2 4 



HINTS AND NOTES 



amount of heat available. This is little liable 

 to vary in open meadows and pastures within 

 several degrees of latitude. But in a wood 

 the temperature is considerably lower than 

 that of the surrounding open country. Heat 

 and colour go together; hence it may be that 

 there is an absence of colour in the wood- 

 lands. 



Moreover, each plant requires a definite 

 amount of heat before it will commence to 

 flower and later ripen seed. If one excepts 

 the bulbous plants that flower before the trees 

 are in leaf, and the trees themselves, the 

 generality of the woodland plants flower late, 

 in spite of their usually perennial character. 



Temperature has also an effect upon the 

 general conditions of plant-life, and this 

 explains the absence of life (lower zones) in 

 a cold dry wood. The absence of moisture 

 with cold prevents the proper balancing of 

 conditions for assimilation ; respiration, tran- 

 spiration, and osmosis are slow. 



Protection of the Woods. The denseness, 

 darkness, and coldness of woods generally 

 are retarding factors which may be well 

 compensated by another feature, and that 

 is their protection. The association of the 

 trees in a close formation, not only serves 

 as a protection in itself to the tree unit, but 

 it has a corresponding conservative effect 

 upon the rest of the flora of a wood. The 

 scrub layer and the ground flora are effectively 

 protected. Wind erosion is almost minimized 

 by the covering tree zone. The effect of frost 

 is also greatly reduced. 



Trees further protect the soil from being 

 worn away by the denuding effect of rain or 

 hail. Where trees drip there is some local 

 erosion, but this is restricted in its work, and 

 the soil is not carried far away. 



In a wood, also, the effect of a drought is 

 far less marked, though a clayey soil suffers 

 more severely in this respect. The scorching 

 heat of the sun in ordinary weather is again 

 moderated by the tree zone. Hence the pro- 

 tective effect of trees is, on the whole, decidedly 

 advantageous to woodland plants. 



Wet and Dry Woods. Whilst the character 

 of the soil determines the type of woodland 

 there being five main types: pedunculate Oak, 

 sessile Oak, Birch, Beech, Ash, with com- 

 binations the water content of the soil has 

 a good deal to do with tree dispersal, and also 

 affects the scrub and ground flora. 



Thus a wet clay is characterized by the 

 pedunculate Oak, whilst a dry, sandy soil is 

 occupied by the sessile Oak. The extent of 

 the effect of soil may be seen in the same 

 tract of wood, for on the siliceous slates of the 

 Charnwood Forest region, which give rise to 



a wet clay, Birch, which is a wet-soil type of 

 tree, is found, with Oak encircling it where 

 those rocks are in turn surrounded by the 

 drier, more sandy red marl. 



The ground flora in a wet and dry wood 

 will differ correspondingly, such plants as 

 Bugle and Tussock Grass indicating a wet 

 wood. The extreme type of wet wood is 

 afforded by the Alder-Willow association, 

 which is characteristic of marshy or aquatic 

 plant formations. 



Effect of Tree -felling on Rainfall. When 

 trees are felled, not only is the shade which 

 they afford at once lost and sunlight able 

 therefore to penetrate near to the surface, 

 but the removal of the trunk and branches, 

 with the numerous leaves, causes the moisture 

 which they accumulate to fall directly upon 

 the earth. Here, on a porous surface, the 

 water percolates and finds its way down to a 

 subterranean reservoir. Water accumulated 

 upon a clayey soil soon evaporates in the 

 open. Radiation is more rapid over a tree- 

 less area than in a forest area. 



The retention of the moisture by the indi- 

 vidual trees may be, moreover, considered 

 apart from the aggregate amount of moisture 

 present in a forest, regarded as a unit in itself. 

 The association of numbers of trees causes 

 the atmosphere itself to remain charged with 

 moisture, and evaporation is consequently 

 slow. The preservation of moisture at the 

 surface by a tree layer, and its retention by the 

 lower strata of plants, are also features of a 

 woodland area that must be considered in 

 estimating the value of forests as water 

 reservoirs. 



The retention of dew is also an important 

 aspect. 



Effect of Woodlands upon Soil. Perhaps the 

 most outstanding feature of a wood or forest 

 is the part it plays in the accumulation of 

 organic matter, plant and animal, upon the 

 surface, which in course of time becomes a 

 valuable asset to the soil. This matter is 

 known as humus, and it is to the presence 

 of this in the soil that the woodland plants 

 owe their distribution to a great extent. 

 Whilst many plants that grow in a wood are 

 able to exist in the open upon other soils, or 

 those not rich (or even deficient) in humus, 

 some that grow in the open do not care for 

 humus. It is suggested that simple experi- 

 ments be made in growing plants in soil with 

 and without humus, and noting the effect. 



Since the original vegetation was woodland, 

 it should be expected that the removal of this 

 from a large area by disafforestation has been 

 the cause of differentiation into meadow and 

 pasture, heath, and other types of vegetation 



