The impacts of elevated CO2 and O3 on stand development processes in northern temperate forests

Background/Question/Methods

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.