Judy Lindow The adjective (and noun) wych is little used these days. The root meaning is 'having flexible branches', from the Common Germanic wik- 'to bend': wych is used of trees, particularly the wych-elm, sometimes (erroneously) written 'witch-elm'. This is the species of elm-tree whose botanic name is ulmus montana or ulmus glabra, and sometimes ulmus glabra montana. Its wood was formerly used to make bows, according to OED.
Sophie I was wondering about this, too! Does that mean "The Wych Elm" is the UK edition and "The Witch Elm" the American one? And are wych elms spelled "witch elms" in America? If not... I'm confused about the alternative spelling.
Melinda UK authors tend to use different covers or even titles for US and UK releases (see Sophie Hannah for an example). It appears the common spelling of this type of tree in Ireland (where French lives) is wych. That spelling was used multiple times throughout the book as well (I received an advanced copy from Edelweiss).