Without a countryside to go to the plant watchers of West Berlin looked within the city and urban ecology was born out of the surprise of what they found. So the story goes. A very nice article is 'Flora and Vegetation Reflecting the Urban History of Berlin' by Herbert Sukopp (2003). The effects of the Blitz on the wild plants of England and especially London has been covered here, here and for WW1 France here. Sukopp writes about Berlin in '45.
In a historical context, sites bombed during World War II have played an important part in the development of the urban flora and its investigation. Only few years after the bombing various plants had colonised the ruined houses. In many cities the war damage and its effects gave rise to special studies of the rubble flora and fauna (Düll and Werner 1955/56, Scholz 1960 and others). During the colonisation of rubble, which offers warmer and drier conditions than natural habitats, plants and animals from warmer regions of the world found suitable conditions. Many plants that had been rather rare have become permanent membersof the urban flora in war-damaged Berlin.
After World War II an intensive and spontaneus development of vegetation began on the rubble, proceeding in more or less rapid succession from short-lived and perennial stages of herbaceous vegetation to shrub and forest-like stands. The variety of species on such inner-city waste land is surprisingly large. Thus a site at Lützowplatz in Berlin-Tiergarten accommodated 140 species of seed plants and at least 200 taxa of insects (Weigmann et al. 1978).
Dispersal of organisms and development of vegetation can be studied on rubble sites on a large scale and in an environment that differs considerably from previously known ruderal places. As Pfeiffer (1957) wrote: “The recolonization of rubble, created in many cities due to the activity of bombers in the last war, has unintentionally become a tremendous natural experiment, which with respect to its size, must be compared to the colonisationof new habitats created by volcanic activity”