Clare island Survey — Tree- Growth. 9 21 



produces fertile seed. In every mountain district Oak scrub may be found 

 well above the 1000-feet contour line, and although this may have 

 occasionally arisen from seed produced at lower levels, the greater part 

 of it must be regarded as relics of a wider extension of that species at some 

 remote period, and under conditions which no longer exist. Moss 1 suggests 

 that the appearance of heather and Vaccinium in the surface flora of Oak 

 woods prevents natural regeneration ; but before this ground flora can 

 establish itself, the leaf canopy of the Oak crop must be weakened by 

 thinning or a reduction in the vegetative vigour of the crop. While 

 thinning alone may alter the character of the surface flora, the appearance 

 of the shade-bearing Vaccinium suggests a deterioration of surface-soil rather 

 than an increase of light, and this may be due to climatic changes such as a 

 lower summer temperature and heavier rainfall, and evidence of changes of a 

 similar nature are found in the west of Ireland. 



But another climatic factor of equal importance to temperature, and 

 one by which the latter can be considerably influenced, is wind. While a 

 low summer temperature may check or retard the growth and propagation 

 of Oak and Hazel, wind can not only check but actually destroy growth 

 already made, and this is especially found to be the case in the west of 

 Ireland. This destructive effect of the wind along the coast is chiefly due 

 to the salt content of the air-currents from the Atlantic, and during the 

 summer and autumn not only partially destroys the foliage of broad- 

 leaved trees, and the needles of many conifers which are usually better- 

 adapted for resisting adverse climatic features, but destroys the woody 

 growth formed earlier in the season. Trees exposed to strong sea-winds 

 acquire a characteristic form, and an examination of these trees shows that 

 the ends of all branches or shoots exposed to the wind are killed back more 

 or less annually for a certain distance, the succeeding year's growth being 

 continued from buds or short spurs below the dead portion. This annual 

 shortening back of each season's growth, which on the west coast takes 

 place usually in autumn, invests the crown with a flattened surface on the 

 top and windward sides, as if the branches had been trimmed with shears, 

 while the leeward side of the crown, being in shelter, develops more or less 

 normally, the tree thus being given an unequally balanced crown and a 

 leaning stem. 



On the extreme west, trees exposed directly to the wind cannot attain 

 more than a few feet in height unless growing in masses, or sheltered by 

 high ground to windward, and for a considerable distance from the coast, 



1 " Woodlands of England." iu Kuw Pliytologisl. 



