Monday, August 31, 2015

Death of a Tree

The death of trees is easier to study than that of animals, just as trees are easier to study in the first place: they move slowly,* and change slowly.  Pay attention in the woods: you'll see trees in every stage of life, death and decay.  Birds and mammals often die out of sight or decay quickly; I can think of only a handful of times I've encountered dead birds (mostly juveniles) in recent weeks. 

A tree dies.  

Perhaps it was weakened by the effort needed to produce a second or even third set of leaves in the face of voracious caterpillars.  Perhaps it was  invaded by borer beetle larvae, which accidentally destroyed the tree's vascular system as they tunneled beneath the bark.  Lightning can explode a tree from within, though the damage may be confined to a strip of bark and not fatal.  Often there is no single cause of death, but a confluence of circumstances: a tree weakened by caterpillars is carried off by drought.

Even dead, the tree probably still stands, and its final act is yet to come. But whether the tree falls when it dies or years later, a gap has opened in the forest canopy, and sunlight streams to the forest floor.

The trees around us are in a long, slow-motion race for light.  Indeed, the whole purpose of a tree's trunk** is to hold the leaves out of the shadows of neighbors.  Sunlight is the power source for making food (carbohydrates), in the process called photosynthesis.  The Car Talk guys used to remind us that, if the engine won't start, it can only be three things: fuel, air or spark.  Likewise, the basic ingredients of photosynthesis are carbon dioxide (pretty abundant in the air), water (sometimes less available) and light.  Photosynthesis, because it allows plants to grow, also feeds those--animals like us, fungi, most bacteria, and a host of other tiny critters--who cannot make food for themselves.  Nearly every food chain on earth's surface begins with a plant or an alga.  Plants are the living world's "producers"****--everyone else is a "consumer."

That hole in the canopy represents an unused energy resource.  Who will come to use it?  There are understory trees just marking time, growing slowly in the shade of their betters, and waiting for the starting gun and their own chance at "life at the top."  But the dead tree's large neighbors are also able to fill it, and may have the best chance.  

Of course, trees have neither minds nor intentions, their lives have evolved to be fully automatic.  The tree does not "try" to reach or fill the canopy gap.  Leaves in sunlight make food and grow; it is, therefore, the food-rich twigs near the sunny gap that will grow fastest, filling that gap. 

Whether by canopy neighbors or understory trees, the gap is finally filled, "waste" of sunlight reduced, and the canopy restored.  

Until the next death.

This juggernaut nearly took out a neighbor as it fell
Such vulnerability is one of the hazards of the rooted life.

Death comes to the young (like this white pine sapling) 
as well as the old (like this much larger white pine).

It looks as if the gaps created by these deaths are still filling in.

How many years have these corpses lain here?

It's a little unusual for a tree to break in the middle like this.
I couldn't confirm either of my suspicions: insect damage or lightning.

From death, new life: dead black oak with stump sprout.

 Sometimes doom is written early: white pine seedlings have sprouted
in a thin layer of duff atop a boulder.

But surprising things can happen:
this tree also rooted on a boulder, but is now full sized.
On the other hand, with so little root area, isn't it vulnerable to wind?

What happens when my twin dies?  If you're a tree, perhaps nothing!

A dead tree a few inches in diameter with a stump sprout is one sign of a chestnut tree (Castanea dentata) facing another generation of death by chestnut blight (Cryphonectria parasitica). From its accidental arrival in the US in 1904, the fungus nearly wiped out this important native tree over its entire range in a few decades.  Its spores invade the trunk through splits in the maturing bark, and and grow to girdle the tree and kill it.  Fortunately, chestnut grows stump-sprouts, each sprout maturing until it, too, is attacked and killed.  Thus the species barely hangs on, decade after decade, seldom producing nuts.  Confirm the identity by looking for long, tapered leaves with large, curving teeth.

Another chestnut.

Here is a tall, full white pine.  But a cancer grows lower down: the wood of the lowest few yards of the trunk in almost completely rotted out.  The tree is half girdled, but the bark edges have healed.  Even so, even a moderate wind from the wrong direction may fell it.
The full crown.

A stripe of rot is begins quite high up.

The remaining bark has healed somewhat,
but the damage to the tree's strength is irreparable.

This "standing dead" tree won't be for long.

*Mainly as pollen and fruit or seeds carried by wind or animals.  Only the adults mainly stay put.

**Vines cheat: by climbing other trees they needn't spend the resources for a strong trunk of their own.  But because they cannot grow taller than their supports, they can't entirely "win" the race, but only "draw."  

***Extremely Cool Free-floating Factoids: Lignin, the key ingredient in stiffening wood, is a complex polymer with building blocks that can be cross-linked in random ways, making it very difficult for fungi and bacteria to break it down.  The evolution of lignin in the Devonian is likely at least partly responsible for the vast coal deposits of the Carboniferous Period, in which millions of years of forest growth were buried without decomposing.  Because these ancient trees pulled carbon dioxide out of the air (while freeing oxygen), and stored it in resistant lignin, enormous insects evolved taking advantage of the high O2, while the low CO2 may have been a cause of a major ice age (via reverse greenhouse effect) in the Devonian.  The coal-forming period came to an end about 300 million years ago, probably with the evolution of white rot fungi with new enzymes able to digest lignin. 

 ****Producers make their own food by assembling biomolecules using an outside energy source, but photosynthesis is only one way to do this.   A variety of tiny Bacteria and Archaea  are able to do "chemosynthesis," that is, use (non-biological) chemical energy to fuel their food production.  Some use hydrogen sulfide given off by deep-sea vents, others use hydrogen from the breakdown of rocks deep in the earth.  Because the study of these creatures is relatively recent, we don't yet know how prevalent they are, or how much of the biosphere depends on them for food.  They are, however, EXTREMELY COOL and found in such unlikely places and inhospitable conditions that their existence fuels much optimism about the existence of life on other planets!

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