Trees are producers, so they are on the first trophic level. They gather their energy from the sun. Any depletion of trees will affect all the other trophic levels, as less energy becomes available for animals who eat trees, and for animals who eat animals who eat trees. Trees who are cut down and used for other purposes do not return their nutrients to the ecosystem when they die, depleting food for detrivores. Also, loss of forest means a loss of habitat for animals living in the forest. If the number of animals living in the forest decrease, this will also affect the organisms these animals eat, and the other animals who eat these animals.