86 PLANT SOCIOLOGY 



Examples are seen in the north and in high mountains. In the cold and 

 relatively snowless winter (in Europe) of 1924-1925, many low shrubs, 

 like Empetrum, Ardostaphylos uva ursi, and Vaccinium uliginosum, 

 suffered severe frost injury on naked, unprotected places in the eastern 

 Alps. In 1925 their growth was decidedly reduced. The alternating 

 but less sensitive areas of Loiseleuria procumbens and tufts of Festuca 

 halleri, however, suffered hardly at all. They grew luxuriantly the 

 next summer and extended their territory at the expense of the frost- 

 bitten low shrubs. 



The duration of low temperatures seems often as important as the 

 degree of frost. Shreve (1914) has shown that the giant cactus, 

 Cereus giganteus, will survive 19 hr. exposure to a temperature of 

 — 8.3°C. but is killed if this temperature persists for 29 hr. or longer. 



Injury from extremes of temperature is enhanced by other unfavor- 

 able factors. For example, the low winter temperatures of high moun- 

 tains and the far north have increased biological effect on account of 

 strong drying winds. The high summer temperatures of subtropical 

 deserts are combined with intense light, radiation from the bare ground, 

 and extreme drought. In such cases it is wholly futile to attempt to 

 evaluate the effect of the temperature factor alone. Injury to proto- 

 plasm by heat coagulation very rarely occurs in nature, except in the 

 case of fire. Heat and drought usually work together, and the latter 

 often deals the death blow. 



In central and southern Europe the winter minima seem often to 

 reach the threshold of sociological importance. But this needs more 

 precise determination. Communities of gregarious evergreen shrubs 

 and trees, such as Cistus spp., Buxus sempervirens, Ilex aquijolium, 

 Ruhia peregrina, and Quercus ilex, would be favorable objects for 

 study (Fig. 46). Besides the absolute minima, the duration and dis- 

 tribution of minimum temperatures are important. Late frost is so 

 harmful because it catches the plant in full activity, at a time when the 

 sugar of the tissues is mostly converted into starch. Foresters and 

 agriculturists are therefore especially interested in the limits of the 

 frost period and the number of frosty days. 



Frost Pockets. — Especially unfavorable temperature conditions 

 may result from lack of air circulation and accumulation of cold, 

 stagnant air in valleys and troughlike depressions. The frost pockets 

 {Frostlocher) of the southwestern Swiss Jura are well known and 

 shunned. Similar conditions cause the Seches — shallow troughs which, 

 although in the midst of the forest zone at 1,000 to 1,300 m., are entirely 

 treeless and at best covered with unshapely dwarf spruces, which 

 yield to neither ax nor saw. The spruce branches are beset with 



