I recently flew into New Jersey over the Delaware river and took this photo of the ridgeline at the Delaware Water Gap. The trees are mostly green (early October) but on the south-east side of the ridge there is a bright streak of reddish foliage along the entire length of the ridge. (I noticed this on other hills in the area as well.)
What could be causing this color change? My first guess would be that it is related to sun exposure.