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It has long been known that sycamore readily germinates on old walls even as high as 2m above ground level (Thurston 1930). The young plant often manages to send a root down to the ground. Such individuals can grow at least up to a DBH of 20cm and even reach sexual maturity. The position is obviously unstable; for instance one specimen at Murlough Bay, Co. Antrim (ca 6m high) growing on a wall 1.5m above ground level was blown away with the upper part of the wall during a storm and landed upright 2m away. Individuals also become established in cracks on the face of walls in towns and along canals and this is very common on both city and country walls. On Durham City walls sycamore was the seventh most abundant species (Woodell & Rossiter 1959) while in south-eastern Essex sycamore occurred on 8% of the walls surveyed (Payne 1978).

In Glenariff Glen (Co. Antrim) a few sycamore saplings were found growing in a gutter. The gutter had not been cleaned for several years and was shaded by some large trees. The oldest sapling was six years old and just over 10cm tall, the average yearly growth rate was about 0.5cm and only one pair of very small leaves was produced every year.

Similarly, 'epiphytes' may be found in the fork of some large trees. In the Birch-Rowan-Hazel woodland at Murlough Bay, Co. Antrim two saplings (two and four years old) were found where the main trunk divides at about 1.5m above ground level accompanied by Oxalis acetosella. Their statures and growth rates were similar to saplings found in the gutter. Such individuals can occasionally send roots to the ground if cracks in the host tree bark are present and may grow into large individuals (see Lüdi 1941).

Sycamore is commonly found growing in disused quarries and can even be found in cracks on rock faces. In abandoned chalk pits sycamore and several other tree species were found to become rapidly established on the floor area and appeared a decade later on unstable scree slopes (Locket 1946). Out of 48 surveyed chalk and limestone quarries in England 22 contained sycamore but only seven were found to have individuals in the youngest age classes. Eleven species of shrub and trees were recorded and only ash (59%) and another invader Buddleia davidii (50%) were more widely distributed (Davis 1982, 1983). Because sycamore, like the majority of other woody species, becomes established in very young quarries Davies (1983) describes the tree as a pioneer! On slate waste tips in North Wales natural recolonization is slow although sycamore and Quercus petraea are able to establish themselves on these inhospitable sites (Sheldon 1975). Following coal and clay excavation sycamore became widely established on the flat top of these mining tips in Shropshire (Tobin et al. 1987). 

In cities sycamore becomes established on flower beds, neglected alleyways and odd spots (Edlin 1967) and is a striking feature of railway banks. However, it only becomes established if seeds are readily available. In bombed areas of the City of London areas sycamore was scarcely recorded in the flora prior to reconstruction (Fitter & Lousley 1953) whereas in Dublin it is regenerating and is probably the commonest tree (Wyse Jackson & Sheehy Skeffington 1984). 

Gilbert (1989) has reported several instances of establishment of woody vegetation and subsequent succession in abandoned and disturbed urban habitats. In all instances sycamore is present but the timing of its establishment varies with the site. Gilbert (1989) points out that the presence or the timing of appearance of a particular woody species is very much dependant on seed availability and that regeneration is negligible once a dense ground cover develops. 

Sheffield urban commons are colonized for the first 3-4 years by light windborne seeds (including Salix spp, Betula spp and buddleia) which are then followed by species such as sycamore and ash.

At a Bristol site undeveloped for 40 years buddleia dominated but was starting to be overtopped by sycamore which Gilbert describes as being probably its natural successor in towns.

On abandoned allotment plots woody plants (broom, gorse, willow, sycamore) become established after 2-3 years following the cessation of cultivation in an open vegetation dominated by Holcus lanatus and Agrostis stolonifera.

Neglected areas of Metropolitan cemeteries created about 150 yrs ago, where sycamore was one of the most commonly planted species, have been invaded by secondary communities of sycamore-bramble-ivy, but ash is also common. Succession includes stages dominated by grasses, bramble and trees. Woody plants become established throughout the first two stages for about 10 years.

A disused railway yard (South Yorkshire) 35 yrs after site abandoned consisted of a closed canopy (height 7m) dominated by Salix caprea with saplings of ash and sycamore underneath which will eventually take over. 

Gilbert (1989) points out that although many people imagine that such secondary urban woodlands are dominated by sycamore, such woodlands are rare and only found in areas where the seed supply of other tree species is absent. Also he notes that although a very large number of seedlings and saplings are produced in urban areas few turn into mature trees.

Although in most of the above examples sycamore regenerates below other woody species (e.g. buddleia, willow, birch) it is not clear whether any form of facilitation is involved.

In rubbish tip areas the vegetation is first dominated by shrubs (Sambucus nigra dominant) and the succession leads to a ruderal forest dominated by sycamore (Fischer 1975).

Abandoned farmland may be quickly invaded by sycamore (e.g Halberg 1984). In an experiment at Silwood Park Crawley (1990) ploughed an acidic grassland on sandy soil. The following year he found that many sycamore seedlings became established. However, they were far more abundant on grazed plots than on ungrazed areas (mean density respectively 51.3 and 3.2 seedlings/m²). The reverse was true for Quercus robur.

It is clear that both disturbance and lack of plant competition favour the establishment of sycamore in all these man-made habitats, and Crawley's work indicates that a certain degree of grazing limits grass competition. It must be noted that it does not do well on all soil types. In areas of china clay sand waste sycamore extension growth was second worst of 20 species grown (Capel 1980). When Bradshaw (1989) grew sycamore close to nitrogen fixing Alnus glutinosa and A. incana, sycamore was able to grow at a normal rate. This clearly shows that some types of soils, e.g. china clay soil, are too deficient in nitrogen for successful sycamore establishment without some form of facilitation.