Saturday, December 3, 2016

The Life of a Tree: 4. Can't Run, Can't Hide, Only Endure

  Sixty-foot white ash (center) and white oak (right) in a strong wind on July 2, 2014, and today.

One consequence of a rooted life is that endurance is worth a lot.  Regarding a tree bowed under the weight of ice following an ice storm, I have wondered at this.  Some events--this year's severe drought and simultaneous gypsy moth caterpillar outbreak, for example--seem almost unsurvivable.  And in our sixteen winters in this house I have seen the thermometer dip into negative temperatures many times.  Yet the ash tree above that may be decades my senior has stood all that time, without shelter or escape, taking whatever chance has sent its wayAnd still it stands, wounded, bowed, but unbroken--the limits of its tenacious grip on life never once exceeded. 

Can trees live forever?*  Having no fixed size, and growing all their lives has a downside.  Because they must collect sunlight to produce their food, trees grow ever upward in competition with those around them.  But in a large tree the mass of trunk and branches needed to support its leaves in sunlight begins increases faster than the leaf area needed to feed all that tissue.  In effect, a large tree's growth begins to slow as the needs of new wood begin to equal the food produced, and parts must begin to starve and die.  Height itself can also be a problem in the tallest trees, such as redwoods: the force required to bring water upward from roots to treetop has a practical limit, beyond which taller shoots must die.  Both of these problems increase susceptibility to disease and insect damage, making life tenuous.  Most tree species have accepted life spans,** though it is a concept fuzzier than most animal life spans.  And the longest-lived trees, such as bristle-coned pines, are often the slowest-growing.  Finally, the longer a tree lives, the longer it is subjected to the slings and arrows of outrageous fortune, such as lightning or fire.  Eventually something must fell it.

The "energy pyramid" shows how energy enters and moves through most ecosystems.  A small fraction of the 697 Watts per square meter of sunlight that strikes the earth's surface is actually fixed in the sugar produced by photosynthesis by producers.  Animals that eat plants (primary consumers) get a fraction of this, those that eat these animals (secondary cons) a fraction of that, and so on.  Producers (on land, at least) will always outweigh other organisms partly because they have first crack at the energy, and so have the most.

I have always appreciated the out-sized role plants play on Earth (another reason I like them): they along with algae and cyanobacteria in the oceans are the producers for all the rest of life.***  Only they can capture sunlight and bring that energy into ecosystems where it becomes available to us consumers--everybody from germs to people.  Science textbooks sometimes treat the photosynthesis that plants do as part of a "cycle" that we complement by our respiration: breaking down the food we eat back into the CO2 and water that are the raw materials from which sugars are made.  This makes it seem that plants need us as much as we need them.  But this is a false conceit: CO2 and water are common substances that are belched out by volcanoes as readily as by living things, while only plants make food and oxygen in any significant amounts.  I tell students that we need plants very much more than they need the animals, fungi, and microbes that depend on them.

Plants define their ecosystems.  A biome --the largest ecological unit, such as rainforest, or desert--is defined by its climate and vegetation.  The plants of any terrestrial biome or ecosystem far outweigh all the other life in them. 

One thing I like best about plants is their abundance, diversity and approachability: I don't have to go through the gymnastics that zoologists sometimes do to simply find the animals they want to study.  On my walks around the neighborhood I greet the trees just the way I would neighbors on their porches, always right where they're supposed to be.

Here ends my essay, The Life of a Tree.

*An interesting side issue is that "lifespan"  is a meaningless concept in organisms such as bacteria that do not necessarily ever die, or species in which the individual is hard to define.  Trees such as aspens fit here: since new trees may sprout from the roots of older trees, a whole grove may consist of genetically identical individuals.  (Plants were expert at cloning before it ever occurred to humans.)

**Life spans in the hundreds of years are often given for longer-lived trees such as white and black oaks, but most typically don't exceed the human life span--perhaps because time and chance happeneth to all things.

***The idea that plants & algae (photosynthesizers) are Earth's only producers is actually too broad a generalization.  A wide variety of bacteria and archaea produce food using inorganic chemical energy (called chemosynthesis), rather than sunlight.  For example, the life surrounding deep sea hydrothermal vents rely on bacteria that extract energy from hydrogen sulfide in hot water flowing from these vents.  These bacteria support their own ecosystems in the pitch darkness at the bottom of the sea, without any reliance on light or photosynthesis.  Other chemosynthetic bacteria have been discovered in even more unlikely places--including inside rock deep in mines.  It remains an open question how much of Earth's life is supported not by photosynthesis, but by chemosynthesis.  But an intriguing hypothesis gaining evidence is that life first arose on earth in such a place, rather than in"some warm little pond" as traditionally assumed.

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