Bayreuther Institut für Terrestrische Ökosystemforschung
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Wedler, M; Heindl, B; Hahn, S; Köstner, B; Bernhofer, C; Tenhunen, JD: Model-based estimates of water loss from "patches" of the understory mosaic of the Hartheim Scots Pine plantation, Theoretical and Applied Climatology, 53, 135-144 (1996)
During the Hartheim Experiment (HartX)1992 conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation "patch" level, direct estimates were made with small weighing lysimeters, and water loss was scaled-up to the stand level based on vegetation "patch-type" distribution. At the leaf level, transpiration flux was determined with a CO2/H2O-porometer for the dominant understory plant species, Brachypodium pinnatum, Carex alba, and Carex flacca. Measured leaf transpiration was scaled-up to patch level with a canopy light interception and leaf gas exchange model, and then to stand level as in the case of lysimeter data, but with further consideration of patch-type leaf area index (LAI). On two days, total understory latent heat flux was estimated by eddy correlation methods below the tree canopy. The understory vegetation was subdivided into five major patch-types which covered 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 average tree canopy LAI of 2.8. The remaining 38% of ground area was unvegetated bare soil and/or covered by moss (mainly by Scleropodium purum) or litter. The evapotranspiration from the understory and unvegetated areas equaled approx. 20% of total forest stand transpiration during the HartX period. The understory vegetation transpired about 0.4 mm d-1 (13%) estimated over the period of May 13 to 21, whereas evaporation 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 model together with estimates of evaporation from moss and soil areas and a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mm s-1 as compared to 5.5 mm s-1 for the tree canopy.
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