THE BEHAVIOUR OF NORWAY SPRUCE 



147 



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Time oF day 



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Fig. 3. A comparison of needle and air temperatures on four successive days in 



July 1957. 



nutrients or, as we have seen, may be caused by heat. In all cases the colour 

 change probably consists of a reduction of the chlorophyll concentration 

 of leaf and needles. 



It is possible to construct an equation relating thermal radiation and 

 transmission to and from an area and the total evapotranspiration from the 

 same area, an equation in the sense of Penman (1956). The equation can 

 naturally only be formulated for a fully transpiring, green crop which 

 covers the soil completely. Such endeavours can, no doubt, be of great use 

 for certain regional estimations of ideal, maximal values of evapotrans- 

 pirations, but they cannot teach anything about evapotranspiration from 

 different vegetation types. The influence and extent of changes in colour 

 or leaf area of the vegetation and the occurrence of imperfect transpiration 

 (see the transpiration curve in Fig. 3b) can only be elucidated through 

 detailed ecological field observations. 



SOIL MOISTURE 



Soil moisture was measured in an old-fasliioned manner by digging out 

 the soil samples, opening a new profde at every new examination. 



