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

Summary 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 CO 2 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 CO 2 concentration. Stand transpiration reached 5.6 and 4.4 mm d −1 for the ambient and elevated CO 2 treatments, respectively. Stratification of the data showed that significant differences in stand transpiration were observed between ambient and elevated CO 2 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 CO 2 may reduce canopy transpiration, the apparent dependency of such an effect on prevailing weather makes detecting a CO 2 ‐induced impact on forest water use difficult.