Sap velocity and canopy transpiration in a sweetgum stand exposed to free-air CO2 enrichment (FACE)

Abstract

Canopy transpiration rates, as a major component of forest hydrologic budgets, are reported for 12-yr-old sweetgum (Liquidambar styraciflua) trees growing in a free-air CO2 enrichment (FACE) study in eastern Tennessee, USA. The compensated heat-pulse technique was used to measure rates of sap velocity, and stand transpiration was estimated as a function of measured sap velocity, total stand sapwood area and the fraction of sapwood functional in water transport. Sap velocity averaged 13% less for trees in elevated compared with ambient CO2 concentration. Stand transpiration reached 5.6 and 4.4 mm d-1 for the ambient and elevated CO2 treatments, respectively. Stratification of the data showed that significant differences in stand transpiration were observed between ambient and elevated CO2 only at mean daily radiation levels > 400 J m-2 s-1 and at vapor pressure deficits > 1.0 kPa. These data illustrate that while elevated CO2 may reduce canopy transpiration, the apparent dependency of such an effect on prevailing weather makes detecting a CO2-induced impact on forest water use difficult.