90 PLANT SOCIOLOGY 



common observation that individuals and communities of less hardy 

 plants occur under the protection of trees and shrubs and are the more 

 restricted to this protection the nearer they come to the limits of their 

 range. Thus in the Rocky Mountains of Colorado many compara- 

 tively delicate plants, such as Polemonium yulcherrimum, Pedicularis 

 grayi, Adoxa moschatellina, and Pirola secunda, reach their altitudinal 

 limits under the protection of a layer of alpine spruce and pine at 

 3,500 m. 



This need of protection is very pronounced in the case of Ilex 

 aquifolium and Daphne laureola in their outposts in the alpine foot- 

 hills, whereas Ilex stands completely in the open in oceanic western 

 Europe and becomes a magnificent tree. In southern Europe the 

 mouse thorn, Ruscus aculeatus, and Viburnum tinus react similarly. 

 If they are exposed by lumbering operations, they freeze out in regions 

 with winter minima of —7 to — 8°C. 



Temperature in the Forest. — An abundance of data on temperature 

 in forests and in the open is given in Mtitterich's annual reports of 

 observations of the forest meteorological stations. Mean tempera- 

 tures from records of 18 years at 17 different places in Germany give 

 2.2°C. less range of temperature in the forest than in the open. The 

 mean maximum was 1.6° below, the mean minimum 0.6° above the 

 simultaneous readings in the open. In spruce forest the daily range 

 was 2.6°; in pine woods, 1.9°C. less than in the open. 



The mean annual temperature, according to the observations of 

 Biihler (1918), is lower in forest than in the open: in spruce wood 0.9° 

 to 1.3° lower; in beech wood, 0.1° to 0.7°C. Temperature of the 

 lower layers of air is influenced by soil temperature : it is cooler under 

 the trees in summer and during the day, warmer in winter and during 

 the night, than in the open. Stebler and Volkart (1906, p. 76) found 

 similar conditions under separate fruit trees. In February and March 

 they found night temperature near the ground 1.0° to 2.0°C. higher 

 under fruit trees than in the open. For this reason and also on 

 account of the absorption of radiant energy by trees, every tree and, 

 still more, every stand of trees forms in winter a local center of warmth. 

 This explains also why the snow melts sooner in orchards and the 

 grass greens earlier under the trees than elsewhere.^ 



Sociologically the habitat under protection of fruit trees forms a 

 sharply bounded inclusion within the central European Arrena- 

 theretum elatioris grassland. The floristic peculiarity of this pro- 

 tected plant community {Gagea lutea-Corydalis cava association) is 



' The rapid melting of the snow is favored by the drip from the branches, 

 which makes the snow denser and therefore a better conductor of heat. 



