Barking?

The bark of some trees (e.g. Aspen, Beech and Silver birch) is thin and contains chloroplasts (https://www.theguardian.com/science/2021/dec/15/plantwatch-how-in-winter-bark-can-act-like-leaves-for-trees). The area just under the bark is, of course, the only living part of a tree trunk. Bark-based chloroplasts means that, in the winter, these trees can photosynthesize, in spite of losing their leaves. This winter photosynthesis may be more important to saplings (young trees). Saplings will have less stored carbohydrate and need to grow quickly, to reduce their chance of becoming overshadowed by bigger plants.