NATURALISATION OF FOREST TREES. 227 



Usually a winter-rainfall climate means a very hot dry summer, 

 which summer-rainfall species are likely to be unable to stand, 

 and, on the other hand, winter rainfall species are often unable ta 

 endure the dry winters and particularly the hot dry windy springs 

 which so often characterise summer-rainfall climates. 



This, however, does not seem to apply to conifers, and at any 

 rate Pinus pinaster, Pinits halepensis and Cnpressus sempervirens, 

 all of which come from a winter rainfall climate, do not seem tO' 

 object in the least to a dry winter climate. The reason no doubt 

 is that they do not start growth in the same sudden way that 

 deciduous trees do, and that they are inherently more hardy to 

 drought than most broad-leaved species, owing to their less rapid 

 transpiration. On the other hand, deciduous trees retain their 

 inherited characteristic of budding out with the warmth of spring, 

 irrespective of the quantity of moisture in the soil, the water 

 stored in the wood and roots being sufficient to start growth, and 

 subsequently they suffer severely from drought when the roots 

 are unable to keep up the water-supply. However, we may perhaps 

 expect to find exceptions to this in some deciduous species, 

 such as Gleditsia and Madura, which thrive in the summer-rainfall 

 climate of the American prairies. The indigenous mimosa is 

 adapted to the dry springs here in that it comes into leaf late. 

 Some species in the South-West of America, where there are two 

 distinct wet seasons in winter and summer, come into leaf in early 

 spring, then lose their leaves as provision against the drought in 

 early summer and come into leaf again on the advent of the summer 

 rains. 



A winter-rainfall climate, however, does not always include a 

 dry, hot summer. For instance, though the rainfall of the home 

 of Pinus insignis occurs in winter, yet it has a very high atmos- 

 pheric humidity in summer when it is frequently bathed in sea 

 mists. This species is much more out of place in a truly winter- 

 rainfall climate such as the Cape Flats or the interior of California 

 with their intensely hot dry summers, than in the mist-belt of 

 mountains of Eastern South Africa, where it promises to thrive 

 just as well, if not better, than in its native home. Thus is also 

 explained its at any rate partial success in some of the better- 

 watered parts of the high veld, when planted on cool aspects and 

 in soil which is deep and retentive enough to compensate for the 

 absence of winter and spring rains. 



Light. 



The factor of light is so intricately connected with those of 

 temperature and moisture that we cannot deal with it here, but 

 probably it is one of the important factors in the distribution of 

 species. A difference in it can also perhaps be compensated for 

 by a difference in one of the other factors. For instance, it is 

 said that the moister the locality, the less shade can a species en- 

 dure. The Douglas Fir is light-demanding in Oregon but shade- 

 bearing or even shade-demanding in the Rocky Mountains. Such 

 cases, however, do not prove that there is any change in the 

 absolute intensity of light required by the species, for the varying 



