A simple hypothesis of how leaf and canopy-level transpiration and assimilation respond to elevated CO2 reveals distinct response patterns between disturbed and undisturbed vegetation

Abstract

Elevated CO2 increases leaf-level water-use efficiency (ω) almost universally. How canopy-leveltranspiration and assimilationfluxes respond to increasedωis currently unclear. We present a simple,resource-availability-based hypothesis of how equilibrium (or mature) leaf and canopy transpiration andassimilation rates, along with leaf area index (L), respond to elevated CO2. We quantify this hypothesis in theform of a model and test it against observations from eight Free Air CO2 Enrichment sites that span a widerange of resource availabilities. Sites were grouped according to vegetation disturbance status. Wefind themodel adequately accounts for the responses of undisturbed vegetation (R2= 0.73, 11% error) but cannotaccount for the responses of disturbed vegetation (R2= 0.47, 17% error). At undisturbed sites, the responsesofLand of leaf and canopy transpiration vary predictably (7% error) with resource availability, whereas theleaf assimilation response is less predictable. In contrast, theLand transpirationflux responses at thedisturbed (mostly forested) sites are highly variable and are not strongly related to resource availability. Initialanalyses suggest that they are more strongly related to regrowth age than to resource availability. Weconclude that (i) our CO2response hypothesis is valid for capturing the responses of undisturbed vegetationonly, (ii) that the responses of disturbed vegetation are distinctly different from undisturbed vegetation, and(iii) that these differences need to be accounted for when predicting the effects of elevated CO2 on landsurface processes generally, and on leaf area and waterfluxes in particular. Show more