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.
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 treeless area than in a forest area.
The retention of the moisture by the individual 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.
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 experiments 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 derived from woodland vegetation. It is probable that moisture and altered light-and-heat conditions have played as important a part as that of the absence of humus.
Woodlands also affect the water content and physical character of the soil, preventing it from becoming pulverized. All these points should be carefully explained.
When the original area of woodland is compared with its extent to-day (there are fourteen national forests of insignificant total acreage) it is obvious that the cause of the reduction in forests has been multiple.
Primarily there was the need for wood for fuel. In Saxon or Norman times, or later even, only the churches, castles, etc., were built of stone, and wood was used for dwellings. From Alfred's day, also, till the time of Nelson there was a constant demand upon the forests for ship-building. Incidentally, hunting and similar causes were responsible for the clearing of forests; and the need for cultivating, especially from the Conqueror's time, has finished the work of depletion.