5.2. Atmospheric and soil conditions during the investigated years
5.2.1. Steigerwald-sites Steinkreuz and Großebene
Meteorological conditions recorded in a large forest gap close to the study site Stein-kreuz were considered representative of the nearby site Großebene as well (see Chap. 3.1 and Chap. 4.5). Table 5.2.1.1 gives an overview of important variables and characteristics of both the Steigerwald and Fichtelgebirge sites.
Table 5.2.1.1: Meteorological variables at the investigation sites in the Steiger-wald and the Fichtelgebirge. Data from this study, plus data courtesy of G. Lischeid (Dept. of Hydrogeology, Bayreuth Institute for Terrestrial Ecosystem Research), of the German Weather Service DWD (DWD 1998–2003), and of the Bavarian Forest Institute LWF Bayern (pers. comm.). n.d. = not determined.
Long-term averages (1961–1990 for the Fichtelgebirge, 1962–1990 for the Steigerwald) from the nearest DWD-stations were adjusted to the local me-teorological stations by means of scaling factors (“adj”; cf. caption of Fig.
5.2.1.1). Precipitation data for Farrenleite were supplemented with monthly data from the DWD-station Fichtelberg-Hüttstadl and adjusted to the site using a scaling factor of 1.18 (cf. caption of Fig. 5.2.1.1) for the whole year of 1998 and for the dormant season in 1999 and 2000.
Period
1961–1990 14.0 (adj to ST) 10.6 (adj to FA) Average air temperature of
Table 5.2.1.1, continued.
temperatures 1998–2000 -9.8 to 25.0 -12.7 to 24.0
Annual sum of precipitation
1961–1990 859 (adj to ST) 1306 (adj to FA) Precipitation during growing
1961–1990 464 (adj to ST) 631 (adj to FA) Annual Rn [MJ m-2 yr-1] 1998
b defined as visibility below 1000 m for 10 minutes; data from Foken (2003) for Waldstein-Weidenbrunnen, 775 m a.s.l., ca. 12 km direct distance from Farrenleite.
c long-term average from BayFORKLIM (1996).
Air temperature and phenology. The year 1998 was the coldest at the climate station Steinkreuz (mean annual air temperature Tair 8.3 °C, Tab. 5.2.1.1), with 2000 being the warmest year (8.8 °C) and 1999 intermediate (8.6 °C). The average mean Tair of these three years was 0.9 K higher than the long-term average (1962-1990, DWD-station Ebrach, 3.7 km direct distance to Steinkreuz) of 7.7 °C (DWD 1999–2000; see Fig. 3.1.2) or 0.6 K higher than the long-term average of the DWD-station adjusted to the climate station at Steinkreuz (see caption of Fig. 5.2.1.1 and Tab. 5.2.1.1). Aver-age Tair during the growing season (see below) was lowest in 1998 (13.5 °C), highest in 1999 (14.4 °C) and intermediate in 2000 (13.8 °C), and the average of the three years was 0.1 K lower than the site-adjusted long-term average of 14.0 °C (Tab.
5.2.1.1). The number of days per year where Tair was ≥ 10 °C reflects the ranking of average Tair during the growing seasons (Tab. 5.2.1.2) and is often used as a proxy for the duration of the growing season. The length of the growing season (defined here as the time span between the beginning of leaf emergence and the beginning of leaf colour change of F. sylvatica) as observed in phenological gardens and inter-polated region-wide by DWD (DWD 1998-2003) and the order among the three years closely matched these values (Tab. 5.2.1.2). The time of leaf emergence and leaf colouration of beech as published by the DWD (for the regions “Fränkisches Keuper-Lias-Land” and „Thüringisches-Fränkisches Mittelgebirge“) and observations from this study at the sites in the Steigerwald and Fichtelgebirge coincided within a few days (Tab. 5.2.1.2).
In 1998, the first five months of the year were warmer than the site-adjusted long-term average mean monthly values of Tair (Fig. 5.2.1.1a), especially January and February were much warmer (+2.3 K and + 3.6 K, respectively), and monthly devia-tions of current Tair from long-term Tair added up to +9.3 K. June, August, October and December showed “typical” mean monthly values of Tair, whereas July, Septem-ber and NovemSeptem-ber were much cooler (deviations of -1.9 K, -1.1 K, -2.8 K, respec-tively; Fig. 5.2.1.1a). The cool months of July (typically the warmest month, cf. Fig 3.1.2a) and September made this the coldest of all three growing seasons, and to-gether with the cold November the coldest year as well (Tab. 5.2.1.1).
