PARENT SESSION
Poster Session 2: Forest Ecology
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Temporal and spatial variation of nearground atmospheric carbon dioxide in a mixed hardwood-hemlock forest in central Massachusetts.

Sipe, Timothy^*,1, Clowers, Jennifer¹, Sanchez Sierra, Alejandro¹, Vuong, Julie¹, ¹ Franklin & Marshall College, Lancaster, PA, USA

ABSTRACT- We measured ambient CO₂ concentrations in the herbaceous stratum of a mixed hardwood-hemlock forest in central Massachusetts as part of a study on the combined effects of nearground enriched CO₂ and sunflecks on photosynthesis by herbaceous and woody species. CO₂ was sampled simultaneously every 30 minutes at two heights (10, 30 cm) each in 23 locations spaced regularly across a 30 x 50 m permanent plot. Soil temperature (5 cm, thermistors) was measured at 8 of the 23 locations and windspeed (40 cm, cup anemometers) at 3 to provide data on the primary factors controlling soil CO₂ flux and mixing. Ambient CO₂ averaged 475 ppm (range 356-1024) across the two heights, the 23 sample locations, and all sampling events during five successive 24-hour cycles (July 1-5). Daytime (6 am-8 pm) values were considerably lower (mean 447, range 356-829) than nighttime (8 pm-6 am) values (mean 511, range 394-1024). The diurnal cycle was pronounced, with peaks near midnight and lows near solar noon on most days. The cycle was phase-shifted with soil temperature by about 7 hours, so ambient CO₂ correlated poorly (R² = 0.12) with estimated soil respiration based on soil temperature. Windspeeds ranged as high as 3.0 m/s, peaked in early afternoon, and were not measurable on most nights. The difference in CO₂ between 10 and 30 cm averaged 20 ppm (range 4-70) across the 23 sample locations, and showed an exponential relationship to overall CO₂ concentration (R² = 0.74). CO₂ concentrations at the two heights were strongly correlated across the 5-day run (R² = 0.98). Global spatial variation expressed as the standard deviation across the 23 sampling locations averaged 23 ppm and showed an exponential relationship to mean ambient CO₂ for both 10 cm (R² = 0.68) and 30 cm (R² = 0.62). The two most divergent sample locations differed by an average of 94 ppm (range 21-306 ppm) at 30 cm. These horizontal and vertical differences in nearground CO₂ environments are large enough to yield marked differences in carbon gain by plants in the herbaceous stratum, based on measured photosynthetic responses to ambient CO₂ for three herbaecous species.

Key words: atmospheric carbon dioxide, herbaceous stratum, temperate forest, spatial-temporal variation