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Ambient CO2 concentrations in terrestrial ecosystems vary substantially on several spatial and temporal scales as numerous soil, plant, and atmospheric processes respond to irradiance, temperature, moisture and wind. There is one widespread microhabitat in terrestrial ecosystems, the nearground zone, in which CO2 is naturally enhanced above average background levels. CO2 produced by soil respiration diffuses through the litter and boundary layers and dissipates fairly rapidly into the overlying bulk air. However, a marked vertical profile of nearground enriched CO2 (hereafter NEC) is usually present in the first 0-50 cm above ground. The degree of enrichment varies primarily with soil respiration rate and turbulent mixing, secondarily with photosynthesis by plants in the herbaceous stratum, and usually shows marked diel and seasonal variation. References to this CO2 "subsidy" and its effects on plants have occurred occasionally in the literature since 1939, but there have been few detailed studies of either the nearground profile or plant responses in the field, particularly for species that consistently occupy the nearground stratum.

Considerable research over the last twenty years in both controlled and field environments has shown that co-occurring plant species may respond differently to artificially elevated CO2. But in contrast to light, temperature, water, and nutrients, plant community ecologists have generally not considered CO2 among the factors that regulate species’ distribution and abundance, except indirectly as it may affect water balance.

We have documented differences in forest composition (woody and herbaceous), soil characteristics, microclimates, and nearground CO2 levels among six sites that were formerly plowed, pastured, or continuously forested woodlots in Prospect Hill. We selected three perennial herbaceous species (Aralia nudicaulis, wild sarsaparilla; Clintonia borealis, blue-bead lily; Medeola virginiana, Indian cucumber root) and two dominant tree species in the Harvard Forest system (Acer rubrum, red maple; Quercus rubra (northern red oak) and measured their photosynthetic light responses to ambient CO2 variation within the range commonly encountered in the field (350-450 ppm) to address five questions: (1) What is the overall effect of NEC on net carbon assimilation? (2) Do species differ overall (land use sites combined) in their responses to NEC? (3) Do the land use sites differ overall (species combined) in plant responses to NEC? (4) Are there site x CO2 or species x CO2 interactions in response to NEC?

Light response curves were measured at three CO2 levels (350, 400, and 450 ppm inside the cuvette) on 3 randomly-selected, healthy replicates of each species in each of the three sites, generating a total of 135 curves. Gas-exchange measurements were made with a LI-6400 infrared gas analyzer (Li-Cor Inc., Lincoln, NE, USA) during ~7:30-12:30 a.m. solar time in late July and early August 1998. The analyzer was calibrated daily just prior to measurements. Air temperature in the cuvette was maintained at 23 deg C (mean morning air temperature in the sites), and relative humidity was maintained at either constant or slowly rising levels (typically less than 5% increase overall) during the 20-25 minutes required for each curve.

Rectangular hyperbolic curves were fitted to the scatterplots and curve parameters (daytime respiration rate, Rday; apparent quantum efficiency, AQE; maximum assimilation rate, Amax; curve convexity; light compensation point, LCP; and light saturation point, Lsat) were estimated using Photosyn Assistant software v. 1.1 (Dundee Scientific, Dundee, Scotland, UK). Six of the curves produced questionable parameters in the quantum yield region and were excluded from further analyses, leaving a total sample size of 129.