, largely due to the enhanced production of phenolic glycosides. This suggests that other primary metabolites may be responsible for explaining a higher proportion of the spectral variation. Analysis of NSC dynamics is ongoing, but we expect to see differential shifts in tissue-specific NSC pools (specifically, starch) in response to herbivory, which are likely to be related to foliar defensive chemistry both prior to and following the defoliation event.

How to cite: Trowbridge, A. M., Dao, P. D., Hills, W. B., Friedman, M. S., Oliveira, C., Zierdan, M., Lindroth, R. L., and Townsend, P. A.: Using image spectroscopy to assess the genetic and environmental controls governing tree chemical defense responses to an irruptive herbivore, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10182, https://doi.org/10.5194/egusphere-egu22-10182, 2022.

Modeling forest drought-induced mortality is critical for predicting the impacts of climate change on ecosystems, natural resources, and global carbon- and water-cycles. The incorporation of mechanistic representations of how water moves through plants (i.e., plant hydraulics) in vegetation, land surface, and Earth system models has enabled estimating the degree of vascular damage that plants experience due to drought stress, with the possibility of mechanistically relating vascular stress to plant mortality. We used forest inventory data and a plant hydraulics model for predicting forest mortality across Western United States. We found that incorporating plant hydraulic model outputs of vascular damage and photosynthetic assimilation in generalized linear models and random forest models improved forest mortality predictions. Nevertheless, the variance explained by these models was relatively low. We use this study to highlight which are the challenges for predicting forest drought-induced mortality at landscape scales. We also propose future research lines that will help close existing knowledge gaps and improve mortality predictions.

How to cite: Venturas, M. D., Anderegg, W. R. L., Trugman, A. T., López, R., and Gil, L.: The challenges of predicting drought-induced forest mortality using plant hydraulic models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7688, https://doi.org/10.5194/egusphere-egu22-7688, 2022.

European forests are an important source for timber production, human welfare, income, protection and biodiversity. During the last two decades, Europe has experienced a number of droughts which were exceptionally within the last 500 years both in terms of duration and intensity and these droughts seem to left remarkable imprints in the mortality dynamics of European forests. However, systematic observations on tree decline with emphasis on single species together with high-resolution drought data has been scarce so far so that deeper insights into mortality dynamics and drought occurrence is still limiting our understanding at continental scale.

Here we make use of the ICP Forest crown defoliation dataset, permitting us to retrospectively monitor tree mortality for four major conifers, two major broadleaves as well as a pooled dataset of nearly all minor tree species in Europe. In total, we analysed more than 3 million observations gathered during the last 25 years and employed a high-resolution drought index which is able to assess soil moisture anomaly based on a hydrological water-balance and runoff model every ten days across the continent.

We found significant overall and species-specific increasing trends in mortality rates accompanied by decreasing soil moisture. In particular, previous-year soil moisture anomaly had a stronger influence on mortality rates than current-year soil moisture, suggesting that legacy effects (either physiological or caused by secondary biotic agents) play a keyrole in actual forest decline. Remarkable peaks in mortality occurred simultaneously in Norway spruce and Scots pine (2004, 2018, 2019), but were largely asynchronous in broadleaves. Mortality rates in Norway spruce and Scots pine have increased by 60% and 40%, respectively (period 2010-2020 compared to 1995-2009). Oak (Quercus robur and petraea) as well as Silver-fir (Abies alba) showed much lower mortality and only a weak upward trend.

We conclude that mortality patterns in European forests are currently reaching a concerning upward trend which could be further accelerated by future global change-type droughts.

How to cite: George, J.-P., Lang, M., Neumann, M., Sanders, T., Cammalleri, C., and Vogt, J.: Are European forests currently experiencing a shift in climate-related mortality? A retrospective analysis across the last 25 years., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1071, https://doi.org/10.5194/egusphere-egu22-1071, 2022.

