So the trees without leaves in the winter don’t get any sunlight. Trees need sunlight to make new food to feed all of the living cells in the tree. However, many trees will make all of their food when it’s sunny, then when winter comes, they will go into dormancy and not make any food anymore. Trees go dormant sort of like how bears hibernate in the winter. The idea is that the organism makes a bunch of food before winter, and then slows down all of its usual processes to survive through the winter on the stored up food.

The reason the tree loses the leaves in the winter is that it takes a lot of energy to maintain the leaves. The tree wouldn’t get much sunlight in the winter anyways so it makes more sense for the tree to not waste energy maintaining leaves and instead just go dormant for the winter. Also leaves make it easier to lose water which could be a huge problem for a tree in the winter.

There are trees, like cacti, that don't have any leaves. They live in such sunny places that they don't need extra surface area from leaves to collect the sun. Their trunk is enough. So the chloroplasts, the organs that take up sun and make energy for the plants, are found on the trunk of the tree, which is why the trunk looks green. Other types of trees have leaves and green photosynthetic trunks so that they can catch a little extra sun.

There are a few trees in the desert that have green bark, but most trees need leaves to get sunlight. Trees in the winter often lose their leaves to conserve energy during the winter months, but they're burning the sugar reserves they built up from the previous summer to stay alive during the winter.

Good question! Some trees wait until conditions are nicer outside before they grow new leaves to capture sunlight. Other trees have photosynthetic bark so they actually capture sunlight on their trunk and branches! Aspen trees, for example, have photosynthetic bark but they are covered in a protective white dust that acts like sunblock. If you go up to an aspen tree you can wipe off the "sunblock" that was used by native americans and see the green bark!