Paper Mulberry (broussonetia papyrifera, 构树).
The fruit is sweet and edible (otherwise it would hardly matter that is sweet) but bruises easily so that it is best to devour it straight after picking. The young leaves are eatable too, when cooked, though it would be better to wait until next spring to try them. As a book-lover, one must respect this tree's long and ancient part in the production of paper.
