Management Considerations For

Management Considerations for

Environmental Stress

Howard L. Eyre, Assistant Professor

Delaware Valley College

Doylestown, PA

To manage trees requires an understanding for how trees grow and respond to their site conditions. Fortunately, there are resources available for increasing our understanding for trees, their functions, and their habitats.

Carl Whitcomb, 2006, Establishment and Maintenance of Landscape Plants II,

Lace Bark Inc., Stillwater Ok ISBN: 0-9613109-7-9

Alex Shigo, 1986. A New Tree Biology, Shigo and Tree Associates,

Snohomish, WA ISBN:0-943563-04-6

Alex Shigo, 1998. Modern Arboriculture, Shigo and Tree Associates,

Snohomish, WA ISBN:0-943563-09-7

Lincoln Taiz & Eduardo Zeiger, 2010, Plant Physiology 5th Ed.

Sinauer Associates, Inc. ISBN:978-0-87893-866-7

Carl Whitcomb, in his text, discusses eight processes that must be accomplished by a plant in order for it to function in its environment. They are:

Capture Light Energy – A unique capacity of green plants is being able to gather in the energy from the sun and convert the sun’s energy into chemical energy, carbohydrates, which may be stored and utilized for various plant functions.

Convert Energy to New Cells – Plants will utilize the chemical energy gathered from the photosynthetic process and convert that energy into new cells, a plant that is not growing is dying.

Absorb Water and Elements from the Soil – The tree must utilize some of its energy to absorb the water and elements from the soil environment in which it is growingto allow for the structure and the functions of the tree.

Transport Energy – The tree will make sugars, carbohydrates, primarily in its leaves, but this chemical energy must be moved throughout the plant to get to the areas where that energy will provide for growth and other plant functions.

Metabolize Nitrogen – Nitrogen,an essential building block of proteins, must be absorbed from the soil and bonded to the sugars made through photosynthesis to create the proteins that are essential to all life forms.

Hormone Synthesis – The tree makes hormones or plant growth regulators which will stimulate the tree to respond to its environment within specific parameters and are produced by combining essential elements with basic carbohydrates.

Enzyme Functions – The tree utilizes a variety of enzymes which are utilized to regulate various plant functions. Lacking certain enzymes does not cause the tree to die, but rather requires it to utilize greater quantities of energy to perform the required plant functions.

Respiration – This is the process by which the chemical energy that is created by the photosynthetic process (carbohydrates)to be released to complete all tree functions. Respiration is critical to all plant functions.

Interestingly and independently, Alex Shigo has created a list of eight factors that are required for tree growth:

Energy – The tree must have a source of energy to be able to complete all of its functions. This energy comes from the sun and is converted into chemical energy by the plant.

Space – For a tree to grow, it requires sufficient space for its crown and its roots.

Temperatures – Trees have evolved to grow within a specific range of temperatures. The variable temperatures regulate the plant functions so that they occur in specific sequences.

Water – Water isa raw material in photosynthesis, it transports materials throughout the tree, it is a component of every live cell, and provides for the transpirational cooling of the tree,

Essential Elements – Trees require approximately 20 elements (the quantity changes with research) to be able to efficiency complete all of its life functions.

Genetic Code- Each tree has a genetic code that allows it to respond within certain parameters to the conditions surrounding the tree.

Time – To fully understand how trees respond to their environment requires understanding the time required by the tree to respond to changes within its site.

Concentration of Factors – For the tree to function at its highest potential on any site requires that, not only must the factors for survival be present but also they must be available in correct relative proportions to each other.

Taiz and Zeiger have produced a text on Plant Physiology that allows the tree care professional to more fully understand how plants function at a cellular level.

Stress is the response by a tree to conditions that are less than ideal and can occur from both Biotic and Abiotic sources. It is commonly held that the majority of biotic stressors – insects, fungi, bacteria, etc. - occur secondarily to the abiotic or environmental stress factors. An infestation of biotic pests is often an indicator of the presence of abiotic stress factors.

As an example, the author has a hemlock tree (Tsuga canadensis) on his property that had a major infestation of Hemlock Wooly Adelgid (Adelges tsugae) which is commonly held to kill this tree species. Rather than spraying the pest for control, additional trees were planted around the hemlock so that replacements existed when the tree died. What has occurred is the creation of a small ecosystem which has improved the health of the hemlock and the Adelgid is essentially gone. This small habitat not only improved the soil conditions but also provided other plant species that can support the populations of predators that do control the Adelgid population.

