Clare Island Survey — Tree- Growth. 9 19 



two species which demand fairly good soil and growing seasons, with tempera- 

 tures of at least 55°-60° Fahrenheit, for the production of fruit on a scale 

 which would bring about their extension on bare land, or that occupied by 

 a competing flora. 



If the geographical distribution of the Oak and Hazel over Europe is 

 investigated, it is found that the former species occurs in Scandinavia up 

 to latitude 61° or thereabouts, although occasional trees may be found still 

 further north. This latitude corresponds in that region to a July mean of 

 about 60° Fahrenheit, and a period during which the temperature remains 

 over 50° Fahrenheit for about 100 days, or from three to four months. From 

 Valencia in the south of Ireland to Malin Head in the north, the July mean 

 steadily decreases from 59° Fahrenheit to 57'2° Fahrenheit ; while means 

 over 50° Fahrenheit prevail from May to October, or for a period of about 

 180 days, the means for the above-named months being slightly over 

 50° Fahrenheit in the south, and slightly under in the north. In all the 

 western Irish stations, however, the means for August are slightly higher 

 than for July, and indicate a relatively warmer autumn than summer, as 

 compared with most parts of the country. While the maximum mean 

 temperatures for the summer months are higher at the northern limit of the 

 Oak and Hazel region in Scandanavia than in western Ireland, the length of 

 the growing season is very much longer in the latter, and this fact must he 

 borne in mind when assuming the climatic conditions necessary for the pro- 

 pagative functions of the species in question. It is a matter of common 

 observation that the normal vegetative development of the Oak in the British 

 Isles corresponds to a July mean of about 59° Fahrenheit. By normal 

 development is meant an average height-growth on average soils and situa- 

 tions, and in close woods, of from 80 to 100 feet, with diameters of 20 to 

 30 inches at breast height. Where the summers are cooler than those 

 represented by this mean, good specimens of Oak are rarely found except on 

 good soils and aspects. This fact is brought out fairly clearly if the July 

 isotherm of 59° Fahrenheit is followed from the west of Ireland to the east 

 coast of Scotland. So far as scanty meteorological records go, this isotherm 

 passes a little north of Valencia Island, and thence in a north-easterly 

 direction along the basin of the Shannon to about Londonderry. Skirting 

 the northern and eastern coast-line, it crosses the Irish Sea somewhere about 

 the Isle of Man, and then again turns north, and crosses Scotland about the 

 latitude of Edinburgh. Along the western seaboard of Ireland few stations 

 appear to go above 58° Fahrenheit, and the records for Blacksod Point 1 for 



1 Clare Island Survey, Part 6. 



C 2 



