135 



All these unfavorable conditions are overcome by the forest, the tops of 

 the trees catching the rain and partially retaining it. Nevertheless the water, 

 which passes through and runs down along the trunks, is retained by the 

 moss and the dry leaves in deciduous forests, forming the upper surface of 

 the soil or the humus, thus becoming of benefit to the vegetation. Furst's^ 

 "lUustriertes Forst- und Jagdlexikon" gives some positive figures on these 

 theoretical discussions. Based on the observations of the forest meteorologi- 

 cal stations, it is stated that the temperature of the air in the annual average 

 is possibly o.8°C. lower under the close roof of tree crowns of the forests, 

 than in the open. The difference is greatest in summer (up to 3°C.) while 

 it approximates the annual average in spring and autumn and almost dis- 

 appears in winter. "The fluctuations in temperature are less under the 

 shelter of the tree crowns than in the open." 



The temperature of the forest soil is from i to 3°C. lower at all seasons 

 of the year than that of open land. The absolute moisture does not differ 

 in the forest and in the open ; but, on account of the lower temperature, the 

 relative moisture in the forest during the winter, spring and autumn is from 

 4 to 8 per cent, higher than in the open, and in summer from 12 to 20 per 

 cent. The evaporation from a free surface of water in the forest is from 

 50 to 60 per cent, less than in the open; "the evaporation of the water from 

 the soil is reduced from 80 to 90 per cent." Of the precipitated moisture, 

 10 to 50 per cent, will be retained by the crowns of the trees, according to 

 the species, the age and dimensions of the forests as well as the amount of 

 precipitation, and in light rains it often amounts to 100 per cent. In general 

 60 to 80 per cent, reaches the soil in the forest. "In Central Europe 

 the annual and the summer temperature will be lowered 1° and 2° to 3°C. 

 by the dimensions of the forest and the relative moisture raised ca. 5 per 

 cent, and 15 per cent." 



Since the amount of the distant action from extended forestration has 

 not yet been determined, the question as to the influence of the forest on 

 chmate must remain open. But one effect of the forest on the immediate 

 vicinity cannot be denied and this phytopathologists must consider. 



Differences in insolation are felt slightly in the forest, but very 

 quickly and strongly in the open field. The soil is warmer; the layers of the 

 air lying above it must necessarily produce an equalizing air current which 

 is most significant in spring when vegetation awakens. 



Hesselmann's- investigations give an insight into forest vegetation. 

 He observed the regular dying of the twigs which takes place within the 

 crowns of the trees and found that in birch and mountain ash the leaves 

 were still strongly active in assimilation ; but in the hazle-nut markedly less 

 so. If well-lighted branches die, phenomena of correlation are at fault. 

 Trees which can live in shade develop distinct sun and shade leaves; trees 



1 Illustriertes Forst-und Jagdlexikon, 2nd. Ed., revised by Dr. Hermann Fiirst, 

 Berlin 1904, Paul Parey, p. 384. 



2 Hesselmann Hendrik, Xur Kenntnis des Pflanzenlebens scliwedischer Laub- 

 wiesen. Jena, Fischer, 1904. Cit. Bot. Centralbl. v. Lotsy, 1904. No. 49. 



