146 E.B.OKSBJERG 



observation. In order to save space I have tried to present the many 

 scattered measurements by diagrams, from four days in succession, but am 

 well avvrare of the fact that the observations were made under such different 

 conditions that strictly speaking they are not comparable and, therefore, 

 should not appear in the same diagram. In representing the observations 

 the question of eventual differences in needle temperatures in older and in 

 sprouting shoots is left out of account (see Fig. 3). 



All measurements show that needle temperatures under the experimental 

 conditions in question (moderate shelter in a 2-4 m high spruce plantation) 

 are higher than air temperature from dawn to sunset whether in sunshine, 

 cloudy weather or mist. No measurements are made when needles are 

 wet from rain but a few minutes after the needles had dried up after a 

 shower the needle temperature was again higher than air temperature. 

 Immediately after sunset the needle temperature wiU be lower than air 

 temperature when the sky is clear. 



The difference in temperature is an important factor governing trans- 

 piration and is the reason for a shght transpiration which takes place when 

 spruce plants (or probably all green plants =) are put into a transparent vessel 

 saturated with water vapour (for example a glass flask the inside of which 

 is covered by permanent wet filter paper) , when the flask is illuminated only 

 by diffuse dayhght, not sunshine. 



Unfortunately no opportunity occurred of measuring needle tempera- 

 tures during the drought of 1955 nor in the warm summer of 1959. The 

 difference between needle temperature and air temperature is biggest in 

 the early afternoon with high sun radiation intensity and decreasing 

 transpiration (see Fig. 3 b) when the water content in the shoots is reduced. 

 Lack of water naturally will increase the needle temperature while thermal 

 loss to evaporation diminishes it. 



The widest difference between needle temperature and air temperature 

 appearing in Fig. 3 is about 5 centigrade and in no reUable observation 

 during the summer of 1957 is this value exceeded. But in a dry and hot 

 summer, no doubt needle temperatures of over 3 5 centigrade will occur. 

 At such temperatures the respiration is very high and the balance of 

 carbohydrates in the needles is certainly negative. Probably the loss of 

 nitrogen in the needles is caused by proteolysis also causing decomposition 

 of chlorophyll. 



In many circumstances in a cold and wet August, or especially a dry 

 and hot summer, the colour of the vegetation changes from dark to hght 

 green in connection with flowering and ripening of seed (e.g. in grasses). 

 In many summers this may also occur as a result of a fall in available 



