BG3.10

Forest health, although not yet unanimously defined, has been monitored in the past forty years assessing tree vitality, trying to estimate tree photosynthesis rates and productivity. Used in monitoring forest decline in Central Europe since the 1980s, crown foliage transparency has been commonly believed to be the best indicator of tree condition in relation to air pollution, although annual variations appear more closely related to water stress. Although crown transparency is not a good indicator of tree photosynthesis rates, defoliation is still one of the most used indicators of tree vitality. Tree rings have been often used as indicators of past productivity. However, long-term tree-growth trends are difficult to interpret because of sampling bias, and ring-width patterns do not provide any information about tree physiological processes. In the past two decades, tree-ring carbon and oxygen stable isotopes have been used  to reconstruct the impact of past climatic events, such as drought. They have proven to be useful tools for retrospectively understanding physiological processes and tree response to  stress factors. Tree-ring stable isotopes integrate crown transpiration rates and photosynthesis rates and may enhance our understanding of tree vitality. They are promising indicators of tree vitality. We call for the use of tree-ring stable isotopes in future monitoring programmes.

How to cite: Cherubini, P., Battipaglia, G., and Innes, J. L.: Tree vitality and forest health: any better indicators?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15377, https://doi.org/10.5194/egusphere-egu21-15377, 2021.

Climate change in the Mediterranean region leads to an intensification of summer droughts. These episodes of extreme water stress threaten the survival of tree species and, by the same token, would affect the structure and ecosystem services of woodlands. Indeed, in conditions of prolonged and intense drought, one of the major risks for trees is the hydraulic failure due to high embolism level. Xylem embolism risk depends essentially on various leaf and hydraulic traits including (i) the vulnerability of their xylem to cavitation, (ii) the turgor loss point (a surrogate for stomatal control) and (iii) their cuticular transpiration (gmin). The two former traits can be used to compute hydraulic safety margins (HSM).

In order to assess whether trees will survive future climatic conditions, it is necessary to quantify and assess the plasticity of these traits to intensified drought. In this study, we used three rainfall exclusion experiments established in mature forests in south-eastern France (Font-blanche, Puéchabon and O3HP experimental sites) to measure these traits and evaluate their ability to adjust to aggravated drought conditions for three Mediterranean widespread species: Quercus ilex, Quercus Pubescens, and Pinus halepensis. We performed pressure-volume curves of trees from rainfall exclusion and control plots to see if adjustments of gmin and leaf hydraulic traits involved in stomatal regulation occurred in these three species. Using the optical method and cavitron, we also quantified the plasticity of xylem vulnerability to cavitation by comparing the values of water potential leading to a 50% reduction in plant hydraulic conductance (P50).

Our results show that Quercus pubescens has the lowest HSM while Quercus ilex has the highest. In addition, gmin is higher for Quercus pubescens than for the other two species. All together these results suggest that Quercus pubescens is the most vulnerable to drought among the three studied. Globally, for most traits and species no significant difference was found between treatments. The only exception was for Quercus ilex that exhibited lower turgor loss point (Ψtlp) in the dry treatment. Drought acclimation for these species may rather depend on other traits, such as leaf area reduction or rooting depth. To integrate the role of these traits to estimate the historic and future mortality risk for these species, the use of hydraulic based models will be of interest.

How to cite: Moreno, M., Simioni, G., Limousin, J.-M., Rodriguez-Calcerrada, J., Ruffault, J., Cochard, H., Torres, J., Delzon, S., Tournant, A., Dumas, P.-J., Davi, H., and Martin-StPaul, N.: Mediterranean trees vulnerability to climate change will not be minimized through hydraulic safety traits adjustments., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3850, https://doi.org/10.5194/egusphere-egu21-3850, 2021.

Forests play a key role in mitigating greenhouse gases and fighting climate change. However, numerous environmental stressors threaten the integrity and ecological functionalities of forests. In recent decades, the increase of drought events and fires occurrence is negatively influencing forest health, causing dieback events and higher rates of mortality, especially in the Mediterranean environments.

