Effects of manipulated atmospheric carbon dioxide concentrations on carbon dioxide and water vapor fluxes in southern California chaparral.

摘要

This research took two different approaches to measuring carbon and water vapor flux at the plot level (2 x 2 meter and 1 x 1 meter plots) to help understand and predict ecosystem responses to elevated CO2 concentrations and concomitant environmental changes. The first measurement approach utilized a CO2-controlled, ambient lit, temperature controlled (CO 2LT) null-balance chamber system run in a chaparral ecosystem in southern California, with six different CO2 concentrations ranging from 250 to 750 PPM CO2 concentrations with 100 ppm difference between treatments. The second measurement approach used a free air CO2 enrichment (FACE) system operated at 550 PPM CO2 concentration. These manipulations allowed the study of responses of naturally-growing chaparral to varying levels of CO2, under both chamber and open air conditions.; There was a statistically significant CO2 effect on annual NEE (net ecosystem exchange) during the period of this study, 1997 to 2000. The effects of elevated CO2 on CO2 and water vapor flux showed strong seasonal patterns. Elevated CO2 delayed the development of water stress, enhanced leaf-level photosynthesis, and decreased transpiration and conductance rates. These effects were observed regardless of water availability. Ecosystem CO2 sink strength and plant water status were significantly enhanced by elevated CO2 when water availability was restricted. Comparing the FACE treatment and the FACE control, the ecosystem was either a stronger sink or a weaker source to the atmosphere throughout the dry seasons, but there was no statistically significant difference during the wet seasons. Annual average leaf transpiration decreased with the increasing of the atmospheric CO2 concentration. Although leaf level water-use efficiency (WUE) increased with the growth CO2 concentration increase, annual evapotranspiration (ET) during these four years also increased with the increase of the atmospheric CO2 concentrations. These results indicate that chaparral or other similar ecosystems, under future elevated CO2 concentrations, might be even more water stressed than they are under current conditions.