484 SUBANTARCTIC ISLANDS OF NEW ZEALAND. [Plant Formations. 



of a high percentage of salt in the air probably also has a prejudicial effect on the 

 vegetation.* 



Were there a large number of deciduous trees in the Southern Hemisphere, it is, 

 of course, possible there might be found some species amongst these which would be 

 able to grow in a place like Campbell Island in spite of the westerly gales ; and in 

 that case the absence of trees in this area might be attributed to the fact that it does 

 not lie in a zone of deciduous trees ; but experience elsewhere is against this suppo- 

 sition. It has been shown that even in the Northern Hemisphere, where deciduous 

 trees are the rule and not the exception, strong winds will prevent tree-growth. 

 Two cases differing in character may be cited in illustration of this. Thus, accord- 

 ing to Mr. F. J. Lewis, who writes of the Shetlands,f " Trees are now entirely absent 

 from the land, although many attempts have been made to form plantations in 

 sheltered spots. Planted in gardens, ash, rowan, and birch grow as high as the 

 surrounding wall, and are then stunted by the winds, so that the trees appear 

 to be shorn off at the level of the protecting wall. ... As the mean annual 

 temperature of the Shetlands is perhaps higher than many parts of Britain which 

 are tree-clad, the absence of trees in these islands must be due not to temperature, 

 but to the force of salt-laden winds." 



Again, the absence of trees from the American prairies, in situations which in 

 most respects are eminently suitable to their growth, has long been a puzzle to 

 American botanists. The original theory to account for their absence was that the 

 prairies were liable to be fire-swept, and that their forests had disappeared in con- 

 flagrations. This explanation, however, is not accepted at present, and Professor 

 Coulter says, J " A possible explanation is as follows : The extensive plains of the 

 west develop the strong and dry winds which prevail over the prairie region. And 

 this brings about extremes of heat and drought, in spite of the character of the soil. 

 In such conditions a tree in a germinating condition could not establish itself." Here 

 again a wind, but in this case a dry one, is the cause of treelessness. 



Taking all facts into consideration, it may, I think, be said with a fair amount 

 of certainty that the absence of trees on Campbell Island is due to the westerly gales 

 which sweep over it almost unceasingly. Possibly a thorough investigation of the 

 peat would give information as to the past climatic history of the island, as similar 

 excavations have done in Europe ; but we had no time for any such systematic 

 digging and excavation as this would require, and so it is impossible at present to 

 say whether trees have at one time occurred on Campbell Island, as in Kerguelen 

 Island and elsewhere, in districts where they are now absent.§ 



* Vide page 487. t Science Progress, October, 1907, p. 307. t " Plant Relations," p. 236. 



§ Mr. J. Crosby Smith asks me to insert the following observations made by him : He made several 

 examinations of the soil, and found it to consist, almost in every case on the lower levels, of layers of 

 brown peaty vegetation. A striking peculiarity was the formation of these layers. One layer would 

 be 1| in. to 2 in. thick, and then a very thin film of loamy soil, sufficient to keep the peaty layers from 

 adhering, so that each layer could be almost as easily separated as the leaves of a book ; the next layer 

 below would be only | in. thick, and the one below that 1 in. ; then again a thicker one, and so on for 

 the whole depth of the spade-cut. The inference which he drew from this was that each layer repre- 

 sented a period of continuous good growing-conditions, followed by a period of bad weather, when 

 a stoppage of growth and a dying-down of vegetable life would occur, and before any further growth 

 ensued a deposit of sandy loam was deposited by wind and rain from the mountain-tops, and thus layer 

 after layer was found in alternating periods. 



