BG3.22

With ongoing climate change, the predicted increase in climate variability is likely to increase the intensity of extreme drought events. This could significantly amplify the consequences of drought, because ecological responses are often non-linear. The importance of functional thresholds has been widely recognized, where comparably small changes in the stressor can have disproportional large consequences for ecosystem functioning. However, very few studies have actually tested for the functional thresholds of drought responses, which creates large uncertainties in our understanding of drought effects. Here, we aimed to determine the effects of drought intensity on plant productivity and to identify potential thresholds underpinning these responses.

We studied the effects of drought intensity on different measures of plant productivity using a gradient design. In a common garden experiment, we performed an experimental pulse drought of 3.5 weeks on planted monospecific mesocosms composed of the common grass Dactylis glomerata and the common forb Plantago lanceolata, respectively. We imposed a drought intensity gradient, which ranged from well-watered to extremely dry soil conditions. During drought and post-drought recovery we repeatedly measured productivity-related parameters, including gross primary productivity (GPP), Normalized Difference Vegetation Index (NDVI) and vegetative height, and assessed aboveground net primary production (ANPP) at peak drought and after recovery.

Drought intensity had non-linear effects on all studied parameters both during the resistance and the recovery phase. At peak drought we observed threshold responses at two stages of drought intensity. The first threshold occurred at moderate drought intensity and was related to a distinct downregulation of GPP and plant height. The second threshold was reflected in a steep decline of NDVI and leaf water content. During the recovery phase, high drought intensity stimulated productivity and regrowth rates. This resulted in an overshooting of biomass production by up to 100% in mesocosms previously exposed to severe drought, the effect being related to the first drought intensity threshold, which had led to a downregulation of GPP during drought. The overcompensation following exposure to high drought intensities was more pronounced for Plantago than for Dactylis. However, highest drought intensities clearly suppressed the recovery capacity of Plantago but not of Dactylis, which demonstrates species-specific differences in the effects of drought intensity on plant resilience.

We conclude that functional thresholds in drought and recovery responses of productivity are related and that with increasing drought intensity plants compensate decreased resistance with increased recovery to optimize overall resilience of productivity to drought.

How to cite: Ingrisch, J., Umlauf, N., and Bahn, M.: Functional thresholds of plant resistance and recovery to drought, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8333, https://doi.org/10.5194/egusphere-egu21-8333, 2021.

Manipulative experiments typically show a decrease in dryland biocrust cover and altered species composition under climate change. Biocrust-forming lichens, such as the globally distributed Diploschistes diacapsis, are particularly affected and show a decrease in cover with simulated climate change. However, the underlying mechanisms are not fully understood, and long-term interacting effects of different drivers are largely unknown due to the short-term nature of the experimental studies conducted so far. We addressed this gap and successfully parameterised a process-based model for D. diacapsis to quantify how changing atmospheric CO₂ , temperature, rainfall amount and relative humidity affect its photosynthetic activity and cover. We also mimicked a long-term manipulative climate change experiment to understand the mechanisms underlying observed patterns in the field. The model reproduced observed experimental findings: warming reduced lichen cover, whereas less rainfall had no effect on lichen performance. This warming effect was caused by the associated decrease in relative humidity and non-rainfall water inputs, which are major water sources for biocrust-forming lichens. Warming alone, however, increased cover because higher temperatures promoted photosynthesis during early morning hours with high lichen activity. When combined, climate variables showed non-additive effects on lichen cover, and effects of increased CO₂ levelled off with decreasing levels of relative humidity. Our results show that a decrease in relative humidity, rather than an increase in temperature, may be the key factor for the survival of the lichen D. diacapsis under climate change and that effects of increased CO₂ levels might be offset by a reduction in non-rainfall water inputs in the future. Because of a global trend towards warmer and drier air and the widespread global distribution of D. diacapsis, this will affect lichen-dominated dryland biocrust communities and their role in regulating ecosystem functions worldwide.

