In East Sussex where I live I know of many places, usually abandoned fields, that have regenerated to secondary woodland surprisingly quickly and, judging by the size of the saplings I have seen in some televised tree planting schemes (maybe only from seed this year), regeneration may be almost as fast, if not faster than planting.  Though it does not do much for the tree nursery trade, or other human engagement with tree planting schemes.

I have long thought that we are failing to appreciate the diversity and complexity of wildlife if we compartmentalise the landscape too much: that is a wood, that is a heath, that is a field.