212 



ANALYSIS OF THE ENVIRONMENT 



total of meteorological factors within a 

 habitat. In this connection, a habitat means 

 a forest, a marsh, a sand dune, and the like. 



Plant climates have been studied prima- 

 rily in western Europe and in the middle 

 reaches of North America, both relatively 

 humid regions. A given plant cUmate in 

 these areas is determined primarily by the 

 nature of the surface, that is, by the various 

 kinds of bare ground or of vegetation, by 

 slope, by exposure, and by such meteoro- 

 logical phenomena as cloudiness, dew for- 

 mation, and evaporation. 



In the macroclimate, temperature de- 

 creases by 0.2° to 1.0° per 100 meters 

 above ground, and temperature inversions 

 are relatively rare. The temperature gradi- 

 ent is normally much steeper in the plant 

 climate in general, and it may become es- 

 pecially steep in the climate of crawling 

 insects. Inversions are much more common 

 in microclimates than in macroclimates. 



Two patterns of plant cUmates are clearly 

 distinguished: the radiational type, charac- 

 teristically developed on clear nights and 

 (Fig. 53), the insolational type, diagram- 



Fig. 53. Types of vertical distribution of 

 temperature in the zone of plant climate. For 

 each graph, distance to the right represents the 

 higher temperature. (Modified from Geiger. ) 



matically developed on warm sunny days 

 in summer. As these graphs indicate, the 

 surface of the ground (the insect cHmate) 

 is the point of greatest interest in plant ch- 

 elates. In sunny daytime, it is the location 

 of highest temperature with a sharp tem- 

 perature decrease in the air above and the 

 soil below. Even in humid, temperate cH- 

 mates, the atmosphere above the soil sur- 

 face in bright sunlight may show a drop of 

 3° in the first 8 mm. of air, and insects 

 readily escape to cooler air even by low 

 flights. At night, the surface becomes cooler 

 than the air in the soil, but the gradients 

 are less steep. These and other tempera- 

 ture gradients of aerial microclimates are 



shown in Figure 53. Graph a gives con 

 ditions at the time of minimum temperature 

 in early morning, when outward radiation 

 is dominant. When dew is being formed 

 (graph a), heat of condensation of water 

 vapor raises the temperature of the insect 

 chmate without aflFecting the remainder of 

 the plant chmate; the air mav be warmed at 

 the ground level as much as 0.6°. As the 

 sun rises and becomes eflFective, the layer 

 in the insect chmate is warmed first, and 

 conditions can again be summarized bv 

 graph a, although the causal relations are 

 diflFerent. With increasing insolation, the 

 temperature profile of the plant cHmate 

 changes to the insolation type shown in 

 graph b after a heavy dew; the thin layer 

 of chilled air in the insect climate comes 

 from the rapid consumption of heat used 

 in the evaporation of dew; early morning 

 warming may produce conditions shown in 

 graph b'. 



An insolation plant chmate in pure form 

 (graph b') is often disturbed during the 

 midday hours by the formation of a layer of 

 air, about a meter above ground, in which 

 the temperature may actually rise somewhat 

 and certainly does not continue the steady 

 fall that again sets in at an altitude of about 

 1.5 meters (graph b"). This upper limit 

 sets a convenient, and perhaps a natural, 

 upper limit for the microclimate (plant cli- 

 mate). The causes of these phenomena are 

 unknown. 



Other environmental conditions show 

 special development in the zone of micro- 

 climates. Humidity relations differ near the 

 ground from those higher in the air. Except 

 when dew is forming, there is nearly al- 

 ways a maximum in absolute humidity iust 

 above the ground surface as compared with 

 higher levels of the plant climate. Maximum 

 absolute humidity varies between day and 

 night, and relative humiditv is normallv 

 decidedly greater at night. Wind velocity is 

 much reduced near the ground, even in 

 level country lacking vegetational cover. 

 There are more hours of absolute calm in 

 the zone of insect climate than a few centi- 

 meters higher, and wind movement is still 

 more constant above the limits of plant cli- 

 mates. The rate of air movement to which 

 ground-dwelling insects are normally ex- 

 posed probably does not normally exceed 

 one-tenth that which man experiences (cf. 

 p. 147). A few of the other relations be- 

 tween macrochmates and microclimates 