Studying the mechanisms of plants in response to these events and relating them to the duration and intensity of stress can be the key to understand the vulnerability and sensitivity at individual and regional scale. Currently, most of the available studies are severely limited in time and space, providing information with a relatively poor temporal resolution.

In this context, our research aims to examine the effects of these events on the ecophysiology of Pinus pinaster Aiton, a very common conifer species in the Mediterranean environment, through the use of the innovative TreeTalker device (TT+). This instrument is able to monitor multiple physiological and environmental parameters of the tree such as sap flow, the amount of light absorbed by the canopy, meteorological information etc. The study is conducted in Southern Italy, more precisely at the Vesuvius National Park, affected in recent years by severe drought conditions and where a large wildfire occurred in July 2017. To evaluate the incidence of stress conditions, during the spring of 2020, 10 TT+ devices were installed in a pine stand affected by fire (Burned Site -BS) and 10 TT+ devices in a second stand called Control Site (CS) in which plants were not affected by the 2017 fire.

The preliminary monitoring data show interesting information about the hydraulic and stomatal strategies implemented by the trees on both stands according to the variation of the climatic conditions. While in the spring a rather regular sap flow trend was observed in both stands, during the summer months (July, August and good part of September), the trees show a reduction in their stomatal activity during the hottest hours of the day (11 am -15 pm), predictably as a mean to avoid episodes of xylem cavitation and to contrast the high temperatures. In the autumn months of October and November, however, vegetative activity has continued uninterrupted although a considerable decrease in hydraulic flow was registered. Finally, from the data collected it emerges that the severe reduction of the crown suffered by the plants of the BS has determined a lower absorption capacity of photosynthetic light, exposing these individuals to a greater possibility of carbon starvation.

The monitoring activities will continue for the next few years, allowing to understand better the eco-physiological dynamics leading the individuals of this species to overcome or succumb to stress events and/or extreme climatic conditions.

How to cite: Niccoli, F., Pacheco-Solana, A., Castaldi, S., Valentini, R., and Battipaglia, G.: Mediterranean forests responses to drought and forest fires: continuous monitoring through the TreeTalker system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7512, https://doi.org/10.5194/egusphere-egu21-7512, 2021.

Current research is presented on global-scale patterns and trends of forest responses to increasingly hotter droughts, particularly extensive tree mortality and forest die-offs involving a range of interactive disturbances (e.g., water stress, insect outbreaks, high-severity wildfire), illustrating emerging ecological changes and growing tipping-point risks to forests worldwide from hotter-drought extremes.  Cross-scale observations and empirical findings – ranging from controlled experiments in pots and local plot-level vegetation data, to networks of carbon flux monitoring sites and globally synoptic remote-sensing data – increasingly indicate that amelioration of hotter-drought stress via fertilization of photosynthesis from elevated atmospheric CO²concentrations may soon be overwhelmed by heat and accelerated atmospheric drought.  These findings highlight some current challenges in realistically projecting the future of global forest ecosystems (and their associated carbon pools and fluxes) with process-based Earth system models.  In particular there is substantial evidence that ‘historical forests’ – established by circa 1880 and dominated by larger, older trees – may be disproportionately vulnerable to increased growth stress and mortality under hotter-drought conditions from ongoing anthropogenic climate change.  The fates of the world’s biggest, oldest trees and historical forests in response to global change are of vital importance, given that they are essential as:  a) disproportionately large carbon sinks;  b) among the most biodiverse and rare terrestrial ecosystems;  c) irreplaceable archives of environmental history;  and d) venerated for many spiritual, aesthetic, and other cultural reasons.  Key scientific uncertainties that impede modeling progress are outlined, and examples of promising empirical modeling approaches are illustrated.

How to cite: Allen, C. D. and Hammond, W. M.: The Global Emergence of Hotter-Drought Drivers of Forest Disturbance Tipping Points, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12096, https://doi.org/10.5194/egusphere-egu21-12096, 2021.

