PARENT SESSION
Symposium #26: Gasping for CO₂: Ecological Effects of Past Variations in Atmospheric CO2.

Organized by: JW Williams and S Cowling
Thursday, August 8. 9:00 AM to 11:30 AM. Turquoise Ballroom, TCC.

Quaternary carbon dioxide concentrations: a comeback for cold-adapted conifers.

SMITH, WILLIAM^*,1, JACKSON, STEPHEN², ¹ Wake Forest University, Winston-Salem, North Carolina² University of Wyoming, Laramie, Wyoming

ABSTRACT- Since the Late Cretaceous expansion of angiosperms, coniferous trees have been dominant primarily in boreal and montane forests, and locally in unusually wet, dry, or nutrient-poor environments. The sole exception to this pattern occurred during much of the Quaternary. The extensive glacial phases of the Quaternary were dominated by conifers (predominantly boreal, montane, and cool-temperate species), in contrast to the angiosperm-dominated forests of interglacial periods and the Tertiary. This glacial dominance of conifers may be attributable, in part, to cooler temperatures, but does not explain the low representation of boreal and cool-temperate deciduous species (e.g., Betula, Alnus). Quaternary glacial phases were also unique in having lower CO₂ concentrations than any time in the Cenozoic. Stomatal frequency may not be appropriate for indicating past atmospheric CO₂ concentrations, although several other morphological and anatomical features characteristic of the needle-like leaves of conifers may favor photosynthetic CO₂ uptake in cold, dry, and low CO₂ conditions of the Cenozoic. These structural features include the leaf surface area-to-volume ratio, a more cylindrical shape, and the degree of cross-sectional radial geometry. Functionally, these leaf characteristics can substantially enhance mechanical strength and CO₂ uptake capabilities in a cold environment, acting to reduce needle damage and low temperature photoinhibition of photosynthesis. These same features may negate the benefit of changing stomatal frequency in conifers in response to changing atmospheric CO₂ concentration. Thus, the needle-like leaf form of conifers may have provided a preadaptive advantage for survival and proliferation in the low-CO₂ , glacial phases of the Quaternary.

KEY WORDS: conifers, Quaternary, carbon dioxide, leaves