332 



ECOLOGICAL FACTORS: 



hand, mostly because of low temperature, or perhaps 

 wind, the vegetation may be stunted and reduced in 

 species and number. Also, there may be a general 

 upslope transition of vegetation from arid to humid 

 and finally alpine types, or there may be temperature 

 inversion and contrasting belts of vegetation showing 

 no such "normal" transitions. In addition, north- 

 facing slopes usually have denser and taller plants 

 than do south-facing slopes. Actually, there is often 

 a progression of decreasing plant development from 

 north to east, to west, and to south exposures, be- 

 cause sunlight causes the greatest drying on south 

 and west slopes and precipitation comes from the 

 north. The general exception to this slope orientation 

 exists when the pattern of precipitation is difTerent. 



Increasing altitude can have consequences that are 

 very similar to those of increasing latitude. Plants 

 might be reduced in number of species and individu- 

 als and may be stunted. Trees might become shrubs 

 or very poorly developed shrubs and little more than 

 ground cover. Also, there might be changes in species 

 of the same genus or races of the same species. When 

 this situation involves species of the same genus, the 

 species are called substitution species. In addition, there 

 is a tendency for vegetation zonation, or the develop- 

 ment of distinct altitudinal tiers of plant communi- 

 ties. A unique effect of altitude is the flag-like form 

 of woody plants, especially trees (Figure 17.19). This 

 deformation probably is related to many aspects of 

 the tree limit environment but is most closely associ- 

 ated with wind. 



Other topographic influences upon environment 



flagging 

 desiccation 



abrasion 



Figure 17.19 Wind effects upon plants. 



were outlined previously. Of most importance are 

 variations in soil depth, amount of precipitation, and 

 velocity of winds. Soil depth is inversely related to 

 the amount of slope; the greater the slope, the thin- 

 ner the soil. Precipitation is low on the leeward side 

 of mountains in the so-called rain shadow. Winds are 

 greatest at the highest elevations and along ocean 

 coasts. 



Although there is a definite relationship between 

 amount of slope and soil depth, flat lands have varied 

 soil conditions. In considering the possible variations, 

 the soil of most plains is used as the average type, or 

 point of reference. Plateaus may possess thick soils 

 that are excessively leached and have thick clay hard- 

 pans in the B layer. Poorly drained plains also have 

 thick soils, but the main material that accumulates is 

 humus. This accumulation is due to water satura- 

 tion, a factor that inhibits organic decay because 

 oxygen is not readily available. 



VEGETATION ZONATION 



The correlation between altitude and latitude can 

 be appreciated further by examination of the distinct 

 altitudinal bands of plant communities (Figure 17.20). 



Lowlands have vegetations no difTerent from that 

 of the surrounding area, because this terrain is the 

 prevalent one. A California example of such lowlands 

 is the Foothill Woodland Community of the western 

 foothills of the Sierras. The Montane ^one is cooler, 

 cloudier, and more humid; but the growing season is 

 shorter and less effective in stimulating plant growth. 

 In the Sierras this zone is represented by the Montane 

 Forest, usually in the form of a Yellow Pine Commu- 

 nity. The Subalpine ^one is still cooler, but is typically 

 more arid than the Montane Zone as a result of its 

 clear, dry atmosphere. The increased aridity causes 

 Subalpine trees to be smaller and less dense than 

 Montane ones. The Subalpine Zone has its upper 

 margin at the limit of trees where trees display de- 

 crease in numbers, stunting, and modification in 

 shape. In the Sierras the Subalpine contains a Sub- 

 alpine Forest Community. The Alpine ^one is colder 

 yet. In spite of its frequent cloud cover, trees cannot 

 survive here, but herbs or small, stunted shrubs can. 

 The herbs regularly predominate and are of tufted 

 and cushionlike form, so the over-all vegetation has 

 the appearance of Tundra or modified grassland. In 

 the Sierras this zone contains the Fell-fields Com- 



