To quantify atmospheric controls of canopy transpiration, meteorological data were recorded concurrently with sap flow measurements. At Steinkreuz, a climate station located in a large forest gap adjacent to the plot has been operational since the end of 1994. Within a circle of about 25 m in the vicinity of the small meteorological tower, all regrowing trees were periodically removed. Photosynthetical photon flux density (PFD) was measured with a quantum sensor (LI-190SA, Li-Cor Inc., Lincoln, Ne-braska, USA), net radiation (Rn) with a pyrradiometer (8111, Schenk, Vienna, Aus-tria), both at 5 m height above ground, air temperature (Tair) and relative humidity (rh) with a humidity-temperature-Probe (HMP45A, Vaisala, Helsinki, Finland) at 2 m height, and wind speed and direction, using a switching anemometer with 3-cup rotor (A100R) and a potentiometric wind vane (W200P, both Vector Instruments, Wind-speed Ltd, Rhyl, UK), installed 5 m above ground. Bulk precipitation (PPT) was measured with an aerodynamic tipping bucket rain gauge (ARG100, Environmental Measurements Ltd, Sunderland, UK, 0.2 mm resolution). All sensors were scanned every 30 seconds and readings averaged or integrated over 10 minutes and stored by a data logger (DL and DL2e with LAC1-cards, Delta-T Devices Ltd, Cambridge, UK). Power was supplied by solar-charged 12 V-lead-acid batteries (see Chap.
4.1.3). For Großebene, meteorological data from nearby Steinkreuz were taken as representative.
At Farrenleite, a climate station was set up in a small canopy gap some 20 m uphill from the trees instrumented with sap flow gauges. From spring 1999 the same vari-ables as at Steinkreuz were recorded, except for wind speed and direction. Tair and rh were measured with a SKH 1011 (Skye Instruments Ltd, Powys, UK), at about 2 m height like PFD and Rn. Precipitation was measured only during the growing season, since servicing of the station during winter was hindered by snow. Temperature measurements started in spring 1998. The water vapour pressure deficit of the air (D) was calculated from air temperature and relative humidity for both climate stations using the Magnus formula.
Additional air and soil temperatures were measured inside the three stands with small thermistors (M841/S1/3K, Siemens, Germany) at 2 m and 0.1 m above the ground (both with radiation shields), in the litter layer, and in the mineral soil (-0.1 m, -0.25 m, -0.5 m). Inside the Steinkreuz stand, under the tree canopy, global radiation was measured at 2 m height with a pyranometer (LI-200SA, Li-Cor Inc., Lincoln, Nebraska, USA) from late July 1999 till the end of October 2000.
In order to follow the seasonal change of soil water supply to the vegetation and to identify periods of potential soil water limitation, soil matrix potential Ψs (in MPa) and volumetric soil water content θ (in m3 m-3) have been monitored with tensiometers and time domain reflectometry (TDR) probes (resolution 0.015 m3 m-3), respectively, (both IMKO GmbH, Ettlingen, Germany) at Steinkreuz since autumn 1994 by the Dept. of Hydrogeology at BITÖK. Tensiometers were installed at 0.2 m, 0.9 m and 2 m soil depth, TDR-probes at 0.2 m, 0.35 m and 0.9 m depth, at five and three dif-ferent locations inside the fenced plot, respectively, and read out hourly by data log-gers. Tensiometers operated down to approx. -0.08 MPa. Data were made available through G. Lischeid, formerly Dept. of Hydrogeology, BITÖK, Univ. of Bayreuth.
Relative extractable water θe in the soil was calculated as the ratio of actual extract-able water to maximum extractextract-able water (Black 1979), the latter term being equal to the difference between maximum and minimum soil water content. The maximum water content, or soil water content at field capacity, was evaluated as the average of the ten highest daily mean values found in winter and early spring after periods of free drainage over the three years under consideration. The minimum water content was estimated from water retention curves established at -1.5 MPa (permanent wilting point) on soil cores from the respective soil horizons at the Steinkreuz site (Langusch and Kalbitz 2001, see Tab. 5.2.1.3). θe was computed as (Black 1979):
θe = (θa - θP)/(θF - θP) (Eq. 4.5.1)
where θa, θP, and θF denote actual soil water content, soil water content at permanent wilting point, and at field capacity, respectively (all in m3 m-3). Minimum θe is 0 (dry soil), maximum θe can exceed 1 (θe at field capacity) after heavy rainfalls as hap-pened occasionally. Only daily mean θ was used in calculations, which minimised the transient effect of over-saturating rain. Values of θe exceeding 1 were excluded. No data were available for approximately 75 days during May–October in 1999 and 2000, for both tensiometers and TDR-probes.
At Großebene, non-automated tensiometers (Oikos, Göttingen, Germany) were in-stalled at the beginning of August 1998 at depths of 0.2 m, 0.35–0.4 m and 0.45–
0.65 m. The position of some gauges was changed at the end of March 1999. Values of soil matrix potential Ψs were recorded in irregular intervals (twice a week to monthly) during the vegetation period 1998 and 1999 with a hand-held read-out device (Oikos, Göttingen, Germany). Minimum Ψs measured with this equipment was approx. -0.075 MPa.
At the Farrenleite site in the Fichtelgebirge, volumetric soil water content θ was moni-tored with a FDR (Frequency Domain Reflectometry) probe (ThetaProbe ML2x, Delta-T Devices Ltd, Cambridge, UK) at 0.3 m soil depth and recorded half-hourly.
Voltage outputs were converted to values of θ afterwards, using the generalised
cali-bration for organic soils provided by the manufacturer (Delta-T 1999). Readings from the year 2000 were erroneous as was found out only later after intense evaluation of the data, probably due to electrical problems in the circuitry. Soil water retention characteristics were not available for this site, so data were evaluated on a relative basis only: Actual soil water content θa was related to soil water content at field capacity θF, the latter being measured at the beginning of the growing period (see above; approx. 0.20 m3 m-3).
Daily, monthly and annual averages, amounts and patterns of climatic variables (especially precipitation, air temperature, and radiation) were compared to data from nearby weather stations of the German Weather Service (DWD) or the Bavarian Federal Institute of Foresty (LWF), both for the sites in the Steigerwald and the Fichtelgebirge, for plausibilty checks and to compare site-specific results to long-term values available from these institutions only, or to supplement the on-site measure-ments to fill data gaps.
Statistical calculations were carried out in Microsoft Excel 97, PV-Wave 6.21, SigmaPlot 4.0 and 8.0, and Systat 8.0.