Effects of elevated air CO2 concentration on loess grassland vegetation as investigated in a mini FACE experiment

Stand and leaf scale responses of loess grassland swards to elevated air CO2 concentration had been investigated in a mini FACE system during 1998–2000. The study concentrated on biomass, leaf area index (LAI) and vegetation surface temperature (stand scale) and on diurnal carbohydrate pattern and gas-exchange responses (leaf scale). Leaf net CO2 uptake under prolonged exposure to elevated CO2 showed an upward response in the dicotyledonous and a downward one in the monocotyledonous species. Dawn and evening carbohydrate levels in leaves suggested growth stimulation of the dicot under elevated CO2 and the opposite for the grass species and indicated sink limitation as a major factor determining photosynthetic acclimation at the species level. The smaller LAI as well as the insignificant biomass response to elevated air CO2 was a compounded response by multi-species stands. Under mild water shortage, elevated air CO2 concentration partly alleviated the drought effect shown by the higher relative growth rate of LAI. Canopy surface temperatures of the vegetation in the CO2 enriched rings were higher than those in the ambient rings suggesting that decreased leaf conductance and transpiration were responsible for the temperature difference between the treatments. Increased canopy surface temperature under elevated air CO2 concentration will probably lead to increased sensible heat flux and therefore enhanced convection at larger spatial scales.