The natural habitat of the fig tree (Ficus carica) is the dry rocky hillsides of the Mediterranean. In Britain its now successfully naturalised on a number of mainly urban rivers in the industrialised heartlands, although an exception to this is Devon and Cornwall where it is now also widespread.
The origin of fig trees, at least on urban rivers, is attributed to seeds derived from sewage. During rainy periods combined storm and foul sewers become overloaded and raw sewage enters the river. According to an item in the Botanical Society of the British Isles (BSBI) journal Watsonia there's a suggestion its occurrence on the River Don in Sheffield is linked to heavy industry whereby elevated water temperatures due to its use in the cooling processes has provided the right conditions for germination. The Flora of Monmouthshire shows a scattering of records for the vice county including this one on the upper reaches of the Afon Lwyd near Forge Side.This mature tree appears to have successfully developed in the gabion baskets of a previous river engineering scheme. Another site on the same river at Pontnewynydd is also close to former industry activity.