Carbon and water dynamics during combined heat, drought and elevated atmospheric CO2CO_{2} in Pinus halepensis\textit{Pinus halepensis} seedlings

Bäume müssen auf ungünstige Umwelteinflüsse mit Kompromissen reagieren. So werden Kohlenstoffaufnahme und Wachstum durch ihren Wasserhaushalt und ihre Wärmedosis begrenzt. Diese Umweltstressoren nehmen an Häufigkeit und Intensität zu und greifen daher stark in grundlegende Pflanzenprozesse ein, indem sie die Verfügbarkeit von Bodenwasser verringern und den Wasserbedarf der Atmosphäre durch einen Temperaturanstieg erhöhen. Der Anstieg der atmosphärischen Kohlendioxidkonzentration ist die Ursache dieser Veränderungen, aber zusätzlich wird auch darüber diskutiert, dass ein hoher $CO_{2}$-Gehalt den physiologischen Stress der Pflanzen durch Sättigung der Photosynthese, Steigerung der Biomasseakkumulation, Verringerung der Photorespiration und Reduzierung der Wasserverluste durch Optimierung der Spaltöffnungen mildern könnte. Man geht davon aus, dass semiaride Regionen besonders anfällig für klimatische Veränderungen sind, aber gleichzeitig könnten sie auch stark auf hohe $CO_{2}$ Konzentrationen reagieren, da die Pflanzen bereits stark Ressourcen limitiert wachsen. Diese Arbeit soll Licht in die weniger beachteten multiplen Stress-$CO_{2}$-Wechselwirkungen von $\textit{Pinus halepensis}$-Sämlingen bringen. Im Folgenden wurde in kontrollierten Gewächshausexperimenten eine Kombination aus Trockenheit und Hitzewellen in Verbindung mit hohen $CO_{2}$ Konzentrationen untersucht. Wachstumskammern ermöglichten eine Trennung von Spross und Wurzel der Pinien Setzlinge. Pflanzenphysiologische Parameter, Gasaustausch, Primärmetaboliten, Emissionen flüchtiger organischer Verbindungen, Wachstum und Mortalität wurden analysiert und kritisch diskutiert. Schließlich konnten folgende Forschungsfragen beantwortet werden: 1.) Wie verändern sich Gasaustausch, Kohlenstoffallokation und Regeneration der Setzlinge nach wiederholten isolierten Hitze- oder Dürreperioden oder einer Kombination aus Beidem (Kapitel 2 und 3)? 2.) Welchen Einfluss hat sehr hohes atmosphärisches [$CO_{2}$] auf Hitzestressreaktionen und die Kohlenstoffbilanz von trockenheitsakklimatisierten gegenüber gut bewässerten $\textit{P. halepensis}$-Setzlingen (Kapitel 4)? Zusammenfassend konnte gezeigt werden, dass selbst eine sehr trockenheitstolerante Kiefernart schnell an ihre Grenzen stößt, wenn sie höheren Maximaltemperaturen ausgesetzt ist, und dass Trockenheit zusammen mit Hitzewellen ihre Stresswirkung unverhältnismäßig verstärkt. Selbst Effekte, die durch sehr hohe $CO_{2}$-Konzentrationen hervorgerufen werden, werden durch Hitzetrockenstress schnell überdeckt, obwohl sie möglicherweise den individuellen Wasserverbrauch der Bäume entlang eines Temperaturgradienten verringern. Unter zukünftigen Bedingungen mit häufigeren Hitzewellen wird dies möglicherweise dazu führen, dass Bäume immer häufiger zu Netto-$CO_{2}$-Quellen werden

Leaf Shedding and Non-Stomatal Limitations of Photosynthesis Mitigate Hydraulic Conductance Losses in Scots Pine Saplings During Severe Drought Stress

During drought, trees reduce water loss and hydraulic failure by closing their stomata, which also limits photosynthesis. Under severe drought stress, other acclimation mechanisms are trigged to further reduce transpiration to prevent irreversible conductance loss. Here, we investigate two of them: the reversible impacts on the photosynthetic apparatus, lumped as non-stomatal limitations (NSL) of photosynthesis, and the irreversible effect of premature leaf shedding. We integrate NSL and leaf shedding with a state-of-the-art tree hydraulic simulation model (SOX+) and parameterize them with example field measurements to demonstrate the stress-mitigating impact of these processes. We measured xylem vulnerability, transpiration, and leaf litter fall dynamics in Pinus sylvestris (L.) saplings grown for 54 days under severe dry-down. The observations showed that, once transpiration stopped, the rate of leaf shedding strongly increased until about 30% of leaf area was lost on average. We trained the SOX+ model with the observations and simulated changes in root-to-canopy conductance with and without including NSL and leaf shedding. Accounting for NSL improved model representation of transpiration, while model projections about root-to-canopy conductance loss were reduced by an overall 6%. Together, NSL and observed leaf shedding reduced projected losses in conductance by about 13%. In summary, the results highlight the importance of other than purely stomatal conductance-driven adjustments of drought resistance in Scots pine. Accounting for acclimation responses to drought, such as morphological (leaf shedding) and physiological (NSL) adjustments, has the potential to improve tree hydraulic simulation models, particularly when applied in predicting drought-induced tree mortality

Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2_{2}

Drought‐induced tree mortality is expected to occur more frequently under predicted climate change. However, the extent of a possibly mitigating effect of simultaneously rising atmospheric [CO$_{2}$] on stress thresholds leading to tree death is not fully understood, yet. Here, we studied the drought response, the time until critical stress thresholds were reached and mortality occurrence of Pinus halepensis (Miller). In order to observe a large potential benefit from eCO$_{2}$, the seedlings were grown with ample of water and nutrient supply under either highly elevated [CO$_{2}$] (eCO$_{2}$, c. 936 ppm) or ambient (aCO$_{2}$, c. 407 ppm) during 2 years. The subsequent exposure to a fast or a slow lethal drought was monitored using whole‐tree gas exchange chambers, measured leaf water potential and non‐structural carbohydrates. Using logistic regressions to derive probabilities for physiological parameters to reach critical drought stress thresholds, indicated a longer period for halving needle starch storage under eCO$_{2}$ than aCO$_{2}$. Stomatal closure, turgor loss, the duration until the daily tree C balance turned negative, leaf water potential at thresholds and time‐of‐death were unaffected by eCO$_{2}$. Overall, our study provides for the first‐time insights into the chronological interplay of physiological drought thresholds under long‐term acclimation to elevated [CO$_{2}$]

Hot drought reduces the effects of elevated CO₂ on tree water use efficiency and carbon metabolism

- Trees are increasingly exposed to hot droughts due to CO2-induced climate change. However, the direct role of [CO2] in altering tree physiological responses to drought and heat stress remains ambiguous. - Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO2]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gasexchange measurements were supplemented by primary metabolite analysis. - Elevated [CO2] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31– 32°C) than drought (33–34°C) treatments unaffected by growth [CO2]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO2] was subtle and mainly reflected in enhanced root protein stability. - The impact of elevated [CO2] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO2] cannot counterbalance the impacts of hot drought extremes in Aleppo pine

Better safe than sorry: Non-stomatal mechanisms delay drought stress and hydraulic failure in Scots pine saplings

Background/Question/Methods There is no more vital connection than the tight linkage between water and organic carbon, and there is no more paradigmatic example for that than plant photosynthesis. In plants, carbon uptake is done at elevated expenses in terms of water transport from soil to the atmosphere. Under limited water supply, transpiration increases the tension of the within-tree water column. This will eventually lead to emboli formation and loss of hydraulic conductivity, and may result in tree death. The main mechanism by which trees slow down such tension increases is by actively closing their stomata. However, even if stomata are fully closed, some water loss can still occur through cuticular evaporation. Therefore, non-stomatal mechanisms exist that additionally reduce water losses, and hence increase hydraulic safety. Among these, leaf shedding as well as non-stomatal limitations over photosynthesis (NSL, combining increases in mesophyll conductance and biochemical down-regulation on photosynthesis), are well-known but poorly quantified mechanisms that trees may trigger to save water under drought stress. In order to better describe such mechanisms quantitatively, we conducted a severe two-month-long dry-down experiment on potted Scots pine (Pinus sylvestris L.) saplings (n = 6) and under controlled conditions. We measured tree transpiration, photosynthesis and leaf shedding. Based on our observations we trained a state-of-the-art tree hydraulic model and we quantified the impact of the above-mentioned processes on whole-tree percent loss of conductance. Results/Conclusions We found that NSL play a key role in tree drought response by further reducing conductance, which subsequently reduces transpiration and delays dehydration. If sap flow was reduced below a given threshold, saplings responded by shedding leaves. Noteworthy, this threshold was uncorrelated to soil water content. Leaf shedding buffered reductions in xylem water potential and loss of whole-tree conductance in the mid-term. This indicates a hierarchy of active acclimation processes involving a continuous NSL response, and a threshold-based leaf area reduction when P. sylvestris is in danger to lose water to dangerous degrees without any counterpart in form of photosynthetic gain. Combined, both mechanisms reduce whole-plant C uptake, but contribute to tree survival under drought stress

Heat waves alter carbon allocation and increase mortality of Aleppo pine under dry conditions

