I bet the salt is a big factor. Also, and because of that, there's probably a huge difference between park-and-plaza trees and regular street trees.

The eucalyptus (http://www.shapingsf.org/ezine/ecology/presidioframe.html) in the Presidio (http://www.presidio.gov/About/Post/AugSep2003/ForestFall2003.htm") are more like a hundred years old. The whole forest there, having been planted in one big flurry of planting in the 1880s-1890s, is reaching the end of its lifespan. Before the Army planted the trees, the Presidio was a magnificent bleak landscape of dunes and scrub.