The spatial interweaving of urban woodlands and developed areas favors the exchange of species in both directions. Numerous cultivated plants disperse themselves as escapees from gardens and parks into neighboring woodlands or are carried into such areas as garden waste (Hodkinson and Thompson 1997). Urban woodlands are generally rich in non-native species especially along their edge areas (Asmus 1981; Moran 1984; Walther 1999). In the other direction, attractive forest plants are usually transplanted into urban gardens (Kosmale 1981) where they survive, but generally are not able to spread, in contrast to the escapees. Forest species with the capacity for long-distance dispersal, for example ferns, orchids and flying ground beetles, may also colonize urban-industrial sites (Dickson 1989; Keil et al. 2002; Weiss et al. 2005). A few highly mobile animal species profit as well from attractive food offerings in bordering gardens and parks. In this way, in a few peri-urban woodlands of Germany, North American raccoons have reached population sizes comparable to those in their home territories (Hohmann and Bartussek 2001). In Berlin, wild boars use gardens and parks near woodlands to search for food; foxes are found in city centers as well.
Impacts on forest species
Recreation use influences the populations of forest species in very different ways. For peri-urban woodlands with good access, frequent disturbance by recreationalists and above all, by dogs, is a given. Predatory animals such as dogs and house cats can have negative effects on bird populations (Marzluff 2001). They can also cause game to retreat farther into the forest, which can, in turn, have positive effects on plants that are preferentially browsed. The yew (Taxus baccata) has become established in parts of Berlin’s Grunewald and can grow large there, something that is usually hindered by browsing (Seidling 1999). Dogs can also disperse forest plants, whose seeds are transported on the dogs’ coats (Graae 2002).
In peri-urban woods, the silvicultural management strategies are often aimed at an aesthetically attractive forest structure. The presence of old and mature trees and stands is regarded as an important feature in the aesthetic perception of forests (Ode and Fry 2002). Through promotion of old individual trees and increase in the percentage of dead wood, specific, mostly threatened, animal species will be encouraged, namely insects that require decomposing wood as a habitat or sustenance basis and tree-cavity residents such as bats and cavity nesters.