How to cite: Porada, P., Baldauf, S., Raggio, J., Maestre, F., and Tietjen, B.: Strong effect of relative humidity on dryland lichens under climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12245, https://doi.org/10.5194/egusphere-egu21-12245, 2021.

The increase of CO₂ concentrations implies direct and indirect (by changing climate) impacts on the terrestrial ecosystem. Several Dynamic Global Vegetation Models (DGVMs) have been developed to better understand the response of vegetation to climate change. However, the representation of plant diversity through a small set of Plant Functional Types (PFTs) adopted by the majority of DGVMs undermines their ability to represent functional diversity and fundamental interactions between these different life strategies of plants, like competition, which has been shown to be paramount in determining ecosystem functioning. Studies have shown that increasing CO₂ concentration may determine the outcome of vegetation competition and, as a consequence, the ability to adapt to the environment, functional diversity, and community assembly mechanisms. Thus, the inclusion of competitive dynamics in these models becomes strategic to improve predictions and understanding the effects of climate change on vegetation and how it affects change in carbon fluxes and stocks in the community. In that sense, this project aims to contribute to the development of a light competition module within CAETÊ model (CArbon and Ecosystem functional Trait Evaluation model) which involves the implementation of allometric relations between plant organs. As a trait-based model, CAETÊ seeks to represent plant functional diversity in a less discrete way through the usage of variant values for functional traits. For this purpose, two key functional traits that are closely related to competition for light are employed as variants: wood density (WD) and specific leaf area (SLA). The main objective is to understand how light competition related to plant functional traits alters the response of Amazon plant communities under changing environmental conditions. As preliminary results, the algorithms containing the allometric and competition equations were developed outside the main model code and represent plant dynamics trade-offs between the variant functional traits and plant physiology and survivorship: WD relates to strategies of mortality and height growth. For example, high values of WD [1g/cm^-3] are related to low heights [~30m.] and, low heights incur higher mortality rates; SLA relates to light competitive effect, Leaf Economics Spectrum, and LAI (leaf area index) determination, one of the most important parameters that determine the absorption of light by different life strategies. These trade-offs allow the representation of different plant life competition strategies. We expected that the light restriction for some functional strategies may incur a decrease in functional dominance and photosynthesis rate, consequently changing net primary productivity and after all the functional structure of the community. For functional diversity, it is expected changes in functional richness and functional divergence (related to the strength that competition exerts in the community) in order to favor strategies that better deal with the new environmental conditions simulated by CAETÊ with increasing [CO₂] to 600 ppmv, for example. Finally, it is expected that this approach may contribute to improving the representation of competition for light in DGVMs to more assertively obtain the effects of climate changes on vegetation and ecosystem dynamics. Final results will be obtained until the EGU Congress takes place.

How to cite: Rocha Cardeli, B., Fazio Rius, B., Fascina, C., Darela-Filho, J. P., Martins Sophia, G., Sanna Freire Silva, T., and Montenegro Lapola, D.: Modeling light competition in the Amazon forest: the effects of high CO2 in functional diversity and biogeochemical cycle., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3514, https://doi.org/10.5194/egusphere-egu21-3514, 2021.

Global climatic changes which are expected in the 21^st century are likely to create unparalleled disturbances on vegetation. In addition, human activities also increase the risk of fire disturbances and insect epidemies. We investigate the resilience of different biomes by examining their behaviour during the Holocene using a taxonomically harmonized and temporally standardized global fossil pollen datasets,synthesized from 2821 palynological records from the Neotoma Paleoecology Database and additional literature. Specifically, we study the composition variability on millennial time-scale and timescale-dependant scaling of variability from centennial to multi-millennial timescales. A principal component analysis was performed in order to characterize the principal modes of variability of the pollen assemblages. We find coherent regional signals of vegetation variability and scaling of variability from the pollen assemblages, indicating significant millennial scale variability which can be related to vegetation taxa and climates. Particularly, we observe more stability in North America and Northern Europe in areas dominated by boreal forest and deciduous forests. This may be linked to the greater stability of forest ecosystems and also a more stable climate over these areas which may be the result of stabilizing feedbacks. We find that diversity plays a key role in vegetation composition and that more diverse regions allow for greater variability. 

