Trees are terrific in every way but one: they make lousy carbon offsets. That was the point of the "First rule of carbon offsets." But a number of comments and some media queries have led me include two rare exceptions: certified urban trees and certified tropical forest preservation. The word "certified" is key in both cases.
For these two rare cases, I would allow trees to comprise no more than 10 percent of an overall offset portfolio (which should be heavily weighted toward efficiency, renewables, fuel switching, and perhaps carbon capture and storage). Also, their offset value should probably be discounted over time (because urban trees are unlikely to be permanent and tropical forest accounting is quite uncertain).
