118 Water 



In swampy woodland, for example, measurements have shown that 

 a high rate of evaporation characteristically exists near the tops of 

 the trees. At half this height the evaporation rate was reduced to 

 30 per cent in one instance. Lower down, near the damp soil, the 

 animals and plants were subject to an evaporation rate of only 7 per 

 cent of that near the treetops. We are consequently not surprised to 

 find that this vertical change in moisture, as well as vertical gradients 

 in light and in other factors, contributes toward the establishment 

 of a characteristic vertical layering by species of the vegetation and 

 sometimes also of the arboreal animal life in forests. Among animals 

 the difference in evaporation rate with distance above the ground is 

 of particular importance to soft-bodied insects. Many of these are 

 known to move higher in the vegetation at night, but during the day 

 they are forced by excessive evaporation to retreat to lower levels. 



Contrasting values for the rate of water loss in two microclimates 

 were reported in an interesting experiment performed in the Middle 

 West. Two sets of green ash seedlings were planted in pots; one set 

 was placed in the midst of a sumac thicket, and the other set was put 

 out on the open prairie only a short distance away. At the end of 8 

 days the first group of seedlings was found to have lost an average of 

 0.38 grams of water per sq cm of leaf surface. The seedlings set out 

 in the adjoining prairie had lost 0.88 grams of water per sq cm, or 

 more than twice as much during the same period. 



A traditional method of finding the approximate direction of north 

 in the woods is by observing on which side of tree trunks the growth 

 of green "moss" is thickest. In the North Temperate Zone in habitats 

 in which the atmospheric moisture is near the critical point, algae and 

 other non-vascular plants are characteristically more abundant on the 

 north surfaces of tree trunks, because the higher evaporation due to 

 insolation effectively retards growth on the south side. However, in 

 many situations differences in exposure to wind may outweigh the 

 effect of the sun in causing excessive water loss. Thus, although the 

 "north side" rule does not always hold, direct measurements show 

 that the growth of epiphytic moss is a very sensitive indicator of micro- 

 climate, and the striking differences in the growth of algae on op- 

 posite sides of tree trunks can be seen by anyone who walks through 

 the woods. An average value for the moisture condition of the forest 

 would give no inkling of these small-scale ecological differences of 

 crucial importance to the species concerned. 



The burrows of desert animals furnish another example of a micro- 

 climate in which the physical conditions are radically different from 

 those of the region as a whole. When the kangaroo rat retreats 



