CLIMATIC CAUSES. 57 



frost, but can rarely be wholly destroyed by it, because of the persistence of 

 perennial species with undergroimd parts. A single case of the destruction 

 of native communities by frost was found in the dune areas of Medanos Spit, 

 near San Diego, in southern California. The severe freeze of 1912 had com- 

 pletely killed many large families of Mesembryanthemum, and these still per- 

 sisted as blackened areas in the spring of 1914. Such areas had been essen- 

 tially denuded by frost, and were already being invaded by other pioneers 

 (plate 9 b). 



The denuding action of hail is often very great. In some parts of the Great 

 Plains destructive hailstorms are so frequent that they have caused the aban- 

 donment of farms and sometimes of whole districts. As with frost, the effect 

 upon cultivated plants is very much greater than upon native vegetation. It 

 is not infrequent to see the fields so razed by hail that not a single plant is left 

 alive. Native communities often suffer great damage, especially broad-leaved 

 forests and scrub, but the effect rarely approaches denudation. Grassland is 

 sometimes mowed down also, but the effect is merely to favor the grasses at the 

 expense of species with broad leaves or rigid stems. 



Bare areas due to lightning. — The r61e of lightning in causing fire in vegeta- 

 tion has come to be recognized as very important (Bell, 1897; Clements, 1910; 

 Graves, 1910; Harper, 1912). The majority of Ughtning strokes do not set 

 fire to trees or other plants, and the attendant rain usually stops incipient 

 burns. Even in such cases forest fires have actually been seen to start from 

 lightning, and the number of such cases in the aggregate would apparently be 

 large. In regions with frequent dry thunderstorms, i. e , those imaccompanied 

 by rain, such as occur especially in Montana and Idaho, lightning is the cause 

 of numerous, often very destructive, fires. Once well started there is no differ- 

 ence in a forest fire due to lightning and one due to other agents, such as man, 

 volcanic eruptions, etc. 



Bare areas due indirectly to climatic factors. — These are due almost wholly 

 to the effect of physiography in exceptional cases of rainfall, of run-off due to 

 melting snow, or of wind-driven waters. In all three the process is essentially 

 the same. The normal drainage of the area is overtaxed. The flood-waters 

 reach higher levels than usual and are ponded back into depressions rarely 

 reached. Moreover, they cover the lowlands for a much longer period. In 

 the one case they form new water areas for invasion. Since these are usually 

 shallow and subject to evaporation, the development in them is a short one. 

 In the case of the lowlands, the vegetation of many areas is washed away, 

 covered with silt, or killed by the water, and the area is bared for a new devel- 

 opment. This is of course essentially what must have occmred at the end of 

 each period of glaciation. The ponding back of glacial waters and the fluvio- 

 glacial deposits were the outcome of the interaction of climate and physi- 

 ography, just as can be seen in miniature at the foot of a glacier to-day. 



Sudden changes of climate. — It is probable that there is no such thing as a 

 sudden change of climate, apart from the striking deviations from the normal 

 that we are so familiar with. If the criteria of evolution and of historical 

 geology are applied to climatology, it seems evident that even the climates of 

 the past are largely to be explained in terms of present climatic processes 

 (Huntington, 1914). If we consider the causes which are thought to produce 

 the most striking and sudden deviations at present, namely, sun-spot maxima- 



