As you may know, trees are used to make things like furniture, cottages and paper. Unused paper is then 'thrown in the recycling bin' and recycled (go through the same system to be reused). Deciduous tree leaves specifically, fall off in Autumn so the sticks, trunk, and remains of the tree can be used to make products with the nutrients in the wood. I think this is part of an ecosystem - hope I help! :D