How to cite: Hébert, R., Li, C., Laepple, T., and Herzschuh, U.: Characterization of the resilience and variability of vegetation during the Holocene using a large database of pollen data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15259, https://doi.org/10.5194/egusphere-egu21-15259, 2021.

The rate at which forests take up atmospheric CO₂ is critical with regard to their potential to mitigate climate change as well as their value for wood production. The allocation of carbon fixed through photosynthesis into biomass is crucially dependent on tree carbon use efficiency (CUE), which is determined by gross primary production (GPP) and plant respiration (Ra) via the relation CUE=(GPP-Ra)/GPP. The effect of future climate on CUE is unclear due to the unknown response of plant respiration to more severe increases in temperature. This motivates assessing spatial patterns in CUE across climatic gradients with marked temperature variations.  

Within the ”Improving tree carbon use efficiency for climate-adapted more productive forests” (iCUE-Forest) project, we aim to develop novel data-driven estimates of plant respiration, net primary production (NPP=GPP-Ra) and tree CUE covering the northern hemisphere boreal and temperate forests. These will be based on recent satellite-driven maps of tree living biomass, databases of N concentration measurements in tree compartments (leaves, stem/branches, roots) and the relationships between respiration rates and tissue N concentrations and temperature. Such estimates will enable the detection of spatial relationships between CUE and environmental conditions and facilitate the parameterization of dynamic global vegetation models which allow predicting the change in CUE in response to future climate and forest management.

Here we will present an extensive database of N concentration measurements in tree stems/branches and roots that we have compiled in addition to data available mainly for leaves from databases like TRY. More than 5000 measurements have been collected from the literature covering all common boreal and temperate tree species. Currently, we are exploring how the variation in tissue N concentrations is influenced by climate and tree species. Subsequently, we apply the derived tree-level relationships between tissue N concentrations and underlying drivers in combination with tree species distribution maps and estimates of tree compartment biomass based on satellite remote sensing products. In this way, we will derive novel estimates of the spatial distribution of N content in northern boreal and temperate forests that will in turn be used to assess CUE variations.

How to cite: Thurner, M., Beer, C., Manzoni, S., Prokushkin, A., Wang, Z., Yu, K., and Hickler, T.: Improving tree carbon use efficiency for climate-adapted more productive forests (iCUE-Forest), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10299, https://doi.org/10.5194/egusphere-egu21-10299, 2021.

Tree killing by spruce bark beetles (Ips typographus) is one of the main disturbances to Norway spruce (Picea abies) forests in Europe and the risk of outbreaks is amplified by climate change with effects such as increased risk of storm felling, tree drought stress and an additional generation of spruce bark beetles per year^[1]. The warm and dry summer of 2018 triggered large outbreaks in Sweden, the increased outbreaks are still ongoing and affected about 8 million m³ forest in 2020^[2]. This is the so far highest record of trees killed by the spruce bark beetle in a single year in Sweden^[2]. In 1990-2010, the spruce bark beetle killed on average 150 000 m³ forest per year in southern Sweden^[3]. Bark beetles normally seek and attack Norway spruces with lowered defense, i.e. trees that are wind-felled or experience prolonged drought stress^[4]. However, as the number of bark beetle outbreaks increase, the risk of attacks on healthy trees also increase^[5]. This causes a higher threat to forest industry, and lowers the possibilities to mitigate climate change in terms of potential decreases in carbon uptake if the forests die^[4,5]. Norway spruce trees normally defend themselves by drenching the beetles in resin^[6]. The resin in turn contains different biogenic volatile organic compounds (BVOCs), which can vary if the spruce is attacked by bark beetles or not ^[4,6]. The most abundant group of terpenoids (isoprene, monoterpenes and sesquiterpenes), is most commonly emitted from conifers, such as Norway spruce^[7,8]. The aim of this study was to enable a better understanding of the direct defense mechanisms of spruce trees by quantifying BVOC emissions and its composition from individual trees under attack

To analyze the bark beetles’ impact on Norway spruce trees a method was developed using tree trunk chambers and adsorbent tubes. This enables direct measurements of the production of BVOCs from individual trees. Three different sites in Sweden, with different environmental conditions were used for the study and samples were collected throughout the growing season of 2019. After sampling, the tubes were analyzed in a lab using automated thermal desorption coupled to a gas chromatograph and a mass spectrometer to identify BVOC species and their quantity.

