Answer:
Trees turn a near two dimensional ecosystem three dimensional. Without trees there is only one layer of habitat for a terrestrial ecosystem but with trees, especially with huge trees like in a rainforest or old growth forest, there can be habitats in the air. Monkeys birds bugs lizards and other arboreal creatures live in the canopy and sometimes never come down. With the height the biomass per square meter of forest floor will increase 100 fold and the trees protect new creatures in the leaf litter.
