4 Application of a 3D-light model to the 3D-representation of beech Gr12
4.1 Methods
4.2.3 Comparison of climate and transpiration data
4.2.3.1 Daily courses
Absolute values of climate variables above a clear-cut near the BITÖK investigation site Steinkreuz were measured by G. LISCHEID, University of Bayreuth and are shown in Fig. 120.
The days during the two weeks from 19.6. -2.7.1998 were mostly cloudy, though not very rainy:
Only the days 21.6., 22.6., and 25.6. had permanently clear, sunny conditions. These days were also the warmest days, so that the first week was generally warmer than the second week. Main rain events occurred during the evening or night hours of 21.6., 26.6., and 27.6., while smaller rain events took place on 19.6., 23.6., 26.6., and 1.7.1998. The daily courses of VPD, PPFD, and temperature were more or less parallel on the three clear sunny days, on 24.6., and during
A C B E D F
Fig. 119: The LMA of leaf clouds is given as their whole leaf mass divided by their leaf area and equals the average LMA of their leaves. Leaf cloud LMA has been set in relation to the average of simulated relative irradiance values in all matrix points directly above the leaf cloud (left side, filled squares). Equation (55) is plotted in the same graph for comparison. Error bars represent the range of LMA values occurring in each leaf cloud apart from leaf cloud B, where this was not measured.
Modelled leaf cloud LMA on the base of averaged simulated relative irradiance and the corrected LMA vs. relative irradiance relationship is well correlated to the measured leaf cloud LMA values (right side, r²=0.87). The model slightly overestimates leaf cloud LMA values by 1.5 g/m² (mean absolute error), equalling 2% of the mean of measured values. The root mean square error was 10.28g/m².
0 0.2 0.4 0.6 0.8 1
relative irradiance 0
20 40 60 80 100 120
faelduolcAMLHgêm2 L
y = 0.99x R2 = 0.87
0 30 60 90 120
0 30 60 90 120
leaf cloud LMA measured (g/m²)
leaf cloud LMA modelled (g/m²)
(53)
26.6. - 29.6.. Opposite tendencies in the courses of VPD and PPFD were due to temperature and were observed on 19.6., 24.6., 30.6., and 2.7..
Some observations may be summarised about the course of transpiration rates of sun and shade leaf clouds during the investigation period:
• The daily course of transpiration rates of sun leaf clouds was generally smoother than that of shade leaf clouds: while sun leaf clouds D, E, and F had wide and round daily peaks of transpiration (Fig. 122), the peaks in the course of transpiration rates from shade leaf clouds A, B, and C were more pointed (Fig. 121).
• When a continuous increase in irradiance was given, transpiration rates of leaf cloud D and the other sun leaf clouds steeply increased early in the morning and then approached to a maximum value at noon. Discontinuities in the irradiance increase with parallel discontinuities of the usual VPD increase in the morning - probably due to dew fall or smaller rain events - caused a decrease in transpiration rates even if irradiances just stayed constant, which may be observed on 19.6., 22.6., and 26.6. on all sun and shade leaf clouds.
• The shade leaf clouds often did not yet start to transpire on these days, when such a discontinuity occurred, which may indicate that a critical light or VPD level was not yet reached before that time. Their later start is one reason for the more pronounced peaks in transpiration.
• Another reason was that the course of PPFD above these leaf clouds consists also of pointed peaks, thereby inducing high irradiances for a short time.
Fig. 120: Synopsis of climate variables during the investigation period 19.6.1998 - 2.7.1998. While temperature and VPD are scaled on the left y-axis, PPFD (in mmol/(m²*s)!) and precipitation are scaled on the right side. All measurements were performed at the BITÖK investigation site Steinkreuz, 5m above a clear-cut approximately 1300m in distance to the Großebene stand byG.
LISCHEID, University of Bayreuth.
0 5 10 15 20 25 30
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
temperature(°C)|VPD(hPa)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
PPFD(mmol/(m²*s))|precipitation(mm)
temperature (°C) VPD (kPa) precipitation (mm) PPFD (mmol/(m²*s))
• The decrease of irradiance after this maximum peak is often (leaf clouds A and C during 19.6.-26.6.) but not in all cases (leaf cloud B, 20.6., 21.6., and 25.6.) accompanied by a sharp decrease of transpiration rates. In opposite to the other shade leaf clouds, irradiance above leaf cloud B again increases in the afternoon and in opposite to A and C, the transpiration rates
-10 0 10 20 30 40 50 60 70
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
-40 0 40 80 120 160 200
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
-30 0 30 60 90 120 150
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
A
C B
Fig. 121: Time course of measured transpiration rates and average of calculated PPFD of the matrix point layer directly above the investigated shade leaf clouds (clouds A, B, and C) during the investigation period. The time course of both variables is often parallel, but not strictly and not on every day.
above leaf cloud B stay high or even increase (21.6., 25.6.) during the time of decreasing irradiance, thereby ignoring the sharp but short decrease in irradiance.
• In sun leaf clouds, a series of high transpiration rates was achieved on most days between 10:00 and 14:00, which was partly independent on light: The transpiration rates of leaf
-200 0 200 400 600 800 1000
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
Fig. 122: Time course of measured transpiration rates and average of calculated PPFD directly above the investigated sun leaf clouds (clouds D, E, and F) during the investigation period. The time course of both variables is often parallel, but not strictly and not on every day. Transpiration “peaks”
are generally wider than those of the shade leaf clouds. A gap exists in the sapflow measurements of leaf cloud F due to technical problems.
D
-100 0 100 200 300 400 500 600
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
E
-300 0 300 600 900 1200
19/06/98 21/06/98 23/06/98 25/06/98 27/06/98 29/06/98 01/07/98 03/07/98 Date
PPFDµmol/(m²*s)]
0 0.5 1 1.5 2 2.5 3
Transpiration[mmol/(m²*s)]
calculated PPFD above leaf-cloud measured transpration
F
cloud D for example do not completely follow the course of irradiation on 22.6, 24.6., and 28.6..
More or less strong decreases in irradiance and VPD did not lead to reductions in transpiration on these days.
• If irradiance increases in the afternoon parallel to VPD, a reaction of transpiration rate is often visible in sun and shade clouds, which is best visible on 24.6. (all leaf clouds).