[Illustration: Fig. 61. Mixed Hardwoods on Lower Levels. Spruce and Balsam Dominate on Higher Elevations. Mt. McIntyre, Adirondack Mountains, New York. _U. S. Forest Service._] [Illustration: Fig. 62. Scrub Growth on Mountain Top. Mt. Webster, New Hampshire. _U. S. Forest Service._] LIGHT. More than by any other factor, the growth of trees in a forest is determined by the effect of light. All trees need light sooner or later, but some trees have much more ability than others to grow in the shade when young. Such trees, of which maple and spruce are examples, are called _tolerant_, while others, for instance, larch, which will endure only a comparatively thin cover or none at all, are called _intolerant_. The leaves of tolerant trees endure shade well, so that their inner and lower leaves flourish under the shadow of their upper and outer leaves, with the result that the whole tree, as beech and maple, makes a dense shadow; whereas the leaves of intolerant trees are either sparse, as in the larch, or are so hung that the light sifts thru them, as in poplar and oak. The spruces and balsam fir have the remarkable power of growing slowly under heavy shade for many years, and then of growing vigorously when the light is let in by the fall of their overshadowing neighbors. This can plainly be seen in the cross-section of balsam fir, Fig. 63, where the narrow annual rings of the early growth, are followed by the wider ones of later growth. A common sight in the dense woods is the maple sending up a long, spindly stem thru the trees about it and having at its top a little tuft of leaves, Fig. 64. By so doing it survives. The fact that a tree can grow without shade often determines its possession of a burnt-over tract. The order in the North Woods after a fire is commonly, first, a growth of fire weed, then raspberries or blackberries, then aspen, a very intolerant tree whose light shade in turn permits under it the growth of the spruce, to which it is a "nurse," Fig. 65. In general it may be said that all seedling conifers require some shade the first two years, while hardwoods in temperate climates, as a rule, do not. [Illustration: Fig. 63. Cross section of Balsam Fir, Showing Fast Growth After Years of Suppression. Notice the width of the annual rings in later age compared with early. _U. S. Forest Service._] [Illustration: Fig. 64. Tolerant Maple. The trees are too slender to stand alone. _U. S. Forest Service._] [Illustration: Fig. 65. Intolerant Aspen, a "nurse" of Tolerant Spruce._ U. S. Forest Service._] This matter of tolerance has also much to do with the branching of trees. The leaves on the lower branches of an intolerant tree will not thrive, with the result that those branches die and later drop off. This is called "cleaning," or natural pruning. Intolerant trees, like aspen and tulip, Fig. 66, clean themselves well and hence grow with long, straight boles, while tolerant trees, like spruce and fir, retain their branches longer.