21 22 

 °C 



FIG. 9-2 Gradient of air temperatures 

 medium height at night (after Waterhou 



and 

 1955). 



23 24 25 



)ve (a) tall grass and (b) short grass on 



26 TALL 

 GRASS 



SHORT 10 

 GRASS 



perature ; beneath the litter in the central Ohio area 

 temperatures do not usually go below freezing 

 (Christy 1952) . A covering of snow gives added pro- 

 tection against freezing of the leaf litter. Another 

 study (Wolfe et al. 1949) revealed differences be- 

 tween temperatures above and below a snow covering 

 2 to 10 cm deep during a period of two months aver- 

 aged 8.9°C, and on one occasion reached 15.5°C. 



Relative humidity decreases from the ground 

 stratum upwards. In a young elm-maple forest in 

 Tennessee, the relative humidity from mid-February 

 to mid-August averaged 77.9 per cent at the surface 

 of the leaf litter, 75.2 per cent in the herb stratum 

 0.5 meter above the ground, 72.5 per cent in the 

 shrub stratum at 0.9 meter above ground, and 67.4 

 per cent in the trees at 7.6 meters above ground 

 (Adams 1941). In this same forest, the rate of 

 evaporation between May and November in the four 

 strata respectively averaged 29.4, 60.7, 72.8, and 

 99.2 cc per week. In a spruce-fir forest in Wyoming, 

 the average weekly evaporation at 0.1 meter was 50.5 

 cc, at 1 meter 7S.2 cc, and at 3 meters 103.9 cc 

 (Fichter 1939). 



In the Tennessee elm-maple forest mentioned a 



nd (c) 



2 13 



in grass of 



moment ago (Adams 1941), the average daily mid- 

 summer light intensities measured with a MacBeth 

 illuminometer for ground, herb, shrub, and tree (be- 

 neath the canopy) levels were respectively 52.3, 60.3, 

 60.4 to 76.2 foot-candles; in early May, before the 

 foliage was fully developed, intensities of 65.8, 78.3, 

 104.4, 119.1 foot-candles were measured. Under the 

 leaf litter and in the soil, the light intensity was, of 

 course, zero. Above the trees it was doubtless several 

 thousands of foot-candles. Ma.ximum light intensities 

 from the sun occasionally reach 15,000 foot-candles. 

 There are, therefore, three distinct sections in the 

 vertical gradient : below ground surface, between 

 ground and tree canopy, and above the canopy. 



Ground insects, millipedes and isopods, when 

 placed in experimental gradients, show a prefer- 

 endum for lower light intensity, higher humidity, and 

 lower temperature than do insects taken from the 

 lierb or shrub startum (Table 9-2). In grassland, 

 motile organisms can quickly vary the microhabitat 

 to which they are exposed by changing their vertical 

 position only a few centimeters, and they do shift in 

 position as the gradient varies at different times of 

 day or from day to day. To obtain an equivalent 



Grassland, forests, and forest-edges 1 23 