Forest ecosystems are known to be paramount in maintaining the terrestrial carbon sink by storing nearly half of all terrestrial carbon, with European beech dominating these ecosystems across many parts of Europe. As such, the state of the carbon sink is mediated to a large degree by the productivity of European beech forests. Dynamic global vegetation models (DGVMs) can be useful tools to explore changes in forest productivity caused by climate extremes. However, DGVMs often lack implementation of processes pertaining to specific extremes, such as spring late-frost.
Though counterintuitive, temperature increases associated with climate change may exacerbate spring late-frost risk in European beech. Defoliation from late-frost can have detrimental effects on forest productivity. Affected trees lose the ability to effectively perform photosynthesis until the canopy is regrown and must also rely on the expenditure of stored carbohydrate reserves to do so. Consequently, late-frost events often result in stark reductions of secondary growth in affected trees. This effect has been widely observed, primarily through tree rings. Tree rings provide a localized, retrospective examination of secondary tree growth in response to frost events, yet they do not allow for quantification of subsequent changes in tree productivity over larger areas let alone for future scenarios.
We bridge this gap by using a network of tree ring data covering past frost events in conjunction with a new version of the DGVM LPJ-GUESS which has been expanded to include representation of late-frost events. The tree ring data covers 30 sites across Bavaria and includes four separate, past late-frost events for which varying degrees of late-frost damage have been previously documented. Using historical climate data (LFU BayObs 5km x 5km spatial resolution) we run simulations at each of the tree ring sites to reproduce the observed data. Subsequently, we run simulations with two versions of LPJ-GUESS, one including the new late-frost module and one without, to directly quantify the changes in tree productivity (NPP) as a result of spring-late frost. Lastly, we force LPJ-GUESS using climate projections to examine how spring late-frost will govern European beech forest productivity under different future scenarios.
Our findings indicate that (1) we are able to accurately reproduce observed late-frost events using the new, late-frost capable version of LPJ-GUESS, (2) spring late-frost events alter productivity dynamics across geographic and climatic regions, and (3) will continue to play a role to varying degrees under different future climate scenarios.

How to cite: Meyer, B. F., Buras, A., Principe, A., Kreyling, J., Rammig, A., and Zang, C. S.: Modeling changes in European beech productivity precipitated by spring late-frost, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11957, https://doi.org/10.5194/egusphere-egu22-11957, 2022.

In recent decades, die-off events in Pinus sylvestris populations have been on the rise. The causes of these phenomena, which are usually related to local and regional extreme hot droughts, have been extensively investigated from a physiological viewpoint. However, the consequences of the die-off process in terms of demography and vegetation dynamics have been less thoroughly addressed. Here, we projected P. sylvestris plot dynamics after a die-off event, under climate change scenarios, considering also their early stages (i.e., seedlings, saplings and ingrowth from the sapling to adult class), to assess the resilience of P. sylvestris populations after such events. We used IPM methodologies to project future plot structure under three climate scenarios (current climate, RCP4.5 and RCP8.0 projections), using climatic suitability – extracted from Species Distribution Models – as a covariable in the vital rates over time. Field data feeding IPM were obtained from two successive surveys, at the end of the die-off event (2013) and four years later (2017), undertaken on populations situated across the P. sylvestris range of distribution in Catalonia (NE Spain). Plots affected by die-off experienced a loss of large trees, which results in their basal area, tree diameter and tree density remaining lower than those of unaffected plots for decades. This situation is partially counterbalanced after the event in affected plots by a greater increase in the basal area and in seedling recruitment into the tree stage, thus promoting resilience. However, resilience is delayed under the climate-change scenarios with warmer and drier conditions involving additional physiological stress, due to a reduced abundance of seedlings and a smaller plot basal area. Overall, the study shows the lagged effect of drought-induced die-off events on forest structure and reveals stabilizing mechanisms which enhance resilience, such as recruitment and tree growth release. These mechanisms are apparently jeopardized, however, by regional warming.

How to cite: Margalef-Marrase, J., Molowny-Horas, R., Jaime, L., and Lloret, F.: Modelling the dynamics of Pinus sylvestris forests after a die-off event under climate change scenarios, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8770, https://doi.org/10.5194/egusphere-egu22-8770, 2022.

