Seasonal variability in leaf and whole-tree responses of Populus tremula L. to elevated CO2 and drought

Trees are exposed to unprecedented climate change, characterized by rapidly rising atmospheric CO2 concentration and longer and more intense drought episodes. Despite research efforts to predict effects of elevated CO2 (eCO2) on tree functioning, its temporal and spatial variability and the interaction between eCO2 and drought remain intensely debated. To address these knowledge gaps, this PhD dissertation investigates the effects of eCO2 on one-year-old European aspen trees at the beginning and the end of the growing season, and under well-watered and drought conditions. For this, leaf and whole-tree water use, carbon gain and carbon loss were monitored during two consecutive growing seasons under ambient or elevated atmospheric CO2 concentration. The conducted literature review and experiments highlight three important insights. First, the magnitude of the effects of eCO2 is highly variable over time, even within a single growing season, as a likely result of the different seasonal carbon requirements throughout plant development. Second, tree responses to eCO2 should not be derived from observations made at the leaf level. Finally, the alleviating effects of eCO2 when facing drought were limited to the leaf level and the late season in European aspen, suggesting a negligible role of CO2 fertilization in mitigating detrimental effects of drought

Simulation of long-term stem diameter variation of Ficus benjamina based on simulated transpiration

Greenhouse microclimate (light, temperature, relative humidity and CO2) and irrigation are important factors for plant growth, development and quality in ornamental horticulture. To optimize plant growth, actual stem diameter growth can be measured and compared with a desired growth pattern. Using the deviation between measured and simulated stem diameter growth, growers can decide whether and in which way the microclimate or irrigation needs to be adjusted. Together with this decision, costs associated with climate control and irrigation must also be taken into account. This will help growers to find a proper balance between cultivation costs and plant growth. In this study, Ficus benjamina was grown from cutting to mature plant in a controlled greenhouse environment. Growing conditions, microclimate as well as plant spacing, closely resembled the ones used in commercial greenhouses. Microclimate, soil water content, leaf temperature, sap flow, stem diameter variation and leaf thickness were continuously measured on three plants. In addition, discrete measurements of leaf area, projected crown surface area, stem water potential, photosynthesis, transpiration and stomatal conductance were performed. These measurements were used to further extend a mechanistic plant model, which allows simulation of long-term stem diameter variation

Use of leaf thickness sensors in horticultural crops

Changes in leaf thickness can be a rapid indicator of the plant’s water status and can therefore serve as an alarm signal for potential water deficits. Combining the use of continuous leaf thickness measurements with a mechanistic plant model describing optimal leaf growth and diel variations, would allow growers to optimize greenhouse growing conditions by adaptation of the microclimate and applied irrigation. Recent development of new sensors offers the possibility for real time measurements of leaf thickness on small plants, including ornamentals. However, the accuracy of leaf thickness variation measurements needs to be assured. In this study, the temperature influence on 12 LeafSen (Netafim, Tel Aviv, Israel) sensors has been tested in a temperature range from 16 °C to 31 °C by installation of the sensors on aluminium plates. Temperature variations in the investigated range resulted in sensor signal differences of up to 48 μm, indicating that temperature response can exceed the expected diel leaf thickness variation. Two typical temperature responses were distinguished, pointing to the need for a sensor specific temperature correction. The practical use of leaf thickness sensors and the established temperature corrections has been demonstrated by installing the sensors on the stem and leaf of three Ficus plants (Ficus benjamina) and three pot roses (Rosa chinensis cv.) starting from cutting stage in a commercial greenhouse environment

CurieuzenAir: Data collection, data analysis and results

This report provides a description of the scientific research during the citizen project CurieuzenAir, which has mapped the air quality (atmospheric NO2 concentrations) across the Brussels Capital Region in October 2021. The report describes (1) the data collection with the help of citizens, (2) the quality control and processing of data, and (3) an overview of the resulting dataset.info:eu-repo/semantics/publishe

