Welcome to GoY! When you say 'on top of them' do you mean around the base of the trunks? It is best not to have anything too close to any tree, but stones are not likely to be harmful in any way. In this dry weather it's important to water any new plants very thoroughly and regularly - maybe the dry weather is causing the wilt?

Are they in containers or the ground, and are they recently planted? As Sheila says the most likely cause is dryness. willows are thirsty trees.

This weather is murderous to even established plants. I have a Hydrangea which is donkey's years old and was wilting like crazy yesterday, never seen it do it before. A bucket of water when the sun was off it perked it up, but a newer bush, shrub, tree would probably need help.