A recent study of Portland, Ore., rental properties, led by Geoffrey Donovan of the U.S. Forest Service, found that the presence of trees can subtly increase rental prices, even once you control for factors like how popular the neighborhood is. A tree in the yard increased rent by $5.62; trees on the street increased it by $21.
Donovan's previous research has found that there's a much bigger effect when you're trying to sell a home, rather than rent it. A paper from 2010 (comfortably after the real estate bubble) found that Portland homes with a tree on the property sold for $7,000 more than homes without trees. (Although weirdly, a tree in your neighbor's yard increases your home value much more — $13,000.)
Other value-boosting factors, like walkability, mattered more in tree-heavy neighborhoods. A house in a walkable Portland neighborhood without trees is valued at $3,500 more than a home with fewer businesses nearby — but areas that are both walkable and tree-lined increase home values by $22,000.