Climate change will potentially modify wind characteristics related to the passage of extra-tropical cyclones over Europe, and the increasing frequency of strong winds will increase forest damage. Negative impacts will be significantly higher in managed forests. Analyzing data about past forest damage can provide a better understanding of the complex relationship between tree/forest features, wind climate properties, and the rate of forest damage. We present results of forest damage modelling and prediction using machine learning techniques. We applied five machine learning methods to data on the volume (m³) of damaged trees between 2007-2010. In a second step, logistic regression was applied to these data expressed as a rate of damaged forest area. We focus on two case studies: the Sudety Mountains region in SW Poland and the catastrophic damage caused by windstorm Klaus in SW France in 2009. We found that the best predictors of tree damage were tree age and volume, the distance from the windstorm track, the normalised difference vegetation index (NDVI) and wind exposure. For the second case study, we tested several combinations of data splitting cut-off levels and various definitions of the damage class during the training stage of the modeling. We obtained models' good predictive power (accuracy and AUC > 0.7) for training and test sets in both cases.

The study has been supported by the Polish National Science Centre (project no 2018/28/U/ST10/00075 and 2019/35/O/ST10/00032) and the ERC Advanced Grant (project no 694481).

How to cite: Pawlik, L., Harrison, S., Godziek, J., and Zawolik, L.: Forest damage caused by the impact of strong wind during extra-tropical cyclones – modelling and prediction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3057, https://doi.org/10.5194/egusphere-egu22-3057, 2022.

Forest management can play an important role in addressing the issue of generalized canopy dieback and increase in tree mortality rates after severe droughts in both plantations and natural stands. In particular, there is interest in establishing the effects of thinning as a strategy to improve growth recovery after drought across tree species and climates. For that reason, we need a robust framework to determine general forest growth and drought resilience changes after thinning, and their temporal legacy. To that end, we designed a regression model to determine differences in radial growth, sensitivity to previous-year growth and drought, and long-term growth trends, as well as individual random variability. Once fitted based on basal area increment records, the model allows simulating post-drought and post-thinning growth trajectories based on the observed parameters. We computed drought resistance, resilience and recovery indices based on these trajectories, obtaining more reliable estimates than computing these indices based on the raw tree-ring records. Moreover, the simulations allowed us to calculate the time to recovery after a drought. We tested this analytical framework on five pine plantations of three species (Pinus halepensis Mill., Pinus nigra Arn. and Pinus sylvestris L.) under different thinning intensities, classified based on the average ranges of basal area removed as moderate (20-35%) and heavy (>35%) thinned. We found that thinning enhanced growth between +85 and +150%, and reduced previous-year growth dependence (between –13 and –26%) and climatic dependence (–23 to –49%). We interpret these effects as a result of competition reduction by thinning and a transitory alleviation of growth climatic constraints, particularly water shortage. Thinning consistently improved drought resistance (+4 to +20%) and resilience (+1 to +4%). Growth recovery, on the contrary, was reduced (–1 to –15%). Since the growth loss during the drought was reduced due to higher drought resistance, the recovery was proportionally lower. Thinning reduced the time to recovery by one to two years, and the thinning legacy effect persisted up to 15 to 20 years after thinning. Taken together, these findings enhance the benefits of adaptive silviculture in making pine plantations less vulnerable to unfavourable extreme climate events such as droughts. We present a novel and robust analytical framework to assess drought-thinning interactive effects on tree growth.

How to cite: Manrique-Alba, À., Beguería, S., and Camarero, J. J.: Forest management effects on post-drought growth resilience: a new analytical framework applied to pine plantations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4059, https://doi.org/10.5194/egusphere-egu22-4059, 2022.

