Does anyone have any tips or referene suggestions for identifying tree species when the leaves are off?

There are a bunch of ways to do it. Most tree classification books will explain it for you.  You will have to look at bud scars, and bark.  Its not hard once you figure out the signs. i recommend the Audubon society books.

Audubon Tree Guide is an excellent reference.    Pat

MAD Cap Horse is Opposite!

M for Maple
A for Ash
D for Dogwood
Caprifoliaceae
Hosechestnut

These species have an opposite branching pattern.

More common is an alternate branching pattern as found in oaks.

As mentioned, there is a lot to learn about buds, bundle scars and bark that can help ID winter trees.  Of course the ones with green needles are pine trees, but even there you can distinguih by the size, shape and number of needles in each bundle.

Bark recognition is actually quite easy once you get the hang of it.

Here is a site that may be helpful:   http://www.cnr.vt.edu/DENDRO/dendrology/main.htm

Thanks....and sorry about the spelling, I was in the woods typing on my phone. I'm pretty good with the oaks and maples, but I ran into a few trees I couln't figure out today.