Maple species alone (in addition to Acer saccharum and its subspecies grandidentatum and nigrum) include A. circinatum, A. macrophyllum, A. pensylvanicum ("abundant producer of very sweet sap which yields more sugar per gallon than larger maples. It is said to have a better flavor than other maples"), A. pseudoplatanus,
A. rubrum, A. saccharinum and A. negundo.

Ref: Stephen Facciola, 1998, CORNUCOPIA II

I'm not a tapper myself, but as Bboy says all the maple species produce good sap, including box elder. I suspect that because of the large size (and so volume of sap) sugar maple is your best bet because you need to boil down a lot of sap to get a sweet syrup. That's an interesting fact about the sweetness of Moosewood (A pensylvanicum)sap. Sadly it doesn't get too big. I've seen squirrels biting off twigs of it though to suck the oozing sap in spring, they are probably hard-wired to go ofter the best sources.

Concerning birches, I would try Black or Cherry birch, the source of birch beer. I once had some of this at a Pennsylvania wedding many years ago - the memory stays with me. It has somewhat of a wintergreen flavor as well - I can vouch at least for the fact that the leaves make a pretty good tea.

questions...

how come tapping maples for syrup doesnt hurt the trees?

when you see sap dripping down a maple, like today i saw sap coming out of a large weedy Box Elder and a Shindojo Japanese Maple... is this a sign of damage to the trees? or just something natural? I have been butchering the Box elder, it is too close to my house. might cut it down all the way this year anyway

I don't think tapping it for sap does the tree any good. The sap which runs out of wounds is a pretty natural process, which this tapping simulates. The tree is expending energy, but then I suppose honey bees would make the same complaint against humanity. In both cases there is apparently a surplus available which the organism can do without.

Butchering boxelder is pretty natural too. Almost everyone around here keeps trying to chop them back, which results in some interesting specimens. I suppose I wont win any points from those in the horticultural know but I love these trees. When I see the frequent bent over trunk, some so horizontal you can almost walk on them, encouraging a luxuriant growth of moss, I am inspired. I also admire the quality of a more upright specimens form, which is unique to box elders. When it is clothed in summer green, it's somewhat amorphous billowyness reminds me of a childs simple crayon rendering of a generic tree, and simple is good. The showers of spent keys it gradually rains down through the winter are ratty looking and annoying, but every tree has some bad points. Here it is a common and natural part of the flood plain flora, and it would be presumptuous for anyone to say it didn't belong here.

Judging from their prevalance in nurseries those in the horticultural know would agree with you that cultivars of the species have useful attributes. Male clone 'Violescens', for example produces a remarkable effect in flower.

That should've been 'Violaceum'. For a photo...

Here is a link that might be useful: Forestfarm plant nursery

I've seen references to tapping black walnut and butternut for syrup making. Wouldn't be a stretch to think you could also tap pecan or any of the hickories, since they're also members of the Juglandaceae, cousins of the walnuts. And, I can attest from years of grafting attempts, walnut and pecan can and do 'bleed' profusely in the early spring.

I've made hickory syrup by boiling hickory nuts/nutshell fragments, and some folks make it by pulling off strips of hickory bark, from shagbark hickories, boiling it for a few hours, straining, adding sugar, then cooking it down to the desired consistency.

I've been looking for information substantiating the notion that moosewood (Acer pensylvanicum) can be used [practically] in making maple-syrup.. are you sure??

Btw, apparently black tupelo/black gum (Nyssa) can be used.