Why Do Evergreen Needles Look So Different From Deciduous Leaves?
Ask anybody to draw a leaf and damn few would draw a pine, spruce, hemlock, or fir needle, and even fewer would draw the green scales of a northern white-cedar leaf. Most would choose maple or oak, right? But leaves they all are, despite so many differences in their size, shape, and approaches to the very important business of being a leaf.
Let’s begin there. There are certain things that all leaves do, regardless of their shape or size or whether they are evergreen or deciduous. Notably, all living leaves absorb sunlight and carbon dioxide from the atmosphere and mix them with the water and elements they wick from the soil in that marvelous free lunch called photosynthesis. They do this to make carbohydrates, which fuel their own existence and growth. It is an imperative shared by all leaves. Accordingly, both evergreen and deciduous leaves make and use the pigment chlorophyll, and so both appear green. Both release oxygen as a byproduct of their self-feeding ways, and both leaf types provide food and homes for all manner of animals, great and small.
From there, though, it’s mostly differences. For starters, and with rare exceptions like tamarack, evergreen leaves remain on the tree much longer than their deciduous counterparts. Despite the misleading name, evergreen leaves are not green forever. Leaves remain on the tree continuously for several years – as many as 10 for some spruces – but eventually whole cohorts turn yellow, orange, and brown and are shed, replaced by new leaves in spring. In this way, the tree remains “ever green,” but it does so with different age classes of leaves.
We also know that evergreen leaves look, feel, and even smell very different than deciduous leaves. They tend to be linear and smaller, more compact, and less flat. They appear, well, more needle-like than leaf-like. They are sticky to the touch when broken, and typically have a pungent, “evergreen” smell.
There are additional, if less visible, differences. Evergreen leaves often have a blue-white, wax-like coating. And the pores through which they exchange oxygen, water vapor, and carbon dioxide with the atmosphere – their stomates – tend to be sunken below that surface. Unlike the obvious veins of broad leaves, evergreen leaves typically run their vascular systems buried deeper within and surrounded by the photosynthetic cells closer to the surface.
Taken together, these traits suggest a tree built to make do in harsh environments. Think winter, when water can be very hard to come by and there is nearly continuous abrasion from wind, snow, and ice and relentless browsing by hungry animals. If a tree is to keep its leaves through winter, they had better be very tough and good at minimizing water loss. The tree’s life is at stake.
Thus, our evergreen intrigue. The hardwoods go out in a blaze of color at the first hint of cold, and nobody can blame them. (Imagine what would happen to those flimsy leaves in winter.) Evergreens persist and we’ve got to ask: how?
An evergreen’s tendency toward conical form and clustered leaf arrangement allows it to shed snow while still capturing light. Compact needle shape and waxy coverings minimize water loss. And not having to make new leaves in the spring seems to be an effective cost-saving measure. The tree gets several years of production to pay off the costs of its leaf making.
Being able to photosynthesize during winter whenever temperatures, sunlight, and moisture allow can be a real advantage. Many studies indicate that while northernmost evergreens develop such deep cold tolerance that they do not photosynthesize in winter, many of our temperate evergreens, such as red spruce and balsam fir, do. This is only an advantage if the trees can balance production with protection, but given climate change and a future where weather might become more extreme, it’s an advantage that someday may really pay off.
Michael Snyder, a forester, is commissioner of the Vermont Department of Forests, Parks, and Recreation.