Understanding Tree Biology through Arboriculture
By Liz Sellors, Master Gardener and Arborist
Caring for trees is a complex subject. When asked to respond to questions about tree care, I generally suggest that a discussion about caring for trees should be accompanied by a demonstration.
For years, I had a love of tree biology, and in 2018, I made the decision to take the Arboriculture course offered through the School of Agriculture at the University of Manitoba. Taking the course confirmed that there are few, if any, short answers to questions about tree care. In 2019, I wrote the three-hour Arboriculture exam that required a 70% pass rate. Passing the exam entitled me to call myself an Arborist. Distinction: not Certified Arborist. That title involves additional study and examinations.
Arboriculture
The distinction between arborists and foresters is arborists maintain trees for their aesthetic value in urban settings, as opposed to logging or the fruit industries. By definition, arboriculture is the cultivation, management, and study of trees, shrubs, vines, and other perennial woody plants. The practice of arboriculture encompasses: selection, planting, training, soil and fertilization, pest/pathogen control, pruning, shaping, and removal. Arboriculture is defined by how woody plants grow and how to care for them through tree biology, diagnosing health issues, assessing risks and potential, and making informed decisions to enhance their well-being. Arborists conduct risk assessments and prescribe treatment as needed. Arboriculture is also about informing the public of the importance of trees, their benefits, and the correct practices for their care and preservation.
Arboriculture provides arborists with skills and knowledge about tree biology and is considered both a science and an art. An arborist understands how trees grow and how to manage and support their growth and development. Plant classification, or taxonomy, is based on biological characteristics that identify tree species and is the first step in understanding tree care. Morphology: the size, shape, and appearance of tree parts must be assessed in order to treat diseases and manage pests. Arborists must be able to identify tree species in all seasons through such characteristics as form, bark, buds, twigs, leaves, fruit, flowers, and scent.
The partnership of trees with soil
Trees and soils are ecologically interdependent. Urban development often disrupts this ecological balance. Appropriate soil is crucial for root distribution and maintaining plant health. Arborists must have a basic understanding of soils and be able to correct problems when they arise. Often, tree decline is attributable to soil stress. Arborists understand soil composition along with its delicate balance of insects, earthworms, nematodes, fungi, bacteria, and microbes. Soils vary greatly. The microorganisms below a turf lawn will be very different from the soil found beneath a forest floor. Arborists understand how the physical, chemical, and biological properties of soils influence tree health and productivity, soil and water management, and soil fertility. Soil testing is important to nutrient management. Urban soil challenges require soil improvement strategies, such as dealing with heavy metals or landscape sites where new trees will be planted in soil that differs physically and chemically from the original tree farm location where the trees were raised.
Nutrient requirements
Arborists must understand both biological tree nutrient requirements and fertilization—both of which are commonly misunderstood by the general public. Trees require specific essential elements or chemical constituents to function, grow, and maintain health. In forests, nutrients are typically present in the soil in sufficient quantities and are recycled through fallen foliage. Landscape trees often grow in modified soils that lack sufficient available essential elements required for satisfactory growth and development. Nutrient-rich topsoil is often removed at construction sites or, if left, becomes depleted of nutrients and gradually deteriorates. Nutrient-recycling (the continuous breakdown of elements that are exchanged or reused between living and non-living components in an ecosystem), is seriously depleted when fallen foliage is removed. This limits the population and activity of earthworms and soil organisms that normally break down plant material. Fertilization, or other forms of adjusting the soil structure, biology and chemistry become necessary to increase nutrients in the soil.
Fertilization can increase trees’ growth, reduce susceptibility to specific diseases and pests, and, in some cases, may reverse declining health. But, if fertilizer is unnecessary or applied incorrectly, it may adversely affect both the tree and its environment. Where trees grow in turf that is fertilized regularly (typically by a lawn-service), additional fertilization should not be applied to those trees. Tree fertilization should be in accordance with ANSI A300 standards guidelines for woody plants. Guidelines indicate 30-10-7 provides nutrients to landscape trees and benefits microorganisms.
Trees must have essential nutrients to be able to grow, develop, and produce. These essential elements occur naturally, are dissolved in water, and are taken up through tree roots. Each element has a specific role and cannot be replaced by other elements. Macronutrients are critical to photosynthesis and are required in the largest quantity. Nitrogen is the most likely to be deficient in trees. Macronutrients such as phosphorus, potassium, and sulfur are required in relatively large quantities but are often present in adequate amounts in the soil. Oxygen, hydrogen, and carbon are also considered macronutrients. Micronutrients, boron, chloride, copper, iron, manganese, molybdenum, nickel, and zinc appear naturally in soil in relatively small but sufficient quantities for trees. Micronutrients can be toxic in high levels, resulting in a profound negative effect on tree health. Secondary nutrients (calcium, magnesium, and sulfur) are required in moderate quantities.
Fertilizers are available in many forms and combinations. Complete fertilizer contains the three primary nutrients: nitrogen, phosphorus, and potassium. A complete fertilizer is often unnecessary because nitrogen may be the only limiting element, which can be determined through a soil test.
Selecting the right tree
Tree selection should only be made with an understanding of tree needs. Trees are the most dynamic, longest living, and largest components of the urban landscape. They beautify landscapes in all seasons through a variety of forms, flowers, fruit, seasonal leaf colour, and interesting bark. They sequester carbon, create ecosystems, provide oxygen and shade, and filter some pollutants from the air. They deflect noise and wind, block undesirable views, and protect soil from erosion. They capture rainwater and reduce rain runoff. They reportedly have positive psychological effects such as reducing stress, reducing crime rates, and reducing recovery time for patients.
When selecting a tree for a residential lot, it must be a good match with respect to the tree’s needs. Every tree has specific environmental requirements for light, water, drainage, soil conditions, soil pH, and growing space. Selecting the right tree for a site gives it a good chance of survival, plus reducing ongoing maintenance costs for pruning, fertilizing, and pesticide applications. Cold hardiness, heat tolerance, sun/shade, drought or flood tolerance, growth rate, size at maturity, form, insect and disease resistance, nuisance potential, and maintenance requirements must be considered. Desirable characteristics include: bird-attraction, exfoliating bark, branching habit, flowers, fruit, and leaf colour throughout the seasons. Fast-growing trees often tolerate poor soils and general neglect, but may have weak, brittle wood and are often short-lived. A well-selected and cared-for tree will be around for many generations.
Arborists know that urban trees require care to maintain proper form and aesthetics. Generally, trees are pruned for corrective or preventive measures. Pruning is common maintenance but must only be carried out with an understanding of how trees respond to branch removal. Most pruning is done to remove dead branches, suckers, and water sprouts in order to improve form, reduce risk, and increase light penetration. Each cut has the potential to change a tree’s growth. Branches should not be removed without a reason or proper understanding of pruning. Improper pruning can create lasting damage and possibly shorten the tree’s life. Routine thinning rarely improves a tree’s health. The timing in a year of pruning is important. Growth and wound closure are maximized if pruning is carried out as prescribed.
In conclusion, the benefit derived by trees include a combination of aesthetic functions. Trees improve air quality, provide wildlife habitats, lower greenhouse gas emissions, provide shade and cooling through evapotranspiration, and beautify surroundings.
Feature image: ISA Certified Municipal Arborist James Kinder