MODEL-BASED ESTIMATES OF WATER-LOSS FROM PATCHES OF THE UNDERSTORY MOSAIC OF THE HARTHEIM SCOTS PINE PLANTATION

During the Hartheim Experiment (HartX) 1992 conducted in the Upper Rhi ne Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation ''patch'' l evel, direct estimates were made with small weighing lysimeters, and w ater loss was scaled-up to the stand level based on vegetation ''patch type'' distribution. At the leaf level, transpiration flux was determi ned with a CO2/H2O porometer for the dominant understory plant species , Brachypodium pinnatum, Cares alba, and Carex flacca. Measured leaf t ranspiration was scaled-up to patch level with a canopy light intercep tion and leaf gas exchange model, and then to stand level as in the ca se of lysimeter data, but with further consideration of patch-type lea f area index (LAI). On two days, total understory latent heat Aux was estimated by eddy correlation methods below the tree canopy. The under story vegetation was subdivided into five major patch-types which cove red 62% of the ground area and resulted in a cumulative LAI of approx. 1.54 when averaged over total stand ground area and compared to the a verage tree canopy LAI of 2.8. The remaining 38% of ground area was un vegetated bare soil and/or covered by moss (mainly by Scleropodium pur um) or litter. The evapotranspiration from the understory and unvegeta ted areas equaled approx. 20% of total forest stand transpiration duri ng the HartX period. The understory vegetation transpired about 0.4 mm d(-1) (13%) estimated over the period of May 13 to 21, whereas evapor ation from moss and soil patches amounted 0.23 mm d(-1) (7.0%). On dry , sunny days, total water vapor flux below the tree canopy exceeded 0. 66 mm d(-1). Using the transpiration rates derived from the GAS-FLUX m odel together with estimates of evaporation from moss and soil areas a nd a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mms(-1) as compared to 5.5 mm s(-1) for the tree canopy.