Abiotic Factors Contributing to Tree Development and Condition
Urban environments pose an array of challenges to tree growth and survival. Many of these challenges arise from nonliving factors that disrupt or impede plant sustenance, growth and defence. This can, in turn, render trees more susceptible to living threats such as plant pests and pathogens. Intense and prolonged instances of stress can disturb the balance of water and carbon in trees, contributing to the main causes of tree mortality.
This chapter's content focuses on identifying nonliving variables that provoke plant stress, affecting one or more physiological processes necessary for tree development and survival.
This week's readings will look at how those variables interact with trees at the whole tree, organ and molecular level. Literature is effective at explaining how plants are equipped to respond to stress, through the mechanisms of avoidance, tolerance and, to some extent, escape. Nevertheless, the capacity of trees to overcome environmental threats is limited. When introducing trees to sites, and maintaining them on the landscape, one cannot rely only on stress response as a long term strategy for tree preservation.
One aspect that requires more research and analysis are the impacts of multiple abiotic factors on tree development and condition. More often than not, several variables contribute to a spiral of tree decline and mortality. Knowing how to effectively manage these nonliving variables is one of the biggest challenges urban forestry professionals face today.
Moving forward, professionals and stakeholders in the urban forest will need applicable frameworks to accomplish the following goals:
Understanding how urban sites can be appropriately designed, built and maintained to support trees
Assessing the effectiveness of engineered urban systems in preserving trees, over time
Recognizing which tree species as well as which selected varieties and cultivars can best withstand urban conditions and their abiotic characteristics
A case study is available in this chapter!
You are invited to explore the commercial district revitalization project on Bloor Street WestLinks to an external site., undertaken by the City of Toronto, Ontario in 2010-2011. See this chapter's optional readings for more information on this case study.
Learning Objectives
By the end of this topic, you should be able to:
Identify and describe abiotic factors that influence the growth, development and survival of trees
Explain how abiotic factors can provoke stress in plants
Understand the different stress resistance mechanisms of trees (avoidance and tolerance) as well as examples of these mechanisms
Topic Readings
This week, it's all about the nonliving factors that can impact tree development and condition.
Several aspects of the urban environment pose a threat to the physical, chemical and biological functions of trees. It is useful to recognize the most critical abiotic variables that affect trees as this information could contribute to smarter urban sites and the selection of trees better suited to grow in specific environments.
Required Readings (Considered for the exam!)
Percival, G. (2017). Abiotic Stress. Routledge Handbook of Urban Forestry. Routledge, London, (pp. 237–246) (Course textbook).
Hirons, A. & Thomas, P. (2018). Applied Tree Biology. Wiley Blackwell - Chapter 10 Environmental Challenges for Trees (Course textbook).
Optional Readings
Looking to get up to speed? Consider this optional reading for more background context and terminology.
Lilly, S.J. (2010).ISA Arborists’ Certification Study Guide. International Society of Arboriculture, Champaign IL (Chapter 4 Water Management and 11 Plant Health Care).
Want to dive a little deeper? Consider a case study to help reinforce the concepts we explored in class.
Case of Bloor Street West Revitalization Project in Toronto, Canada 2010 to 2015
This project incorporated underground soil cells as well as open and landscaped tree beds for 133 new trees. Nevertheless, only one species of tree, Platanus x acerifolia, was planted. This site was visited by participants of the International Society of Arboriculture's Annual Conference on August 7, 2013 and lauded as a victory for tree-friendly urban design (The course instructor was impressed by this investment!).
In 2015, all of the trees were removed and replaced following massive decline of virtually all of the trees on this streetscape. This case was the subject of a peer reviewed research project and highlights the complexity of abiotic factors in urban environments. For more information, see the following reference:
Ordonez, C., Sabetski, V., Millward, A., Steenberg, J.W.N., Grant, A. and Urban, J. (2018) The Influence of Abiotic Factors on Street Tree Condition and Mortality in a Commercial-Retail Streetscape. Arboriculture and Urban Forestry. 44 (3): 133-145.
Abiotic factors are nonliving variables that can reduce the rate of physiological processes in trees, causing plant stress.
Acclimation is a gradual process in which trees adjust aspects of their physiology and/or morphology, in response to their environment, to compensate for an initial decline in performance. Trees undergo this process within their lifetime, some forms of acclimation occurring over a matter of minutes while others over several years.
For example, drought acclimation involves various physiological responses to maintain water potential (Ψ) even during times of drought.
Adaptation is an evolutionary response to stress that occurs over multiple generations. This process involves genetic changes in the population that compensate for the decline in performance associated with stress. Different populations of trees growing in similar climatic zones can evolve with similar adaptations over time.
