Fall cartography

Via Mapas, Mapas, I came upon this handy Google Map of fall foliage. What I found particularly interesting was what happens when you scroll down away from New England proper, where the fall colors are in full swing. The remainder of the eastern US is colored in green, with a smattering of counties in yellow (indicating that the foliage is starting to turn). But these turning counties are not distributed in any ecologically sensible fashion -- rather, they're disproportionately the home of large cities. For example, the only two non-green counties in Tennessee are Davidson (home of Nashville) and Shelby (home of Memphis). It's possible that there's some ecological reason that urban areas would provoke trees to change their leaves sooner (some brief Googling didn't turn up anything, and I haven't noticed any significant tree color gradient in my commute between Pittsburgh and Slippery Rock). But if you click to see the underlying data, the explanation is clear -- big cities have more people, and therefore a higher likelihood that someone in the city would have started thinking about fall leaves, heard of this site, judged the leaves to have crossed from "green" to "turning," and submitted a report. Such are the perils of crowdsourced data. It would be interesting to compare the underlying data from this map, particularly dates at which counties were switched over, to population size and some more systematic measurement of foliage (perhaps from satellite data). I would guess that in addition to the urban-rural difference, there would be a general tendency for the Google Map to switch counties over quicker, because people like to be the one to announce something new and see it marked on the map. On the other hand, if there's any tendency for foliage in areas accessible to people to change faster or slower than in inaccessible areas, a crowdsourced map like this would be more appropriate, since the reason people would look at a map like this is to see where they can observe fall colors -- it's irrelevant to map users if trees in the deep woods are a different color.