Fichtelgebirge-site Farrenleite

Although some measurements were carried out in 1998, the main period of investi-gation at Farrenleite was 1999-2000. Therefore, most of the following descriptions focus on these two years and are compared with the Steigerwald-sites.

Air Temperature and phenology. The temperatures at the site Farrenleite in the Fichtelgebirge are in general lower during the whole year compared to the sites in the Steigerwald (cf. Fig. 3.1.2; note that the DWD-station Fichtelberg is about 300 m lower than Farrenleite), due to its higher elevation and more eastern location (cold continental air masses in winter, see Chap. 3.2). The mean annual air temperature Tair at Farrenleite, averaged over the three years of investigation, was 2.7 K lower (5.9 °C) than at Steinkreuz (and 0.4 K higher than the site-adjusted long-term aver-age) and the mean growing season Tair (May–October) 2.8 K lower compared to Steinkreuz (and again 0.4 K higher than the site-adjusted long-term average, Tab.

5.2.1.1). As at Steinkreuz, 1998 was the coldest, 2000 the warmest year at Farren-leite (Tab. 5.2.1.1). At the latter site, the order was the same for the growing seasons, in contrast to Steinkreuz, where the growing season of 1999 was the warmest (Tab. 5.2.1.1).

Steigerwald, 1999

-100 -80 -60 -40 -20 0 20

0.2 m soil depth a

-100 -80 -60 -40 -20 0 20

May Jun Jul Aug Sep Oct Nov Dec

below 0.3 m soil depth [kPa]soilΨΨΨΨ b

Corresponding to the higher location, phenological phases are shifted in the Fichtel-gebirge. Leaves at the site Farrenleite emerged about ten days later in spring and changed colour about five days earlier than at the sites in the Steigerwald (the author’s own observations 1998–2000; see Tab. 5.2.1.2). The number of days with average Tair ≥ 10 °C corresponded to the order of average Tair during the growing seasons (Tab. 5.2.1.1), but in contrast to the findings from the Steigerwald did not agree with the absolute duration of the growing season as given by the DWD. How-ever, in contrast to this, dates of leaf emergence and of leaf colour change observed in this study closely matched the dates observed and interpolated by the DWD (1998-2003) for the whole region (“Thüringisches-Fränkisches Mittelgebirge”; Tab.

5.2.1.1).

Deviations from the long-term averages of monthly mean Tair basically followed the same patterns as at Steinkreuz and for many months deviated in almost the same way (Fig. 5.2.1.1a; data in the graph adjusted from German Weather Service-station Fichtelberg at 659 m a.s.l., see figure caption). Tair in February and March was closer to the long-term average (i.e. lower) than at Steinkreuz in 1998 and 2000, October 1998 and 1999 were colder than the long-term average, unlike at Steinkreuz where Tair was normal or almost average, respectively. In contrast to October, the deviation of Tair was slightly higher than at Steinkreuz for all of 1999, particularly in July and August. The same holds true for the growing season of the year 2000, when devia-tions were always higher (or less negative: July, September) than in the Steigerwald, which continued into the following winter (Fig. 5.2.1.1a). The highest Tair at the site Farrenleite was recorded on 21 June, 2000, with 28.9 °C (30 minute-average), also the day with the highest minimum Tair (20.1 °C; 30 minute-average) and highest average Tair (24.0; Fig. 5.2.2.1, below).

Precipitation. The average annual precipitation PPT for 1998–2000 was estimated to be almost two times higher at Farrenleite than at Steinkreuz, namely 819 mm at the latter and approx. 1577 mm at the former site (own data supplemented with data from the DWD, cf. Tab. 5.2.1.1). Precipitation with fog was not accounted for but could be estimated to an additional 9 % of rain as found by Wrzesinsky (2004) for the nearby Waldstein peak (April 2001–March 2002).

During the vegetation period (mean 1998–2000) PPT was more than 1.5 times higher at Farrenleite (850 mm, Steinkreuz 473 mm, cf. Tab. 5.2.1.1). There were slightly more rainy days (precipitation > 0.2 mm d^-1) at Farrenleite in 1999 (two), and consid-erably more rainy days in 2000 during the growing season (21, data not shown) com-pared to Steinkreuz. It rained on about 45 % of the days at Steinkreuz in 1999 and 2000 and at Farrenleite in 1999, whereas there was rain on 56 % of the days at Farrenleite in 2000 (data not shown). Rain events were usually more intense (on a daily basis) at Farrenleite, and more than one quarter of all rainy days showed more than 10 mm here, compared to 8 - 16 % at Steinkreuz (cf. Figs. 5.2.1.2, 5.2.2.1). Fog may have contributed to approx. 8 % of rainy precipitation to the total precipitation during the vegetation period (Wrzesinsky 2004, see “Radiation”, below).

