ESTIMATES OF WATER-VAPOR FLUX AND CANOPY CONDUCTANCE OF SCOTS PINE ATTHE TREE LEVEL UTILIZING DIFFERENT XYLEM SAP FLOW METHODS

During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhi ne Valley, Germany, three different methods were used to measure sap n ow in Scots pine trees via heating of water transported in the xylem: (1) constant heating applied radially in the sapwood (''Granier-system ''-G), (2) constant heating of a stem segment (''Cermek-system''-C), a nd (3) regulated variable heating of a stem segment that locally maint ains a constant temperature gradient in the trunk (''Cermak/Schulze-sy stem''-CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estim ates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between in serted electrodes and reference point. The C- and CS-systems assume th at all transported water is encompassed and equally heated by the elec trodes. In this case, flux rate is determined from temperature differe nce or energy input and the heat capacity of water. Active sapwood are a need not be determined exactly. In contrast, the G-system requires a n empirical calibration of the sensors that allows conversion of tempe rature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the n atural fluctuation in temperature of the trunk near the point of inser tion of heating and sensing elements is the same as that where referen ce thermocouples are inserted. Using all three systems, 24 trees were simultaneously monitored during the HartX campaign. Tree size within t he stand ranged between 18 and 61 cm circumference at breast height, w hile sample tries ranged between 24 and 55 cm circumference. The small est trees could only be measured by utilizing the G-system. Sap flow r ates of individual trees measured at breast height increased rapidly i n the morning along with increases in irradiance and vapor pressure de ficit (D), decreased slowly during the course of the afternoon with co ntinued increase in D, and decreased more slowly during the night. Ign oring potential effects introduced by the different methods, maximum f low rates of individual trees ranged between 0.5 and 2.5 kg H2O h(-1) tree(-1) or 0.3 and 0.6 mm h(-1) related to projected crown area of tr ees and daily sums of sap flow for individual trees varied between 4.4 and 24 kg H2O tree(-1) d(-1) or 1.1 and 6.0 mm d(-1). Maximum sap now rates per sapwood area of trees varied least for the G-system (11-17 g cm(-2) h(-1)) and was of similar magnitude as the C- (8-21 g cm(-2) h(-1)) and CS-system (4-14 g cm(-2) h(-1)). Regressions of total tree conductance (g(t)) derived from sap flow estimates demonstrated the sa me linear increase of conductance with increasing irradiance, however decrease of conductance with increasing D under non-limiting light con ditions was different for the three systems with strongest reduction o f g(t) measured with the CS-system followed by the C- and G-system. Th is led to different estimates of daily sap now rates especially during the second part of the measurement period. Variation in sap flow rate s is explained on the basis of variation in leaf area index of individ ual trees, heterogeneity in soil conditions, and methodological differ ences in sap flow measurements. Despite the highly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and so il depth at the scale of square meters still make it difficult to appr opriately and efficiently select sample trees and to scale-up water fl ux from individual tries to the stand level.