The development of young forests from open-grown plants to closed-canopy stands is associated with a variety of biological and physical processes. These processes, such as self-thinning, changes in carbon (C) allocation, shifts in species abundance, and the increased draw-down of soil water content have important interactions with forest C cycling and other biogeochemical processes. Similarly, predicted increases in atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) concentrations will also impact forest C cycling. However, few studies have examined how these changes in atmospheric composition will affect and interact with forest developmental processes. Here, we use results from the Aspen free-air CO2 enrichment (FACE) experiment, which exposed three different young northern temperate forest communities to elevated concentrations of CO2 and/or O3 for 11 years, to understand these interactions.
Results/Conclusions
In the community composed only of aspen, stand density decreased dramatically from an initial density of 10,000 trees/ha to an average of approximately 6,200 trees/ha. Exposure to elevated O3 significantly increased the rate of self-thinning, with steeper declines in density as average tree size increased. In contrast, elevated CO2 increased the amount of leaf area per tree without a concomitant decrease in stem density. There was significantly less mortality in the two other communities (aspen-birch: 32%, aspen-maple: 22%) and greater variability within treatments. In the two mixed species communities, elevated CO2 shifted the relative abundance of tree species, increasing the relative abundance of birch over aspen by 4% and decreasing the abundance of maple relative to aspen by 6%. The relative fraction of tree C in high-turnover components (leaves, fine roots, and groundcover plants) was greater at the end of the experiment in plants exposed to elevated O3 and lower in plants exposed to elevated CO2. However, this appeared to be a function of differences in plant size, with increasing wood and coarse root fractions in larger trees. Late summer volumetric soil moisture decreased by 45% from years 2 and 3 of the experiment to years 8 and 9 of the experiment. This decrease was smaller under elevated CO2, consistent with decreases in stomatal conductance. Overall, CO2 and O3 appear to have altered developmental processes in ways that will likely have lasting impacts on forest biogeochemistry.