A white oak tree |
By Toby Hemenway, Gaia's Garden: A Guide to Home-Scale Permaculture, 2009
[The following is taken from pages 120-123].
As
I’ve said, when we look at a plant, we often see it as doing one thing. Take the hypothetical white oak I
referred to above. Some homeowner
placed that tree in the backyard to create a shady spot. But even this single tree, isolated in
a lawn, is giving a rich performance, not simply acting as a leafy
umbrella. Let’s watch this oak
tree to see what it’s doing.
It
is dawn. The first rays of
sunlight strike the canopy of the oak, but most of the energy in these beams is
consumed in evaporating dew on the leaves. Only after the leaves are dry does the sunlight warm the air
within the tree. Above the oak, however, the air has begun to heat, and a cloud
of just-awakened insects swirls here.
Below the canopy, it’s still too chilly for the bugs to venture
out. They roil in a narrow band
within the thin layer of warm air above the tree. Together the sun and the oak have created inset habitat, and
with it, a place for birds, who quickly swoop to feast on the swarm of bugs.
In
the cool shades of this tree, snow remains late into the spring, long after
unprotected snow has melted. Soil near the tree stays moist, watering both the
oak and nearby plantings and helping to keep a nearby creek flowing. (Early miners in the West frequently
reported creeks disappearing once they’d cut nearby forests for mine timbers.)
Soon
the sun warms the humid, night-chilled air within the tree. The entrapped air dries, its moisture
escaping to the sky to help form clouds.
This lost moisture is quickly replace by transpiring leaves, which pull
water up from roots and exhale it through puffy-lipped pores in the leaves,
called stomata. Groundwater,
whether polluted or clean, is filtered by the tree and exits through the leaves
as pure water. So trees are
excellent water purifiers, and active ones. A full-grown tree can transpire 2,000 gallons of water on a
hot, dry day. But this moisture
doesn’t just go away—it soon returns as rain. Up to half the rainfall over forested land comes from the
trees themselves. (The rest
arrives as evaporated from bodies of water.) Cut the trees, and downwind rain
disappears.
Sun
striking the leaves ignited the engines of photosynthesis, and from these green
factories oxygen streams into the air. But more benefits exist. To build sugars and the other
carbon-based molecules that provide fuel and structure for the tree, the leaves
remove carbon dioxide from the air. This is how trees help reduce the level of
greenhouse gases.
As
the leaves absorb sunlight and warm the air within the tree, this hot, moist
air rises and mixes with the drier, cool air above. Convection currents begin
to churn, and morning breezes begin. So trees help create cooling winds above
them.
Closer
to the ground, trees block the wind and make excellent windbreakers. Wind streaming past a warm building can
carry off a lot of heat; so one or more trees on a house’s windward side will
substantially reduce heating bills.
The
oak’s upper branches toss in the morning breeze, while down below the air is
still. The tree has captured the
energetic movement of the air and converted it into its own motion. Where does this energy go? Some scientists think that captured
energy is converted into the woody tissue of the tree, helping to build tough
but flexible cells.
The
morning breeze carries dust from the plowed fields of nearby farmland, which
collects on the oak leaves. A
single tree may have ten to thirty acres of leaf surface, all able to draw dust
and pollutants from the air. Air passing through the tree is thus purified—and
humidified as well. As air passes
through the tree, it picks up moisture exhaled from the leaves, a light burden
of pollen grains, a fine mist of small molecules produced by the tree, some
bacteria, and fungal spores.
Some
of those spores have landed below the tree, spawning several species of fungus
that grow symbiotically amid the roots, secreting nutrients and antibiotics
that feed and protect the tree. A
vole has tunneled into the soft earth beneath the tree in search of some of
this fungus. Later this vole will leave manure pellets near other oaks,
inoculating them with the beneficial fungus—that is, if the owl who regularly
frequents this oak doesn’t snatch up the vole first.
This
tree’s ancestors provided Native Americans with flour made from acorns, though
most contemporary people wouldn’t consider this use. Now, blue jays and squirrels frolic in the oak, snatching
acorns and hiding them around this and neighboring yards. Some of these acorns, forgotten, will
sprout and grow into new trees. Meanwhile, the animals’ diggings and droppings
improve the soil. Birds probe the
bark for insects, and yet other birds and insects depend on the inconspicuous
flowers for food.
Later
in the day, clouds (half of them created by trees, remember) begin to build.
Rain droplets readily form around bacteria, pollen, and other microscopic
debris lofted from the oak. These
small particles provide the nucleation sites that raindrops need to form. Thus,
trees act as cloud seeders to bring rain.
As
the rain falls, the droplets smack against the oak leaves and spread out into a
fine film, coating the entire tree (all ten to thirty acres of leaves, plus the
branches and trunk) before much rain strikes the ground. This thin film begins to evaporate even
as the rain falls, further delaying any through-fall. Mosses and lichen on this old oak soak up evermore of the
rain. We’ve all seen dry patches beneath trees after rain: A mature tree can
absorb over a quarter inch of rain before any reaches the earth, even more if
the air is dry and the rain is light.
