Drought and Plant Stress

Another shower, another eight-inch of water that makes no difference, and the drought goes on.  We in Massachusetts have had four continuous months of low rainfall.  In June, at our nearest recording station, less than two inches of rain fell altogether, and only two days brought a quarter-inch or more.  July was as bad or worse, with just over two inches, and no day with even half-an-inch, never reaching more than the top-most inch-or-so of soil.   Three weeks ago the state declared a drought watch; Worcester's reservoirs are down a third or more, Brockton draws increasingly on a desalinization plant on the Taunton River to help preserve the main reservoir of Silver Lake.  Walking raises dust--even crossing the lawn--groundwater recedes, plants are stressed.  

I have watched for some time for signs of drought stress in the trees.  Trees and most shrubs are not yet in obvious trouble, but there are exceptions. 

 European buckthorn, an alien invasive, is wilting even on a drizzly, overcast day.

 Wild grape, Vitus labrusca, wilts in sunshine, but has recovered turgor in the overcast.

 Gray dogwood, Cornus racemosa, a thicket-forming shrub, is wilted all over my little wood.

 Garlic mustard, a prolific alien invasive, is suffering; but its a tough plant and will recover.

 Enchanter's nightshade, Circaea quadrisulcata (above), and especially heart-leaved aster, Aster divericatus (below) are looking poorly.

 The cucumbers in my garden have wilted nearly every day--even when watered
--but they recover out of direct sun. 

 White ash, Fraxinus americana (above), and 

ash-leaved maple, Acer negundo (below), have leaves curling with wilt.

 Of the three paper birches I saw this day, only this one was losing leaves prematurely.

The same stretch of Salisbury Brook in early May above, and mid-July below.

Like all living things, plants are critically dependent on water.  With carbon dioxide from the air, water is a raw material for photosynthesis: the building of sugar molecules (and many other compounds) using the energy in sunlight.  Photosynthesis is a trick that every other creature around you, from mushroom to man,  depends on for life: it is the ultimate source* of all our food and oxygen.  

Of the enormous amount of water consumed by plants, only a tiny fraction is "split" into hydrogen, for making sugar, and "waste" oxygen; most is used to maintain turgor pressure in soft tissues, to expand their cells as they grow, and to transport minerals upwards in the plant.  If you've ever marveled at how soft a football is when deflated, you will understand turgor pressure: just as the pressure of air can turn flabby pigskin into a nearly-rigid football, the pressure of water does the same for plants.  Typical plants first draw water into their roots by maintaining a higher concentration of dissolved substances in their root cells than are present in soil water (osmosis).  From these root cells, water is drawn upward in the tube-like vascular tissue of the stems in a kind of tug-of-war by evaporation of water from leaf cells.  (These tubes--"vessels" in flowering plants and "tracheids" in conifers-- are small enough that the attraction of water molecules for each other causes them to act as if the water were in "strands" that could be pulled upward.)  Plants are forced to lose water at high rates by their need for carbon dioxide from the air: the same pores (stomata) in leaves that allow carbon dioxide to diffuse in and oxygen to diffuse out, also allow water to evaporate from cell surfaces and difuse away.  (The evaporation of water from plant surfaces is called transpiration.)

Bad news for plants begins when water is not entering the roots quite fast enough to replace that evaporating from leaves.  Plants respond to this by closing their stomata: these pores (usually in the undersides of leaves) are flanked by banana-shaped "guard cells" that are spring-loaded to close automatically as their turgor pressure declines.  Since the closing of stomata reduces water loss, you may not see any change in the plant at this point, but the plant is feeling it.  With stomata closed, carbon dioxide  becomes scarce inside the leaf tissue, so photosynthesis declines.  Then a funny thing happens--called photorespiration--yet another fact that argues against living things being "designed".  

In the first step in building a sugar molecule by photosynthesis, an enzyme called ribulose 1,5-bisphosphate carboxylase** ("rubisco" for short) grabs a carbon dioxide molecule.  The rubisco enzyme has a peculiar trait: it has a small tendency to grab oxygen (O2) instead of CO2, probably because the enzyme has a hard time telling these compounds apart.  As long as the stomata are open, there will be a good supply of CO2 inside the leaf, so that photosynthesis usually occurs normally.  But with stomata closed and CO2 scarce, more and more often rubisco will grab oxygen and engage in photorespiration--a sort of "reverse photosynthesis" that is very wasteful of energy and creates nothing good for the plant.  In dry conditions under bright sun, it is possible for a plant to waste by photorespiration much of the sugar it already fixed.  My guess is that many even of those plants that are not visibly wilting are photorespiring*** furiously in the bright sun, and at best marking time until there is enough moisture in the soil to go back to the business of making the food our local ecosystems depend on.

Micrograph of the underside of a fern leaf, showing seven stomata.  Besides the flanking guard cells, some of the choroplasts that do the work of photosynthesis are visible as tiny green hockey pucks.

Source: https://www.flickr.com/photos/epingchris/5152579683

Micrograph of cross-section through a privet leaf shows different cell types (mesophyll) that do photosynthesis, the air spaces in the leaf, the vein that brings water and minerals, and stomata.

Source: http://researchguides.library.vanderbilt.edu/c.php?g=69346&p=809936

*Our understanding of another of way of making food--extracting energy by chemosynthesis from inorganic chemicals such as the hydrogen sulfide (think ocean floor "black smokers") as certain bacteria can--is still evolving.  Some believe chemosynthesis is as important a food source for life as photosynthesis, and it may well have preceded photosynthesis in life's early history.

**Rubisco is fairly slow enzyme and so plant chloroplasts need them in high concentration--making it possibly the most abundant protein on Earth, and responsible for "fixing" more than 100 billion tons of atmospheric carbon dioxide each year.

***A small fraction of plant species have solved this problem.  They have evolved a way of isolating their rubisco from the air and using a powerful added enzyme (PEP carboxylase) to scavenge CO2 even at low atmospheric concentrations to provide rubisco with a high CO2 levels even with stomata closed.  Such plants can do better in dry conditions, and also in dense stands of plants that have drawn down the CO2 concentration in the air around them.  Among these C4 plants are corn and  crabgrass.

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!