Tree mortality in dryland forests is pronounced following extreme drought events. Moreover, long-term monitoring showed that the die-off rate of oaks in a Mediterranean woodland increased exponentially with decreasing annual rain amounts. In forests and woodlands of low water availability, trees are subjected to competition for water, and likely also for nutrients, by neighboring woody and herbaceous vegetation. Increasingly arid conditions induced by a hotter and drier climate intensify the “battle” for resources among plants. Trees might be partly released from competition by nearby vegetation when subjected to livestock grazing, a common forestry practice in many regions worldwide. Grazing not only reduces competing vegetation, but also modifies the chemical and physical soil environment. While the impact of grazing on herbaceous biomass, species composition and diversity has been extensively studied over the past decades, its influence on trees remains largely uncertain. We investigated tree growth, water relations and nutrient status as affected by livestock grazing in semiarid forests and woodlands subjected to extreme seasonal drought conditions. Livestock grazing alleviated drought stress of evergreen oaks (Quercus calliprinos) at the dry edge of their distribution and enhanced their growth 2-3 fold. Oaks in grazed plots showed higher leaf gas exchange and a lower drop in leaf water potential under very dry conditions than oaks in plots from which grazing was prevented. Grazing exclusion also shifted the trees’ water use strategy to be more isohydric. In addition, the size of oaks increased along a gradient of increasing grazing intensity, a gradient which did not include extreme overgrazing intensities. Grazing did not affect tree water relations and even tended to increase drought stress of Pinus halepensis trees in a very dry semiarid forest. Yet, the environmental growing conditions appeared to be improved for pines in grazed compared to pines in non-grazed plots, according to longer needles of trees in grazed plots. Needle length has previously been proven to act as a reliable index of tree health in this species. In a more moist Pinus pinea forest, grazing influenced soil moisture and leaf water potential negatively. However, tree-ring analyses showed a higher growth rate of P. pinea trees in grazed compared to trees in non-grazed plots, which may be related to higher nutrient availability (nitrogen, phosphorus) in soils under grazing. Livestock grazing intends to generate economic revenue and to reduce fire risks in forests. Here we showed that this type of forest management may decrease the vulnerability and potentially increase the resistance and resilience of trees to drought in forests and woodlands subjected to sparse water supply. Consequently, grazing may benefit trees in mesic forests experiencing increasing frequency and severity of extreme droughts and heatwaves.

How to cite: Grünzweig, J., Hasson, O., Burrows, L., Navon, Y., Klein, T., and Osem, Y.: Saving forests from climate change – can livestock grazing reduce the vulnerability of trees to drought?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9367, https://doi.org/10.5194/egusphere-egu22-9367, 2022.

Bark beetle infestation is a major causal agent of tree mortality that may be critical for forest persistence under future climates with increased warming and drought episodes. Such persistence, in terms of resistance to bark beetle disturbance, could be determined by the location of host tree populations in the species’ climatic niche space, increasing close to the host climatic optimum and reducing close to the beetle climatic optimum. Therefore, we analyzed the resistance of European coniferous forests to bark beetle attack and its derived tree mortality, using successive censuses of forest damage surveys in relation to the climatic niche characterization of both host tree and bark beetle species. Specifically, we modelled the responses of forest resistance in relation to the distance to the niche optimum of the host tree and beetle species, stand attributes and previous drought characteristics. Regional patterns of recent beetle disturbance evidenced that forests in Central, North, and East of Europe could be at risk under the attack of multivoltine bark beetle species. In addition, we found that forest resistance to beetle attack was determined by several driving factors. The environmental position of the affected forest within the host and beetle species’ climatic niche and the stand attributes mediated the influence of drought on the resistance to beetle attack. In particular, monospecific stands with a high frequency of drought in previous years and located close to both host tree and beetle climatic optimum showed low resistance to beetle attack. In turn, forest resistance to derived tree mortality was exclusively determined by the intensity and duration of previous drought. Once the forest resistance is exceeded to be infested, the mortality of host tree populations may be enhanced with the severity of drought events. These findings revealed that the resistance of European coniferous forests to bark beetle disturbance is modulated by the joint host-insect climatic suitability and by beetle-drought interactions. Moreover, the expected increase of extreme drought events in the coming decades, particularly under the threat of multivoltine bark beetle species activity, may amplify beetle-induced tree mortality threatening forest persistence.

