Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short-term direct and long-term indirect effects of elevate d CO2 on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 mu mol mol(-1) atmospheric CO2 using free-air carbon dioxide enrichment (FACE) technology. After correcting for measureme nt artefacts, a shortterm 200 mu mol mol(-1) increase in CO2 reduced leaf r espiration by 7-14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO2 con centration under which the trees were grown. Growth under elevated CO2 did not appear to have any long-term indirect effects on leaf maintenance respi ration rates or the response of respiration to changes in temperature (Q(10 ), R-0). Also, we found no relationship between mass-based respiration rate s and leaf total nitrogen concentrations. Leaf construction costs were unaf fected by growth CO2 concentration, although leaf construction respiration decreased at elevated CO2 in both species for leaves at the top of the cano py. We conclude that elevated CO2 has little effect on leaf tissue respirat ion, and that the influence of elevated CO2 on plant respiratory carbon flu x is primarily through increased biomass.