The pattern was similar in 1999 in that the first five months were rather warm again, except for the somewhat cooler than average February. June was cold this year, in-stead of July as in 1998 (- 1.2 K deviation from long-term Tair, Fig. 5.2.1.1a), as well as November (-1.4 K). September was remarkably warm (+ 3.2 K), which made it, in concert with a somewhat above-normal July, the warmest of the three growing sea-sons (Tab. 5.2.1.1).
In the year 2000, Tair in late winter and early spring was above normal again (Febru-ary deviated +3.3 K from the long-term average), particularly in April (+2.6 K, due to a warm last third of the month, Fig 5.2.1.1a, 5.2.1.2). July was outstandingly cold (de-viation -3.3 K) which made it the second coldest of the investigated growing seasons (Tab. 5.2.1.1). Yet since September was the only other month with below-average Tair, the whole year was the warmest observed (Fig 5.2.1.1a, Tab. 5.2.1.1).
The highest air temperature during the period 1998-2000 was 36.3 °C (10 minute-average), measured on August 12, 1998, also the day with the highest minimum (19.5 °C; 10 minute-average) and highest average (25.0 °C) air temperature (Fig.
5.2.1.2).
Table 5.2.1.2: Phenology. Observations from the present study and data from DWD (1998–2003) are listed. DWD-data are interpolations for a whole region, where the Steigerwald is part of the “Fränkisches Keuper-Lias-Land”, and the Fichtelgebirge part of the “Thüringisches-Fränkisches Mittelgebirge”.
b beginning of leaf emergence and leaf colour change of F. sylvatica.
c no earlier observations available; on day 284 leaf colour change was still at initial stage.
d no earlier observations available; on day 280 leaf colour change was already rather advanced.
Figure 5.2.1.1: Deviation of mean monthly air temperatures (Tair, a) and precipitation (PPT, b) at Steinkreuz, Steigerwald (filled diamonds), and Farrenleite, Fichtelgebirge (shaded squares), for the three years of investigation from the long-term average.
Since no long-term observations were available from either investigation site, the re-spective data from the nearest German weather service (DWD) stations (Ebrach and Fichtelberg-Hüttstadl, cf. caption to Fig. 3.1.2) were multiplied by a scaling factor de-rived from linear regression between monthly averages of presented and DWD-data (DWD 1999–2000). Regression equations used are for air temperature at Steinkreuz:
y = 1.0397x; R2 = 0.991, p < 0.0001; for air temperature at Farrenleite: y = 0.9401x -0.80; R2 = 0.993, p < 0.0001; for precipitation at Steinkreuz: y = 1.0625x; R2 = 0.900;
p < 0.0001. For Farrenleite only a few values of monthly precipitation were available so these were used to extract a scaling factor of 1.18. The same factor was derived when using a precipitation gradient of 60–70 mm/100 m, as detailed by Gerstberger et al. (2004) for another mountain in the Fichtelgebirge. Using this factor, deviation from long-term mean precipitation was estimated from DWD-data from the station Fichtelberg for months when the rain gauge at Farrenleite was not operational (open squares).
Precipitation. At Steinkreuz, in the year 1998 the highest annual sum of precipitation (PPT) was observed (916 mm), in 1999 the lowest (744 mm), and in 2000 intermedi-ate amounts (798 mm). The long-term average for the DWD-station Ebrach (1962-1990; DWD 1999–2000) is 808 mm and adjusted to the study site Steinkreuz (see Fig. 5.2.1.1b) amounts to 859 mm (Tab. 5.2.1.1). Thus the average annual PPT of the three years studied was 40 mm below the site-adjusted long-term average. 1998 was also the year with the highest amount of precipitation during the growing season (May-October), namely 586 mm, followed by the year 2000 (483 mm) and 1999 (350 mm). So in 1999, 25 % of the long-term average precipitation (464 mm, site-adjusted) was missing during this period (Tab. 5.2.1.1). The highest daily rate of precipitation of 41 mm was recorded on August 5, 1999 (see Fig. 5.2.1.2).
