Temporal variability in tree responses to elevated atmospheric CO2.

At leaf level, elevated atmospheric CO2 concentration (eCO2 ) results in stimulation of carbon net assimilation and reduction of stomatal conductance. However, a comprehensive understanding of the impact of eCO2 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 eCO2 , including carbon and water relations at the leaf and the whole-tree level. Spring and early season leaf responses are most susceptible to eCO2 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 eCO2 . These large temporal and spatial variabilities in tree responses to eCO2 highlight the risk of predicting tree behavior to eCO2 based on single leaf-level point measurements as they only reveal snapshots of the dynamic responses to eCO2 .