A. Site-to-Site Comparisons
Several factors were responsible for differences in stand transpiration between sites including: growing season duration, incident radiation, species composition, and the development of soil moisture deficits. Shorter growing seasons limited stand transpiration at MMSF and FEF, and incident radiation loads were lower at FEF. Drought and reduced canopy conductance were a significant limitation to stand transpiration at BSFNRA, and the WB-Oak and -Pine sites during 1998. The predominance of Yellow-poplar trees at the GSMNP site dramatically increased stand transpiration (select graph). Values for daily stand transpiration (colored symbols) and relative leaf area index (Rel. LAI) for each site are shown in the figure below.
B. Canopy Conductance By Species
Average canopy leaf conductance for all measured species can be derived from continuous sapflow and weather data sets for model comparison and validation. Such data indicate clear differences in the magnitude of mean canopy conductance among species, but show a consistent pattern of response to key environmental driving variables (i.e., vapor pressure deficit (VPD) and soil water potential (SWP)). Similar response patterns imply a general mechanism that can be modeled with only minor adjustments across species.
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