I had seen articles explaining the increase in property values based on distance to parks, but I had not seen one explaining the effect of mature trees, vacant lots, yard versus street trees, etc. This article from research of Kathleen Wolf at the University of Washington breaks down the impact pretty well.

She might be a good speaker.

the article
http://www.cfr.washington.edu/research.envmind/Policy/Hedonics.pdf

Price Increase Condition
2% mature yard trees (greater than 9-inch dbh)
3-5% trees in front yard landscaping
6-9% good tree cover in a neighborhood
10-15% mature trees in high-income neighborhoods