The pattern of rainy periods during the vegetation period in general was the same at Farrenleite and Steinkreuz, both being mainly under a maritime influence during the vegetation period and reflecting their location within the same climatic region in summer (cf. Figs. 5.2.1.2, 5.2.2.1). Also, at Farrenleite the patterns of deviation from the long-term average monthly PPT (Fig. 5.2.1.1b) was similar to that of Steinkreuz.

In 1998, the deviation was mostly higher (or less negative) than at Steinkreuz, which was also true for the following winter. Over the course of the growing season of 1999, Farrenleite did not experience substantial below-average PPT as Steinkreuz did (cf.

Chap 3.1 and “Soil Moisture”, below). During the growing season of the year 2000, monthly PPT at Farrenleite was somewhat closer to the long-term average than at Steinkreuz, corresponding to the more positive deviations of Tair during that time, July deviated even higher than at Steinkreuz (> 90 % positive deviation, Fig. 5.2.1.1a).

Figure 5.2.2.1: Seasonal changes in climatic variables (photosynthetic photon flux density, PFD; air temperature, Tair; 24 h-average of water vapour pressure deficit, Davg; daily maximum D of 10‘-values, Dmax; pre-cipitation, PPT) and volumetric soil water content, θ, in 0.2 m depth at the site Farrenleite during 1999 and 2000. Tair

alone was measured year-round, other variables only during the growing period.

Data of θ are available for 1999 only. Data of D and PPT missing for July 4-12, 2000.

Precipitation amounted to approx. 75 mm on July 6, 1999, and to 55 mm on July 14, 1999.

0 10 20 30

D [hPa]

D avg D max -20

-10 0 10 20 30

Tair [C°]

0 10 20 30 40 50

PPT [mm d-1 ]

0 0.1 0.2 0.3

θθθθ [m3 m-3 ]

1999 2000

instrument failure Farrenleite, Fichtelgebirge 1999-2000

0 20 40 60

PFD [mol m-2 d-1 ]

Soil moisture. The wetter site conditions in the Fichtelgebirge are also reflected in soil moisture (Fig. 5.2.2.2; note that expression of θ as θe was not possible for Farrenleite as detailed in Chap. 4.5). At the beginning of the vegetation period in 1999, the volu-metric soil water content θ in 0.3 m soil depth was so high after the snowmelt that the frequent rain events at that time led to values of θ above field capacity(θF). Even later in summer θ reached saturation.The lowest values of θ were recorded towards the end of September 1999 (0.14 m³ m^-3). Thus θ decreased by only about 30 % during the vegetation period (minimum θ/θF = 0.68, Fig. 5.2.2.2). At Steinkreuz in contrast, the θ decreased during the same period more than twice as much, by about 66 % (minimum θ/θF = 0.34), corresponding to an θe min of 0.09 (Tab. 5.2.1.3, Fig. 5.2.2.2).

The θ very probably declined even further at Steinkreuz (see chap. 5.2.1, above).

Refilling of the soil water reserves was already completed at Farrenleite at the beginning of autumn (Fig. 5.2.2.2) whereas it took much longer at Steinkreuz (cf. Fig.

5.2.1.2). This also allows the assumption that deeper soil layers were also already near saturation at Farrenleite in autumn 1999, while they were not (yet) at Steinkreuz (cf. Fig. 5.2.1.2). When only considering days for which data from both Steinkreuz and Farrenleite were available, the seasonally integrated soil water depletion (calcu-lated from day-to-day changes in θ/θF) reached approximately 120 % of θ/θF at Stein-kreuz and 70 % of θ/θF at Farrenleite, or at Steinkreuz almost 1.8 times the amount at Farrenleite (data not shown). Frequent data gaps in data from Steinkreuz (see Chap.

4.5) hindered a more detailed analysis of differences between the two sites. For the year 2000 no data of θ were available for Farrenleite due to instrument failure, but it could be estimated that soil water conditions during the vegetation period should have been even moister than in 1999, due to more frequent rains (though only slightly larger amounts), lower irradiance (-4.5 % PFD, -4 % Rn), and longer duration of fog (+14 %, during day-time; see Tab. 5.2.1.1). Even if measured θ in 30 cm soil depth had been low enough to potentially cause drought effects in tree water use, additional water reserves in surely root-penetrated soil layers deeper than the as-sessed upper 30 cm of the soil profile (deeply weathered rock and hence soil depth of up to 100 cm, Tab. 3.3.1) would most likely have prevented such effects.

