PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

An examination of the relationship between sapflow rate and measured stem CO₂ efflux in trees.

Clinton, Barton¹, Maier, Chris², Sullivan, Neal³, ^{1 2 3}

ABSTRACT- The aqueous transport of CO₂ within the xylem my increase or decrease the gaseous transfer of CO₂ from the stem surface such that under certain conditions, traditional chamber-based measurements of stem respiration become equivocal. We examined the relationship between transpiration, sapflow, xylem [CO₂] and stem respiration in several hardwood and softwood species. We hypothesized that the degree of correlation within a species is strongly influenced by a variety of biotic and abiotic factors, bark diffusivity, and xylem sap [CO₂]. In mature scarlet oak (Quercus coccinea) and yellow poplar (Liriodendron tulipifera) we found a general inverse relationship between stem CO₂ efflux and leaf level measures of transpiration. Stem surface CO₂ efflux and sapflow velocity (Granier method) were measured over three consecutive summer days in a 40-yr-old eastern white pine (Pinus strobus) and yellow poplar stand. There was a significant but negative correlation between average daily stem CO₂ efflux and sapflow velocity in yellow poplar (r²=0.48) but not in white pine (r²=0.11). In planted cottonwood (Populus deltoides) on dry sites in central Texas we found a significant positive relationship between stem CO₂ efflux and VPD (r²=0.32). These results suggest that there may be a species-specific relationship between stem CO₂ efflux and water use. Hence, understanding diurnal and seasnal variation in stem CO₂ efflux within and among species would provide a more complete characterization of whole-plant C cycles.

KEY WORDS: respiration, sapflow