II] WOODLAND ASSOCIATIONS 69 



distinction is sometimes made inaccurately. Further, none of 

 these maps attempts to distinguish between natural and semi- 

 natural woods on the one hand and obviously artificial planta- 

 tions on the other. 



Trees and Shrubs 



The ash (Fraxinus excelsior) is dominant throughout the 

 length and breadth of the ash woods (see figure 9); and in 

 them it is not confined, as it is in the oak woods, to the damper 

 situations. It seems -clear that, in any given natural station, 

 the abundance of the ash is due to one of two causes, either to 

 a high water-content or to a high lime-content. Some of the 

 local foresters are of opinion that the timber of the ash grown 

 on the limestone soils is harder and more durable than that 

 grown on the wet, non-calcareous soils. 



The two most frequent arboreal associates of the ash are the 

 wych elm (Ulmus glabra-= U. montana) and the hawthorn 

 (Crataegus Oxyacantha), both of which are here more generally 

 distributed than in the oak or birch woods. The elm is more 

 abundant at the lower altitudes and in the damper situations 

 (see figure 10), the hawthorn in the drier situations and at 

 the higher altitudes. When the ash, the most valuable timber 

 tree of the dales, is removed or dies out in a degenerating 

 wood, the elm or the hawthorn, as the case may be, becomes 

 locally subdominant ; and societies of elm and hawthorn are as 

 characteristic of the ash woods as birch and alder societies are 

 of oak woods. On the vegetation maps, these societies are in- 

 dicated by the same colour as the ash association of which they 

 form a part ; but, where practicable, the initial letter or letters 

 of the genus of the locally subdominant tree is printed on 

 the general woodland colour. An example of a society of wych 

 elms occurs in upper Middleton Dale; and hawthorn societies 

 are typical of most of the upper parts of drier dales. 



Two conifers are native in the ash woods. One of these, 

 the juniper ("Juniperus communis") is very rare, and ap- 

 parently confined to one place: the other, the yew (Taxus 

 baccata) is not common ; but small specimens occur here and 

 there on the ledges of limestone cliffs in the ash woods. It is 

 rather curious that these plants should be so uncommon here, 