Temporal variability in tree responses to elevated atmospheric CO2

At leaf level, elevated atmospheric CO2 concentration (eCO(2)) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the impact of eCO(2) at larger temporal (seasonal and annual) and spatial (from leaf to whole-tree) scales is still lacking. Here, we review overall trends, magnitude and drivers of dynamic tree responses to eCO(2), including carbon and water relations at the leaf and the whole-tree level. Spring and early season leaf responses are most susceptible to eCO(2) and are followed by a down-regulation towards the onset of autumn. At the whole-tree level, CO2 fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO2-induced reductions in leaf stomatal conductance do not systematically translate into limitation of whole-tree transpiration due to the unpredictable response of canopy area. Reduction in the end-of-season carbon sink demand and water-limiting strategies are considered the main drivers of seasonal tree responses to eCO(2). These large temporal and spatial variabilities in tree responses to eCO(2) highlight the risk of predicting tree behavior to eCO(2) based on single leaf-level point measurements as they only reveal snapshots of the dynamic responses to eCO(2)

Curieuzenair zet Brusselse luchtkwaliteit op de kaart : burgeronderzoek legt verschillen in Brusselse luchtkwaliteit bloot

In oktober 2021 onderzochten 3.000 Brusselaars de luchtkwaliteit in hun eigen straat. Nooit eerder werd de luchtkwaliteit in een grote Europese stad zo gedetailleerd in kaart gebracht. En het goede nieuws is: de luchtkwaliteit gaat erop vooruitinfo:eu-repo/semantics/publishe

Leaf and tree responses of young European aspen trees to elevated atmospheric CO2 concentration vary over the season

Elevated atmospheric CO2 concentration (eCO2) commonly stimulates net leaf assimilation, decreases stomatal conductance and has no clear effect on leaf respiration. However, effects of eCO2 on whole-tree functioning and its seasonal dynamics remain far more uncertain. To evaluate temporal and spatial variability in eCO2 effects, one-year-old European aspen trees were grown in two treatment chambers under ambient (aCO2, 400ppm) and elevated (eCO2, 700ppm) CO2 concentrations during an early (spring 2019) and late (autumn 2018) seasonal experiment (ESE and LSE, respectively). Leaf (net carbon assimilation, stomatal conductance and leaf respiration) and whole-tree (stem growth, sap flow and stem CO2 efflux) responses to eCO2 were measured. Under eCO2, carbon assimilation was stimulated during the early (1.63-fold) and late (1.26-fold) seasonal experiments. Stimulation of carbon assimilation changed over time with largest increases observed in spring when stem volumetric growth was highest, followed by late season down-regulation, when stem volumetric growth ceased. The neutral eCO2 effect on stomatal conductance and leaf respiration measured at leaf level paralleled the unresponsive canopy conductance (derived from sap flow measurements) and stem CO2 efflux measured at tree level. Our results highlight that seasonality in carbon demand for tree growth substantially affects the magnitude of the response to eCO2 at both leaf and whole-tree level

Woody tissue photosynthesis increases radial stem growth of young poplar trees under ambient atmospheric CO2 but its contribution ceases under elevated CO2

