What a great time of year! When I lived in Michigan, I tapped sugar maple, silver maple, and even the box elder trees. To cut down on the cooking, I would put the sap in big bowls, and put those out in the open overnight. With all the radiative heat loss, the sap would freeze. Since ice is fresh water, the sugars in the sap would be more concentrated in the non-frozen liquid. I could get the sap pretty concentrated that way. Then I'd cook it down over an outdoor camping stove hooked to propane. It worked pretty well. Enjoy all that sap! :D