Distribution of light within the crown of an open-grown Douglas-fir Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/41687k35r

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  • A systematic investigation was made of the distribution of short wave radiation within the crown of an open-grown Douglas-fir tree. The study tree was 23 years old, 14 meters tall and had a maximum crown diameter of about ten meters. Global radiation was measured every 15 cm along eight different horizontal transects through the crown. All radiation measurements taken within the crown were measured by four Kipp pyranometers mounted on a trolley-track system. Three different levels of the crown were examined : the lower 10%, 25%, and 40% vertical position of crown height. At each of these three levels, two horizontal transects were made at right angles to each other along compass directions (i.e., North-South, East-West). Two additional transects were made parallel to the East-West transect at the 25% level but horizontally displaced from the center of the tree. Global radiation measurements were taken along these transects approximately every two hours throughout the day during cloudless sky conditions. Measurements were taken during various months of the season over a period of two years. Global radiation was also taken on several overcast days throughout the season. The penetration of global radiation into the crown on cloudless days was found to be highly variable and was a function of solar position and branch foliage distribution. The greatest penetration occurred when the zenith angle of the sun was equal to the branch angle. The penetration of diffuse radiation was deeper and less variable than that of direct radiation. Since the study tree could not be destructively sampled, a control experiment using detached branches from other Douglas-fir trees was performed. From the control experiment, it was shown by a photographic technique that a modified form of Beer's Law of radiation penetration was valid for Douglas-fir branches. A simple geometrical model using solid circular cones is presented. This model can be used to predict the distribution of radiation within the tree crown.
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