J. Ross, M. Mõttus Agricultural and Forest Meteorology 104 2000 215–231 217
The accuracy of determination of the linear dimen- sions of sunflecks and umbrae is ±5 mm. The height
of the horizontal bar inside coppice on which the sen- sor is mounted, can be changed from 0.4 to 6 m. One
measurement scan consisted of a transit of the carriage perpendicular to willow rows from one end of the bar
to the other and back with a total scan length of about 12 m. Because of leaf flutter caused by the wind, the
data obtained from the two transits were not identi- cal; to increase the statistical reliability of the sample,
the data from the two transits were considered as one measurement. The passage took about 400 s, and dur-
ing each scan 2400 readings were recorded.
As an example, Fig. 2 shows the recordings of the sunfleck sensor on a cloudless day, 14 September
1998, at three heights relative heights zz
U
0.85 a, 0.65 b and 0.36 c, with the coppice height z
U
= 3.1 m. Fig. 2 shows how the large variation in the
recordings of the sunfleck sensor depend on measur- ing height within the coppice.
3. Coppice architecture
As mentioned above, measurements with the sun- fleck sensor were performed during 1995–1998. The
years 1995, 1996, 1997 were the third, fourth and fifth growing years, respectively. At the end of 1997, the
plants were cut and 1998 was the first growing year after cutting.
Some phytometrical characteristics of the planta- tion Fig. 3 show the growth dynamics of coppice
architecture. The architecture of willow coppice is de- scribed in more detail in Ross and Ross 1998, Ross
and Mõttus 2000 and Ross et al. 2000, where it was demonstrated that it would be reasonable to di-
vide willow coppice into three layers.
The upper foliage layer z
U
− z
V
consists of current- year stems and branches modelled as differently ori-
ented foliage cylinders with leaves distributed along their axes; we assume random spatial distribution of
cylinders. Cylinders start growing from the apices of previous-year stems and branches. In the first year, this
layer consists of nearly vertical foliage cylinders, in the following years, of shoot and branch foliage cylin-
ders of the current year. So, starting from the second growing year, the character and structure of the upper
foliage cylinder layer does not change considerably. The lower foliage layer z
U
− z
A
consists of branch and shoot foliage cylinders with greatly varied orien-
tations and dimensions. This layer is almost homo- geneous, and for its architecture, the turbid medium
model Ross, 1981 will be used. The architecture of this layer is characterised by the vertical distribution
of the leaf area density u
L
z and probability distribu- tion function of the leaf area orientation g
L
rrr
L
2π . The spatial distribution of leaves is assumed to be ran-
dom. Since in both foliage layers the share of stems, branches and twigs is less than 10 of that of leaves,
their area will be neglected.
The lowest leafless coppice layer consists of nearly vertical stems Fig. 4a. Orientation of cylindrical
branches Fig. 4b varies over a wide range, the domi- nant inclination angles ϑ
S
being between 10
◦
and 30
◦
. In accordance with the measurements performed by
Ross in 1997 and 1998, an S. viminalis plantation is characterised by a large number of leaves per stem and
a great variability of leaf length Fig. 5a and leaf area S
L
Fig. 5b. In midsummer, an S. viminalis stem with a height of 500 cm has about 7000 leaves, the area of
the smallest leaves being 0.3 cm
2
and the area of the largest ones, 27 cm
2
. The size of a leaf depends on its age: the smallest leaves are the youngest, located
at the top of the shoot, and the largest ones are about 50 cm lower down the stem.
4. Statistical treatment
