THE HARTX-SYNTHESIS - AN EXPERIMENTAL APPROACH TO WATER AND CARBON EXCHANGE OF A SCOTS PINE PLANTATION

In May 1992 during the interdisciplinary measurement campaign HartX (H artheim eXperiment), several independent estimates of stand water vapo r Bur were compared at a 12-m high Scots pine (Pinus silvestris) plant ation on a fiat fluvial terrace of the Rhine close to Freiburg, German y. Weather during the HartX period was characterized by ten consecutiv e clear days with exceptionally high input of available energy for thi s time of year and with a slowly shifting diurnal pattern in atmospher ic variables like vapor pressure deficit. Methods utilized to quantify components of stand water flux included porometry measurements on und erstory graminoid leaves and on pine needles and three different techn iques for determining individual tree xylem sap flow. Micrometeorologi cal methods included eddy covariance and eddy covariance energy balanc e techniques with six independent systems on two towers separated by 4 0 m. Additionally, Bowen ratio energy balance estimates of water flux were conducted and measurements of the gradients in water vapor, CO2, and trace gases within and above the stand were carried out with an ad ditional, portable 30 m high telescoping mast. Biologically-based esti mates of overstory transpiration were obtained by up-scaling tree sap Bow rates to stand level via cumulative sapwood area. Tree transpirati on contributed between 2.2 and 2.6 mm/day to ET for a tree leaf area i ndex (LAI) of 2.8. The pine stand had an understory dominated by sedge and grass species with overall average LAI of 1.5. Mechanistic canopy gas exchange models that quantify both water vapor and CO2 exchange w ere applied to both understory and tree needle ecosystem compartments. Thus, the transpiration by graminoid species was estimated at approxi mately 20% of total stand ET. The modelled estimates for understory co ntribution to stand water flux compared well with micrometeorologicall y-based determinations. Maximum carbon gain was estimated from the can opy models at approximately 425 mmol/(m(2) day) for the tree needles a nd at 100 mmol/(m(2) day) for the understory. Carbon gain was suggeste d by the modelling analysis to remain relatively constant during the H artX period, while water use efficiency in carbon fixation increased w ith decreasing vapor pressure deficit. Biologically- and micrometeorol ogically-based estimates of stand water flux showed good general agree ment with variation of up to 20% that reflects both errors due to the inherent assumptions associated with different methods as well as natu ral spatial variability in fluxes. The various methods support a relia ble estimate of average ET from this homogeneous canopy during HartX o f about 2.6 mm/day (a maximum of about 3.1 mm/day) with an insignifica nt decreasing trend in correlation with decreasing vapor pressure defi cit and possibly soil moisture. Findings during HartX were embedded in local scale heterogeneity with greater roughness over the forest and much higher ET over the surrounding agricultural fields which results in weak but clearly existant circulation patterns. A variety of measur ements were continued after the HartX campaign. They allow us to exten d our findings for six months with changing environmental conditions, including shortage of soil moisture. Hydrological estimates of soil wa ter extractions and micrometeorological estimates of ET by the one-pro peller eddy covariance (OPEC) system were in very good agreement, supp orting the use of this robust eddy covariance energy balance technique for long-term monitoring.