welcome to GoY.

There are several ways to tackle this problem. You can go for lots of spring bulbs that will thrive before the tree comes into leaf. What type of tree is it? I have underplanting under a large beech tree and grow Ajuga [bugle] Lamium Geranium macrorrhizum, Geranium phaeum, as well as snowdrops crocus early flowering narcissus cyclamen coum in spring and cyclamen hedrifolium in autumn. To wards the edge of the tree line I have several ferns that are happy in dry shade Polystichium and Asplenum.

Initially you will have to water what ever you plant regularly to get them established.

Google RHS plants for dry shade and you will get a list of many more.

I have a patch of cyclamen hederifolium and another of lamium under a birch. I had to keep them well watered to begin with. You could try geranium phaeum too.

Many epimediums will take dry shade as well