One of the older oaks on Tooting Common has a comically bright beefsteak fungus bursting out of its trunk that recalls nothing so much as a clown's nose:
And at its base there's a more delicate, almost painterly, earth-toned cluster of hen-of-the-woods fungus, too:
We had strolled over to the trees retrieve our dog, who was circling the trunk in high excitement having just chased a squirrel across the grass and up into the oak's branches. As we stopped to examine the two fungi, parakeets screeched at us from above: the old tree really was bustling with life, even as its leaves were changing colour and beginning to drop.
English oaks are home to over 300 species, making them the most biodiverse of our trees. Yet the reason is not some innate quality of the oaks themselves, but the simple fact that they have been here for so long, and so have had time to build up relationships with a huge range of other organisms. And that is why native species are so important when it comes to preserving and promoting biodiversity: not because of some misguided sense of nationalism or xenophobia, but simply because the longer living things have coexisted, the more time they have had to evolve in tandem and find ways to profit from one another – whether that's spangle galls on oak leaves, parakeets taking over woodpecker nest-holes or fungi living on tree trunks. Imported species to the UK often support far less life, but that is usually simply because they have been taken out of their own ecosystems. Back at home, they may well be just as biodiverse as our English oaks.