They flower in early spring, before the trees have grown back all their leaves, thus blocking all the sunlight. This means that the bluebells get all the sunlight they need to photosynthesise.
(Deciduous) Trees
They typically have broad leaves to absorb as much sunlight as possible because sunlight is weak.
But this means there's a lot of potential water loss through the stomata, so they shed their broad leaves in autumn, in response in reductions in light and heat, which enables them to conserve water.
Leaf fall comes early in years when there's been a shortage of water in late spring and summer.
They have both shallow roots to access nutrient-rich humus at the top of the soil, and deep roots to access nutrients that have been leached due to rainfall.
Thick bark protects the tree from frost and animals.
Moss
They've adapted to live in dark and dank places, perfect for the ground layer of temperate deciduous forests. In the northern hemisphere, they also grow on trees, but only on the north-facing side where the sun doesn't shine directly.