THE SANITARY INFLUENCE OF FORESTS 15 



Shelter belts are of great economic importance, but this 

 view of the subject scarcely comes within the scope of this 

 book. I may advert to one or two points of interest. 

 The utilisation of mountain pasture by sheep is a large 

 industry. In the Highlands of Scotland extensive areas 

 of the sheep farms are covered with bracken and are 

 consequently of no value at present for grazing. Bracken 

 land can always be covered with trees. It is believed that 

 of the total acreage of mountain pasture in the Highlands, 

 at least 10 to 20 per cent could be afforested without 

 producing any reduction in the number of sheep, merely 

 by planting up the bracken areas. Furthermore, the 

 shelter provided would enable the grass to grow better and 

 the sheep to thrive more, and would actually in the end 

 increase the output of mutton considerably. 



Throughout Ireland in the central plain and other low- 

 lying districts, and in parts of Scotland, there are extensive 

 tracts of peat, which were formed after the retreat of the 

 Ice Sheet, by the gradual filling up of the shallow lakes 

 that were due to the disturbance of the natural drainage 

 by glacial deposits. The chilly effects of these peat-bogs is 

 well known (6). They act as centres of cold, often giving 

 rise to severe spring frosts in the adjoining farm land, so 

 that early kinds of potatoes cannot be grown, while grass is 

 later in spring in the pasture fields. In order to mitigate 

 these effects, peat-bogs should be surrounded with plantations 

 which can be easily and cheaply established in the cut- 

 over margin of the bog. This part of the peat moss is 

 either already drained or can be drained at a small expense ; 

 and on it spruce, maritime pine, Scots pine, and larch can 

 be profitably grown (7). 



It was formerly believed that the leaves of the trees, by 

 their absorption of carbon dioxide from the atmosphere and 

 their exhalation of oxygen, made the air in a forest healthier ; 

 but Ebermayer showed that the oxygen exhalation of a 

 forest in proportion to the needs of human respiration is 

 insignificant, and is perhaps offset by the increase of carbon 

 dioxide which results from the decomposition of organic 



