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Harvard Forest Symposium Abstract 2006

• Title: Herbaceous Stratum Effects on Nearground Enriched CO2 Profiles
• Primary Author: Timothy Sipe (Franklin and Marshall College)
• Additional Authors: Stephanie Strouse (Franklin and Marshall College)
• Abstract:

  Soil respiration generates a naturally enriched zone of atmospheric CO2 near the ground in terrestrial ecosystems that may enhance photosynthesis by herbs and woody seedlings. Numerous factors affect the nearground CO2 profiles, including plants in the herbaceous stratum. Herbs block air movement, reduce mixing, and thus may allow CO2 concentrations to increase compared to exposed areas. Conversely, they photosynthesize and may deplete the effluxed CO2, depending on herbaceous stratum composition, structure, and physiological activity in response to factors such as incident radiation. We examined these processes in summer 2005 through two experiments and additional measurements in mature hemlock and mid-successional hardwood stands in Prospect Hill.
  
  
  In the hemlock site, we explored the impacts of physiologically inert herb structure on windspeeds and CO2 profiles. We established 12 circular 1-m radius plots in areas without ground cover and measured vertical CO2 profiles (5, 10, 20, & 40 cm) every 10 minutes at the centers of all plots over a 3-week period. Soil temperature (5 cm) and moisture (upper 15 cm, TDR) were also measured continuously, and soil respiration was measured several times in all plots. After baseline measurements, artificial (silk) ferns with size and form similar to hay-scented fern (Dennstaedia punctilobula L.) were “planted” in some of the plots to yield several treatments: (1) control, (2) average density, equal to the mean density measured along transects in the hardwood site, (3) high density, 1.5x the average density, and (4) disturbance control, where coated horticultural wires were inserted to ground level.
  
  
  We examined the relative contributions of ferns as barriers versus photosynthetic sinks for CO2 in the hardwood site, which has a substantial hay-scented fern stratum. We established 12 plots in heavy fern cover and measured CO2 and soil variables in the same way as in the hemlock site. After baseline measurements, the plots were divided into three treatments: (1) control, (2) exposed, with all shoots clipped to ground level, and (3) replacement, with all shoots clipped to ground level and replaced immediately with silk ferns at densities that matched the natural fern leaf area. We also measured fern densities, leaf area indices, vertical photosynthetic photon flux gradients, and photosynthetic light response curves across all combinations of three air temperatures (15, 20, 30 C) and 6 ambient CO2 levels (375-500 ppm) in the hardwood site to estimate potential impact of hay-scented fern on plot-level CO2 assimilation.
  
  
  We documented marked diel cycling of CO2 profiles in both sites, with peaks shortly after midnight and troughs near solar noon. Both sites showed substantial within-site spatial variation in CO2 profiles among plots that was poorly correlated to soil temperature, moisture, or soil respiration. Both natural and artificial fern strata reduced windspeeds and altered the shape of the nearground wind profiles compared to exposed plots. Spatial variation in all processes reduced our ability to detect treatment effects on CO2 profiles in both sites. Overall, hay-scented fern showed remarkable acclimation to the shade, with low dark respiration rates, very low light compensation points, and modest maximum assimilation rates (5-6 µmol C m-2 s-1). Fern leaf area indices ranged from 1.8 to 2.4 in areas with continuous fern cover. Results thus far suggest the fern stratum assimilates on average approximately half the CO2 flux from soil respiration on clear days, with maximum values during sun patches that can exceed the highest soil fluxes. Consequently, the ferns probably compete for the CO2 and prevent other species from realizing the benefit of exposure to elevated CO2 concentrations.
  
  
  

• Research Category: Physiological Ecology, Population Dynamics, and Species Interactions