Trees evolved in diverse forest communities and in these communities typically there will be required factors that are either deficient or limiting (Least Limiting Factor). Since this is the common condition, trees have evolved with the capacity to complete their life functions with some limiting conditions. Within any ecosystem there may be air pollution, light limiting factors, moisture problems, soil environmental conditions that are not conducive to growth, extremes of weather, and many other potential challenges.

To manage stress, requires understanding the potential sources of stress that may exist within any environment. In their writings both Whitcomb and Shigo point out the unique situation of trees within the environment, they are incapable of moving to more favorable conditions. Therefore, trees have evolved with the capacity to continue to function in situations that are less than ideal for tree growth. Understanding these capacities is central to the management of trees on any site. Whitcomb introduces the concept of “Tolerance” or the capacity of a tree to grow in conditions which are not ideal. Shigo introduces the concept of “Survival” which is defined as the capacity of a tree to grow in conditions with the potential to kill the tree. These are two differing terms, but they address the same issue, trees have adapted with the capacity to function within very wide ranges of variables of their environmental conditions. These capacities may be observed when the same species may be found over a diverse range of environmental conditions. Managers of trees need to fully understand this capacity. It may this very capacity to tolerate adverse conditions that has led to many of incorrect horticultural practices. Trees produce and store carbohydrates very efficiently so that energy is available for plant functions when conditions become less than ideal. This means that there is typically a time delay between the event which is causing stress and the appearance of the visible symptoms of that stress thereby confusing the process of understanding cause and effect in tree care.

Whitcomb speculates on the desire for having a scale for gauging how well any tree may be growing on any specific site. He imagines a scale of “1 to 10” in which “1” is totally dead and “10” is a perfect tree. Within this discussion he speculates that the majority of our trees are functioning somewhere around a “5” or “6” on this scale; meaning that on any site there is the potential for improving the growth of the tree. He points out that being “green” is not a good indicator of the tree’s health since the tree will respond to site conditions to provide the greatest photosynthetic capacity that is possible on that site. While the plant may be green and appear to be “healthy”, there are likely factors limiting its capacity to efficiently complete all life functions of a truly healthy plant. It should also be noted that the rate of growth is not a reliable measure of health. Shigo demonstrates that applications of excessive nitrogen to a site may accelerate growth but this is most likely at the expense of the compounds that provide the tree with the defensive chemicals that are required for protection from insects and fungi.

Whitcomb presents results from a research project where various limiting factors were identified and then compensated for; this provided growth rates for seedling trees that were at very high rates. This can only be achieved if all of the limiting factors are identified and remediated, otherwise the accelerated growth will only be at the expense of the overall health of the plant.

Shigo introduces the concept of “Sudden and Repeated” to aid in explaining how trees handle stress. Should a tree experience an event that reduces its capacity to function, stress, to overcome this event requires that the tree use stored energy, energy that may otherwise be used for defense or other plant functions. Through time, the tree may return to a stable growth condition. However, if before the tree is able to regain its health another stressing event occurs, then the tree may move from being “stressed” to a condition of “strain” which begins a long period of slow decline and the eventual death of the tree. Therefore, it is essential to manage trees to reduce the potential of Sudden and Repeated stressing events.

Stress Resistance Syndrome is an explanation for how a tree may respond to poor growing conditions. Trees that are growing in conditions that are less than ideal will typically have a low Shoot to Root ratio. This means that the tree is allocating energy to produce roots so that the roots may extend outward and/or downward from the core of the tree so that a greater soil mass may be harvestedfor the required factors for growth. It also means that the tree typically will be shorter with shorter internodal lengths than may be common for the species. Plants that are growing in less than ideal conditions also tend to allocate energy to the production of defensive chemicals as a priority so that there are fewer problems with pest populations. Before any attempt is made to remediate any limiting factor, all limiting factors must be known and understood. (vonWettberg, et al. 1993)

Managing environmental stress is most successful if a process is followed in the management of trees. The process begins with diagnosing the symptoms that are observable. The presence of wilted foliage may be indicative of moisture stress. The discoloration of foliage can be a symptom of a multitude of potential stress factors. There are many lists that attempt to systematize the appearance of chlorosis in leaves as indicators of elemental deficiencies. These listings do not provide any accuracy when it comes to correcting elemental deficiencies. Diagnosing for fertilizer requirements can only be done by completing a soil test of the site. When a satisfactory diagnosis has determined which growth factor is limiting growth, then it is important to set dosage rates to reduce the impact of that limiting factor.

