up specific information about the Monterey pine (Pinus radiata), which is how I learned why it had been particularly suited to where I live. About the same time, I had begun a practice of taking early morning walks in Muir Woods. Both led me metaphorically deeper into the trees.
My wonder of trees keeps growing as I learn more about what they are and do. It has also been learning for the sake of it. Trees seem so ordinary and familiar and unmoving: they just stand wherever they took root and, until we know better, don't seem to be doing anything much. Those with the oldest lineage are members of the conifer family. The conifers do nothing showy—no autumn colors, spring blossoms, or glorious fruit—but when they are noticed and we understand how wonderful they are, a depth and poetic appreciation can result. Out of their wonder and love of the trees they study, naturalists have written about them with poetic sensibility. John Muir, America's most famous and influential naturalist, for example, described a juniper as “a sturdy storm-enduring mountaineer of a tree, living on sunshine and snow, maintaining tough health on his diet for perhaps more than a thousand years” (Muir, My First Summer in the Sierra, 1911, p. 146). Muir's ability to describe what he saw in the high Sierras and Yosemite Valley, to write of the awe he felt in the presence of the ancient redwoods, and to influence others had a significant role in preserving them, including Muir Woods.
In The Tree, a comprehensive book on the subject, the English author and naturalist Colin Tudge compares the building of a beautiful cathedral with how a tree grows, a comparison in which the tree comes out ahead:
[A] cathedral or a mosque is built; it does not grow. Until it is complete it is useless, and probably unstable. It must be held up by scaffold. When it is finished it remains as it was made for as long as it lasts—or until some later architect designs it afresh, and rebuilds. A tree, by contrast, may grow to be tall as a church and yet must be fully functional from the moment it germinates. It must fashion and refashion itself as it grows, for as it increases in size so the stresses alter—the tension and compression on each part. To achieve hugeness and yet be self-building—no scaffold or outside agencies required—and to operate for good measure as an independent living creature through all phases of growth is beyond anything that human engineers have achieved. (2006, p. 75)
What Exactly Is a Tree?
Trees are arboreal perennials: they have a columnar woody stem with branches growing from it. The height varies according to the specific species, environment, and various other factors, though normally they reach a height of twenty feet (six meters) or more. The shape and general development of a tree are so characteristic that the category also includes species of lesser size, such as dwarf trees.
In the delightful way that the English have with words, Colin Tudge begins his answer with what every child knows: “A tree is a big plant with a stick up the middle” (The Tree, p. 3) and proceeds to be eloquent and scientific, a small part of which I paraphrase and pass on here.
Some two to three billion years ago, a layer of vegetation grew upon barren rocks—a slime perhaps no thicker than a coat of paint, made of bacteria, molds, mosses, lichens, algae, and fungi. Chlorophyll in algae made the slime greenish and photosynthesis possible: the energy from sunlight (photons) was used to make sugars and stored by algae. This was the significant first step. Stalks formed slowly, slowly over many, many millions of years, grew from nubbin to matchstick to become ferns that proliferated and grew to enormous size in the Carboniferous period, which began about 350 million years ago. This was a time when giant forests of huge tree ferns covered the Earth. These tree ferns removed prodigious amounts of carbons from poisonous gases, storing it in their leaves and stalks. After millions of more years went by and layer upon layer fell into decay, pressure and time transformed these vast fern forests into coal. Removing carbon dioxide and releasing oxygen, these giant tree fern forests made the air breathable. They also made it possible for more sunlight to reach the surface of the Earth through the clearer air.
The fern forests became the womb and the nursery of the first trees. As John Stewart Collis, another English author phrased it, “In these glades was matured the idea of not falling down” (Collis, The Triumph of the Tree, 1954, p. 10). The ferns rose and fell, over and over again, producing stalks and branches that grew eventually to be the size of trees. In their midst, some 290 million years ago, a more energy-efficient form of plant life, which had woody trunks and branches, appeared. Wood tree trunks are structurally stronger than stalks, and they have roots that anchor the tree in the ground. Tree trunks provide a two-way conduit of water and nutrients from roots to leaves, and from leaves to the whole tree. As a tree grows above the ground, its root structure grows also. In good, deep soil, some species of trees can have as large a circulatory root system below ground as the visible branches and leaves.
The root system of trees continues to have a key role in transforming rock into soil. This process began when the planet was lifeless rock, with a thin layer of algae, mold, lichen, and fungi. Soil is made from rock that disintegrates into dust and releases minerals, plus decaying organic matter, oxygen, and water. Trees draw from and contribute to making more soil. Their roots break up and aerate rock and hard clay. Dropping leaves provide organic matter. Their leaves release water vapor and oxygen into the atmosphere, drip water into the ground below, and provide shade that prevents evaporation. Trees create the conditions for ground-covering plants to grow under them. Tree roots hold the soil down, preventing runoff after rain and keeping strong winds from carrying it away. Trees create watersheds, the source of water to feed streams and rivers. When huge areas of forests are clear-cut for timber or burned down to raise cattle, the ecological systems supported by trees—from roots to leafy canopies—are also destroyed, affecting all forms of life that once thrived there, as well as the quality of the air, soil, and water in the immediate area and far downstream.
Every large tree has an ecosystem of its own, a sphere of influence in its immediate environment. I began to think about this after my Monterey pine was cut down. There were observable consequences, beyond its absence. The resident squirrel got displaced. More direct sun instead of partial sun and shade changed what would thrive in the half dozen terra-cotta planters that I planted with annuals. Direct sun in spring and fall, morning fog in the summer had been ideal for the bright, colorful impatiens that I had planted for years, exchanging them for cyclamen as autumn approached. The tree had also sheltered many plants from the wind, which I next discovered. For the first time, in the absence of shade, I planted sun-loving petunias, which initially grew very fast and had to be watered often. Then came the summer fog, and the petunias became immediately pathetic, the blooms overnight becoming limp and mildewed. A slow-growing vine went into overdrive, sending out waving tendrils by the foot that now needed to be cut back often, before they could cover or strangle nearby rhododendrons. Now unprotected from the direct sun, rhododendron and camellia leaves became sunburnt in unusually hot weather. The side of the hill on which this particular tree had thrived for forty or so years has very poor soil; the dirt is mainly gravel and sand and very hard. Yet the ground cover and established shade-loving flowering plants and a maple tree did well, with virtually no watering. The pine needles had been a water-dripping system. Not just for itself, but also for its tree neighbors. So much so that when I went out to get the morning newspapers, the walk beneath its branches often looked as if it had rained during the night. The pine tree had been the center of an ecologically sustainable little island, which now requires watering.
Invisible to me was the ecosystem underground. Trees are part of a mutually beneficial community in all directions. Trees are a habitat for the plants, insects, birds, and animals in their vicinity, but an even closer bond is formed with the fungi and bacteria that are intimately connected to the metabolism of the tree. They eat the sugars that the tree makes and bind the hydrogen that the tree needs. Bill Mollison, the originator of permaculture, a sustainable ecological design inspired by observing rain forests, described how bacterial colonies on the leaves of trees are carried aloft by the wind high into the clouds, where ice crystals form around them, and as they get heavier and fall, they seed the clouds and cause rain to fall on the trees. The rain that
