Page:Plant succession; an analysis of the development of vegetation.pdf/10

CONTENTS.

VI

III. Initial Causes - Continued

Climatic Causes

 * Róle 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

Ecesic Causes

 * Nature 63

Aggregation

 * Concept and róle 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 róle 68


 * Germination 69


 * Fate of seedling 70


 * Growth 71


 * Reproduction 71


 * Ecesis in bare areas 71

Competition

 * Nature 72


 * Competition and dominance 72


 * Competition in air and in soil 73


 * Róle of competition in succession 73

Invasion

 * Nature and róle 75


 * Kinds of invasion 75


 * Manner of invasion 76


 * Barriers 77


 * Biological barriers 77


 * Changes in barriers 78

Reactions

 * Concept and nature 79


 * Róle in succession 80


 * Previous analyses of reaction 80


 * Kinds of reactions 81

Soil Formation

 * Manner 81


 * Reaction by accumulating plant bodies or parts 81


 * 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 by 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 aërial organisms 95


 * Correlation of reactions 96


 * Quantitative study of reactions 96

Stabilization and Climax

 * Stabilization 98


 * Causes of stabilization 98


 * Relation to the climax 98


 * Degree of stabilization 99

Life-History Stages

 * Nature


 * Kinds of stages 100


 * Role of life forms 100


 * Reasons why plants disappear 100


 * Reasons why plants appear at certain stages 100


 * Reasons why plants appear before their proper time 103


 * Initial stages 103


 * Medial stages 105