Shade trees contract many kinds of diseases, some of which can kill or greatly disfigure a tree, predispose it to storm or other damage or make it susceptible to other disease agents. All diseases develop over time, and this is what distinguishes them from simple injuries. Diagnosing diseases requires keen observation of symptoms (plant responses) and signs (pathogen propagules, fruiting bodies or other parts). Abiotic disorders lack a pathogen and thus cannot spread as diseases do on wind, in water or with vectors. These disorders are an interaction of the host (tree) and the environment over time. Since there is no pathogen, signs are not usually present – only symptoms.
Since the interaction that results in abiotic disorders is between the tree and its environment, these disorders are about adaptations. When the environment changes, if the environmental tolerances of the tree are exceeded, then symptoms will occur. The well-adapted tree has few abiotic disorders. Dr. Terry Tatter called abiotic disorders “people-pressure
diseases.” People tend to modify the environment in ways that are not tree friendly. Growing conditions in cities often impose limits for tree growth and create environments for which many trees are not well adapted. Reduced water inputs, restricted root development, lack of mulch or litterfall, excessive pruning and reflected heat offer challenges for growing, healthy shade trees in urban landscapes. Abiotic disorders are so harmful that the average lifespan of trees in urban centers is reduced by decades from their natural lifespans.
To understand the impact of urban life on trees, it is important to consider all the functions of the various tree organs. Roots provide anchorage and physical stability to a tree, while also providing the water supply for the canopy as well as minerals necessary for all the biochemical reactions that go on in trees. Roots also interact with the soil microbial community and with the roots of other trees. Roots produce signaling molecules, known as phytohormones, and act as storage tissue for starch and other concentrated energy molecules.
Stems provide the architectural framework of a tree’s canopy. The main stem or bole of a tree provides structure for the arrangement of major branches, which in turn branch and rebranch into an extensive network as a platform for leaves, the “solar collectors” of a tree. Stems are also the conduits for water and minerals, as well as for photosynthate that travels throughout the tree and to roots. Stems also contain the meristems responsible for tree growth and reproduction. Leaves conduct gas exchange and the photosynthetic reactions that capture energy from sunlight, producing sugar molecules that move throughout a tree. All these systems interact with each other and are vulnerable to environmental changes.
An obvious dichotomy is that trees have two categories of potential environmental impacts: those to above-ground tree organs (stems, leaves and branches) and those to the soil-dwelling organs (roots). Extremes of light, temperature, humidity, wind and air pollution all impact the above-ground parts. These are modified by practices such as pruning and impacts of the surrounding landscape. Roots are affected by soil moisture content, oxygen content and soil-volume issues. Since the two systems of trees are connected, impact to roots often shows symptomatically in leaves, and, over time, impacts to the canopy can lead to reduced vigor of root systems or make them more susceptible to soil-borne pathogens.
It may seem obvious that trees need light, but too much light can be harmful to some trees and too little light reduces tree or branch vigor and can predispose trees to canker and other pathogens. Deciduous trees are adapted to high levels of light penetrating their canopies during winter, when the angle of the sun is low and leaves are not present. However, some deciduous trees, such as apple, will be damaged by high light levels if they are over pruned and leaves cannot form fast enough to cover branches. Sunburn and sunscald affect thin or green-bark trees when they are exposed to intense sunlight after pruning or damage to the tree crown. Maple, camphor, apple, avocado and crape myrtle all have relatively thin bark that is easily damaged by high light intensities.
Another impact of high light levels is to release latent buds from dormancy. This can result in an abundance of epicormic shoots populating the tree crown. Vigorous epicormic shoots are more susceptible to powdery mildew and will require thinning later to maintain structural integrity in the canopy.
High and low temperatures also can dramatically affect trees. During the last few years, dramatic, record-high temperatures have occurred in the Western United States. These episodes were so extreme that canopies of native trees were severely burned in two separate summer seasons. High-heat events are likely a part of our climate future, and the impact of sudden canopy loss is not entirely clear. While the West endures record-high temperatures, in other areas of the country, sudden and severe onset of freezing temperatures can lead to frost cracks in trees and winter kill of everything, from the tips of conifers to entire shrubs in some landscapes. Temperature extremes usually are notable, and we have some warning before they hit so measures can be taken to limit damage.
