VI 



CONTENTS. 



PAGE. 



III. Initial Causes — Continued. 

 Climatic Causes. 



R61e 55 



Bare areas due to climatic factors directly ... 55 



Bare areas due to drouth 56 



Bare areas due to wind 56 



Bare areas due to snow, hail, and frost 56 



Bare areas due to lightning 57 



Bare areas due indirectly to climatic factors . . 57 



Sudden changes of climate 57 



Biotic Causes. 



General relations 58 



Action and effect 58 



Bare areas due to destruction of vegetation 



alone 59 



Bare areas with dry or drier soils 59 



Bare areas with wet soils or water 60 



Primary and Secondary Areas. 



Distinction 60 



Sterility of primary and secondary areas .... 60 



Denudation 61 



Methods of denudation 61 



Depth of removal or deposit 61 



Rate and extent of removal 62 



IV. EcEsic Causes. 



Nature 63 



Aggregation. 



Concept and r&le 63 



Effects of simple aggregation 63 



Relation to denuded areas 64 



Interaction of aggregation and migration 64 



Migration. 



Concept 64 



Mobility 64 



Seed-production 65 



Influence of the organ used 65 



Influence of the migration contrivance 66 



Role of migration agents 67 



Destructive action of agents 67 



direction of migration 67 



Ecesis. 



Nature and r61e 68 



Germination 69 



Fate of seedUng 70 



Growth 71 



Reproduction 71 



Ecesis in bare areas 71 



Competition. 



Nature 72 



Competition and dominance 72 



Competition in air and in soil 73 



R61e of competition in succession 73 



Invasion. 



Nature and r61e 75 



Kinds of invasion 75 



Manner of invasion 76 



Barriers 77 



Biological barriers 77 



Changes in barriers 78 



PAOE. 



V. Reactions. 



Concept and nature ^^ 



R61e in succession °^ 



Previous analyses of reaction 80 



Kinds of reactions °'^ 



Soil Formation. 



Manner °^ 



Reaction by accumulating plant bodies or 



parts ^^ 



Reaction by accumulating plant concretions . . 83 



Reaction by producing weathering 83 



Reaction upon wind-borne material 84 



Reaction upon water-borne detritus 85 



Reaction upon slipping sand and gravel 86 



Soil Structure. 



Reaction by adding humus 86 



Reaction by compacting the soil 87 



Reaction by preventing weathering or erosion . 88 



Water-Content. 



Reaction b v increasing water-content 88 



Reaction by decreasing water-content 89 



Nutrients and Solutes. 



Reaction by adding nutrients or foodstuffs 89 



Reaction by decreasing nutrients 89 



Reaction by producing acids 90 



Reaction by producing toxins 90 



Soil Organisms. 



Reaction by means of parasites 91 



Reaction by means of saprophytes 92 



Air Reactions. 



Reaction upon light 92 



Reaction upon humidity, temperature, and 



wind 94 



Reaction upon local climate 94 



Reaction upon aerial organisms 95 



Correlation of reactions 96 



Quantitative study of reactions 96 



VI. Stabilization and Climax. 



Stabilization 98 



Causes of stabilization 98 



Relation to the climax 98 



Degree of stabilization 99 



Life-History Stages. 



Nature loo 



Kinds of stages 100 



Role of life forms 100 



Reasons why plants disappear 102 



Reasons why plants appear at certain stages . . 102 

 Reasons why plants appear before their proper 



time 103 



Initial stages 103 



Medial stages 105 



