What if trees never lost their leaves?

In summer, trees are full of green leaves. In autumn, many leaves turn red and yellow and fall down.

But pine trees stay green all winter. What if every tree kept its leaves, all year long?

Walk through a park in autumn and the ground is covered with leaves. Maples, birches, ashes and many other trees with wide, flat leaves drop every one of them. Yet pines and spruces, with their narrow needles, stay green through the winter.

Why do so many trees throw away their leaves, only to grow new ones in spring? This lab is a thought experiment. Imagine broad-leaved trees keeping their leaves all winter, then follow three paths: snow, evergreen needles and autumn colors.

Trees that drop all their leaves for part of the year are called deciduous; trees that keep foliage year-round are evergreen. In cold northern forests, most broad-leaved trees are deciduous, while most conifers keep their needles.

Regrowing a full canopy every spring is costly, yet these trees do it every year. This thought experiment asks what would change if northern broad-leaved trees kept their leaves through winter, using what foresters know about water loss, snow loading and the chemistry of leaf drop. Outcomes for the imagined world are labeled as ideas.

In autumn, a tree takes good things back out of its leaves. Then the leaves let go and fall.

In spring, the tree grows brand-new leaves.

Leaf drop is not an accident. A Michigan State University forester explains that it is controlled mostly by the changing length of the days, and that it starts as early as late July or early August.

First, the tree pulls useful materials back out of each leaf. Then it grows a thin layer of cells where the leaf joins the twig, and the leaf breaks off there. The bright autumn colors are the part of this process we can see. In spring, the tree spends a lot of its stored resources growing a whole new set of leaves.

Bill Cook of Michigan State University Extension describes leaf drop as largely controlled by annual changes in photoperiod (day length), beginning in late July or early August. The tree withdraws materials from the leaf and then forms a thin layer of cells at the base of the leaf stalk, where the leaf eventually separates.

The cost is real: deciduous trees must spend a lot of resources growing new leaves each spring, while conifers can photosynthesize all year, as conditions permit. Dropping leaves only makes sense if keeping them through winter would cost even more.

Now imagine the maple tree keeps its big leaves all winter.

The leaves might dry out in the cold. Snow might pile up on them. Heavy snow might break some branches!

Imagine a maple keeping its wide leaves all winter. Michigan State University's forester explains that broad leaves are a major way trees lose water, and that a lack of liquid water is one of the two big challenges trees face in winter. A tree full of leaves might dry out.

Snow would be a problem too. New Hampshire Public Radio's naturalists explain that trees with no leaves have less surface to carry the weight of snow and ice, and that early snowstorms, when trees still have their leaves, do the most damage. A whole winter of leafy branches might mean many more broken limbs.

Two costs would follow. Water: broad leaves are a major source of water loss, liquid water is scarce in winter, and broad-leaved trees lose most of their ability to move water after the first freeze (MSU Extension). Leafy crowns through winter might lose water they could not replace.

Snow load: bare crowns present less surface area for ice and snow, and New Hampshire naturalists observe the most damage in early snowstorms that arrive while trees are still in leaf; a heavy, wet October storm hits harder than January's dry snow. Year-round broad leaves might turn every snowfall into that kind of storm. Both are ideas, reasoned from present-day observations.

Pine needles are thin and tough. They do not lose much water.

When snow piles on a pine, its branches bend down and the snow slides off.

So how do pines and spruces keep their needles? A needle has much less surface than a broad leaf, and it is better at keeping water in. That lets these trees go on making food all year whenever the weather allows.

Their branches help with snow too. New Hampshire Public Radio explains that conifer branches fold up like an umbrella to shed heavy snow, then spring back afterward. Even so, conifers do drop some needles: the oldest ones each fall. Larches and tamaracks are conifers that drop all of their needles every year.

MSU Extension notes that needles have proportionately less surface area than leaves and better barriers to water loss, and that conifers photosynthesize year-round as conditions permit. Their water-conducting cells also differ: they have valve-like structures that can let water move again in winter, and stronger walls that resist ice expansion.

Conifers still lose older needles each fall; New Hampshire naturalists say they keep about 80 percent of their needles through winter, so snow loading still matters, which their flexible branches handle by folding like an umbrella. Tamarack and larch drop all their needles. The two strategies are trade-offs, not one right answer.

Leaves are green because of something called chlorophyll. In autumn, the green fades away.

Then we can see the yellow and orange that were hiding. Some leaves make bright red too!

Connecticut's forestry agency explains where autumn colors come from. All summer, leaves are full of green chlorophyll, the pigment that helps them make food. They also hold some yellow and orange pigments, called carotenoids, but the green hides them.

In autumn the chlorophyll breaks down, the green fades, and the yellows and oranges show. Some trees also make new red pigments, called anthocyanins, from sugars trapped in their leaves on bright days and cold nights. If broad-leaved trees never shut down their leaves, we might lose much of this colorful show.

Connecticut's Department of Energy and Environmental Protection describes the chemistry. Chlorophyll dominates summer leaves and masks the yellow and orange carotenoids. As the leaf's food production shuts down in fall, chlorophyll breaks down and the carotenoids show. Red anthocyanins are new: on warm days with cold nights, sugars made in the leaf cannot move down the tree, and in bright light they form anthocyanins. Bright, cool days and chilly but not freezing nights give the most brilliant color.

Those colors are a by-product of leaf shutdown. A tree that kept working leaves all winter might never show them, an idea rather than a prediction.