The answer is surface area. The bigger the leaves the more energy can be captured from sunlight and used to help the tree grow. The maple tree has large flat leaves that do this very effeciently while the spruce has needles with a smaller area, so they cannot grow as quickly or survive as well in lower light areas as the maple. The problem is that large leaves are not good in the winter since they would dry out quickly once the tree freezes and then would die. To get around this problem the maple looses its leaves each winter. The spruce has a different solution - it has leaves with a small surface area and a waxy coating that stops them form drying out when frozen. The problem for the maple is that it takes a certain amount of time to grow the leaves in the spring which results in lost growing time compared to the spruce. In southern areas the lost growing time is more than made up for by the larger leaves... but in the north where the growing season is shor! ter the advantage goes to the spruce with its year round needles.