Materials and methods Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol104Issue3Sept2000:

216 J. Ross, M. Mõttus Agricultural and Forest Meteorology 104 2000 215–231 of sunflecks, umbra and penumbra is closely related to the progress of new comprehensive computer mod- els of canopy photosynthesis and evapotranspiration, which require detailed 3D characteristics of different radiation types for input. As the interrelationship be- tween PAR and photosynthesis is nonlinear, the mean values of radiation do not yield correct results. In our opinion, for further calculation of canopy photosyn- thesis, leaves in plant canopies should be divided into three groups according to the type of direct sunlight they receive: sunflecks, umbra and penumbra; and photosynthesis should be calculated separately for each group. Therefore, it is necessary to know the area of sunflecks, its vertical distribution and tempo- ral dynamics in order that different data for umbra, penumbra and sunflecks could be used in modern radiation transfer models. Ross et al. 1998 proposed a statistical treatment of PAR variability and its application to willow cop- pice. In this treatment, statistical distribution of PAR global irradiance, obtained with a LI-COR quantum sensor, was approximated by a normal distribution in sunfleck and umbra areas, and the fractional area of penumbra was approximated by the Beta-distribution and considered to be function of pathlength τ and solar elevation h. This paper is a continuation of the previous work by Ross and Mõttus 2000 on a statistical treatment of umbra length and is focused on the statistical treatment of sunfleck length inside a willow coppice. Experi- mental data were obtained with a new instrument — the sunfleck sensor constructed by Sulev; the method- ology of statistical data processing is analogous to that used in our previous paper Ross and Mõttus, 2000. Sunfleck’s different statistical characteristics, such as sunfleck length distribution, sunfleck fractional area, number of sunflecks, mean sunfleck length, etc. in different canopy layers will be studied as the func- tions of the pathlength of the direct solar radiation 1 τ =Lsin h, where Lz = R z U z u L z dz is the down- ward cumulative leaf area index and u L z is the leaf area density. 1 In our previous paper Ross and Mõttus, 2000, we used the term ‘optical pathlength’ for this quantity. However, to avoid any confusion that might arise from this general term, we use a more comprehensive term, ‘pathlength of the direct solar radiation beam’, in this paper.

2. Materials and methods

Radiation and phytometrical measurements were carried out during the foliated periods during 1995–1998 in a willow coppice of Salix viminalis, clone 78 021, located at Tartu Observatory, Tõravere latitude 58 ◦ 16 ′ N, longitude 26 ◦ 28 ′ E, altitude 70 m above sea level, Estonia. More details of the research area, measuring system and data processing methods are given in Ross and Mõttus 2000. Phytometrical data on leaf, shoot, stem and coppice characteristics used in this paper were published in Ross and Ross 1998, Ross and Mõttus 2000 and Ross et al. 2000. The measurements were carried out on different cloudless days during the whole growth period from June until October for solar elevation of 20–55 ◦ during the years 1995–1999. The data used in this paper were obtained with the sunfleck sensor. The sunfleck sensor Fig. 1 is, in principle, a miniature actinometer with an angle of view of 10 ◦ in the verti- cal direction and 15 ◦ in the horizontal direction. Such an angle of view guarantees that the sun will remain completely in the field of view for 6 min. A silicon photodiode was used as a sensor, and the instrument was calibrated on the pyrheliometric scale using a ref- erence actinometer. Due to the different spectral sen- sitivities of receivers and the special configuration of the field of view, this calibration is less accurate than the calibration of standard actinometric instruments. The sunfleck sensor has a polar mounting and must be directed manually towards the sun before each scan. Fig. 1. The sunfleck sensor. The length and diameter of the sensor are 32 and 14 mm, respectively. 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