Document: ELK-3-29-12

Daily carbon gain of red maple and tulip-poplar growing in ambient and elevated CO₂ in a forest understory environment.

NAUMBURG, E.* ¹, D.S.ELLSWORTH ^1,2 and R.W.PEARCY ¹

Duke University, Durham, NC 27708-3028 USA ¹
Brookhaven National Laboratory, Upton, NY 11973-5000 USA ²

Abstract:
It has long been of interest to determine whether carbon gain differences in species underlie observed species differences in apparent shade tolerance. Further, increasing atmospheric CO₂ could differentially affect carbon gain in species with contrasting shade tolerance. We estimated daily carbon gain under ambient and elevated CO₂ in red maple (Acer rubrum, shade tolerant) and tulip-poplar (Liriodendron tulipifera, shade intolerant) with a physiological and architectural model and allometric relationships for saplings in Duke Forest, NC. Daily courses of PFD (photosynthetic photon flux density) were estimated using canopy photos together with a ray-tracing model (YPLANT) that contained spatial information for branches of 22 of study plants. These PFD courses were used in a dynamic sunfleck photosynthesis model that predicts photosynthesis as a function of light and species-specific photosynthetic parameters. Daily photosynthesis of whole plants was scaled from individual branches using leaf-area-stem diameter relationships. At ambient CO₂, tulip-poplar had slightly higher canopy daily carbon gain than red maple because of slightly higher leaf-level carbon assimilation and higher leaf area in plants < 2 cm diameter (0.5 m height). At larger diameters, however, red maple had higher canopy-level carbon gain because of higher leaf areas. Under elevated CO₂ in FACE, red maple had similar canopy assimilation rates as tulip-poplar despite greater photosynthetic enhancement in red maple under elevated CO₂. The latter was due to slight reductions in specific leaf area that resulted in lower canopy leaf area for red maple under elevated CO₂. These results indicate that while shade intolerant species can have a carbon gain advantage when small, this advantage is not maintained for larger saplings and may contribute to low understory survival. Elevated CO₂ will not change this pattern.

Keywords: shade tolerance, elevated CO2, carbon gain, sunflecks

This abstract is being presented at: 10:30 AM in session:
Poster Session #18: Elevated CO₂.