Extension Communications Extension 4-H Youth Building Ames, Iowa 50011-3630 (515) 294-9915

10/25/04

Chris Feeley, Forestry Extension, (515) 294-6839, cfeeley@iastate.edu
Del Marks, Continuing Education and Communication Services, (515) 294-9807, delmarks@iastate.edu

Garden column for the week of Oct. 29, 2004

Why Trees Die

By Chris Feeley
Forestry Extension
Iowa State University

If someone gave you a plant to take care of, wouldn't you try your best to keep that plant alive? That would require research to find out what basic elements that plant needs to not only live, but also thrive.

I get asked a simple question nearly everyday, "why did my tree die"? Unfortunately, it is a simple question that can be very difficult to answer. In most cases, there are numerous factors that contribute to the decline and eventual death of a tree.

Trees, like all other living organisms, eventually grow old and die. For some trees, death may come suddenly from a lightening strike or being uprooted during one of Iowa's windy days. However, for most trees, death is preceded by a period of natural decline.

Many trees have a fairly predictable life span. For example, a forester might expect an aspen tree planted on a good bottomland site to live about seventy years. Over that time period, flooding, decay fungi, leaf fungi, and the occasional insect will contribute to the slow decline and eventually death of the tree. A bur oak, on the other hand, can live much longer. A bur oak planted properly on a good well-drained soil, would have a life expectancy well over a hundred years.

What we do know is that as trees age they weaken and do not grow as vigorously. Older trees do not photosynthesize as much and therefore produce less food (carbohydrates). We also know that trees planted in urban areas have many more stress factors that contribute to the decline of the tree. For example, trees in the urban environment are constantly recovering from damage caused by lawn mowers, weed whips, construction damage, grade changes, deicing salts and animal damage (rabbits, squirrels and deer).

These extra "stresses" found in the urban environment hinder the movement of water and nutrients and lessen the food production, eventually causing the tree to decline. Declining trees exhibit abnormal growth patterns such as early loss of leaves or needles, abnormal leaf shapes, sparse foliage, little to no growth, and branch die-back. Once the tree starts to decline, it is more vulnerable to additional problems such as diseases and insects that benefit from trees under stress.

What needs to be done to help prevent a tree from declining? First and most importantly, the tree should match the planting site. Many tree species have specific site requirements for growth. Some of these site characteristics include soil texture/drainage, soil pH, and adequate space for the roots and crown to expand.

Consider the location of utilities, existing vegetation, sidewalks, and roads that might restrict tree growth before selecting and planting the tree species. For instance, a Japanese tree lilac would be well-suited for a boulevard that has eight to ten feet between the curb and the sidewalk. On the other hand, that would not be enough space for a silver maple to grow and stay healthy for many years.

Keep in mind that the Japanese tree lilac planted on the site above still has numerous stress factors that can influence the tree's health. The area between the curb and sidewalk has a higher soil temperature, more reflected heat from the road, soil compaction, and a greater chance of deicing salt damage.

What can be done to limit the stress factors that contribute to the decline of trees in the urban forest? Mulching. The simple act of adding organic mulch around your tree prevents lawnmower and weed whip damage, limits temperature extremes, conserves moisture, and adds nutrients back into the soil. Other practices such as proper pruning, removing dead branches, watering, and avoiding construction near the tree can help ensure a healthier, happier tree.

There are a few diseases and insects that can invade a healthy tree. Fortunately, there are very few in Iowa that cause severe damage. When in doubt as to why your tree is declining, consult with a tree professional, and contact your local Iowa State University County Extension Office for publications on tree care. Many of these publications can be downloaded for free at http://www.forestry.iastate.edu/ext/pubs.html.

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ml: isugarden

Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 376K and the bottom picture's fullsize photo is 208K. Caption: Trees planted in urban areas have many more stress factors that contribute to the decline of the tree.
Caption: A good site for a tree to grow is good soil texture/drainage and adequate space for the roots and crown to expand.

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