CO 2 Concentrations in Forests along a Topographic Gradient

Abstract

CO2 concentrations and air temperatures were measured at seven levels at 2-hr intervals for 24 hrs at stations along a slope leading from an upland oak forest into a cedar swamp. Skies were clear and the winds light over the period. Although there was evidence for cold air drainage downslope and concentration of CO2 in low sites during the early evening hours, temperatures tended to become isothermal at all stations along the gradient later in the night while CO-2 concentrations were higher near the ground on the slope but vertically uniform in the swamp. There was little evidence that CO2 concentrations were significantly higher through most of the night in the swamp than on the upland. INTRODUCTION CO2 concentrations fluctuate in nature because of variation in rates of photosynthesis and respiration, and changes in microclimatic conditions. Investigations have shown that CO2 concentrations are generally higher in forests than in fields or grasslands (Fuller, 1948; Wiant, 1964), and that concentrations increase markedly near the ground (Huber, 1952; Mitscherlich et al., 1963; Sparling and Alt 1966; DeSelm, 1952). Near the ground, concentrations may vary spatially depending on sunlight penetration or proximity to C02-rich sources such as decaying logs (Wiant, 1964). Biological and microclimatic changes associated with the diurnal cycle produce large changes in CO2 concentrations at all levels, but especially near the ground. Tabulated data presented by Mitscherlich et al. (1963) for a 24-hour period showed average maximum changes of 51 ppm at the 1 m level, 32 ppm at 5 to 5.5 m level, and 25 ppm at various heights within tree crowns. The purpose of this study was to measure topographic influence on concentrations of CO2. The study was conducted at Cedar Creek Natural History Area, Anoka Co., Minnesota, 48 km north of Minneapolis. The site was on a sandy peninsula nearly surrounded by a peat-filled basin supporting a white cedar (Thu ja occidentalis L.) swamp forest. Upland vegetation on the peninsula was dominated by northern pin oak (Quercus ellipsoidaluis E. J. Hill). A narrow "lagg" (marginal fen) community about 20 m wide was located along the sand-peat intersection at the foot of the slope. This community w~as dominated by black ash (Fraxinus nigra Marsh. ), American elm (Ulmus americana L.) and speckled alder (Alnus rugosa (Du Roi) Spreng). 1 Present address: Dept. of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755.