The preliminary results show a strong increase in BVOC emissions from a healthy tree that became infested during the data collection. The finalized results expect to enable better understanding of how spruce trees are affected by insect stress from bark beetles, and if bark beetle infestation will potentially result in increased carbon emission in the form of BVOCs.

References

[1] Jönsson et al. (2012). Agricultural and Forest Meteorology 166: 188–200
[2] Skogsstyrelsen, (2020). https://via.tt.se/pressmeddelande/miljontals-granar-dodades-av-granbarkborren-2020?publisherId=415163&releaseId=3288473
[3] Marini et al. (2017). Ecography, 40(12), 1426–1435.
[4] Raffa (1991). Photochemical induction by herbivores. pp. 245-276
[5] Seidl, et al. (2014). Nature Climate Change, 4(9), 806-810.
[6] Ghimire, et al. (2016). Atmospheric Environment, 126, 145-152.
[7] Niinemets, U. and Monson, R. (2013). ISBN 978-94-007-6606-8
[8] Kesselmeier, J. and Staudt, M. (1999). Journal of Atmospheric Chemistry, 33(1), pp.23-88

How to cite: Jaakkola, E., Jönsson, A. M., Olsson, P.-O., Linderson, M.-L., and Holst, T.: Impacts of European spruce bark beetle infestations on Norway spruce BVOC emissions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7624, https://doi.org/10.5194/egusphere-egu21-7624, 2021.

Anthropogenic activities resulted in a significant increase in nitrogen (N) compounds in the atmosphere and their deposition back to the biosphere, with important implications on both carbon (C) and N cycles. Indeed, an increase in N deposition can increase forest productivity in N limited forest ecosystems. In addition, it can also increase N loss pathways, leading to soil acidification and nutrient imbalance. Several N manipulation experiments have been carried out for decades till now, though most of them focused on conifer forests. We consider two manipulation experiments established in 2015 on two beech (Fagus sylvatica L.) forests in Italy, Cansiglio and Collelongo sites, located on the Eastern Alps and Central Apennines, respectively. The two forests were chosen along a climate and N deposition gradient. Thus, our goal was to assess the effects of simulated N deposition increase on nutritional, physiological status and growth of beech forests from two contrasting climatic conditions. At both sites, N was added directly to the soil as NH₄NO₃ in two doses: 30 kg N ha^-1 yr^-1 and 60 kg N ha^-1 yr^-1. Moreover, in Cansiglio we also included a canopy N fertilization adding 30 kg N ha^-1 yr^-1. Leaves were collected in 2016 and 2018 for the analyses of nutrients, stable C and N isotopes, and photosynthetic pigments. The aboveground production was periodically monitored with girth band and litterfall collectors. The nutrient stoichiometry analysis showed elevated N concentrations and high N:P in both forests, even in the control plots. N addition significantly increased N:P and N:S ratios in the treated plots. Changes in chlorophyll concentration were mainly related to differences between the two sites, while carotenoids were also influenced by N fertilization. After four years, we did not find an effect of the treatment (regardless of the doses) on both tree growth and leaf biomass. Altogether, our results suggest that both forests were not N limited. Finally, difference between the two manipulation approaches will be discussed in terms of leaf nutrients and C and N stable isotopes in the case of Cansiglio site.

How to cite: Teglia, A., Di Baccio, D., Magnani, F., Giorgio, M., Scartazza, A., De Cinti, B., Mazzenga, F., Muzzi, E., Ravaioli, D., and Marcolini, G.: Effects of simulated nitrogen deposition increase on plant nutritional status and physiological responses at two contrasting Beech forest sites in Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12622, https://doi.org/10.5194/egusphere-egu21-12622, 2021.