-4 -2 0 2 4
Tair [K]
Steinkreuz
Farrenleite a
-100 0 100 200 300
PPT [%]
1998 1999 2000
b
Deviation from long-term average
Figure 5.2.1.2: Seasonal changes in meteorological variables (photosynthetic photon flux density, PFD; net radiation, Rn; air temperature, Tair; 24 h-average of water vapour pressure deficit, Davg; daily maximum D of 10‘-values, Dmax; precipitation, PPT) and relative extractable soil water (θe) at the site Steinkreuz for the years 1998-2000. Climatic data were measured in 5 m height, above regrowth, in a forest gap.
Precipitation was supplemented by data from the German Weather Service (DWD, station Ebrach). Soil water content was measured by TDR-probes in 0.2, 0.35 and 0.9 m soil depth at three below-canopy locations within the experimental plot; fre-quent data gaps exist in the 1999- and 2000-data sets. For calculation of relative extractable soil water see text. Data courtesy of G. Lischeid, Dept. of Hydrogeology, Bayreuth Institute for Terrestrial Ecosystem Research (BITÖK).
1998 started as a “dry” year with more than 100 mm less precipitation during the first five months than the long-term (site-adjusted) average, and in particular May was dry (29 mm, -64 % deviation from the long-term average, Fig. 5.2.1.1b), with a period of more than two weeks without rain (see also Fig 5.2.1.2). June and July were average
Steinkreuz, Steigerwald 1998-2000
and August (34 mm) lacked 57 % of the long-term precipitation again. Very high amounts of precipitation in September (+100 %) and October (+250%) and to a lesser extent November (+37 %) made this the year with highest annual sum of PPT of the three years investigated. Almost half of the annual sum was precipitated be-tween the beginning of September and mid November (Fig 5.2.1.2).
In 1999, precipitation moderately exceeded long-term average monthly sums in Feb-ruary, May and December by 37 %, 24 %, and 53 %. During the other months pre-cipitation was below average, especially in March and April (around -40 % deviation), June (-55 %, typically the month with most rain, cf. Fig. 3.1.2a), September and Oc-tober (approx. -40 %, Fig. 5.2.1.1b). As a result this was comparatively the driest of the years studied (Tab. 5.2.1.1).
The year 2000 showed a somewhat more balanced pattern of monthly sums of pre-cipitation in that positive and negative deviations were spread more evenly over the year (Fig. 5.2.1.1b). During April, May and June the site received less than the long-term average precipitation, meanwhile in July and September the amounts were above mean values (78 and 54 %, respectively). Very little precipitation in December then caused the annual sum to drop below the long-term average (Tab. 5.2.1.1).
Soil moisture. The amount of soil water available to the vegetation within the mixed Steinkreuz stand was described as relative extractable soil water θe. The seasonal minima of θe and associated parameters are given in Table 5.2.1.3 and the seasonal changes of θe in Figure 5.2.1.2.
Table 5.2.1.3: Soil physical characteristics (soil water content at permanent wilting point, θP, and at field capacity, θF) and soil water availability at Steinkreuz during the years studied. θmin is minimum observed annual soil water content, θe min is minimum annual relative extractable soil water. For further explanations see Chapter 4.5.
Values are averages from three sensors per depth. Note that minimum values ob-served in 1999 may not be the lowest values experienced by the vegetation due to a large data gap for all depths from mid-September to mid-October. Original soil water content-data courtesy of G. Lischeid, Dept. of Hydrogeology, BITÖK. Values for θP, determined for this site are from Langusch and Kalbitz (2001).