Earth’s forests face grave challenges in the Anthropocene, including hotter droughts increasingly associated with widespread forest die-off. But despite the vital importance of forests—especially historical forests—to global ecosystem services, their fates in a warming world remain highly uncertain. Critically missing is quantitative determination of hotter-drought climatic drivers at globally-distributed, ground-based, tree-mortality sites. We established a precisely geo-referenced global database documenting climate-induced mortality events spanning all tree-supporting biomes from 154 studies since 1970. Here we quantify a lethal global hotter-drought fingerprint from these tree-mortality sites across 675 locations encompassing 1,303 database plots. Frequency of these lethal climate conditions accelerates under projected warming, up 140% by +4℃. Our database, soon available at tree-mortality.net, provides initial footing for further community development of quantitative, ground-based monitoring of global tree mortality (e.g., still including peer-reviewed observations, but importantly also those from forestry professionals, land managers, and citizen scientists). Furthermore, our database immediately enables critical predictive model validation and improved remote sensing of mortality. While our initial database enabled empirical quantification of a global climate signal for hotter-drought triggered tree mortality, ongoing and online contributions to the database (with efforts to be more spatially representative) will enable myriad future analyses and progress toward understanding the role of hotter-drought in the mechanistically complex process of tree mortality. Our global fingerprint of lethal hotter-drought confirms many of Earth’s forests are increasingly imperiled by further warming.

How to cite: Hammond, W. M., Williams, A. P., Abatzoglou, J. T., Adams, H. D., Klein, T., Rodríguez, R. L., Sáenz-Romero, C., Hartmann, H., Breshears, D. D., and Allen, C. D.: A hotter-drought fingerprint on Earth’s forest mortality sites–warming accelerates risks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6153, https://doi.org/10.5194/egusphere-egu21-6153, 2021.

The climate change has brought better environmental conditions for numerous bark beetles to reproduce in unmet amounts. Large-scale tree mortality events have been witnessed globally due to mass outbreaks of phloem feeding pest insects, such as Ips typographus (L.), that are jeopardizing numerous ecosystem services forests provide. To be able to assess the current and future bark beetle-induced tree mortality, we need more profound understanding of the processes that occur after the infestation of a tree, eventually leading to tree mortality. We measured the diurnal variation in tree stem diameter from four healthy and four infested trees trees during an I. typographus infestation in Helsinki, Finland, of which the four infested trees died during the investigation period between June and September in 2020. The condition of the tree crowns was also visually assessed in the beginning and the end of the study period.

We found that the amplitude of diurnal diameter variation was considerably smaller in the infested trees compared to healthy trees indicating smaller diurnal variation in the water content of the stem. The decrease in diurnal diameter variation was followed by abrupt and irreversible declines in tree diameter likely indicating tissue damage due to hydraulic failure. The declines were triggered largely by increased atmospheric water demand during the hottest days of the investigation period. The condition of the tree crown in the beginning of the study did not reflect the timing of the decline in tree diameter, but one of the most visually vital trees declined first.

The results indicate that hotter summer temperatures will increase and hasten bark beetle-induced tree mortality. This happens because irreversible hydraulic failure seems to occur in a cross pressure of bark beetle-induced stress and increased atmospheric water demand. Trees are likely more vulnerable to bark beetle-related hydraulic failure in the future because of increasing atmospheric water demand and more intense droughts. The triggers and processes that cause bark beetle-related tree mortality need more careful investigation to incorporate them into models that forecast tree mortality.

How to cite: Junttila, S., Hölttä, T., Saarinen, N., and Vastaranta, M.: What makes a tree die? - Bark beetle-induced mortality causes abrupt declines in tree diameter, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7214, https://doi.org/10.5194/egusphere-egu21-7214, 2021.

One foundational assumption of trait-based ecology is that functional traits can predict species demography. Yet, in general, the links between traits and demographic rates are not as strong as usually assumed. These weak associations may be due to two main reasons: the use of easy-to-measure traits as proxies of tree species performance, and the lack of consideration of size-related variations in both traits and demographic rates.

Here, we examined the associations between wood functional traits and mortality rates of 19 tree species from Eastern Amazonia. We measured eleven wood traits (i.e., structural, anatomical and chemical) in sapling, juvenile and adult wood, and related them to corresponding mortality rates.

Both sapling and juvenile mortality rates were best explained by wood specific gravity (WSG) and vessel lumen area (Va), while adult mortality was predicted only by Va. On the other hand, we found that the predictive power of wood trait on mortality rates decreased from saplings to adults.

