PARENT SESSION
Oral Session #13: Plant Ecology: Light Relationships. Presiding: J. Rebbeck.
Monday, August 6, 2001. 1:00 PM to 4:30 PM. Hall of Ideas P&Q.

Diffuse illumination in an eastern hemlock (Tsuga canadensis) stand: Effects on photosynthesis and carbon storage.

HADLEY, JULIAN¹, SCHEDLBAUER, JESSICA ¹, FARGIONE, JOSEPH², ^{1 2}

ABSTRACT- To estimate the importance of photosynthesis in very low illumination, we measured photosynthetic photon flux (PPFD) incident on foliage at 29 canopy locations in an eastern hemlock (Tsuga canadensis L.) forest. From April through mid-June PPFD was <100 mol m^-2 s^-1 (<5% of direct sunlight) during 44, 55, and 90% of the daylight hours in the upper, middle, and lower canopy respectively. From July through September, low PPFD was slightly more frequent (46, 65, and 92% below 100 mol m^-2 s^-1). During the primary photosynthetically active months (April-October), PPFD values <100 mol m^-2 s^-1 were most common in October. Most of the low PPFD values were in diffuse illumination. Below 200 mol m^-2 s^-1 PPFD, net photosynthesis was greater at the same level of illumination if the light source was diffuse rather than a narrow-angle beam. The enhancement of net photosynthesis by diffuse versus direct illumination was only 10 to 20% when PPFD was above 100 mol m^-2 s^-1, but photosynthesis was enhanced 50 to 100% by diffuse PPFD below 20 mol m^-2 s^-1. In an ecosystem model, enhanced photosynthesis under diffuse illumination increased estimated annual photosynthesis by hemlock trees by about 10%, and increased ecosystem carbon storage from about 0.2 to 1.3 Mg ha^-1 y^-1.

KEY WORDS: diffuse illumination, photosynthesis, Tsuga canadesis, ecosystem carbon exchange