Soil depth -0.2 m -0.35 m -0.9 m
θP [m3 m-3] 0.068 0.068 0.111
θF [m3 m-3] 0.246 0.264 0.385
1998
θmin [m3 m-3] 0.084 0.114 0.265
θe min 0.09 0.23 0.56
1999
θmin [m3 m-3] 0.084 0.112 0.252
θe min 0.09 0.23 0.52
2000
θmin [m3 m-3] 0.126 0.140 0.286
θe min 0.33 0.37 0.64
The relative extractable soil water θe reached values of or near saturation only late in the spring of 1998 (Fig. 5.2.1.2), due to comparatively little precipitation during late winter and early spring (see above and Fig. 5.2.1.1b). The decline in θe was rather steep then in 0.2 and 0.35 m soil depth from the beginning of May, the time of leafing out, because of little supplementary rain. The decrease of θe in -0.9 m depth was slower. Frequent rain in late May and in June led to increases in θe in the upper soil layers, yet infiltration did not reach the deeper horizons (-0.9 m). Rainy spells re-sulted in increases of θe around the end of the first third of July and at the end of July (upper two soil levels only). Minimum values of θe were reached on 21.08.1998 (-0.2 m, -0.35 m) and 02.09.1998 (-0.9 m; Fig 5.2.1.2, Tab. 5.2.1.3). Abundant pre-cipitation in September and October caused θe to increase rapidly in all measured soil depths.
Rather little precipitation during March and April 1999 may have caused θe to begin to decrease immediately after the time of leaf emergence, interrupted by intense rains in May. After that, θe declined more rapidly than in the previous year, particu-larly at 0.9 m soil depth. Suspended by a rainy spell around mid-July, continuous rains on August 5, 1999 (41.8 mm in 21 hours, visible in increasing θe down to -0.9 m), and a few rainy days in mid-August, lowest values of θe were observed on Sep-tember 8, 1999 (Tab. 5.2.1.3). θe very probably declined even further, since a long late summer dry spell (daily PPT not exceeding 1.2 mm, beginning in mid-August) lasted until the evening of September 20, 1999, but θ-data are lacking from 09.09.
until 13.10.1999. Replenishment of soil water stores proceeded much slower than the previous year since October was also lacking typical amounts of rain (Fig. 5.2.1.2, Fig. 5.2.1.1b).
In the year 2000, θe decreased continuously from the time of leaf unfolding, initially at a somewhat lower rate though than in 1999. Minimum values in the three depths were already reached on July 2, 2000, which was the lowest values for this time of the year during 1998-2000. Precipitation of 77 % above normal afterwards prevented θe from dropping below the levels reached during the previous two years (cf. Tab.
5.2.1.3). There were only 13 days in July with less than 1 mm of rain and 11 days with more than 5 mm. The upper soil layers repeatedly dried out again in August, September and October, yet soil water supplies from 0.9 m depth remained favour-able (θe > 0.60). A slow recovery to saturation levels was observed, especially in -0.9 m, during late autumn/early winter (cf. Fig. 5.2.1.1b).
Coefficients of variation (CV) of θe ranged from 0.01 to 0.17 (i.e. 1 to 17 %) for 0.2 m soil depth, from 0.01 to 0.19 for 0.35 m soil depth and from 0.01 to 0.17 for 0.9 m soil depth. Obviously the three sensors installed per depth cannot represent the spatial heterogeneity of the whole plot. Nonetheless the CV hint at the within-site variablity which was highest during phases of soil rewetting and soil dry-down and lowest when θe reached minimum values (not shown).
Figure 5.2.1.3 shows profiles of seasonal dynamics of the soil water potential Ψsoil at Steinkreuz. The graphs corroborate the general trends extracted from data of volu-metric soil water content θ and extend it down to 2 m soil depth. Note that in this data set the same data gap exists as in that of θe, so corresponding assumptions can be made as above, namely that Ψsoil probably reached lowest values during the summer of 1999. Changes of Ψsoil in 2 m soil depth were less pronounced than in the upper layers and commenced only towards the end of the summer.
Figure 5.2.1.3: Vertical profiles of soil water potential Ψsoil at Steinkreuz on different dates in 1998 (a), 1999 (b) and 2000 (c). Data are averages of five soil water poten-tial profiles in the experimental plot. Tensiometers were placed in 0.2 m, 0.9 m and 2.0 m soil depth. Dates were chosen to be in monthly intervals (where possible de-spite frequent data gaps), except for the last date of each year, which was to show the lowest seasonal Ψsoil (in 0.9 m and 2.0 m depth); due to a large data gap from mid-September to mid-October 1999, the shown values for 07.09. are probably not the lowest values for that season (see text). The first date of each year is prior to leaf unfolding. Data courtesy of G. Lischeid, Dept. of Hydrogeology, BITÖK.
