The statistical testing is tricky, though. Did you go in with the thesis that there might be a correlation between age and ideal number of trees, or did you first pin down "too many trees" and then search the data for correlation? Or even more problematically, did you search for cross-correlations on all columns and then discover the age-tree link? If either of the second, your definition of "significance" needs to be custom.
We take a fairly pragmatic approach to the multiple comparisons problem you're highlighting. If p-values are < 0.00001 we don't really worry unless we're doing a crazy amount of analyses. And when we do get p-values that are borderline (like .01) and we're doing multiple comparisons we'll mention that there may or may not be something actually happening there (as we did in our post about Baltimore parking tickets: http://blog.statwing.com/baltimore-parking-tickets-revisited...).
For what it's worth, I think ceph_'s comment and link below (http://online.wsj.com/article/SB116165781554501615.html) about the relationship between trees and gentrification is the best available guess as to what's ultimately driving these attitudes. But yeah, it's probably a combination of a lot of things, many of which aren't actually about trees per se.