Effects of Pre-industrial and Future Atmospheric CO2 concentration on Net Ecosystem Exchange on Arid and Semi-Arid Ecosystems

Abstract:

Ecosystem carbon dioxide flux was studied between 1997 and 2000 under six different CO₂ concentrations (250 ppm, 350 ppm, 450 ppm, 550 ppm, 650 ppm, and 750 ppm) using CO₂ LT (CO₂ controlled, naturally Lit, Temperature controlled) null balance chambers in Southern California chaparral dominated by Adenostoma fasciculatum. The purpose of this study is to evaluate possible effects of altered levels of atmospheric CO₂ concentrations on carbon fluxes in a natural chaparral ecosystem. Here we present that the increase of CO₂ from near pre-industrial levels of around 250 ppm to recent past CO₂ levels of 350 ppm are sufficient to increase NEE. These data indicate that chaparral ecosystems will increase carbon sequestration under elevated CO₂ levels and that under elevated atmospheric CO₂ there will be greater sink or reduced source of ecosystem CO₂ to the atmosphere as a result of improved moisture status. The effect of elevated CO₂ on increasing NEE was greatest during the warm and dry season versus the cold and wet season. Further, it appears that increasing atmospheric CO₂ will have greater relative effects in areas of increasing water stress as CO₂ treatment effects on NEE were greater in modestly dry years and with longer periods of drought. The daily maximum NEE difference between the lowest (250 ppm) and the highest (750 ppm) CO₂ concentrations treatments for January was -0.127gC m^-2 h^-1, but for June was -0.267 gC m^-2 h^-1 in this study, which was a 210 percent increase. The differences between the lower treatments and higher treatments were greater in the later years indicating there was an accumulative effect. Cumulative of net ecosystem exchange (gC m^-2) between 1/1/1997 and 1/1/2001 under six different CO₂ concentration is presented in the figure attached.