These results indicate that the associations between traits and mortality rates can change during tree development, and also that hard-to-measure traits, such as wood chemical or anatomical traits, may be better predictors mortality rates than WSG. Our findings are important to expand our knowledge on tree life-history variations and community dynamics in tropical forests.

How to cite: Gonzalez-Melo, A. and Posada, J.: The predictive power of wood traits on species mortality change during tree development, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-618, https://doi.org/10.5194/egusphere-egu21-618, 2021.

Forest demographic processes - growth, recruitment and mortality - are being altered by global change. The changing balance between growth and mortality strongly influences forest dynamics and the carbon balance. Elevated atmospheric carbon dioxide (eCO₂) has been reported to enhance photosynthesis and tree growth rates by increasing both light-use efficiency (LUE) and water-use efficiency (WUE). Tree growth enhancement could be translated into an increase in biomass stocks or could be associated with a reduction in the longevity of trees, thus reducing the ability of forest ecosystems to act as carbon sinks over long timescales. These links between growth and mortality, and the implications for forest stand density and self-thinning relationships are still debated. Scarce empirical evidence exists for how changing drivers affect tree mortality due to existing data and modelling limitations. Understanding the causes of observed mortality trends and the mechanisms underlying these processes is critical for accurate projections of global terrestrial carbon storage and its feedbacks to anthropogenic climate change.

Here, we combine a mechanistic model with empirical forest data to better understand the causes of changes in tree mortality and the implications for past and future trends in forest tree density. Specifically, we test the Grow-Fast-Die-Young hypothesis to investigate if a leaf-level CO₂ fertilization effect may lead to an increase in the biomass stock in forest stands. We use a novel vegetation demography model (LM3-PPA) which includes vegetation dynamics with biogeochemical processes allowing for explicit representation of individuals and a mechanistic treatment of tree mortality. The key links between leaf-level assimilation and stand dynamics depend on the carbon turnover time. In this sense, we investigate alternative mortality assumptions about the functional dependence of mortality on tree size, tree carbon balance or growth rate. These formulations represent typical approaches to simulate mortality in mechanistic forest models. Model simulations show that increasing photosynthetic LUE leads to higher biomass stocks, with contrasting behavior among mortality assumptions. Empirical data from Swiss forest inventories support the results from the model simulations showing a shift upwards in the self-thinning relationships, with denser stands and bigger trees. This data-supported mortality-modelling helps to identify links between forest responses and environmental changes at the leaf, tree and stand levels and yields new insight into the causes of currently observed terrestrial carbon sinks and future responses.

How to cite: Marques, L., Weng, E., Bugmann, H., Forrester, D. I., Hobi, M., Kettunen, H.-R., Rohner, B., Thuerig, E., Trotsiuk, V., and Stocker, B. D.: Does forest growth acceleration lead to denser stands? Insights from Swiss forests and mechanistic modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8832, https://doi.org/10.5194/egusphere-egu21-8832, 2021.

Model predictions about future states of ecosystems under environmental change are uncertain. Understanding which factors drive these uncertainties is of immense value for directing research, but also for their interpretations. Here, we analyse sensitivities and uncertainties of a state of the art dynamic vegetation model (LPJ-GUESS) across European forests. We found that predictions of carbon fluxes are most sensitive to structure-related and mortality-related parameters, but most uncertainty is induced by drivers, nitrogen-, water- and mortality-modules. The uncertainty induced by drivers increases with increasing temperature, decreasing precipitation and from north to south across Europe. Moreover, environmental conditions change the resulting uncertainties in other processes. In this context, we encounter that the stress-gradient hypothesis is implicitly displayed in the model processes. In conclusion, our study stresses the importance of  environmental drivers for ecosystem predictions not only due to their uncertainty contributions but also because they determine the uncertainties of other processes.  

How to cite: Oberpriller, J., Anthoni, P., Arneth, A., Herschlein, C., Krause, A., Rammig, A., and Hartig, F.: Importance of drivers and processes for the predictive uncertainty in vegetation dynamics change across European forests, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3831, https://doi.org/10.5194/egusphere-egu21-3831, 2021.