GeoGraph - photograph every grid square

The Woodland Trust has summarised the mechanism of leaf change thus:

Why do leaves change colour? Early in the year, when there is plenty of light, the leaves produce chlorophyll – the green colour in grass stains. This is used to convert sunlight into energy which the leaf stores as sugar. Chlorophyll isn’t the only pigment, (red) anthocyanin and (yellow) carotene, the main colour in carrots, are also present. Just like a grass stain, the chlorophyll covers up all of the other colours in the leaf making it appear green. However, as light and temperature drop at the end of the summer, the leaves stop making chlorophyll and it breaks down revealing the colours underneath. Beech and maple, which have more anthocyanin have red leaves in autumn while hazel and birch have more carotene and yellow autumn leaves. The intensity of the colour in the leaves is controlled by how concentrated the stored sugar is. The more concentrated the sugar, the better the colours. After a hot, dry summer with cool nights, the sugar is particularly concentrated and we get the benefit in a spectacular autumn.
This being a sycamore it has a few so-called 'tar spots', caused by a fungus (Rhytisma acerinum), where the leaf has already started to decompose.