294 INTRODUCTION TO PLANT GEOGRAPHY 



physiographic ones separately — especially as they are well marked 

 in their effect on vegetation in regions of drastic topography and 

 harsh climate. Strong topographical relief tends to produce marked 

 local climates, summits for example being very different in these 

 respects from sides of mountains, and narrow valleys from open 

 plains. Quite apart from the tendency to greater windiness and 

 exposure at higher altitudes, the air and soil temperatures tend to 

 get lower and the relative humidity greater as we ascend, with 

 atmospheric pressure decreasing and heat-radiation increasing in 

 intensity. Altogether, climatic variation becomes more and more 

 extreme and rapid with increasing altitude. 



As an example of physiographic effects in arid regions, we may 

 rise from unproductive plains to fertile slopes and forests on moun- 

 tain sides, and at very high altitudes reach again an unproductive 

 zone of low absolute humidity and rigorous exposure. The changes 

 are due largely to local climate but would not take place if it were 

 not for the physiography. Moreover, major topographic features 

 often affect the climate at a considerable distance — as, for example, 

 mountain ranges which may cause rainfall locally and decrease it 

 in their lee — while in Chapter IV we saw how such features, and 

 wide expanses of water, can act as barriers to plant migration. 



Another important physiographic effect is aspect : in the northern 

 hemisphere, north-facing slopes tend to be more hygrophilous 

 (adjusted to moist conditions) than south-facing ones at similar 

 altitudes (Fig. 83, A). This is owing to the effect of insolation on 

 air and soil temperatures, and consequently on relative humidity 

 and evaporation and, through them, on the local water situation 

 (even when precipitation is the same). For vegetational differences 

 due to topography are most often correlated with moisture, and 

 naturally tend to be marked chiefly where water is deficient and 

 consequently is a critical factor. Sometimes for this reason there 

 may be entirely different vegetations and even floras on the two 

 sides of a deep valley or steep mountain (Fig. 83, B), as in the dry 

 Mediterranean region, while if similar zones are developed they 

 tend to be at higher altitudes on the south- than on the north-facing 

 slope. The western slopes of mountains may be noticeably warmer 

 and drier than the eastern ones, owing to the sun's afternoon warmth. 

 Quite drastic effects may often be seen already on a microclimatic 

 scale, for example in the shelter of rocks or even stones on exposed 

 sea-cliffs and mountain summits. Here we may refer again to 

 Fig. 79, which shows a striking aspect effect in the Arctic. 



