EDAPHIC OR SOIL FACTORS: PHYSICAL 



221 



Figure 112 illustrates the annual cycle of temperature measured in 

 the forenoon at various depths in the soil at Montpellier, France. 



In multilayered communities — scrub and forest — the soil tempera- 

 ture is more equable and lower than in exposed soil. For this reason, 

 according to Biihler (1918), tree seeds germinate more slowly in the 

 shade of woods. In openings germination is best toward the center of 

 the area. In this case probably light is a factor worthy of consideration. 



■^ iOcm 

 iscn 



Jan. Fet>. Mar. /Iprii May June July Aug. Sept. Ocf. Nov. Dec. 



Fio. 112. — A, The yearly range of mean soil temperature at various depths; and B, the 

 air temperature 20 cm. above the soil, at 9:00 a.m., near Montpellier, France. 



Maximal Soil Temperatures. — The highest soil temperatures are 

 attained in open, single-layered plant communities of the arid regions. 

 While Kraus (1911, p. 109) gives the maximal temperature of the top- 

 most soil layer (2 to 5 cm.) in natural sod communities (Bromion) near 

 Wiirzburg as 38.5° to 44°C., and Allorge (1922) in the Paris basin as 

 49°C., the temperature in the steppe soils of Russia reaches 50° to 60°C. 

 or more. In the desert region of Arizona, Sinclair found a temperature 

 of 71.5°C. at 4 mm., and Buxton (1924, p. 132) observed near Jerusalem 

 63.4°C. on the surface of the soil in August. 



Such extreme temperatures, especially when accompanied by 

 extreme dryness, permit only a few plants to live for any length of time. 

 Chamaephytes and nanophanerophytes endure them, while thero- 