Radiation. Net radiation Rn and photosynthetical photon flux density PFD were high-est in the year 2000, respectively, both integrated to seasonal (May-October) and to annual levels (Tab. 5.2.1.1). Lowest values were observed for both intervals in 1999 regarding Rn and in 1998 regarding PFD (Tab. 5.2.1.1). Rn and PFD closely resem-bled each other (Fig. 5.2.1.2) over the years. The highest radiation was observed during June and July in 1999 and regarded as typical, whereas in 1998, Rn and PFD were highest in August, followed by May and June and much lower in July, in accor-dance with the pattern of Tair (see also Fig. 5.2.1.1a). In the year 2000, the largest radiation was measured in June, followed by May and August, while for July, values were again the lowest of the four months as in 1998 (Fig. 5.2.1.2).
Vapour pressure deficit of the air. As depicted in Figure 5.2.1.2 for the Steigerwald sites, the highest values of vapour pressure deficit of the air D, the driving force of transpiration, in 1998 were observed during the second half of July and especially August, and maximum daily values of Davg (17.5 hPa; 10’-means averaged over 24 hours) and Dmax (46 hPa; 24 hour-maximum of 10’-mean values) were measured on August 11 and August 12, 1998, respectively. During the following year, such high values were not reached again, and the days with higher Davg and Dmax were distrib-uted more evenly over the summer, with dry atmospheric conditions long into the warm September (see also Fig. 5.2.1.1a, b). The highest recordings were made on July 3, 1999 with Dmax = 34 hPa and on August 2, 1999, with slightly higher Davg (13.4 hPa) than on the previous date. In the year 2000, values of high D were found to-wards the first half of the growing season, with high evaporative demand already en-countered in May and maximum values of Dmax (39 hPa) and Davg (19.6 hPa) reached on June 20, 2000 (Fig. 5.2.1.2).
Differences among the Steigerwald-sites. Measurements of soil water potential Ψsoil
carried out at the second Steigerwald-site Großebene in varying intervals are com-pared with continuous readings from Steinkreuz at similar depths in Figures 5.2.1.4–
5 for the vegetation periods of 1998 and 1999, respectively. Apart from the noteworthy spatial variability there may be some indications that at Großebene, Ψsoil
in deeper soil layers (-0.6 m) stayed higher longer, but towards the end of the vegetation period of 1999 became as negative as at Steinkreuz, given that Ψsoil at Steinkreuz was measured in -0.9 m (Fig. 5.2.1.5b).
Figure 5.2.1.4: Soil water potential Ψsoil from May to December 1998 at the sites Steinkreuz and Großebene in 0.2 m soil depth (a; solid line is an average for Stein-kreuz, n = 5, symbols are for Großebene, n = 7), and (b) in 0.35 to 0.5 m (symbols, for Großebene) and in 0.9 m soil depth (solid line, for Steinkreuz, n = 5). Filled and shaded symbols in b) depict values from tensiometers in 0.35 m (n = 5) and in 0.5 m soil depth (n = 5) at Großebene, respectively. Bars (± 1 SD) in a and b depict spatial variability within the plot Steinkreuz. Data from Steinkreuz courtesy of G. Lischeid, Dept. of Hydrogeology, BITÖK.
Steigerwald, 1998
-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]ΨΨΨΨsoil b
Figure 5.2.1.5: Soil water potential Ψsoil from May to December 1999 at the sites Steinkreuz and Großebene, in 0.2 m soil depth (a; solid line is average for Stein-kreuz, n = 5, symbols are for Großebene, n = 3) and (b) in 0.35 to 0.6 m (symbols, for Großebene) and in 0.9 m soil depth (solid line, for Steinkreuz, n = 5). Filled and shaded symbols in b denote water potentials measured in 0.35 m (n = 4) and 0.6 m soil depth (n = 2) at Großebene, respectively. Bars (± 1 SD) in a and b depict spatial variability within the plot Steinkreuz. Data from Steinkreuz courtesy of G. Lischeid, Dept. of Hydrogeology, BITÖK.