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dciepest, freshest, most excellent soil will refuse to support vegetation above the line of 

 eternal snow. That the temperature of the climate generally exerts great influence on all 

 v.'getation may be inferred from the observation of the flora of diS^erent climates. As 

 the temperature declines from the equator towards the pole and from the sea level towards 

 higher elevation, so we find that the forest growth in both directions shows a less 

 div^ersified appearance and species, and at last a decline in the number of individuals. 



This question of climatic influence on tree-growth becomes of practical interest, when 

 the possibility for existence and prosperity of a tree species under certain climatic condi- 

 tions is to be ascertained for the purpose of introducing new species, and it becomes 

 necessary to determine whether the thriving of a species depends on the mean yearly, the 

 mean summer or winter teuiperature, or on the extremes of temperature. The solution 

 of this question is as yet possible only on grounds of observation in regard to the natural 

 distribution of trees, or else must in each individual case be ascertained by experiment. 



It is not probable that the mean yearly temperature influences the growth much, as 

 may be inferred from the fact that localities of equal mean yearly temperature show a 

 very difierent influence on their timber growth. 



Whilst in Siberia the willow grows on frozen soil, which thaws for a few summer 

 months, the St. Gothard Mountain, though enjoying a higher mean temperature than 

 the locality referred to in Siberia, is entirely bare of all vegetation. It is rather more 

 probable that the distribution of species depends mainly on the mean temperature of the 

 summer, or better on the length of the time of vegetation. For tender species the line of 

 distribution will no doubt be determined by the lowest extreme of winter temperature, 

 which may cause death by frost. On the contrary extremes of summer heat, if accom- 

 panied by sufiicient humidity of soil and atmosphere are not opposed to the cultivation of 

 species, which in their natural occurrence belong to northern or elevated localities with 

 lower temperatures. 



It may be mentioned here, that the diSerent temperatures of the soil, depending 

 greatly on its colour, may hasten the revival of vegetation in spring and thus expose the 

 young buds to late frosts. In some localities the phenomenon of frosts rendering tree 

 culture hazardous is due to a rapid evaporation of the moisture of the soil as in the dells, 

 and vales, and on heavy, impervious soils where water is collecting and insufficient circu- 

 lation of air impedes its speedy removal. Plateaus too suflfer often from frosts, when 

 plains with the same mean yearly temperature are left intact. 



This phenomenon is mainly due to the increased radiation of heat during the night 

 because the thinner strata of air in such plateaus ofier less resistance to radiation. 



So far we have considered such conditions of forest growth as are in the majority of 

 cases given and often unchangeable ; we have to accept them as they exist and try to 

 make the best of them. If our soil is a dry sand we shall not be able in most cases to 

 adapt it to the cultivation of elder or ash ; if we live so far north that the period of 

 vegetation is too short for the prospering of the oak, we may as well not attempt its cul- 

 tivation, and so on. There is little scope for changing the conditions of soil, air, climate, 

 or at least the change can be effected only in an extended period of time and by careful 

 forestry. 



But here a condition of forest growth presents itself, which largely, we may say 

 entirely, lies in the hands of the forester ; a condition which he is able to create and control, 

 on the understanding of which a successful management of his plantation must be based 

 throughout. In fact we may say that the most important criterion inforestal operations, 

 is formed by the relation of the forest trees towards light and shade. The conditions 

 created by the existence or absence of the proper amount of light, we should characterize 

 as the principal one for the consideration of any manager of forests. We do not mean 

 here to discuss the physiological influence of light on vegetation in general, which shows 

 itself in the decomposition of the carbonic acid of the air, thus furnishing the means of 

 assimilation of the carbon which is necessary for the growth of the plant, its colouring, 

 the ripening of the seed, etc. ; but the necessity of providing in the forest a proper amount 

 of light or shade according to the wants of different species in their different ages. 



The credit of having drawn the attention of foresters to the importance of this 

 relation of forest trees is due to Dr. G. Heyer, now Professor of Forestry in Munich, the 



