The contributions can be invested, similar to university and other endowment funds. Only the annual income would be used for urban forestry operations. As long as the principal remains intact, there will be an annual source of revenue. Residents can be encouraged to make gifts to mark births, weddings, deaths, anniversaries and other significant occasions. You are limited only by the creativity of the people promoting the program.
Many foundations prefer to fund capital projects, like tree planting.
This residential street was recently planted with treeform lilac, Callery pear and Amelanchier to replace more traditional shade trees. These varieties were selected to reduce interference with the overhead utility wires. Another potentially larger source of funding can be bequests. Residents who appreciate the quality of life in their community may wish to continue that quality of life by remembering your foundation in their will. Like everything else, the contributions realized will be proportional to the effort expended to gain those contributions.