įrom the perspective of the water and carbon cycles, the key vegetation responses are changes in canopy (that is, per unit ground area) transpiration and assimilation rates, in biomass and in soil carbon. Commonly observed responses include increases in assimilation and growth rates (especially for woody and C3 species), the enhancement of general resource use efficiencies, and changes in leaf and/or canopy transpiration rates.
#Disturbed vs undisturbed tremors free#
Free Air CO 2 Enrichment (FACE) experiments-arguably the best means yet of observing vegetation responses to eCO 2 under natural conditions-are covering an ever-increasing range of ecosystem types and resource availabilities, such that some emerging patterns have been identified across disparate sites. While laboratory and chamber-based investigations into this CO 2 fertilization effect have a long history, field-based studies are still gathering pace. Key PointsĮlevated concentrations of atmospheric CO 2 (eCO 2) are known to almost universally boost primary productivity of terrestrial vegetation by increasing leaf water-use efficiency (that is, the amount of carbon gained in photosynthesis per unit of water lost through transpiration).
We conclude that (i) our CO 2 response hypothesis is valid for capturing the responses of undisturbed vegetation only, (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 CO 2 on land surface processes generally, and on leaf area and water fluxes in particular. Initial analyses suggest that they are more strongly related to regrowth age than to resource availability. In contrast, the L and transpiration flux responses at the disturbed (mostly forested) sites are highly variable and are not strongly related to resource availability. At undisturbed sites, the responses of L and of leaf and canopy transpiration vary predictably (7% error) with resource availability, whereas the leaf assimilation response is less predictable. We find the model adequately accounts for the responses of undisturbed vegetation ( R 2 = 0.73, 11% error) but cannot account for the responses of disturbed vegetation ( R 2 = 0.47, 17% error). Sites were grouped according to vegetation disturbance status.
We quantify this hypothesis in the form of a model and test it against observations from eight Free Air CO 2 Enrichment sites that span a wide range of resource availabilities. We present a simple, resource-availability-based hypothesis of how equilibrium (or mature) leaf and canopy transpiration and assimilation rates, along with leaf area index ( L), respond to elevated CO 2. How canopy-level transpiration and assimilation fluxes respond to increased ω is currently unclear. Elevated CO 2 increases leaf-level water-use efficiency ( ω) almost universally.
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