Leaf physiology, production, water use, and nitrogen dynamics of the grassland invader Acacia smallii at elevated CO2 concentrations

Invasion by woody legumes can alter hydrology, nutrient accumulation and cycling, and carbon sequestration on grasslands. The rate and magnitude of these changes are likely to be sensitive to the effects of atmospheric CO2 enrichment on growth and water and nitrogen dynamics of leguminous shrubs. To assess potential effects of increased atmospheric CO2 concentrations on plant growth and acquisition and utilization of water and nitrogen, seedlings of Acacia smallii Isely (huisache) were grown for 13 months at CO2 concentrations of 385 (ambient), 690, and 980 micromole mol-1. Seedlings grown at elevated CO2 concentrations exhibited parallel declines in leaf N concentration and photosynthetic capacity; however, at the highest CO2 concentration, biomass production increased more than 2.5-fold as a result of increased leaf photosynthetic rates, leaf area, and N2 fixation. Measurements of leaf gas exchange and aboveground biomass production and soil water balance indicated that water use efficiency increased in proportion to the increase in atmospheric CO2 concentration. The effects on transpiration of an accompanying decline in leaf conductance were offset by an increase in leaf area, and total water loss was similar across CO2 treatments. Plants grown at elevated CO2 fixed three to four times as much N as plants grown at ambient CO2 concentration. The increase in N2 fixation resulted from an increase in fixation per unit of nodule mass in the 690 micromole mol-1 CO2 treatment and from a large increase in the number and mass of nodules in plants in the 980 micromole mol-1 CO2 treatment. Increased symbiotic N2 fixation by woody invaders in response to CO2 enrichment may result in increased N deposition in litterfall, and thus increased productivity on many grasslands.