How do forest landscapes respond to elevated CO2 and ozone? Scaling Aspen-FACE plot-scale experimental results

Abstract

The Aspen-FACE (Free-Air Carbon Enrichment) experiment was an 11-yr study of the effect of elevated CO2 and ozone (alone and in combination) on the growth productivity of model aspen communities (pure aspen, aspen-birch, and aspen-maple) in the field in northern Wisconsin, USA. Uncertainty remains about how these short-term plot-level responses might play out at landscape scales where climate change, competition, succession, and disturbances interact with tree-level responses. In this study, we used a recent physiology-based approach (PnET-Succession v3.1) within the forest landscape model LANDIS-II to scale the site-scale FACE results to landscape extents by mechanistically accounting for the globally changing drivers of CO2, ozone, temperature, and precipitation. We conducted a factorial simulation experiment to test five hypotheses about the effects of three treatments (CO2 concentration, cumulative ozone exposure, and disturbance). CO2 was clearly the dominant driver of landscape response, with disturbance also having a large effect. Ozone was not a dominant driver of landscape dynamics or total landscape biomass, but its negative effect on mean landscape biomass was nevertheless significant. We found that CO2 mitigation of water stress may not have a major effect on species composition or biomass accumulation. We found that species diversity was somewhat decreased by elevated CO2 as expected, but somewhat increased by O3, contrary to expectations. The spatial pattern of the landscape was minimally affected by the treatments. While rising CO2 concentrations have some mitigating effect on the negative O3 effect on the species studied, additional research is needed to confirm whether researchers and managers can be justified in disregarding O3 as a primary driver of forest dynamics in other ecosystems. Our results also add more support to the growing consensus that projections of climate change effects must include robust, direct links between CO2 and tree growth and competition; temperature effects (as demonstrated elsewhere) appear to be less by comparison.