Clements 



The Nature and Role of Plant Succession 



31 



light and temperature. The associated 

 animals are affected less immediately 

 by these, but find their chief relations 

 with plants as the source of food, ma- 

 terials and shelter. Man has modified or 

 evaded physical conditions to a large 

 degree, but even he is much controlled 

 by differences of climate, soil, raw ma- 

 terials and food. 



PLANTS INDICATE CONDITIONS 



The significant outcome of these 

 relations is that both species and com- 

 munities serve as measures or indexes 

 of conditions and hence are known to 

 the ecologist as indicators. In connec- 

 tion with land classification, agricul- 

 ture, forestr}', grazing, erosion, flood- 

 ing, and water supplies, the use of indi- 

 cators furnishes a method of primary 

 importance. 



They indicate not merely the pres- 

 ent features of climate and soil, but 

 they also possess the clairvoyance of 

 forecasting future changes and the pos- 

 sibility of controlling them, as well as 

 of deciphering past events. Thus, cli- 

 max and succession have not only great 

 practical applications, but also provide 

 the open sesame by which traveller or 

 nature-lover ma)- unlock the pages of 

 nature's book and read the past and 

 present of every landscape, and like- 

 wise its further story. 



The priman' indications have to do 

 with climate and soil and the out- 

 standing changes of the past, but 

 woven into this pattern is the infinite 

 variety wrought by man, directly 

 through fire, settlement, logging, culti- 

 vation and so forth, or indirectly by 

 grazing, erosion, flooding, draining. 

 Each of these processes has its own in- 

 dicator communities, and its major ef- 

 fects can be read with almost as much 

 certainty as though recorded on the spot 

 by an eye-witness. [Lately, the effect 

 of atomic radiation on plants and ani- 

 mals demands serious attention— Ed.] 



THE GREAT PLANT CLIMAXES 



Ever}'one is familiar in a general 

 way with the great climaxes of our 

 country and especially with the two 

 most extensive, the eastern forest of 

 beech, maple, chestnut and oaks, and 

 the prairies of many kinds of grasses. 

 In addition to these are the great trans- 

 continental forest of spruce and fir to 

 the northward and the Barren Grounds 

 of sedge and lichen stretching along the 

 Arctic Circle from ocean to ocean. 



Related to these and hence of single 

 interest as seeming far out of place are 

 the alpine tundras of Mount Katahdin, 

 Mount Washington, and of Pikes 

 Peak, Mount Whitney, Mount Rai- 

 nier, and other high summits of the 

 Rocky Mountains, Sierra Nevada, and 

 the Cascades, all survivals of a distant 

 time of glacial advance when the arctic 

 tundra moved far to the south. 



Each of these great communities 

 consists of certain dominants, a ruling 

 class drawn usually from trees or 

 grasses and best fitted to the climate 

 concerned, and of various subordinate 

 groups, among which the flowering 

 herbs of woodland and prairie are the 

 most conspicuous and familiar. Each 

 climax is the product of its particular 

 climate and hence the indicator of it, 

 and thus serves as the point of depar- 

 ture for all the disturbances brought 

 about bv man and for all projects of 

 utilization, restoration and rehabilita- 

 tion under way or projected in the pres- 

 ent national program. 



KINDS OF SUCCESSION 



Examples of the growth of cli- 

 maxes, of their childhood and adoles- 

 cence are to be found ever)'where within 

 the corresponding climate. Most fre- 

 quently seen are those due to disturb- 

 ances caused by man, but others with a 

 much longer lifespan occur in pond and 

 lakelet, on rocky ridge and cliff, in sand 