Climate extremes are likely to occur more frequently in the future, including a combination of heat waves and drought. However, the responses of trees to combined stress and their post-stress recovery are not fully understood yet. Therefore, this study investigated the responses of semi-arid Pinus halepensis seedlings to moderate drought, heat and combined heat-drought stress, as well as post-stress recovery. The seedlings were grown under controlled conditions and exposed to two 4-days-long heat periods, reaching air temperature maxima of 42°C and vapor pressure deficit (VPD) of 7 kPa. Day- and nighttime canopy gas exchange was measured and differences in shoot and root allocation of non-structural carbohydrate (NSC) compounds (soluble sugars, starch, cyclitols, and carboxylic acids) assessed. Fluorescence parameters, nitrate levels, proline content and shoot water potential (ψ) provided additional indicators for stress severity and recovery performance. During the heat periods, net photosynthesis and stomatal conductance decreased immediately. This decline was modest under well-watered conditions, with transpiration and dark respiration rates remaining high and despite reductions in root NSC content, trees recovered following heat release. This was not the case in the heat-drought treatment, where stress resulted in high mortality rates and the few surviving seedlings showed reduced gas exchange rates and low root NSC content, while leaf nitrate and proline remained elevated even 3 weeks after heat release. Shoot ψ indicated that hydraulic failure was not the reason for mortality in the heat-drought seedlings. Instead, we argue that low transpiration rates, which resulted in needle temperatures >47°C during heat stress (6°C above air temperature) have caused irreversible damage. In summary, it could be demonstrated that heat waves in combination with moderate drought can either result in increased mortality or, if the seedlings survive, in delayed recovery. This highlights the potential of an increase in heat wave temperatures to trigger forest decline in semi-arid regions.German Research FoundationGerman Federal Ministry of Education and Research (BMBF

Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2_2

The cause of reduced leaf-level transpiration under elevated CO$_2$ remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22–40 months under elevated (eCO$_2$; c. 860 ppm) or ambient (aCO$_2$; c. 410 ppm) CO$_2$. We assessed if eCO$_2$-triggered reductions in canopy conductance (g$_c$) alter the response to soil or atmospheric drought and are reversible or lasting due to anatomical adjustments by exposing eCO$_2$ seedlings to decreasing [CO$_2$]. To quantify underlying mechanisms, we analyzed leaf abscisic acid (ABA) level, stomatal and leaf morphology, xylem structure, hydraulic efficiency, and hydraulic safety. Effects of eCO$_2$ manifested in a strong reduction in leaf-level g$_c$ (−55%) not caused by ABA and not reversible under low CO$_2$ (c. 200 ppm). Stomatal development and size were unchanged, while stomatal density increased (+18%). An increased vein-to-epidermis distance (+65%) suggested a larger leaf resistance to water flow. This was supported by anatomical adjustments of branch xylem having smaller conduits (−8%) and lower conduit lumen fraction (−11%), which resulted in a lower specific conductivity (−19%) and leaf-specific conductivity (−34%). These adaptations to CO$_2$ did not change stomatal sensitivity to soil or atmospheric drought, consistent with similar xylem safety thresholds. In summary, we found reductions of g$_c$ under elevated CO$_2$ to be reflected in anatomical adjustments and decreases in hydraulic conductivity. As these water savings were largely annulled by increases in leaf biomass, we do not expect alleviation of drought stress in a high CO$_2$ atmosphere

Vapour pressure deficit was not a primary limiting factor for gas exchange in an irrigated, mature dryland Aleppo pine forest

Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil‒atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g$_{br}$), transpiration rate (E) and net photosynthesis (A$_{net}$) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g$_{br}$ to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the g$_{br}$ and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions

Assessing model performance via the most limiting environmental driver in two differently stressed pine stands

Climate change will impact forest productivity worldwide. Forecasting the magnitude of such impact, with multiple environmental stressors changing simultaneously, is only possible with the help of process-based models. In order to assess their performance, such models require careful evaluation against measurements. However, direct comparison of model outputs against observational data is often not reliable, as models may provide the right answers due to the wrong reasons. This would severely hinder forecasting abilities under unprecedented climate conditions. Here, we present a methodology for model assessment, which supplements the traditional output-to-observation model validation. It evaluates model performance through its ability to reproduce observed seasonal changes of the most limiting environmental driver (MLED) for a given process, here daily gross primary productivity (GPP). We analyzed seasonal changes of the MLED for GPP in two contrasting pine forests, the Mediterranean Pinus halepensis Mill. Yatir (Israel) and the boreal Pinus sylvestris L. Hyytiala (Finland) from three years of eddy-covariance flux data. Then, we simulated the same period with a state-of-the-art process-based simulation model (LandscapeDNDC). Finally, we assessed if the model was able to reproduce both GPP observations and MLED seasonality. We found that the model reproduced the seasonality of GPP in both stands, but it was slightly overestimated without site-specific fine-tuning. Interestingly, although LandscapeDNDC properly captured the main MLED in Hyytiala (temperature) and in Yatir (soil water availability), it failed to reproduce high-temperature and high-vapor pressure limitations of GPP in Yatir during spring and summer. We deduced that the most likely reason for this divergence is an incomplete description of stomatal behavior. In summary, this study validates the MLED approach as a model evaluation tool, and opens up new possibilities for model improvement.Peer reviewe

Gas exchange of Aleppo Pine seedlings including emissions of biogenic volatile organic compounds during repeated hetawaves, drought and recovery period

The data describes plant gas exchange dynamics (CO2, H2O) together with online proton transfer reaction mass spectrometry measurements of biogenic volatile organic compound emissions of Pinus halepensis seedlings exposed to two similar heatwaves together with drought and a recovery period. Measured in a scientific glasshouse facility at KIT IMK-IFU Garmisch-Partenkirchen, Germany, via an automated chamber setup