Every year in Iowa, ornamental woody plants, trees and shrubs, die without showing any glaring and obvious causes. In addition, insects and diseases cause their share of decline and mortality in trees.
In some cases, if the trees are stressed or if they have been wounded (a major cause of tree stress), they may be more susceptible to damage caused by insects and diseases. Stresses in trees may be caused by natural factors and conditions or through the activities of man or animals. These factors may be chronic (recurring and lasting for a long time) or acute (immediate impact). Examples of chronic damage are wet soils caused by site selection, soil compaction, or poor nutrition; acute damage includes flooding, freezing conditions, severe construction damage, and deer browsing.
Causes of stress in trees and shrubs:
- Environmental: nutrient deficiency, drought, wind, flooding, freezing, sun scorch
- Man-caused: pollution, mechanical, soil compaction, excess water, excess fertilizer, improper pruning
- Animal: nematodes, insects, birds, deer, rabbits, mice
- Other organisms: viruses, fungi, vines, weeds, bacteria, mycoplasmas
Tree stresses may be very dramatic and obvious or in many cases not easily observed or recognized. Obvious stresses may include basal damage or storm damage; stresses from grade changes, soil compaction or pollution are not very visible.
Trees often do not display immediate responses to stresses because of their accumulated growth habit. However, with stresses come several changes within the tree depending on the damage caused by the stress. In some cases, the process of photosynthesis, which is the primary supply of carbohydrates for all tree functions, is reduced and the tree’s stored food reserves are depleted. When root systems are damaged by construction damage, compaction, or poor drainage, they cannot supply adequate water and nutrients for the trees growth and survival. When this happens, often the tree is unable to produce sufficient carbohydrates and growth regulating chemicals. When trunks or stems are damaged, the carbohydrates movement to where it is needed for growth and function is stopped, and may result in death of roots or other growing points of the tree. The end result of these reduced processes is that the tree at best, operates at less than peak efficiency and in many cases it begins a downward spiral of all of its growth functions.
As stresses continue, the tree does eventually exhibit external symptoms. Annual incremental growth is reduced and becomes significantly less than normal. Leaves may be fewer in number and smaller in size. Sometimes, the tree produces excess fruit or seed as a survival mechanism. The tree may exhibit summer scorch symptoms because of insufficient water provided to the leaves during weather. With continued stresses, branches begin to die, and at the same time the root system of the tree is reduced because the crown is producing inadequate food for good root expansion and growth. These processed continue into a downward spiral, usually resulting in the continued decline and eventual death of the tree over a period of 2-15 years. In most cases, once the tree has tipped the balance of not providing sufficient carbohydrates for continued growth of the tree, it cannot recover.
If the physical stresses do not kill the tree, it will often be exposed to more stresses through opportunistic diseases and insect attacks. These biotic attacks may speed up and/or complete the demise of the tree.
Much of the survival, growth and health of our woody vegetation in our landscapes is dependent on the homeowner/manager working to prevent stress and provide the optimal growing environment of the tree. This may begin with plant selection to ensure that the selected plant will perform well on the specific site and soil. Avoid injuries to trees and their expansive root systems during construction or when working around trees. Don’t over fertilize trees; excessive leaf production often results in moisture stress during hot dry periods in Iowa. Avoid basal damage to tree trunks because this is the direct connect from the roots to the leaf tissue and area. Lawnmowers are still one of he major causes of damage to trees. Use proper pruning techniques, and avoid pruning during the spring period of leaf expansion. Use mulches to reduce temperature and moisture extremes. Use caution when using lawn irrigation systems; trees often suffer because of too frequent watering resulting in soils which are too wet for good root growth.