Trees & Shrubs In a Sustainable Landscape
Native trees and shrubs are essential components of a well-planned landscape. This is due to the role they play in the natural ecology, which most people know a little of, but don't often think about.
Consider the shape of a tree. It has a crown, supported by a strong trunk, beneath which spread roots deep into the earth. The roots mimic the shape of the crown. A tree with a broad, high crown has broad, deep roots. A shrub is smaller with more branches and less trunk, but otherwise is similar in shape.
Now consider its parts and what they do. There are leaves, branches and twigs, flowers and fruit, bark, trunk, roots, and sometimes rhizomes (like bamboo and aspen). Leaves convert the sun into food for the tree. They also release excess moisture into the air, sweating to keep the tree cool, just like humans do. (In humans it's called perspiration, in plants it's called transpiration.)
The following sections show what each of the parts of a tree does, for the tree itself and also for its environment. Note that the main benefits to using native trees over non-natives are that they thrive in the existing environment, so they shouldn't need much nurturing, and they attract and support local flora and fauna.
How a Tree Survives
A tree's crown - its branches and leaves - are designed to capture sunlight and convert it into food (photosynthesis). It shades the trunk and upper roots, and the ground beneath, so not too much water evaporates, but is left there for the use of the tree. The leaves soften the fall of rain and the twigs and branches direct it down its trunk to the roots beneath, so it has plenty of water when it needs it.
A tree produces flowers, fruit, and seeds to help it reproduce itself. When the flowers and fruit are no longer needed, they fall to the ground and eventually compost, aiding in feeding plants that grow under it. Those seeds that fall nearby are sheltered by the crown and the compost, which protects them, so they can sprout and grow.
The trunk holds the tree up, growing taller and taller each year to rise above the plants around it and capture more sunlight. It also grows thicker and thicker each year, so the crown has plenty of support under it. The trunk is covered with bark that helps protect its innards. Just inside the bark is the growing layer of the trunk. Everything inside that growing layer is old trunk - previous years' growth. The trunk is the fulcrum part of a tree.
The roots both support and feed the tree the water it needs. They spread out far and wide under the ground (usually), matching the shape of the crown for balance, and growing deep in their search for water.
Water pulled up by the roots plays the same role in a tree as water does in our own bodies - it carries nutrients to each of the cells, and also washes out toxins to be excreted. Whereas our own bodies have three different ways to excrete, though, trees only have one - their leaves, through transpiration.
How a Tree Helps the Environment
These functions keep the tree alive and help it thrive, but they also help the environment around them to thrive, if the tree is one that belongs in that ecology. The tree's crown, providing shade for itself, also provides shade and protection for local animals and birds. The branches provide places for birds to build homes. The flowers and fruits of a native tree provide food for animals, birds, and local insects. And old leaves and twigs that fall to the ground are turned into compost by microbes living in it.
The trunk and its bark provide homes for insects and birds, as parts of it wear out or old branches fall off, or insects chew holes in the trunk. Insects and some animals, like rabbits and beaver, also feed on tender bark or the growing layer just inside the trunk, or on the "sap" (the thick, nutritious water inside). Some animals use bark to strengthen their homes with, including termites, which chew the bark into mush and line their nurseries with it.
The roots dig tunnels in the earth that provide passageways for water to infiltrate and be absorbed. Any woods you find with trees interlacing branches, also have roots interlacing under the ground, making the ground spongy and absorbent.
"The clearest way to the Universe is through a forest wilderness."
— John Muir
Tree roots provide one of the earth's most effective ways of capturing water in the soil, helping to fill up an aquafir after a good rain. Without this network of roots underground, the soil compacts and becomes hard and less absorbent. This is one of the greatest benefits of native woodlands to all of its neighboring life that depend on water.
How a Tree Helps Your Landscape
Of course, all of these benefits to the environment also apply to your own landscape. The crown of the tree will provide homes for the songbirds you love to hear. Its flowers provide beauty. Its fruits (including nuts) provide food for humans, as well as animals. Its leaves provide shade for your house and any sun-sensitive plants you position beneath them. Fallen leaves and twigs provide material for your compost pile.
The trunk of the tree and its broader branches provide a great place for kids and cats to climb. And the thicker branches are great for hanging swings or hammocks. The bark can be peeled off by kids curious about the insects that live under it, providing an interesting learning opportunity. Roots keep your garden soil absorbent, just as they do out in the wild.
Enough trees in an area can also create their own rain, when conditions are right, by the intense evaporation that occurs from heated wet debris, surface water, and the transpiration of leaves (proven by meteorological studies of the Amazon Rainforest and related areas). And healthy trees certainly cool the air, making an otherwise hot area much more pleasant for humans to live in.
One last recommendation. If your area is fire prone, as it is in Southern California, some of your native trees will have evolved to resist or survive fires exceptionally well. Look for them, since they can help protect your landscape. If your area is prone to tornadoes or floods or any other kind of natural weather extreme, there will be trees and shrubs that have adapted to protect themselves better than most in that habitat also. These can be special boons to your landscape, if you let them live under the conditions they are used to.