American Larch (Larix laricina)

A Conifer That Thinks It's a Broadleaf

In most of North America, our trees fall neatly into the winter haves and have nots -- those that have green and those that do not. Broad-leafed deciduous trees -- oaks, and maples and the like -- are gray skeletons against the sky, while cone-bearing evergreens safeguard Nature's green all winter. And then there is one tree that stands alone: The Larch.

What makes the Larch an oddball is that it is one of the rare coniferous trees that larchdrops its needles each fall right along with the maples and oaks. In fact, before Larch needles fall off, they turn bright yellow, adding to the spectacle of fall colors for which New England is so famous. And then every Larch becomes as bare as any beech or birch.

Why does this needle and cone bearing tree act so much like the broad-leafed maples and oaks? Doesn't it know that conifers are supposed to be evergreen? Maybe it knows something we don't. Scientists have long wondered why some trees drop their leaves each fall and others don't. Why is it that trees in the far north are generally evergreen, as are most tropical trees, while trees in the middle temperate zones tend to be deciduous, or leaf dropping?

As is often the case in science, oddballs can provide clues to the norms, which is why much of the research on the question of deciduous versus evergreen focuses on the oddball Larch.

"There are many ways to be a successful tree," says Jim Fownes, forest ecologist in the Department of Natural Resource Conservation at the University of Massachusetts in Amherst. "Deciduous tree leaves have a James Dean lifestyle — live fast, die young. Evergreens are more like Bob Hope, their needles live slow, and live long." Oaks and maples and other broad-leaf deciduous trees put a lot of nutrients like nitrogen into their leaves at low cost. Their leaves are relatively thin and have high surface area for their weight. All of this means that they can photosynthesize at high rates. That's the "live fast" part of the James Dean lifestyle.

The cost of this leaf strategy is that the high nutrient leaves are attractive to insects and other leaf eaters. The thin leaves can't take much abuse before they fall apart. And the high surface area means they lose water easily, a real problem in temperate winters when the air is dry and most ground water is tied up in ice. In addition, the large leaf area doesn't shed ice or snow well, leading to broken limbs and downed trees in harsh winters.

The solution: drop the leaves before winter and replace them again in the spring. After all, they weren't that expensive to make in the first place.

Evergreen conifers have a different leaf strategy. They make small, but sturdy leaves, built for structural integrity. The leaves have a relatively small surface area compared to their weight, and they are coated with waxy substances that reduce water loss. The downside of this conservative strategy is that these leaves are relatively costly, in terms of energy and carbon, for the trees to make. And the structural components that makes needles tough allows less room for the chemicals and cellular structures needed for photosynthesis. That may be why evergreen needles tend to photosynthesize at lower rates than the leaves of broad-leaf trees.

The benefit to needles is that they can withstand a great deal more abuse compared to leaves, and they aren't as attractive to insects. Even needles need to be replaced eventually, and every conifer drops some percentage of its needles each year, but needles can last a very long time. Some bristlecone pine needles, for example, have remained green on the tree for 40 years.

The interesting thing is that both strategies work, and work well. What decides which trees live where? That's one of the unanswered questions. Large swaths of the world are covered with deciduous trees, and large swaths are covered with evergreens. Though there is great debate about the details and the whys, the broad answer seems to be that evergreens predominate in cold and nutrient-poor areas such as northern forests and flooded bogs, where deciduous trees can't find enough nutrients for their high-octane existence.

Then there's the Larch. (Fans of Monty Python's Flying Circus, the British television comedy series, will surely remember the episode that carried a running joke, "And now for something completely different ... 'The Larch.'" I'm not sure if the writers knew of the Larch's biological oddity or not, but they were on the mark.) There are ten species of Larch, including our native Eastern Larch, also called the Tamarack or Hackmatack, after the Abnaki Indian word for snowshoe wood. Tamarack and Bald Cypress, another swamp-dwelling conifer that also sheds its needles each fall, were prized for their long-lasting, water-resistant wood. Tamarack roots were even used to sew bark canoes together. A relative, the European Larch, was introduced to North America for use as an ornamental shrub, but it now grows to a height of 70 feet or more.

Larches, with soft needles about an inch long that grow in flower-like clumps, retain some of the nutrient conserving tendencies of their conifer cousins, but larchthey have adopted that broad-leaf trick of turning up the photosynthetic volume during the summer and then discarding attractive, but fragile leaves every fall. The Larch leaf strategy seems to be live somewhat fast, die young. Not what you would intuitively think would be the best strategy (imagine James Dean in a white limo, or Bob Hope on a motorcycle), and scientists are still trying to work out where and why the Larch strategy works, but it seems to offer advantages in some of the most harsh of environments. For example, in areas with extreme winter conditions, where even winter-hardy conifers have trouble with snow damage and water loss, the Larch protects itself by dropping its needles. In fact, our native Eastern Larch lives well into northern Canada and Alaska, and the Siberian Larch lives well above the Arctic Circle, farther north than any other tree in Asia.

In a world of deciduous broad-leafs and evergreen conifers, Larches are the oddballs that prove the rule of Life's enormous variability and potential for adaptation. And if you happen to be walking under some larches in fall and are showered by tiny yellow leaves, fear not. It's just the annual Larch needle drop. Just be grateful no one has to rake them.




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