First-year growth response of trees in an intact forest

Although elevated atmospheric CO2 has been shown to increase growth of tree seedlings and saplings, the response of intact forest ecosystems and established trees is unclear. We report results from the ®rst large-scale experimental system designed to study the effects of elevated CO2 on an intact forest with the full complement of species interactions and environmental stresses. During the ®rst year of exposure to a 1.5 ¥ ambient CO2, canopy loblolly pine (Pinus taeda, L.) trees increased basal area growth rate by 24% but understorey trees of loblolly pine, sweetgum (Liquidambar styraci ̄ua L.), and red maple (Acer rubrum L.) did not respond. Winged elm (Ulmus alata Michx.) had a marginally signi®cant increase in growth rate (P = 0.069). These data suggest that this ecosystem has the capacity to respond immediately to a step increase in atmospheric CO2; however, as exposure time increases, nutrient limitations may reduce this initial growth stimulation.