The countryside is created by human requirements. Factors a

Climate change over the past few thousand years - palaeo-biology indicates how trees have come and gone.
Increased need to feed the populace - an increasing world problem
Use of timber for construction and fuel - necessary for human survival


There is a mistaken belief that Britain and Ireland were coated with trees in the past. As so often, trees grow where it suits them, and where it doesn't there aren't any. Western Ireland is too wet and warm for trees to grow well. The Highlands of Scotland and the Welsh mountains were too exposed, apart from the valleys and gullies where trees were protected from harsh wind. Where there are flood plains, trees with water-resistance can survive such as willows or alders, but tend not to form large groups. In much of the English countryside, trees are found as much in hedges as in woodland.