Root systems experience many abiotic conditions that may or may not cause notable symptoms above ground. Since roots are responsible for supplying both water and minerals to the canopy, they are essential to tree health. A compromised root system will take up less water and loses the ability to selectively import minerals that are required in the canopy.
In urban settings, soil compaction decreases soil oxygen content, making it harder for roots to “respire,” that is, to take in oxygen and release carbon dioxide. Compaction results in the physical destruction of soil structure, crushing and collapsing pore spaces that roots need to thrive. One way to fix compaction is to increase mulching in the root zone, as coarse, woody, tree-chip mulches eventually help to restore soil porosity. Severely compacted soils may need to be physically broken up to allow air to diffuse into lower layers. Over time, the best way to fight compacted soils is to stop the source of the compaction and increase organic matter that supports a healthy soil food web. There is no evidence that aeration tubes benefit shade trees by increasing soil oxygen levels.
Standing water is indicative of compacted soils. It also can be the result of excessive rainfall. Increases in flooding and suffocation of roots that experience long periods of saturation reflect the changing climate in many parts of the U.S. Trees have some genetic tolerance for flooding, but the new norm may exceed some tree-adaptation capabilities. Trees in swampy, low-lying areas or on ground that does not drain well will die back, become infested with insects or root pathogens and gradually be killed out of the landscape. Planting resilient trees is going to be a growing part of urban forestry in coming years.
In places where trees rely less on rain and more on irrigation, salinity is often an issue. Usually in these locations, the amount of evaporation exceeds precipitation, and soils are not well leached. Many Western U.S. soils tend to build up higher quantities of salt and base cations. High salt levels in the root zone require more energy for the tree to take in water, as it must allocate salts to its roots to keep the osmotic gradient favorable for water movement into the tree. When soils dry out, they may exceed the osmotic tipping point in roots and water will move into soil from the roots, exacerbating physiological wilting.
Trees growing in salty water take in a lot of ions with the bulk flow of water. The salty water moves to the canopy and water is transpired, leaving those salts behind in leaves. The result is foliage that develops edge necrosis, or “salt burn.” Salt-affected plants are predisposed to root-rot-causing organisms, because their roots become more “leaky,” attracting zoospores of root-rot pathogens. The best cure for all this is for leaching rainwater to move salts out of the root zone. In absence of rain, it is best to keep soil evenly moist so that roots are not burned by salts as the soil dries down between irrigations.
Another soil-related problem is pH. Soil reaction, or pH, is tuned for optimal nutrient uptake by roots at a pH of 6.8. This value, just below neutral, is where the most plant-required nutrients are fully soluble. Soil reaction is a measure of the ratio of hydroxyl (OH) ions to protons or hydrogen (H) ions. At pH 7.0, the concentration of OH and H ions is equal. As you move up or down the pH scale, each number represents a tenfold increase or decrease of hydrogen ions. The pH scale is a log10 (logarithmic10) scale, so each number in the scale is 10 times greater or less that its adjacent number. So while pH 7.8 does not seem high, it is 10 times higher (more alkaline) than the ideal pH.
As soil reaction becomes more acidic (pH 5.8), metals in soil are 10 times more likely to dissolve and leach. As reaction becomes more alkaline (as in many Western U.S. soils), minerals precipitate into insoluble compounds and are bound from absorption by roots. Nutrient deficiencies in trees may reflect the absence of a necessary mineral in soil or a pH condition that does not allow the needed mineral to be taken up sufficiently by the plant. Diagnosing deficiencies should always involve a test of soil reaction to see if absorption is likely or unlikely. Beyond pH, soil tests for macronutrients and micronutrients should be used to further refine a fertilization program. Trees mulched with fresh woodchips require less fertilization or none at all in Western soils.
Abiotic disorders are caused by extremes in the environment. This is tempered by a tree’s adaptations to its environment. A tree well adapted to soils and climates suffers less from abiotic disorders than species that are marginal in a growing area. A challenge for arborists will be to select trees that match the changing environment, which in the future may be more extreme in temperature, water availability, salinity, wind and humidity.
Arthur James Downer, Ph.D., is environmental horticulture/plant-pathology advisor with the Universality of California Cooperative Extension in Ventura County, California.