Temporal variability in tree responses to elevated atmospheric CO₂

Abstract

At leaf level, elevated atmospheric CO 2 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, CO 2 fertilization causes consistent biomass increments in young seedlings only, whereas mature trees show a variable response. Elevated CO 2 -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 .

Do stomata optimize turgor‐driven growth? A new framework for integrating stomata response with whole‐plant hydraulics and carbon balance

“…While experiments suggest λ indeed increases under eCO 2 over multiple years (e.g. Katul et al ., 2010), increases in λ may reflect other physiological acclimations to eCO 2 over longer timescales (Buckley & Schymanski, 2014), including acclimation of photosynthetic capacities, leaf area, and NSC reserves (Ainsworth & Rogers, 2007; Dietze et al ., 2014; Lauriks et al ., 2021; Fig. S13).…”

“…In acclimated simulations, we increased a L by a factor of 1.25 based on Lauriks et al . (2021), reduced the maximum carboxylation capacity ( V c,max ) by a factor of 0.90, and reduced the maximum electron transport capacity ( J max ) by a factor of 0.95 based on Ainsworth & Rogers (2007).…”

“…We model σ g and σ r similarly to Jones et al . (2020) (Eqns S1.4, S1.8): where C struct is the total dry biomass in carbon equivalents, and γ g and γ r are unitless parameters. We include C struct in Eqns 5 and 6 to signify tree size should influence potential NSC limitations and so σ g and σ r can be related to NSC concentrations rather than whole‐tree NSC reserves and are thus presumably less sensitive to tree size.…”

“…Furthermore, we consider the possibility of long-term acclimation of leaf traits (leaf area, a L , and photosynthetic capacities) to elevated c a (eCO 2 ) when testing our GOSM to increased c a . In acclimated simulations, we increased a L by a factor of 1.25 based on Lauriks et al (2021), reduced the maximum carboxylation capacity (V c,max ) by a factor of 0.90, and reduced the maximum electron transport capacity (J max ) by a factor of 0.95 based on Ainsworth & Rogers (2007).…”

Declining tree growth rates despite increasing water-use efficiency under elevated CO2 reveals a possible global overestimation of CO2 fertilization effect

“…A long-term increase of iWUE indicates increasing net photosynthesis and/or decreasing stomatal conductance (and thus reduced transpiration). Because iWUE is expressed as the ratio of net photosynthesis to conductance for water vapor, it is very likely that the increase in atmospheric CO 2 concentration in the treatment valley has resulted in both increased photosynthesis and reduced transpiration ( van der Sleen et al., 2014 ; Zuidema et al., 2020 ; Lauriks et al., 2021 ). The increase in temperatures observed since the 1980s (Figure S3) may indirectly result in higher iWUE as higher temperatures increase vapor pressure deficits (VPD), to which trees may respond by closing their leaf stomata and therefore reducing g w ( Lloyd and Farquhar, 2008 ; Brienen et al., 2011 ).…”