Symbolism, Imagery, Allegory
Though nature imagery in general is pretty important in The Waves (see "Flowers"), trees in particular seem to "crop up" quite a lot. Sorry, we couldn't let water have all the punny fun.
For example, upon hearing that a man was found dead in the gutter, Neville concludes: "[W]e are doomed, all of us, by the apple trees, by the immitigable tree which we cannot pass" (1b.67). That image returns when Percival dies, and Neville says:
He is dead. […] He fell. His horse tripped. He was thrown. The sails of the world have swung round and caught me on the head. All is over. The lights of the world have gone out. There stands the tree which I cannot pass. (5b.1)
Nature imagery is often used to convey fertility and life (the darling buds of May, etc) but this tree recurs in Neville's narrative as a symbol of death and doom.
Also, in a similar vein, it's worth noting the numerous references to yew trees in particular throughout the text. Yew trees are often found in European graveyards, so perhaps these references are part of the novel's commitment to drawing attention to death in the characters' universe. Because, you know (or should we say "yew know?"), there isn't enough doom 'n' gloom in The Waves already; Woolf wanted to shoehorn in a little extra.