REQUIREMENTS OF THE UNITED KINGDOM 55 



fixation of sand-dunes, shelter from wind and sun, land- 

 scape adornment, etc., is of considerable importance, but is 

 less easily expressed in exact terms than the supply of, 

 and demand for commercial timber. In the tropics, 

 climate may probably be modified as regards temperature 

 and rainfall by forest growth, while the effect of a large 

 area of woodland upon water storage is self-evident. Soil 

 erosion on steep slopes, or the shifting of drift sand, can be 

 prevented by the binding action of tree roots, while 

 their stems and branches obstruct both the formation of 

 water channels, and the passage of wind currents on or 

 over the surface. The effect of trees as wind-breaks, and 

 the creation of shaded air spaces and soil surfaces, have a 

 distinct and easily noticed influence upon local climate, 

 and the aesthetic effect of clumps, belts, and single trees 

 upon a landscape is recognised by all communities pos- 

 sessing the most elementary ideas on civilisation and 

 natural beauty. 



While the consideration of the above facts might be 

 extended to several chapters, it is sufiicient to deal briefly 

 with those which more closely affect the British Isles in 

 their physical, hygienic, or social bearings. The theory 

 that the climate of these Islands generally has been, or 

 can be affected by forests is gradually being destroyed, 

 athough observations have not, perhaps, been carried 

 sufficiently far to prove this conclusively. The storage of 

 rainfall is not entirely dependent upon forest growth, and 

 the uniform distribution of rain throughout the year 

 renders it less necessary than in countries with definite 

 periods of alternating drought and excessive rainfall. 

 This uniformity also decreases the general tendency of 

 heavy rain to wash the finer soil particles into water 

 channels and rivers after prolonged drought, and a 

 further deterrent to this process is provided by the almost 

 continuous growth of grass, moss, etc., throughout the year, 