How to cite: Jaime, L., Batllori, E., Ferretti, M., and Lloret, F.: Drivers of forest resistance to bark beetle disturbance in European forests, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4698, https://doi.org/10.5194/egusphere-egu22-4698, 2022.

Forest fires are becoming more intense and recurring due to climate change and are increasingly threatening the integrity and functionality of forests worldwide. Trees’ resilience is closely influenced not only by direct fire damages, but also by pre-existing climate stress conditions, such as high temperatures and water deficit. Fire wounds and extreme climate events can impair plant physiology triggering tree mortality in the medium and long term. Therefore, understanding the links between fire, climate and tree health is essential to anticipate the impacts of global warming and to plan climate-adapted forest management strategies. In this context, our research aims to study the post-fire effects on a Pinus pinaster Aiton forest growing in Vesuvius National Park, a particularly drought prone area in Southern Italy, comparing plants with severe damage to the canopy with non-defoliated trees. We combined inter-annual analyses of dendrochronology, carbon and oxygen isotope composition in tree rings, and intra-annual monitoring of xylogenesis to explore the effects on tree-growth, ecophysiological processes and wood formation dynamics. The tree-ring approach showed that crown damage compromised the photosynthetic activity of burned trees, with a decrease in tree-growth in the medium term compared to control trees. Moreover, the xylogenesis analysis demonstrated a delay in phenology and a lower xylem productivity and plasticity of the defoliated trees, as well as the negative influence of hot and dry months on cambial production. Our findings suggested that although maritime pine can survive severe forest fires in the short term, a severe crown defoliation and prolonged drought conditions can compromise the species' eco-physiological functions reducing the chances to regain the pre-disturbance productivity rates.

How to cite: Niccoli, F., Pacheco-Solana, A., De Micco, V., and Battipaglia, G.: Monitoring wood formation dynamics of Pinus pinaster Aiton in a burned area of Vesuvio National Park, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4485, https://doi.org/10.5194/egusphere-egu22-4485, 2022.

Forest grown in semi-arid regions are facing severe drought limitations, nevertheless they are still a considerable component of the global carbon cycle. Drought limitations on ecosystem carbon fluxes can be caused either by limited soil moisture supply, mainly expressed through the soil water content (SWC), or enhanced atmospheric demand, expressed through the atmospheric VPD. However, these two parameters are generally strongly correlated and the distinguish of their differential effects on ecosystem level processes is difficult. While soil water content is mainly affected by the local precipitation patterns, VPD is strongly affected by the air temperature. Both quantities are expected to alter under the current climate change scenarios, but the rise in temperature is expected to result in amplified increased in VPD. The ability to quantify the differential effects of SWC and VPD on ecosystem productivity is, therefore, critical for making predictions about future forest productivity and survival in the Mediterranean region under the ongoing climate change.

In this study we attempt to make a distinction between the impact of soil moisture and VPD on water limitations on GPP in Aleppo pine forests in Greece and in Israel. For the aim of the study, we performed GPP estimates for two Mediterranean sites in the two study sites that differ in water supply and local VPD. The separation of SWC and VPD limitations was achieved by the use of generalized additive models. Our results indicate that the parameter that dominates drought limitations in Aleppo pine is SWC, while VPD has a secondary and lesser effect.

Aleppo pine is a conifer species representing one of the major components of Mediterranean ecosystems, which is, in turn, one of the most vulnerable regions to climate change. The results imply, therefore, that the potentially enhanced increase in VPD, caused by rising temperatures in these regions, is likely to have a limited effect on future carbon fluxes, which will depend more strongly on trends in precipitation pattern.

How to cite: Markos, N., Preisler, Y., Radoglou, K., Rotenberg, E., and Yakir, D.: Comparative study of two Mediterranean pine forests demonstrates a dominant effects of soil moisture supply over atmospheric VPD in influencing gross primary productivity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12118, https://doi.org/10.5194/egusphere-egu22-12118, 2022.