Plants growing on metal bridges

[aeshna5]

Over the last few years I've been facinated by the number of plants growing on Hammersmith Bridge over the Thames. What is interesting is that the unique flora is dominated by aliens + not all are the regular species such as Buddleja which is of course ubiquitous around London.

Any soil is from dust + humus from plant decay. Another factor in the flora is that numbers of Starlings often sit on the struts of the bridge + no doubt inadvertently donate some plants in the form of seeds passed through their digestive system. There is a Swedish Whitebeam, Sorbus intermedia, about 1.5m tall Cabbage Palm, Cordyline australis about 1 m high, quite a bit of trailing Virginia Creeper, Parthenocissus quinquefolia + adding some colour Trailing Bellfower, Campanula poscharskyana.

In previous years there was a small Black Mulberry, Morus nigra, which seems to have now perished but present for a few years + there was also a Cape Gooseberry, Physalis peruviana, for at least a couple of years.

Natives are far + few- the odd small Salix sp.

I find it a fascinating insight into urban ecology how all these species come together.

Any other tales of urban habitats that have their interesting exotics.

[Dorts]

Aeshna5, I find that most interesting, but why such a paucity of natives? Is it due to the shortage of habitat in the surrounding area, being so urban? Presumably the majority of plants species in the vicinity are in gardens and of garden origin.
Or is it that Mulberry, Physalis, Sorbus, Cordyline and Parthenocissus all have seeds attractive to birds such as Starlings, certainly more attractive than many of our common 'weeds'.

I haven't lived in an urban environment for very many years, but on my occasional visit's to London, I've always been intrigued by the flora along the old 'cut-and-cover' underground routes of inner London, many strange looking species among the detritus , which unfortunately you don't get a chance to study for more than a brief moment.

I have often though of producing a Flora of Walls. Perhaps one of Metal Bridges would be equally interesting.

It would be fascinating to get similar reports from other metal bridges around the country.
Dorts.

[aeshna5]

I do remember buying a Flora of Walls which was fascinating- not a big publication + possibly in conjuction with English Nature at the time.

As to habitat near Hammersmith Bridge- not too much on the Hammersmith side but on the southern side (Barnes) there is an extensive wild towpath which is quite rural looking with lots of fairly nearby wild places such as Barnes Common, London Wetland Centre, Lonsdale Road Reservoir plus many large houses with spacious gardens.

Starlings I'm sure have an important part as dispersal agent as many of the exotocs present have fruits which birds will feed on. Any soil is minimal + I suspect plants adapted to a more arid climate may have an advantage ( rather similar to the railway clinker with its Oxford Ragwort, Yelllow Corydalis + Purple Toadflax to name 3 common railway plants in London), though presumably a certain amount of moisture must evaporate from the river below providing some humidity.

[Cotham Marble]

Originally Posted by aeshna5

Natives are far + few- the odd small Salix sp.

I find it a fascinating insight into urban ecology how all these species come together.

I suspect that there are certain ecological limiting factors at play. In terms of the expected natives in these type of niches (river valley cliff face analogues ?) Ferns would be expected, but these are notoriously pollution averse, and I would guess, particularly on the metal bridges, that the growing medium was not only hydrocabon polluted but also acidic. The biogenic start of this 'bridge soil' is likely rusticular so that will have a major impact upon what gets a toe hold. The absence of willow herb is surprising though, perhaps periods of drought are a limiting factor that is more resisted by the non native woody plants. Salix are a known pioneer of more acidic sites, while many other UK cliff face specialists perhaps tend to be more lime loving.

CM

[poschiavanus]

Older unmaintained metal bridges (like fences) soon build-up a decent community of lichens (in towns, the usual nitrophilous suspects). I imagine mosses follow pretty soon afterwards. These pioneer species would soon provide a decent substrate for higher plants.

I presume that Hammersmith Bridge is not too rusty or lichenous, but I imagine there are still plenty of places to catch enough of various kinds of detritus to form some kind of proto-soil.

There must be plenty of old railway bridges to investigate for similar things. Personally I buy into the theory that the environment favours species better adapted to more arid conditions. Although zoochorogical () selection by seed size etc. might be a big factor : for instance, I know several places where the exotic elements of the flora have obviously been put there by thrushes.

I think there's been a fair bit done in places like Berlin & Vienna on the more unusual plant communities of cities. Some interesting ruderal plant communities are documented at this site Rheinische Pflanzengesellschaften (text in german).

[aeshna5]

You're right Hammersmith Bridge isn't particularly rusty or lichenicolous.

Certainly some of the railway bridges have a good fern flora which with cleaner air has increased the number of both individuals + species. Hart's-tongue is probably most numerous with also expected species such as Maidenhair + Black Spleenworts, polypodies + some interesting exotics including at least a couple of Pteris spp + Cyrtonium falcatum in a some locations.

[Meta menardi]

Originally Posted by Dorts

I haven't lived in an urban environment for very many years, Dorts.

I have thought it for some time, but this confirms it, you are a wise person.