Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Simultaneous measurements of xylem sap now and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap now was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day(-1)), and daily cumulated sap flow on a ground area basis varied between 2.0 and 1.7 mm day(-1) depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h(-1), i.e., 230 W m(-2). Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a rime lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap now = 0.745* vapour flux, r(2) = 0.86. The difference between the two estimates was due to understory transpiration. Canopy conductance (g(c)) was calculated from sap now measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations of g(c) were well correlated (r(2) = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression for g(c) = f(R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.