Any attempt to correct a limiting factor will always begin by the plant using its stored energy sources. Therefore, the energy that could have been used for defense or supporting soil organisms will be limited until the tree regains satisfactory levels of stored energy. This should be only a minor problem for the tree if the factor identified is what is truly limiting growth.

Following the application of the identified dosages, there should be a period during which the growth of the tree is closely monitored to observe if the intended improvements have been achieved. While most limiting factors are tested for and identified individually, in fact there is a complex interaction between all of the factors impacting growth. For example, if it is ascertained that potassium is a limiting factor in growth, correcting that deficiency may allow the moisture deficiency that has also been functional to become apparent. For long term benefit for the tree, the manager must maintain complete and comprehensive records of all factors that could limit growth on any site to achieve any real improvements.

The management of environmental stress factors requires having some knowledge of the potential sources of stress, the history of the site, and an understanding of human activity on the site. Many of the factors that are limiting may be beyond the capacity of the manager to modify to any real extent. Air pollution, as chemical pollutants, particulates in the atmosphere, and acid rain are not factors that will be easily managed on any one site. Likewise, light problems are not easily addressed, except in situations where auxiliary lighting may be controlled. Unfortunately, the managers of our landscapes arrive on a site long after the engineers and builders have so disrupted the soil environment that makes growing any plant a challenge. Temperatures and extremes of weather are also beyond simple regulation. The history of the site is important for understanding how the tree has responded over time to the conditions that have been present. For these reasons, landscape managers tend to focus upon those factors that may be easily impacted; available water, soil fertility, and plant selection.

Water is essential for tree function. Essentially, all of the water that is utilized by the tree in its various functions is absorbed from the soil environment. For this absorption to occur, the cells of the roots must utilize energy to move the water from the soil environment into the transport tissues of the xylem. The transpiration process not only cools the tree, but it also provides for a negative pressure in the xylem in the crown of the tree helping to pull the water upward from the roots to the foliage in the tops of tall tree. Water stress may occur not only from a deficiency of available moisture in the soil but also when the soil has become saturated.

Of the four primary functions listed earlier, by far the greatest volume of water is for transpiration or the evaporative cooling of the plant and it is this use that will likely cause the greatest stress. When stress is caused by excessive rates of transpiration, then turgor pressure of cells, the rate of photosynthesis, and the capacity of the tree to transport essential elements and energy compounds throughout the plant are all reduced. All moisture stress is manifest in the tree first as wilted foliage. If the stress is not alleviated, then leaves may abscise, leaves die, and possibly branch structures dieback. Moisture stress can occur on both a daily (diurnal) cycle and on a seasonal cycle and the tree has differing strategies for each. With diurnal stress, the soil may have adequate moisture available, but the rate of loss due to transpiration may exceed the capacity of the xylem to move water fast enough to meet the evaporative loses from the foliage. On days such as this, leaves tend to lose turgor pressure, but overnight the cells are restored. While this is a stress event and the tree is utilizing energy to overcome the loss of turgor pressure, this is not typically a life threatening condition.

There may be seasonal moisture stress due to drought and for trees species that are otherwise healthy and truly adapted for their region, drought is not commonly a life threatening condition. While we typically think of droughts occurring within a brief period of time, trees on most sites are slowly responding to the changes in soil moisture in ways we do not see. As a drought develops,surface roots tend to shrivel and die, the hormones of the plant reduce root branching and energy is utilized to have roots grow more deeply into the soil allowing the tree to survive the period of low water availability. The stomates of the leaves close in response to low moisture reducing the rates of transpiration. With the stomates closed, photosynthesis shuts down and the tree continues to function more slowly utilizing stored energy. Trees that have not truly evolved for the potential drought conditions tend to respond by dying.

Shigo has identified “Summer Adjustment Shed” (SAS) to explain a common response to moisture stress by trees. The growth of a tree is the result of the environment it experiences over at least two growth periods. In late summer and into fall, the tree forms new cells in its buds; the number of cells formed is determined by the environmental conditions the tree experiences in that growth period. In the following spring the preformed cells of the buds emerge and expand to the size that is determined by the water available in that growth period. If, however, in the period following cellular expansion, conditions become such that the tree is not capable of supporting all of the leaves formed due to moisture stress, the tree will intentionally abort green foliage to reduce the moisture demand for the entire tree. This is a natural phenomenon and is a sign of stress. However, it is a function utilized by the tree to actively protect itself from damage due to moisture stress.