228 



ANALYSIS OF THE ENVIRONMENT 



vironment, especially the climatic factors, 

 by the biotic elements of the environment 

 are a major influence in setting up micro- 

 climates, the appreciation of which is a rel- 

 atively recent development" (the term 

 "microclimate" has much the same meaning 

 as the so-called plant climates of Geiger, 

 1927; see page 231). The microcUmate is 

 distinguished from the climate in general 

 by the modifications of the component 

 factors within distinguishable zonal or areal 

 formations. These are partly inorganic, as in 



tical temperature gradients, measured in 

 two widely separated tropical forests, are 

 illustrated by the data given in Table 18. 



The dominant trees of both forests are 

 about 120 to 130 feet high; hence the 

 gradient extends over a considerable verti- 

 cal distance. Otherwise there is no difi^er- 

 ence in principle from the temperature 

 stratification to be found under any dense 

 growth of plants. It is noteworthy that the 

 air near the forest floor may have a rela- 

 tively constant temperature for days in suc- 



Table 18. The VeHical Temperature Gradient in the Tropical Forests of 

 Panama and Luzon (Data from Allee, 1926, and Brown, 1919) 



the various inhabited depth zones in soil 

 and broken rock, and partly biotic, as in 

 the grass mat of a prairie or within a forest 

 canopy. The minor niches within major 

 biotic formations have their own distinctive 

 microclimates, with their own seasonal 

 cycles. 



Temperature 



Plant cover produces a conspicuous bi- 

 otic modification of the temperature factor 

 by the combination of shade, altered humid- 

 ity, and lessened wind movement, with an 

 obvious extreme in the forest floor in dense 

 forest. Indeed, in wide areas of forest, the 

 modifications induced by the leaf canopy, 

 the humid air below it, and the root net- 

 work in the soil combine to affect the mac- 

 roclimate over great areas in such other 

 major features as rainfall and soil moisture. 



Temperatures in a variety of microcli- 

 matic niches may be compared with nearby 

 open air temperatures. The steepness of 

 the temperature gradient within the plant 

 matrix and the sharpness of the transition 

 from one temperature to another depend on 

 several factors, of which the height of 

 plant growth, the density of shade, and rel- 

 ative exposure to wind are important. Ver- 



• See K. Friederichs ( 1930) and Uvarov, 

 (1931). 



cession. During the week's record used in 

 Table 18 to illustrate the forest tempera- 

 ture gradient in Panama, the extreme varia- 

 tion at 9 inches above ground level was only 

 1.6°; for most of the week the temperature 

 ranged between 26° and 27° C. In the ab- 

 sence of nearby sun flecks, the air at the 

 bottom of the atmosphere of such a forest 

 tends to approach the uniformity of tem- 

 perature to be found in the water near the 

 bottom of a deep lake, or in the desert soil 

 some distance below the surface. 



Recurrent daily variations in the tree 

 tops, in midforest, and near the ground are 

 recorded in Figure 58, together with graphs 

 of the daily variations in water evaporated 

 from a continually moist filter paper in the 

 tree tops and near ground level. The graph 

 at the bottom of this figure shows the cus- 

 tomary, slight daily variation in barometric 

 pressure that characterizes the trade-wind 

 belts of the world. 



All available records show that the air 

 in March (1924) over Gatun Lake in 

 Panama took up approximately three 

 times as much water as that in the forest 

 canopy and six times as much as that near 

 the forest floor. Sherff (1913) cites evidence 

 and references indicating that the rate of 

 evaporation just above dense stands of tall 

 "sedge vegetation," cattails, and other marsh 

 and swamp plants may run as much as 1500 

 per cent above that found near their base. 