The impact of interacting global change stressors on terrestrial ecosystems is hard to predict due to non-linear, amplifying, neutral or even buffering interaction effects. We investigated the effects of drought and plant invasion on Mediterranean cork oak (Quercus suber L.) ecosystem functioning and recovery with a combined rain exclusion (30-45 % reduction) and shrub (Cistus ladanifer L.) invasion experiment. As key parameter, we determined tree, shrub and ecosystem transpiration in four treatments: 1) cork oak control stands, 2) cork oaks with rain exclusion, 3) cork oaks invaded by shrubs and 4) cork oaks with rain exclusion and shrub invasion. Rain exclusion and plant invasion led to moderate, but neutral reductions of tree transpiration of 18 % (compared to control) during the mild summer drought in 2018. In 2019, the rain exclusion simulated the second driest year since 1950 for Southwestern (SW) Iberia. The interaction effect of drought and plant invasion was strongly amplifying, reducing tree transpiration by 47 %. Legacy effects on shrubs under the rain exclusion treatment led to a non-linear response during recovery from the severe drought in 2019. Invaded trees showed a delayed transpiration recovery (-51 % vs. control) due to strong competition with shrubs, while invaded trees with rain exclusion recovered to 75 % of the control. This buffering interaction response was caused by a weaker competition from drought-stressed shrubs. Given the projected increase in the frequency, intensity and duration of drought, an increasing non-linear impact on Mediterranean cork oak ecosystems is expected. Our results demonstrate that abiotic stressors modulate biotic interactions thereby impacting ecosystem functioning in a highly dynamic manner. Further efforts are thus needed to model and manage the impact of interacting global change stressors on terrestrial ecosystems.

How to cite: Haberstroh, S., Caldeira, M. C., Lobo-do-Vale, R., Martins, J. I., Moemken, J., Pinto, J. G., and Werner, C.: Non-linear response of Mediterranean ecosystem to interaction of drought and plant invasion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1518, https://doi.org/10.5194/egusphere-egu21-1518, 2021.

Stomatal conductance formulations are of great importance to how land surface models predict carbon assimilation and transpiration in vegetation. In this study, novel stomatal conductance formulations based on the CAP optimisation hypothesis (Dewar et al. 2018) are implemented in the land surface model JSBACH. Besides new stomatal conductance functions, the CAP framework enables a computational streamlining of the resolution of photosynthesis rate and leaf internal CO₂ concentration.

The formulations are based on the CAP optimisation hypothesis coupled to different photosynthesis models. Models constructed this way incorporate non-stomatal limitations to photosynthesis through the coupling of carbon assimilation to the soil-to-leaf hydraulic pathway. This entails a direct link from soil water status to stomatal conductance, photosynthesis rate and leaf internal CO₂ concentration. While this construction does away with the need for some previous fitted or empirical parameters, new parameters are required to represent xylem hydraulic conductance and downregulation of photosynthesis during drought stress.

These new models are compared to the widely used USO stomatal conductance model (Medlyn et al. 2011). A standalone version of JSBACH is run for single grid cells representing two boreal Scots pine (Pinus sylvestris) dominated sites in Finland (Hyytiälä and Sodankylä). Climate forcing is done with FLUXNET data from 2001 through 2012 and observations are from eddy covariance measurements from the two sites.

Preliminary results indicate that some of the new formulations give reasonable results. This is very promising, since they are more detailed and theoretically robust than their semi-empirical predecessors, yet streamline the computational process.

References:
Dewar et al. 2018, New Phytol. 217: 571–581
Medlyn et al. 2011, Glob. Change Biol. 17: 2134–2144

How to cite: Mauranen, A., Mäkelä, J., Hölttä, T., Salmon, Y., and Vesala, T.: The CAP Optimisation Hypothesis Provides Improved Formulations for Stomatal Conductance and Photosynthesis in JSBACH, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11072, https://doi.org/10.5194/egusphere-egu21-11072, 2021.