In ww2 Germany right near the end they ate loves of barley mixed with grass as filler. The soviet union mixed saw dust in the bread. etc.
The bad news is that its not digestible...unless you happen to have 3 stomachs like some animals.

The good news is that the INNER bark (cambium layer) of most trees is edible for humans.

The bad news is that its not very palatable. You will have much better luck with weeds.

The good news. Many of the invasive weeds came with the settles as food and were so successful, they invaded from coast to coast. I can count 7 invasive but edible weeds within 15 ft of the window Im looking out of.

sap? sure takes a ton of time to make even a little maple syrup, but then again it is sure good and can be used to make other things better like pine needle tea. I also happen to know you can make birch tree syrup. But does anyone else know of other kinds of syrup from different kinds of tree's? You don't have to boil it right done, and the natives like it as a dark liquid drink to share while hunting.

I know this is not a tree, but cat-tails or sometimes called bull rushes; the ones at the sides of ponds and streams, the roots can be boiled for a while to allow the starches to come out and the water later evaporated off leaving you a white powder like flour substitute. The natives also used the pollen mixed with the root flour when making breads. You could drink the water and root mix but the likely hood of you being able to stomach it; it is very bad tasting and bitter. I have never tried, but I did read a good artical on this, also watched "lee shroud" try and drink the boiled root water mix, way to funny lol If nothing else around I would try and make the flour from the roots but have not tried yet.

Also in the spring I am a huge fan of fiddleheads. They do taste pretty awesome fried up with mushrooms and garlic butter and I think they taste a lot like Brussel sprouts or pea pods. But it is very important to boil these well 1st as they do contain small traces of poison or something and boiling them for 10-15 mins well then straining them throu cold water takes care of this, also it cleans all the dirt and stuff out of them. Then fry with mushrooms and garlic butter until very soft but not mushy. These are awesome 2 thumbs up for sure, you can freeze them after boiling for future use or eat them but frying with butter and mushroms is a great way to go. here is a pretty good video of what to look for http://www.youtube.com/watch?v=cKx00sIo_GA
and how to cook them http://www.youtube.com/watch?v=yUihJOxB7co

Hawthorne have edible leaves before the berries form , and the ripe berries have a compound that is good for the heart . They make a good shelterbelt tree with wicked spiked thorns to keep large stock in and they are a good flowering tree for bees .

Linden tree also known as Lime tree is much talked about as being a tree that offers tasty young leaves and it has been suggested is good for urban boulevard plantings for more edible cities : )

http://www.wildplantforager.com/1/post/2013/05/eating-like-a-giraffe-5-trees-with-edible-spring-leaves.html

A book that I discovered earlier this summer is "Edible and Medicinal Plants of Canada". Published by Lone Pine. This might help expand your list of edibles.

As mentioned the cambium layer is good

ie: you remove the exterior of the bark until you see the white interior. black birch , red birch and many varieties of evergreen are edible , and produce around 500 calries per 1 lb

that book Edible and Medicinal Plants of Canada is very good and you can get one more specific to your area...like central North America

Bearscott / threestorms;
I have the same book. Saw it in a store back in the summer and nabbed it before another customer that was eyeing it. Glad I did too.
Always keep my eyes sharply peeled for new books on the shelves.

I only know pine needle for tea,
spruce tips to eat,
raspberry leaves for tea(cal, iron, and good for 'lady things'),
rose hips, rose petals,
willow bark for pain relief,
birch sap for xylitol,
and maple trees for syrup,