PARENT SESSION
Oral Session # 64: Plant Ecology V: Physiology and Function II.
Presiding: G North
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 102.

Potential for carbon gain by understory plants via phenological avoidance of canopy shade.

Augspurger, Carol^*,1, Cheeseman, John¹, Salk, Carl¹, ¹ University of Illinois, Urbana, IL

ABSTRACT- Understory individuals that avoid canopy shade by leafing out early in spring and/or maintaining leaves into late autumn have the potential to enhance carbon gain. We evaluated the extent to which leaves of five species are phenologically, developmentally, and physiologically capable of exploiting these periods of high light within the understory. Adults of three species of shrubs/treelets and juveniles of two canopy species were sampled. Measurements were made throughout leaf lifespan of phenological stage, leaf area and mass, chlorophyll, nitrogen, and light- and CO₂-saturated net C assimilation (A_sat). Dates of canopy closure in spring and opening in autumn were quantified based on phenological observations of canopy tree species weighted by their relative basal areas. For leaves of all species, A_sat (mol CO₂ m^-2 sec^-1) peaked and plateaued during and immediately following expansion and then slowly declined until senescence. The period of highest photosynthesis paralled highest levels of leaf nitrogen (mg/cm₂), but not of chlorophyll (mg/cm₂), which did not peak until well after leaf expansion. Species differed markedly in whether sufficient phenological avoidance occurred to capitalize on the potential for high carbon gain in spring. Prior to canopy closure, Aesculus glabra juveniles had the highest potential for carbon gain, whereas Asimina triloba had no potential for carbon gain because its major leaf expansion postdated canopy closure. Lindera benzoin, Carpinus caroliniana, and Acer saccharum juveniles were intermediate in response, with minor avoidance of shade and minor potential for carbon gain in spring. In contrast, potential for carbon gain in autumn was non-existent or negligible for all species either because leaf senescence occurred prior to canopy opening or because of low A_sat of senescing leaves. Species differences in carbon gain by phenological avoidance could contribute to variation in resource availability for survival, maintenance, and reproduction of these understory plants.

Key words: photosynthesis, deciduous forest, phenology, saplings