156 WALTER TRANQUILLINI 



vicinity. It will be recognised at once how much cooler it is in the higher 

 region. Especially in summer during the growing season, the monthly 

 average air temperatures at 2000 m are about 8-10° lower than those of 

 the valley station. The temperature ranges, hmited by annual trend of 

 maximum and minimum temperatures, do not overlap, i.e. the average 

 monthly maximum temperatures at Obergurgl just reach the minimum 

 temperatures at Innsbruck. In winter the differences are smaller because 

 temperature inversions occur frequently then. 



The average temperatures of leaves are similar to the air temperatures. 

 We have measured the temperature of needles of young pine trees (Pinus 

 cemhra L.) at 2000 m for a whole year almost without interruption and 

 found (Tranquillini and Turner, 1961) that the average monthly tempera- 

 ture deviates from the air temperature by only 2°C at the most. The average 

 extremes though are twice as far apart as those of the air temperature; thus 

 the average maxima of the needle temperature at 2000 m correspond 

 approximately to those of the air temperature at 500 m (Fig. i). 



Since analogous measurements of plant temperature in lower regions 

 have not been recorded, we have to observe Row other factors important 

 for the heat relations of plants change with increasing altitude, i.e. solar 

 radiation and wind. Only then is it possible to estimate whether plants 

 growing in higher altitude are heated above air temperature more highly 

 than those in lower regions. 



In Fig. 2 the annual trend of average monthly solar radiation measured 

 at the research station near Obergurgl is compared with that found at an 

 altitude of 200 m (Vieima) (Turner, 1961a). Radiation at 2000 m is much 

 more intensive than at lower altitudes. This depends less on the intensity of 

 direct sun radiation than on the intensity of diffuse radiation under cloudy 

 conditions because of the smaller thickness of the cloud layer at 2000 m. 

 Because the horizon in the mountain area is strongly masked by elevations, 

 radiation in summer is only 10-20% higher than that of lower regions. 

 Radiation in winter will be referred to later. 



Wind velocity also increases with higher altitudes especially if we leave 

 the forest area which covers the lower parts of the slopes in the mountains. 

 During a long time we have measured the average wind velocity 3 m above 

 ground in different places along a line being a cross-section of the valley. 

 Auhtzky (1958) has pointed out that the wind velocity next to the timber- 

 Hne is already 40%, and at the tree-line only 200 m above the timber-Kne 

 is 70% higher than on the bottom (1800 m above sea level) of the valley 



(Fig- 3). ^ , 



This increase in wind velocity is probably not important for the water 



