A Conifer That Thinks It's a Broadleaf
(2/18/2000)
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  drops
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  they
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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