The Forest-Atmosphere Carbon Transfer and Storage (FACTS-II) experiment at the Free Air Carbon Dioxide Enrichment (FACE) facility in northern Wisconsin is currently assessing the potential long-term biological and physical impacts of elevated surface carbon dioxide (CO₂) and ozone (O₃) concentrations on aspen, maple, and birch ecosystems. Included in this assessment is an examination of the resultant productivity, sustainability, competitive interactions, and carbon and nitrogen fluxes in ecosystems of this type that have been exposed to elevated CO₂ and O₃ concentrations. A critical component of the FACTS-II experiment is the examination of the indirect microclimate trends resulting from the altered vegetation characteristics induced by elevated near-surface CO₂ and O₃ concentrations. Forest vegetation changes in response to increasing CO₂ and O₃ concentrations under a changing climate have the potential for altering atmospheric environments and the dynamics of forest-atmosphere interactions in forest ecosystems.

Since January 1999, a micrometeorological monitoring network at the FACTS-II study site has been measuring near-surface atmospheric conditions and trends within and above four of the twelve 30 m diameter FACE treatment plots which were planted with aspen (Populus tremuloides), paper birch (Betula papyrifera), and sugar maple (Acer saccharum) seedlings in 1997. The twelve treatment plots represent three replicates of four plots each. Each replicate contains a control plot where CO₂ and O₃ concentrations are not altered, a plot with elevated CO₂ concentrations, a plot with elevated O₃ concentrations, and a fourth plot with elevated CO₂ and O₃ concentrations. Meteorological instrumentation has been installed in all four plots of one replicate as a means of sampling conditions under the different types of FACE-technology fumigation strategies. In addition, a 20 m tower at the FACTS-II study site has been instrumented to measure near-surface ambient conditions outside the treatment plots.

This paper reports on the micrometeorological trends in the instrumented plots and at the ambient tower site at the FACTS-II study site for years 1999-2001. Diurnal and monthly variations in temperature, vertical temperature gradient, relative humidity, wind speed, wind shear, net radiation, photosynthetically-active-radiation, and soil temperature are described in relation to developing vegetation characteristics in a control plot and CO₂-, O₃-, and CO₂+O₃-fumigated plots. The results to date suggest that potential increases in episodic surface O₃ concentrations in conjunction with projected increases in atmospheric CO₂ concentrations will not only have an impact on aspen, maple, and birch vegetation, but the increases will also impact the microclimatic conditions in birch, maple, and aspen ecosystems. These altered microclimatic conditions can, in turn, further influence tree growth and productivity.