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

Abstract

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, 400 ppm) and elevated (eCO2, 700 ppm) 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.

Dynamically optimizing stomatal conductance for maximum turgor-driven growth over diel and seasonal cycles

“…1 ). This coordination explains observations of more aggressive water use and accelerated declines in ψ and water content in NSC-depleted plants ( O’Brien et al 2014 ; Sapes et al 2019 ) and in defoliated trees ( Salmon et al 2015 , which stored fewer stem NSCs; Poyatos et al 2013 ) as well as heightened stomatal sensitivity to elevated CO 2 concentrations (eCO 2 ) at the beginning of the growing season when NSC demands are high ( Quentin et al 2015 ; Urban et al 2019 ; Sanches et al 2020 ; Lauriks et al 2021 b , 2022 ). Alternatively, stomata should close, deprioritizing carbon gain, even when water is available, if growth is sink limited ( Blonder et al 2023 ).…”

“…The carbon-use strategy can explain why maximal photosynthetic stimulation by eCO 2 and coinciding heightened stomatal sensitivity typically occur at the beginning of the growing season (low χ w during the early growing season in Fig. 2 ; Quentin et al 2015 ; Urban et al 2019 ; Sanches et al 2020 ; Lauriks et al 2021 b , 2022 ) when NSCs deplete to meet growth demands ( Palacio et al 2018 ; Tixier et al 2020 ). Since larger seasonal NSC fluctuations occur in more seasonal climates ( Fermaniuk et al 2021 ), we expect seasonal variations in photosynthetic and stomatal sensitivity to eCO 2 to be pronounced in boreal ecosystems.…”

Field phenotyping of ten wheat cultivars under elevated CO2 shows seasonal differences in chlorophyll fluorescence, plant height and vegetation indices

“…The observed rapid growth under e[CO 2 ] likely stimulated the production of NADPH and ATP, which are essential for efficient electron transport, thereby promoting photosynthetic efficiency. The gap between F q '/F m ' under ambient and e[CO 2 ] then narrows, possibly due to increased sink-driven respiration or potential acclimation effects ( Lauriks et al., 2021 ).…”

“…Several studies investigated the effect of e[CO 2 ] on photosynthetic assimilation ( Lauriks et al., 2021 ) and efficiency ( Javaid et al., 2022 ) at different stages of development in diverse plant species yielding various outcomes – a large number of them were devoted to understanding long term effects of tree species and grasslands. A growth chamber experiment with Acacia logifolia showed increased photosynthetic assimilation per unit leaf area under e[CO 2 ], mainly at the beginning of the growing period.…”

Increased wood biomass growth is associated with lower wood density in Quercus petraea (Matt.) Liebl. saplings growing under elevated CO2

“…Under the scenario that has most closely tracked human historical emission trajectories, the atmospheric CO 2 concentrations will reach approximately 550 to 700 ppm by the middle-tolate 21st century [2,3]. Previous studies on the effect of elevated CO 2 on different tree species were conducted mostly based on these concentrations [16,54,55], whereas more pessimistic predictions of 900-1000 ppm, which are expected by the end of 21 century, have been rarely used [13,18]. In our study, ambient CO 2 (hereafter aCO 2 ) and eCO 2 treatment concentrations in the open-top glass domes were 400 and 700 ppm, respectively, and CO 2 was continuously supplied from April to November, from 2017 to 2019.…”