Figure 5.2.2.2: Seasonal course of mean daily soil water content θ relative to soil water content at field capacity θF in 0.2 m depth at Farrenleite (Fichtelgebirge, filled diamonds) and Steinkreuz (Steigerwald, shaded squares) in 1999. Data for Stein-kreuz are averages of three TDR-probes in the experimental plot, at Farrenleite only one sensor was available. Original data of soil water content from Steinkreuz cour-tesy of G. Lischeid, Dept. of Hydrogeology, BITÖK.

1999

0 0.2 0.4 0.6 0.8 1

May Jun Jul Aug Sep Oct Nov

θθθθ/θθθθF

Farrenleite Steinkreuz

Radiation. Photosynthetical photon flux density (PFD) received by the overstory can-opy at Farrenleite (Fig. 5.2.2.1) was on average during the growing season (1999-2000) only 78 % of that at Steinkreuz (cf. Tab. 5.2.1.1). The large number of 80 days with fog (average of growing seasons 1999 and 2000) at the higher elevations of the Fichtelgebirge, measured at 775 m elevation a.s.l. at the nearby (12 km direct dis-tance) Waldstein mountain (Foken 2003, Wrzesinski 2004, Tab. 5.2.1.1) may at least partly explain this difference. The duration of fog was on average (growing seasons 1999 and 2000) 175 hours during day-time (10’-average visibility ≤ 1000 m), of which more than 45 % were dense fog (10’-average visibility ≤ 200 m; data from BITÖK database). It could be expected that fog was even more frequent at the higher loca-tions of the Fichtelgebirge (Foken 2003) like Farrenleite. The reduction of PFD and Rn by approx. 5 % at Farrenleite in 2000 (May-October) compared to 1999 coincided with an increase of day-time fog from 164 to 187 hours at Waldstein; the duration of dense day-time fog increased from 68 to 92 hours. In 2000, the growing season-PFD at Farrenleite was only 76 % of that at Steinkreuz, whereas in 1999 it was 80 % (cf.

Tab. 5.2.1.1). The differences in Rn between the two sites were even more pro-nounced than those in PFD (Tab. 5.2.1.1).

Vapour pressure deficit of the air. The vapour pressure deficit of the air, D, observed at Farrenleite (Fig. 5.2.2.1) followed similar patterns as at Steinkreuz (see Chap.

5.2.1, above), and levels of average daily D (Davg) were mostly comparable as well, whereas maximum daily D (Dmax) were lower than in the Steigerwald as could be expected from the lower Tair at Farrenleite: In 1998, the maximum seasonal values of Dmax (August 12) and Davg (August 18) was 25 hPa and 14.5 hPa, respectively (data not shown). The following year, highest daily values observed were 23 hPa (Dmax) and 13.2 hPa (Davg), both on July 19, 1999. During the year 2000, June 21 was the day with the driest atmospheric conditions with Dmax = 29 hPa and Davg = 21.2 hPa, also the maximum values for all of 1998-2000 (Fig. 5.2.2.1).

General trends in the Steigerwald and the Fichtelgebirge. General trends of climatic change may be seen in mean monthly air temperatures over the three years compared to long-term averages, both in the Steigerwald and Fichtelgebirge (Fig.

5.2.1.1a), namely that winters were warmer, that the warmest month was not July, but August, and perhaps that cold spells occurred at some stage in June or July; but if the latter always occured in either June or July, then it might not be seen in long-term average monthly temperatures. One might also hypothesise that there was a trend towards less precipitation in winter and/or spring compared to the long-term average at the Steigerwald and the Fichtelgebirge sites (Fig. 5.2.1.1b). Foken (2003) found (non-significant) indications for similar trends for the Fichtelgebirge, particularly decreased precipitation in spring and increased precipitatation in July. Significant trends in temperature were observed, namely an increase of the average annual air temperature of 0.34 K • (10 a)^-1 or of 0.5 K • (10 a)^-1 considering only the winter months (Foken 2003). Average air temperature showed a stronger positive trend in August than in July, coinciding with the trend in precipitation (see above). No trend was detectable in relative humidity (Foken 2003).

5.3. Radial within-tree variation of xylem sap flow density Js (with emphasis