The Suburban Ecologist, Part II: Tree Selection

Most tree deaths result from people interfering with trees’ natural ability to survive.  Even under the foulest suburban and urban conditions, trees often thrive and reproduce.  Consider volunteer trees in fencerows: often our best attempts to remove them fail, and they continue to come back year after year.  Why is it then that I often see dead trees in people’s yards?  With young trees, this is most common.  However, many trees that have become established for decades die suddenly.  As a long and persistent illness that has gone untreated, these deaths were predicated by harsh conditions that have existed since the trees’ first two leaves unfolded.

Homeowners often feel helpless to protect and preserve their trees.  In absence of basic tree knowledge, they often consider tree deaths random and causeless.  They become frustrated and assume that nature is cruel and unpredictable.  Nothing could be farther from the truth.  Homeowners, builders and landscapers cause more tree deaths in urban areas than storms, pests, and diseases combined.  Healthy trees seldom die in storms, and trees have hearty natural resistance to pests and disease.  The chemicals that trees produce to resist pestilence are so potent, that tree extracts are sometimes used as medicine for humans, such as oak gall extract and witch hazel.  Before a tree dies in a storm or succumbs to pests or disease, it usually must suffer years of abuse which weaken it structurally and reduce its ability to fight off attackers.

People who plant trees have a vested interest in tree survival.  It stands to reason that if people knew how to take care of their trees, they would prevent many of these deaths.  I have good news.  Preserving the health and stability of a tree is easier than imagined.  We have the power to drastically reduce tree deaths in suburban areas by instituting a few simple practices.

We introduce trees to a domestic landscape in two ways.  The best way to have trees around a new house is to preserve trees that already exist on the property.  This eliminates the need for choosing the right species, since established trees are obviously suitable to the local climate and pests.  Preserving trees during construction is a challenge, which requires a basic understanding of the way trees grow underground.  Half of tree roots grow within the top six inches of soil.  The image of a massive taproot that extends deep into the ground is usually false.  The primary structural support of trees underground is lateral, and roots usually extend far beyond the edges of the tree canopy above.  The reason for the common root structure is simple.  

First, most tree nutrients (decaying plant material) filter through the soil from the surface of the ground.  The more surface area the trees can access, the more access they have to the mineral building blocks of their cells.  Second, tree roots need air.  In fact, around half of the volume of healthy soil is empty spaces filled with air or water.  The deeper roots travel, the less access they have to life-giving air.

The key to maintaining healthy trees during construction is preserving the roots.  If possible, install stakes and caution tape at the edge of the tree canopy.  Heavy traffic causes compaction of the soil, which reduces the empty spaces and suffocates the roots.  Machinery and trucks driving over roots is even worse.  Usually, protecting the entire area under a tree canopy is not possible.  In areas where people or equipment must pass over root systems, designate one path to avoid compaction of the surrounding areas.  If frequent traffic close to a tree is necessary, place plywood over the tree roots to spread out the impact of feet and tires.

While trimming back some roots will not kill a tree, cutting roots should be avoided.  Landscapers must be instructed not to cut roots when they trench for irrigation.  Professionals have access to air spades, which remove soil without removing roots.  Furthermore, irrigation too close to tree trunks can cause them to rot.  Above all, do not raise or lower the level of the soil next to a tree.  To make nearby ground level changes, build retaining walls.  Even if building a wall cuts some roots, the tree has a far better chance of survival than if it is buried under even an inch of additional soil.  Established trees frequently die three to five years after new construction due to compaction and ground level changes.

