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

Canopy transpiration rates, as a major component of forest hydrologic budge ts, are reported for 12-yr-old sweetgum (Liquidambar styraciflua) trees gro wing in a free-air CO2 enrichment (FACE) study in eastern Tennessee, USA. The compensated heat-pulse technique was used to measure rates of sap veloc ity, and stand transpiration was estimated as a function of measured sap ve locity, 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 d ata showed that significant differences in stand transpiration were observe d between ambient and elevated CO2 only at mean daily radiation levers > 40 0 J m(-2) s(-1) and at vapor pressure deficits > 1.0 kPa. These data illustrate that while elevated CO2 may reduce canopy transpirati on, the apparent dependency of such an effect on prevailing weather makes d etecting a CO2-induced impact on forest water use difficult.