Temporal variability in tree responses to elevated atmospheric <scp>CO<sub>2</sub></scp>
Fran Lauriks, Roberto L. Salomón, Kathy Steppe
Abstract
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 .