Of course, I'm already behind the curve, since trees have been sneakily changing for awhile now. But in my defense, much of the color seemed to me a response to drought: we have not had more than a quarter-inch of rain any of the few times t has rained since way back in mid-August, and the plants have been hurting.
In late August our big ash looked sick, with every excuse in the world.
A month later it looked wilted, and I was getting really worried.
Now she is definitely turning, along with other white ash trees in the neighborhood. (10/2)
The trees on a little island in Nippenicket Pond, West Bridgewater a couple of
weeks ago first led me to suspect that autumn was underway: living with their feet
submerged in the pond as they did, the colors of the red maples (above)
and tupelo (below) were likely NOT due to drought!
My neighbor's big red maple well on its way. (10/2)
Big Daddy, a large red maple standing in a row of small females,
is going great guns, but will soon be leafless. (9/29)
Nearby, Little Mama, is still making up her mind to turn, beginning
with the edges of her leaves and her veins and leaf stems. (9/19, 10/2)
The sugar maples in the neighborhood, like the red maples, are all over the place.
A tree deformed by years of hacking by road crews trying to keep it away from power lines
is now in its glory: it began turning weeks ago. Sugar maples sometimes turn an amazing flame-color.
Just around the corner is another tree that is having
an internal disagreement: shall we turn, or not?
In a trio of sugar maples nearby, one tree has already lost half his leaves,
while his neighbors have not yet begun.
A brief explanation of how and why leaves turn the colors they do is here, with the slightly longer and more grown-up version here. Another nice page that makes it clear that leaves don't just "fall," but are actually pushed. Not sure about the reason for leaf drop, though. I have heard two other hypotheses: that trees drop their leaves to avoid drying out, and that it reduces wind stress. Here's a University of Illinois page that delves a little into the chemistry and biology of leaf color.