Woody tissue photosynthesis (P-wt) contributes to the tree carbon (C) budget and generally stimulates radial stem growth under ambient atmospheric CO2 concentration (aCO(2)). Moreover, P-wt has potential to enhance tree survival under changing climates by delaying negative effects of drought stress on tree hydraulic functioning. However, the relevance of P-wt on tree performance under elevated atmospheric CO2 concentration (eCO(2)) remains unexplored. To fill this knowledge gap, 1-year-old Populus tremula L. seedlings were grown in two treatment chambers at aCO(2) and eCO(2) (400 and 660 ppm, respectively), and woody tissues of half of the seedlings in each treatment chamber were light-excluded to prevent P. Radial stem growth, sap flow, leaf photosynthesis and stomatal and canopy conductance were measured throughout the growing season, and the concentration of non-structural carbohydrates (NSC) in stem tissues was determined at the end of the experiment. Fuelled by eCO(2), an increase in stem growth of 18 and 50% was observed in control and light-excluded trees, respectively. Woody tissue photosynthesis increased radial stem growth by 39% under aCO(2), while, surprisingly, no impact of P-wt on stem growth was observed under eCO(2). By the end of the growing season, eCO(2) and P-wt had little effect on stem growth, leaf photosynthesis acclimated to eCO(2), but stomatal conductance did not, and homeostatic stem NSC pools were observed among combined treatments. Our results highlight that eCO(2) potentially fulfils plant C requirements, limiting the contribution of P-wt to stem growth as atmospheric [CO2] rises, and that radial stem growth in young developing trees was C (source) limited during early phenological stages but transitioned towards sink-driven control at the end of the growing season

Limited plasticity of anatomical and hydraulic traits in aspen trees under elevated CO2 and seasonal drought

The timing of abiotic stress elicitors on wood formation largely affects xylem traits that determine xylem efficiency and vulnerability. Nonetheless, seasonal variability of elevated CO2 (eCO(2)) effects on tree functioning under drought remains largely unknown. To address this knowledge gap, 1-year-old aspen (Populus tremula L.) trees were grown under ambient (+/- 445 ppm) and elevated (+/- 700 ppm) CO2 and exposed to an early (spring/summer 2019) or late (summer/autumn 2018) season drought event. Stomatal conductance and stem shrinkage were monitored in vivo as xylem water potential decreased. Additional trees were harvested for characterization of wood anatomical traits and to determine vulnerability and desorption curves via bench dehydration. The abundance of narrow vessels decreased under eCO(2) only during the early season. At this time, xylem vulnerability to embolism formation and hydraulic capacitance during severe drought increased under eCO(2). Contrastingly, stomatal closure was delayed during the late season, while hydraulic vulnerability and capacitance remained unaffected under eCO(2). Independently of the CO2 treatment, elastic, and inelastic water pools depleted simultaneously after 50% of complete stomatal closure. Our results suggest that the effect of eCO(2) on drought physiology and wood traits are small and variable during the growing season and question a sequential capacitive water release from elastic and inelastic pools as drought proceeds

Limited mitigating effects of elevated CO2 in young aspen trees to face drought stress

Elevated atmospheric CO2 concentration (eCO2) is expected to mitigate the adverse effects of moderate drought on leaf and whole-tree functioning. However, tree responses to eCO2 under severe drought and throughout the growing season remain largely unknown. One-year-old Populus tremula L. trees were grown in two controlled treatment chambers under ambient and elevated CO2 conditions, while progressive drought was imposed early (spring/summer 2019) and late (summer/autumn 2018) during the growing season. Leaf level responses to eCO2 (i.e., stomatal conductance, leaf carbon assimilation and leaf respiration) were monitored in concert with whole-tree level responses (i.e., canopy conductance, radial stem growth, stem CO2 efflux, xylem water potential and non-structural carbohydrates (NSC)). At the leaf level, eCO2 lowered the drought susceptibility of stomatal closure and delayed drought-induced reduction in leaf carbon assimilation during late season drought, but these responses were not observed during the early season drought. Drought effects on whole-tree functioning and NSC depletion remained unaltered by eCO2. Under moderate drought, stem volumetric growth ceased earlier than photosynthesis, while leaf and stem respiratory metabolism were maintained at 30 % of well-watered levels even under severe drought, independent of the CO2 treatment and timing of drought. Therefore, the ability of eCO2 to mitigate drought was mainly limited to leaf processes during the late season and under moderate drought (> − 2 MPa), while drought offset any beneficial effect of eCO2 at the whole-tree level. These results urge us to revisit predictions of forests' potential to sequester carbon under climate change scenarios.Depto. de Genética, Fisiología y MicrobiologíaFac. de Ciencias BiológicasTRUEpu