PARENT SESSION
Poster Session #53: Plant Competition.
Thursday, August 8. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Photosynthetic responses of oak seedlings growing along a gradient of evergreen understory shrub density.

Horton, Jonathan^*,1, Nilsen, Erik¹, Beier, Colin¹, Walker, John¹, Miller, Orson¹, Clinton, Barton², ¹ Biology Department, Blacksburg, VA² USDA Forest Service Southern Research Station, Otto, NC

ABSTRACT- Reduced light availability has been implicated in the inhibition of tree seedlings growing under evergreen understory shrubs in the southern Appalachian Mountains. There may also be an interaction between low light availability and the ability of seedlings to attract mycorrhizal symbionts. We investigated the light environment, photosynthetic properties and carbon gain of both pre-inoculated (with a generalist symbiont Scleroderma citrinum) and uninoculated one-year old seedlings of Quercus rubra and Q. prinus planted in split plots along gradients of shrub density at the Coweeta Hydrologic Laboratory in southwestern North Carolina in 2001. Light availability (determined from hemispherical canopy photograph analysis) was variable, but was not correlated with increasing shrub density at either our higher or lower elevation sites. At our higher elevation site, there was no effect of increasing shrub density on gas exchange parameters determined from light response curves of either pre-inoculated or uninoculated Q. rubra seedlings, and total carbon gain of both groups increased with increasing light availability. At our lower elevation site, light-saturated photosynthetic rates, quantum yield and total carbon gain of pre-inoculated Q. rubra decreased with increasing shrub density, but there was no change in photosynthetic parameters of the uninoculated seedlings along the gradient of shrub density. Quercus prinus seedlings showed the opposite pattern; light-saturated photosynthetic rates, quantum yield and total carbon gain of uninoculated seedlings decreased with increasing shrub density while there was no change in these parameters in the pre-inoculated seedlings. There was surprisingly little effect of shrub density on light availability, possibly due to high canopy leaf area index that may have masked the effect of shrub density on light availability. Additionally, seedlings may still be utilizing resources stored while growing in the greenhouse before planting. We will continue to monitor seedling performance and expect seedlings under shrubs to become more resource limited over time.

KEY WORDS: Rhododendron, light availability, Photosynthesis, inhibition