356 STRUGGLE BETWEEN PLANT COMMUNITIES sect, xvii 



including Papaver hybridum, Helianthemum guttatum, Trifolium agra- 

 rium, Galactites tomentosa, and Jasione montana.^ After some years 

 this herbage is suppressed by Cistus monspehensis, and the rest of the 

 w^^Mt-vegetation gradually appears ; first, Daphne Gnidium establishes 

 itself, and is succeeded by the other species : while eventually Cistus 

 monspehensis is consigned to the position that it normally takes in 7naqui. 



Summary of Results. 



It is difficult to make general statements in regard to vegetation 

 appearing on new soil, because very few detailed investigations bearing 

 on the subject have been conducted. Published work ^ seems to justify 

 the following conclusions : 



i. In many cases, possibly always, the first colonists are algae and 

 lichens, as well as mosses (for example, arenicolous algae on the shore, 

 algae and lichens on lava-fields, rocks, and so forth) ; these prepare the 

 way for Vascular plants. The early vegetation is open. Some time elapses 

 before a coherent covering of vegetation is produced. So far as Vascular 

 plants are concerned, the individuals are at first very scattered, but 

 gradually increase in numbers. 



ii. The number of species present is small at first ; it increases, until ! 

 after the lapse of a certain length of time it is greater than ultimately. 

 For, at first, many species find suitable spots, but are subsequently sup- 

 pressed when the vegetation forms a continuous covering and more 

 tyrannous species have entered. Various parts of the recently colonized' 

 ground are often clothed with plants in a very dissimilar manner. Gradually 

 the vegetation becomes more uniform and poorer in species. 



iii. Very frequently annual and biennial species are more numerous 

 at first than later on, because on open ground they find the conditions 

 more favourable to them than on overgrown ground ; many of them belong 

 to the local weed-flora. Afterwards perennial herbs or woody plants 

 preponderate. 



iv. The first species to enter are those which occur in the vicinity 

 and possess the best means of dispersal by wind or by birds. Rubble- 

 heaps in the Alps are first colonized by species with wind-dispersed seeds.^i 

 In Norway, when a coniferous forest is destroyed, the first immigrants 

 are the birch and poplar (with fruits and seeds respectively that are easily 

 conveyed by wind) and Sorbus (which has berry-like fruits).^ 



V. So far as the immigration of trees is concerned, light-demanding 

 trees precede shade-enduring ones ; the reverse is impossible. Shrubs 

 are suppressed by trees that enter subsequently. 



vi. The differentiation of sharply defined communities proceeds 

 gradually. The earliest commingled individuals in reality belong to 

 different natural communities, which only little by little distribute them- 

 selves in the most suitable stations. One can therefore speak of initial,^^ 

 transitional, and final communities. 



To the preceding statements there are naturalh^ exceptions, as isj 

 shown in the succeeding paragraphs. 



\ See Fliche, 1888. 



" See Hult, 1885 ; Grevillius, 1893 ; A. Nilsson, 1902 ; Cowles, 1899, 1901 



Clements, 1904, 1905; Ernst, 1907. See also : Moss, 1906. 



" Kerner, 1863. * Blytt, 1882; Hult, 1885. 