The second way to incorporate trees into a landscape is to plant them.  Choose the right type of tree for each site, and incorporate more than one tree family in every landscape.  Pests and diseases that affect elms are usually different from diseases that affect oaks, and hazards to oaks often do not apply to willows, pecans, or birches.  The best and simplest way to make a choice is to look up the tree species native to your area.  For instance, the tree species native to Allen, Texas, are Shumard red oak and bur oak (Quercus shumardii, Q. macrocarpa), American and cedar elm (Ulmus americana, U. crassifolia), pecan and black walnut (Carya illinoinensis, Juglans nigra).  The above tree pairs are grouped by family, so choose only one from each family until a great variety of tree families is achieved.  Also look for Texas persimmons (Diospyros virginiana), sycamores (Platanus occidentalis), redbuds (Cercis canadensis) and eastern red cedars (Juniperus virginiana).  Another native tree is the red mulberry (Morus rubra), not to be confused with the fruitless mulberry (M. alba), an invasive ornamental from China.  While not native, the baldcypress (Taxodium distichum) is also a tolerable choice.  Many other tree species, such as hackberry (Celtis laevigata), red buckeye (Aesculus pavia), black locust (Robinia pseudoacacia), and willow (Salix nigra) are native but difficult to plant or unavailable from nurseries.  If you find them, leave them alone and they will thrive.

Use caution with one native tree, the cottonwood (Populus deltoides).  While it is a fast growing and beautiful tree, it sheds large branches in the wind.  Never allow a cottonwood to remain within fifty feet of a house or other structure, and never stand under one when it is windy.  This tree is unique, and should not be confused with other tree types.  Under most windy conditions, native trees with proper upkeep are perfectly safe.

In Texas, the easiest way to find your native tree species is go to tpwd.texas.gov and click on your ecoregion.  In other places, try various searches such as “<county name> native trees,” “<state name> native trees,” or find your ecoregion using “<state name> ecoregion” and “<ecoregion name> native trees.”

Noticeably absent from the native tree list for Allen, Texas, is the live oak (Q. virginiana), which is the most popular choice of builders and landscapers across North Texas, or the silver leaf maple (Acer succharinum), another tree often found in front yards.  These trees are prime examples of the importance of selecting native trees.  At the slightest hint of ice or snow, streets lined with beautiful arching live oaks become piles of snapped branches bordered by shattered trunks.  Live oaks are coastal trees that never lose their leaves, and the added weight of ice or snow literally destroys them.  The bigger they are, the harder they fall, crushing houses, cars, blocking roadways, and snapping power lines.  The danger with silver leaf maples is more insidious.  These trees will grow for up to thirty years, but the shortened winter season in north Texas do not allow a long enough period of recovery for the long summers.  They invariably rot from within and will never reach full size or maturity.

The Bradford pear (Pyrus calleryana) also deserves special mention.  Prized for their lollypop shape and snowy flowers, this ornamental tree imported from China is a great gift for your enemies.  Unless carefully pruned from a small size, it will develop rot in the center where the branches divide, drawing ants and eventually splitting the tree.  I have never seen one live beyond thirty years.  They quickly outgrow their cute lollypop shape and transform into a top heavy vee that sheds large branches with zero provocation.  This tree is such a hazard I recommend cutting them to the ground and replacing them immediately, since the cost of removing them increases with size.  Furthermore, they have become an invasive species in some parts of the US.* Along with live oaks, they are the most common tree choice of builders in this area.

Just as most builders and landscapers perpetuate horrible tree selection, many landscapers and tree trimmers practice the worst imaginable tree care.  This will be the topic of the next article.  For now, I hope that this information has removed some of the uncertainty about tree deaths.  Many tree deaths and expensive property damage are preventable through proper tree selection.  Please share what you have learned to your friends or to the world by just planting trees.  We must all take responsibility for the environment around us, or senseless tree deaths will continue.

This is the second installment (See The Suburban Ecologist, Part I: Albedo) of a series of articles on better environmental practices in the suburbs of America.  Soon to come, “The Suburban Ecologist, Part III: Tree Care.”  Feel free to click the Follow icon in the lower right corner of your screen and enter your email to receive a notification when I post a new article.  The International Society of Arboriculture has free podcasts on tree selection and much more.  If you are interested in delving deeper into the science behind tree care, the Arborists’ Certification Study Guide by Sharon J. Lilly is a user-friendly compendium of tree knowledge.

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