-MONDAY v/ TUESDAY- 



ai Mt BI 



-WEDNESDAY- 

 SI 



-THURSDAY- 



xn Mt 



- FRIDAY - 



xn 



\ 



fc 



FIG. 9-1 A weekly chart fr 



hygrofhermograph placed at shrub level in a decic 



entral Illinois. 



was reduced to a minimum of 10 per cent when the 

 trees were in leaf and 25 per cent when not (Williams 

 1936). With a protective edge of shrubs, the wind 

 velocity would doubtless have been decelerated more 

 quickly. 



Summer light intensities are much less under 

 foliage than out in the open. Noontime illumination 

 under shrubs in Iowa averaged 26 per cent of full 

 sunlight; within the forest interior, 6 per cent (Aik- 

 man and Smelser 1938). The forest floor is not uni- 

 formly illuminated because small openings in the 

 canopy admit sun-flecks of varying intensity. In the 

 Cottonwood, pine, black oak, and sugar maple stages 

 of the sand sere at the lower end of Lake Michigan, 

 the percentages of the forest floor shaded during the 

 midday hours were 68, 87, 75, and 90 per cent, re- 

 spectively (Park 1931.). There may be some change 

 in the quality of light that filters through the forest 

 canopy, as there is of intensity, as some wavelengths 

 are used more than others in photosynthesis ; green 

 is transmitted or reflected and not absorbed. Where 

 a stand of trees abruptly confronts an open field, 

 light penetrates laterally under the forest canopy and, 

 the typical edge configuration reversed, the light per- 

 mits shrubs to extend 40 meters or more into the 

 interior. 



Vertical gradient 



There is a gradient in microhabitat factors from 

 above the grasses down to the ground. In one study 

 of virgin prairie (Weaver and Flory 1934), light 

 intensity varied from 100 per cent in full sunlight to 

 25 per cent at one-half the pile depth of the grasses 



to 5 per cent at the base of the stems, and, of course, 

 zero per cent in the subterranean stratum. The rela- 

 tive humidity above the grass was 20 per cent ; in the 

 grass, 31 per cent. The wind velocity above the 

 prairie grasses was 14.5 km/hr (9 mph) ; at the top 

 level of the grasses, 6.0 km/hr (3.7 mph) ; at the soil 

 surface, zero. The rate of water evaporation from 

 white spherical atmometers was 55.3 cc/day above 

 the grasses, 33.3 cc at top surface of the grasses, 15.1 

 cc at one-half the pile depth of the grasses, and only 

 13.4 cc just above the soil surface. The temperature 

 gradient varies with the height of the grass and be- 

 tween day and night. 



The vertical gradient of temperature in a decidu- 

 ous forest in central Ohio varies with the season and 

 with the height of macroclimatic temperature (Table 

 9-1). In the summer, the greatest extremes of tem- 

 perature occur in the canopy, but at other levels, both 

 above and below the ground, summer daily mean 

 temperature is more stable than at any other season. 

 Because the canopy largely controls the air tempera- 

 ture beneath it, there is little or no thermal stratifica- 

 tion between it and the ground. Summer soil tem- 

 peratures are always lower at 1.2 meters below the 

 surface than at the surface. For comparison, air tem- 

 peratures in a coniferous forest in Wyoming during 

 July and August averaged 12.3°C at 0.1 meter above 

 the ground and 7.6 °C 0.1 meter below the surface 

 litter (Fichter 1939). 



During the winter, temperatures in deciduous for- 

 ests are lowest near the ground and more uniform at 

 all higher levels than during the summer, since the 

 absence of a canopy permits greater turbulence, hence 

 less stratification, of the air. Soil temperature at 1.2 

 meters depth is generally higher than surface tem- 



122 Habitats, cominunities, succession 



