The most important factors affecting the colonization of open areas by plants are: the year and season of abandonment; the physical state of the site; climate; soil; the existing flora and fauna; proximity and position of source material; opportunities for vegetative regeneration;

and the presence, within a range possible for seed dispersal, of an efficient generative reproduction and a rapid, rich and long-distance dispersal of seeds (Falinski, 1980; Harmer et al., 2001). Reviews by Osbornova et al. (1990) and Myster (1993) report many studies of tree generation on abandoned farmland. Natural colonization by trees and other species have been recorded since 1882 at the Broadbalk Wilderness, UK, which has established on former farmland (Harmer et al., 2001). The first tree plants were recorded 30 years after abandonment, i. e. in 1913. The main species regenerating in the area were: common ash (Fraxinus excelsior L.); sycamore (Acer pseudoplatanus L.); field maple (Acer campestre L.); suckers of wild cherry (Prunus avium L.); blackthorn (Prunus spinosa L.); pedunculate oak (Quercus robur L.) and hazel (Corylus avellana L.). In 1998 the dominant and most frequent tree species were pedunculate oak, common ash, wild cherry and sycamore.

The area of farmland no longer in agricultural production increases as land owners cease activities or direct their energies towards other forms of management. When farmland is abandoned it is invaded by herbs and broadleaved tree species (alder, aspen and birch). In general, one species dominates in the new stand. Most such farmland areas are owned by private individuals. In Sweden, Johansson (1999a) found up to 10,000 broadleaved tree stems ha-1 on about 100,000 hectares of former farmland.

Natural tree establishment in an open area is a slow process, and it may be 5-10 years before trees 2-5 old are seen (Werner and Harbeck, 1982). Most such areas in Northern Europe are small, amounting to 0.5-2.0 ha. In the initial phase, the areas are not noticeable from the surroundings, but later a dense stand is established and the landscape is changed. In general, these areas continue to develop unnoticed by the owner or the public. Eventually, former open areas become covered by forest. Such ingrowth can be the result of natural seeding, sprouting or suckering (Fig. 2).