We have so many uses for trees, that one can determine multiple career opportunities for him/her self by asking "what is a tree's function?" question.
Trees help clean the air we breathe, filter the water we drink and provide habitat to over 80% of the world's terrestrial biodiversity. Forests provide jobs to over 1.6 billion people, absorb harmful carbon from the atmosphere, and are key ingredients in 25% of all medicines.
Trees are a part of us, and our growing economy consumes trees as tools for growth. Everything around you exists because someone cut trees years ago at the same location.