Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO₂ across four free‐air CO₂ enrichment experiments in forest, grassland and desert

Abstract

The magnitude of changes in carboxylation capacity in dominant plant species under long-term elevated CO 2 exposure (elevated pC a ) directly impacts ecosystem CO 2 assimilation from the atmosphere. We analyzed field CO 2 response curves of 16 C 3 species of different plant growth forms in favorable growth conditions in four free-air CO 2 enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO 2 assimilation (A) by 1 40 AE 5% in elevated pC a (49.5-57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC a in some species. Photosynthesis at a common pC a (A a ) was significantly reduced in five species growing under elevated pC a , while leaf carboxylation capacity (V cmax ) was significantly reduced by elevated pC a in seven species (change of À19 AE 3% among these species) across different growth forms and FACE sites. Adjustments in V cmax with elevated pC a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC a treatment did not affect the mass-based relationships between A or V cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO 2 responses can only be measured experimentally on a small number of species, understanding elevated CO 2 effects on canopy N m and N a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO 2 in different species and plant growth forms. NomenclatureA 5 light-saturated net CO 2 assimilation at chamber pC a A a 5 light-saturated net CO 2 assimilation per unit leaf area at current pC a of 36 Pa A m 5 light-saturated net CO 2 assimilation per unit leaf mass at current pC a (36 Pa) A a-56 5 light-saturated net CO 2 assimilation per unit area at elevated pC a of 56 Pa A m-56 5 light-saturated net CO 2 assimilation per leaf mass at elevated pC a of 56 Pa FACE 5 free-air CO 2 enrichment f N-Rub 5 the apparent fraction of N allocated to active Rubisco enzyme in situ Correspondence: David S. Ellsworth, tel. 1 1 734 615 8817; fax 1 1 734 936 2195, J max 5 in situ maximum electron transport capacity per unit leaf area M a 5 leaf mass per unit area N a 5 leaf N content per unit area N m 5 leaf N content per unit mass pC a 5 partial pressure of CO 2 in air pC i 5 partial pressure of CO 2 in intercellular air spaces V cmax 5 in situ leaf maximum CO 2 carboxylation capacity per unit leaf area V cm-m 5 leaf maximum CO 2 carboxylation capacity per unit leaf mass

Modelled effects of rising CO2 concentration and climate change on native perennial grass and sown grass-legume pastures

“…This allows for [CO 2 ] downregulation of photosynthesis, e.g. through reduced carboxylation capacity due to declining leaf N as evidenced by Ellsworth et al (2004). There is also a decline in stomatal conductance in response to elevated [CO 2 ] that has the potential to affect plant water relations.…”

Photosynthetic ecophysiology of evergreen leaves in the woody angiosperms – a review

“…When a diverse sample of only evergreen species was analyzed, LMA was not correlated with the wall nitrogen fraction (Harrison et al 2009). The fraction of nitrogen allocated to rubisco, on the other hand, has been shown to decline with LMA in some studies (Ellsworth et al 2004;Harrison et al 2009;Hikosaka and Shigeno 2009), while in Australian sclerophylls it was highly variable and was not lower than in other, more mesophyllic C3 species (Warren et al 2000). In contrast to metabolism-associated proteins, the concentration of wall proteins does not decrease with leaf age because the insoluble wall proteins are resistant to degradation and undergo hardly any resorption.…”

The effects of CO2 and nutrient enrichment on photosynthesis and growth of Poa annua in two consecutive generations

“…There was also evidence for photosynthetic acclimation at elevated CO 2 at the end of the experiment at this nutrient level. The results allow us to suggest that photosynthetic rates were downregulated by the redistribution of nitrogen within the plant away from Rubisco and into the formation of additional reproductive biomass and leaf tissue (Sage and Pearcy 1987;Stitt 1991;Ellsworth et al 2004). Plants grown at elevated CO 2 in nonfertilised soil were able to convert the limiting amount of nutrients available into biomass more efficiently than plants grown at ambient levels of CO 2 .…”