This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
A condition that regulates the distribution of plants is the 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 woodlands.
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, transpiration, and osmosis are slow.
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 protective effect of trees is, on the whole, decidedly advantageous to woodland plants.
Whilst the character of the soil determines the type of woodland - there being five main types: pedunculate Oak, sessile Oak, Birch, Beech, Ash, with combinations - 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.