Succession. 



24 INDIAN FOREST MEMOIRS. 



probably be recognised in (a) the xerophilous light-demanding woodlands of A cacia Catechu 

 in the stony, sandy, riverbeds of Northern India, {b) the dry miscellaneous forests of the- 

 same region, in which the proportion of A . Catechu has diminished and a number of addi- 

 tional species have made their appearance of which several have a marked power of increas- 

 ing the humus content of the soil, e.g. Ehretia Icevis, Holarrhena and Adhatoda Vasica. 

 Such woodlands are clearly less xerophilous than the pure woods of Acacia, (c) the markedly 

 mesophilous shade-bearing Sal forest in which the sal predominates. This type of vege- 

 tation also extends, in parts of Bengal and Assam, into the region of the hygrophilous 

 tropical evergreen forest which must be regarded as the highest type of woodland. 1 

 Regressive On the other hand the reverse succession from a highly developed to a more simple type 



may be termed regressive. An example of such a succession is seen when mesophilous 

 forest is cleared, or more gradually destroyed by fire and grazing, on steep slopes, the 

 resulting erosion converting the area into a rocky hillside only capable of supporting the 

 poorest and most xerophilous types of vegetation. Fire is a very potent factor in causing 

 regressive successions, for not only is it capable of temporarily depriving the soil more or 

 less completely of its covering of vegetation, but it also directly dries the soil and destroys 

 the humus. Fire may in this way be responsible for the existence of xerophilous grassland, 

 or woodland, in localities which once supported mesophilous, or possibly hygrophilous, 

 vegetation. 



Grazing again, by destroying the undergrowth and keeping a forest open, may so reduce 

 the humus content of the soil as to render impossible the reproduction of the mesophilous 

 species constituting the forest and may thus cause a regressive succession. 



The effect of artificially clearing an area of its covering of vegetation in this connec- 

 tion is often insufficiently appreciated although this is of vital importance to the Forest 

 Officer. The entire removal of the tree-growth on an area, such as takes place under the 

 treatment of clear-cuttings, or coppice-fellings, results in the drying of the soil, the rapid 

 disintegration of the humus already contained in it and a temporary interruption in the 

 supply of additional humus. In this way the capacity of a sandy soil to retain moisture 

 may be greatly reduced, while a clayey soil becomes denser, less aerated, liable to water- 

 logging in the rainy season and to become very dry at other seasons owing to the moisture 

 which is evaporated not being replaced rapidly, on account of the sluggish movements of 

 water by capillarity in the dense soil. Conditions may therefore be thus created which are 

 unsuitable for the development of young plants of the species which composed the original 

 woodland. It is therefore easy to understand from this point of view why, in certain 



1 Nothing said above regarding the details of a succession which under certain circumstances may occur must be taken to mean that 

 succession must necessarily always follow the same sequence, or that a Sal forest, for instance, can only establish itself on o-ronnd previously 



re 

 .— i locality (1) directly on 



sandy or well drained clayey soil, provided sufficient moisture is available by percolation or otherwise, (2) on sandy soil after a erowth of 

 plants like Adhatoda has considerably increased its humus content, (3) on clay which has supported a growth of plants like Adhatoda' MallotiLt 

 Trewia nndi flora, and others. The texture of the clay is improved by the draining action of the roots of these plants and by the admixture 

 of humus so that the aeration of the soil is improved and the movement of water from the subsoil to the surface by capillarity is fa militated 



[ 14 } 