The
leaves and branches act as a funnel, channeling much of the rain to the trunk
and toward the root zone of the tree. Soil close to the trunk receives two to
ten times as much rain as that in open ground. And the tree’s shade slows evaporation, preserving this
moisture.
As
the rain continues, droplets leak off the leaves and splatter on the
ground. Because this tree-drip has
lost most of the energy it gathered during its fall from the clouds, little
soil erodes beneath the tree. Leaf litter and roots also help hold the soil in
place. Trees are supreme
erosion-control systems.
The
water falling from the leaves is very different from what fell from the
sky. Its passage through the tree
transmutes it into a rich soup, laden with pollen, dust, bird and insect
droppings, bacteria and fungi collected by the leaves, and many chemicals and
nutrients secreted by the tree.
This nutritious broth both nourishes the soil beneath the tree and
inoculates the leaf litter and earth with soil-decomposing organisms. In this way, the tree collects
and prepares its own fertilizer solution.
The
rain eases toward sundown, and the sky clears. The upper leaves of the tree
begin to chill as night falls, and cold air drains down from the canopy,
cooling the trunk and soil. But
this chill is countered by heat rising from the day-warmed earth, which warms
the air under the tree. The leafy canopy holds this heat, preserving it from
escaping to the night sky. So
nighttime temperatures are warmer beneath the tree than in the open.
The
leaves, however, radiate their heat to the sky and become quite cold, often
much colder than the air. All these cold surfaces condense moisture from the
air, and the resulting dew drips from the leaves and wet ground, watering the
tree and surrounding plants. Leaves can also gather moisture from fog: On foggy
days the mist collects in such volume that droplets trickle steadily from the
leaves. On arid but foggy coasts,
tree-harvested precipitation can triple the average rainfall. By harvesting dew and fog, trees can
boost available moisture to far beyond what a rain gauge indicates.
As
we gaze at this huge oak, remember that we’re barely seeing half of it. At least 50 percent of this tree’s mass
is below the ground. The roots may
extend tens of feet down, and horizontally can range far beyond the span of the
tree’s branches. We’ve already learned how these roots loosen and aerate soil,
build humus as they grow and die, etch minerals free from rocks with mild acid
secretions, and with sugary exudates provide food for hundreds or even
thousands of species of soil organisms that live with them.
Roots
gather nutrients from deep in the ground, and the tree uses them to fashion leaves.
When these leaves drop in the fall, the carbon and minerals collected from the
immense volume of air and earth around the tree are concentrated into a thin
layer of mulch. Thus, the tree has
harvested a diffuse dusting of useful nutrients, once sprinkled into thousands
of cubic yards of soil and air, and packed them into a rich, dense
agglutination of topsoil. In this
way, trees mine and concentrate the sparse ores that surround them to build
fertility and wealth. This wealth
is shared with many other species, which root and burrow, feed and build, all
nourished by the tree’s gatherings.
But
there is more: This tree’s roots have threaded toward those of nearby oak trees
and fused with them. A tree’s roots, researchers have shown, can graft with those
of its kind nearby, exchanging nutrients and even notifying each other of
insect attacks. Chemical signals
released to secrete protective compounds that will repulse the soon-to-invade
bugs. If an oak has grafted to its
neighbors, does it remain an individual tree? Perhaps trees in a forest are more like branches from a
single subterranean “tree” than a group of individuals. One of the largest organisms in the
world is a forest of aspen trees that is in fact a single individual. Above ground, it looks like a grove of
separate trees, but beneath the surface, they are all connected via their
entwined roots. Each of these
aspen trees is genetically identical.
The
ways in which a single tree interacts with other species and its environment,
then, are many. I’ve barely
mentioned the swarms of insects that this oak supports: gall wasps and their
hymenopteran relatives, beetles that tunnel into twigs and bark, and all manner
of sucking and chewing bugs and their many insect predators. Then there are the birds that feed on
these bugs. And we shouldn’t
forget the myriad nearby plants that benefit from the rain and nutrients
collected by this tree.
Through
this tree, we glimpse the benefits f ecological thinking. Instead of viewing a
tree simply as something that looks nice or provide a single offering such as
apples or shade, we can begin to see how deeply connected a tree is to its
surroundings, both living and inanimate.
A tree is a dynamic element embedded in and reacting to an equally
dynamic landscape. It transforms
wind and sunlight into a variety of daily and seasonally changing
microclimates, harvests nutrients, builds soil, pumps and purifies air and
water, creates and concentrates rain, and shelters and feeds wildlife and
microbes. Add to all this the better known benefits for people: fruit or nuts,
shade, climbing and other fun for kids, and the beauty of foliage, flowers, and
form. We start to see how tightly
enmeshed is a simple tree with all the other elements in a landscape. Now we can begin to imagine the
richness of a landscape of many plant species, all interconnected by flows of
energy and nutrients, nurturing and being nourished by animals and microbes
that flap and crawl and tunnel among them.
Each
plant modifies its environment. These changes in turn support or inhibit what
lies nearby, whether living or not.
Recognizing that plants don’t stand alone can radically affect the way
we place the features of our garden.
No comments:
Post a Comment