Stress resistance mechanisms for plants are organized into 3 distinctive categories, 2 in particular are more relevant for trees:
Stress avoidance: Strategies to prevent stress exposure or prevent stress from affecting plant tissues
Stress tolerance: Permits plants to withstand stress and survive
Escape from stress: The ability of plants to alter their phenology and complete their lifecycle before stress factors intensify (relevant for ephemeral plants)
In urban environments, abiotic factors of plant stress that pose the greatest threat to tree function and survival are the following;
Water availability - either deficit of water (drought, soil compaction) or excess water (waterlogging, flooding)
High temperature
Oxygen availability - deficiency of oxygen for tree roots (hypoxia) or total depletion of soil oxygen (anoxia)
Light availability (duration, quality and quantity)
The following variables can also impact tree health and survival, depending on the intensity, frequency and duration of these incidents/events:
Low temperature and chilling/frost injury (in relation to the tree's cold hardiness)
Excess salinity in the soil - De-icing salts (Sodium chloride NaCl), overfertilization (chlorides of calcium, magnesium, sulfates and carbonates)
Exposure to herbicides
Mechanical injury
Limited availability of mineral nutrients (macro and micronutrients required for tree growth and development)
Air pollution
It is useful to understand these factors as well as examples of ways in which trees acclimate and/or evolve to survive under these conditions. (see readings)
Water deficit can be caused by a lack of available water in the soil or by the physical loss of fine roots of the tree. Water deficits provoke loss of leaf turgor, stomatal closure, reduced or inhibited photosynthesis and decreased metabolic function. If water transpiration exceeds what a tree's fine roots are able to absorb, the tree experiences a decline in water potential (Ψ) as the water in the xylem is placed under increasing tension. Turgor pressure is reduced as cells dehydrate and leaves wilt. Water stress also affects the movement of water in the xylem leading to cavitation and air embolisms. Moreover, vital cellular processes involving protein synthesis and enzyme activity are impaired in the absence of water.
High temperature (Heat) can provoke leaf and wood desiccation in trees, inducing water stress as transpiration exceeds absorption. At 40°C, the carbon balance is effected and damage can occur in temperate trees, at over 50°C temperate trees sustain significant injury, the same can happen for tropical trees at up 60°C. High heat can damage the photosystem II (PSII) protein complex in trees, reducing the light-dependent reactions of photosynthesis. Heat stress also reduces electron transport and enzymatic processes necessary to fix carbon and convert it into sugars. Moreover, heat stress can inhibit shoot and root growth.
Oxygen availability: Soil "deoxygenation" can be caused by excess water (waterlogging or flooding), compaction or soil sealing. Tree roots consume oxygen during respiration and can deplete oxygen in the soil if levels are not maintained through proper aeration. Tree physiology can be impaired when soil oxygen concentration falls below 15%. This can cause cell damage and death in different tissues and organs, contributing to leaf damage, reduced photosynthesis, root damage/death and stem hypertrophy. Lack of oxygen can lead to the establishment of anaerobic microbes in the soil. They produce phytotoxic substances and can potentially reduce soil pH through denitrification, leading to additional abiotic factors (reduced nutrient availability).
Light availability: All trees require minimum amounts of light to survive, but different species have developed adaptive responses to high and low light. Excess light can cause photoinhibition, reducing photosynthesis and affecting net carbon gain between the processes of photosynthesis and respiration.
Low temperature (Cold and Frost): This environmental variable establishes the territorial limit of where certain tress can grow, at a given time, but hardiness maps are changing.
Canada Cold Hardiness MapLinks to an external site. Canadian zones not only take into account monthly mean of daily minimum temperatures by mean frost free periods of 0°C in days, but also amount of rainfall, maximum snow depths, maximum wind gusts and monthly mean of the daily maximum temperatures (°C) of the warmest month among other climatic variables.
Severe frosts can produce vertical frost cracks and splitting in trees, partially caused by the tension generated between cold sapwood shrinking and warmer heartwood retaining its form. This physical form of damage can also be caused by the dehydration of the outer sapwood as the water freezes, causing the outer parts of wood to contract. These cracks tend to close with warm weather and can re-split when cold conditions return.
Excess salinity: This occurs specifically in nordic climates where de-icing salts are used repetitively. The chloride ion is toxic to plants, reducing chlorophyll concentration in leaves and causing leaf necrosis, bud failure and branch die-back, ultimately affecting photosynthesis. Sodium can disrupt cation exchange in soils leading to increased soil pH and affecting mineral nutrient availability. High concentration of salts can disrupt the osmotic balance, lowering soil water potential and making it difficult for roots to extract water from the soil. Some trees are able to re-establish osmotic balance by modifying the structure of their cells and tissues.
Exposure to herbicides: The general affects of herbicides on non-target species is not always well understood. Selective herbicides acting on broadleaf plants use growth regulators to kill target species. If these products come into contact with adjacent vegetation, exposure can result in dysfunction, injury or death. For this reason, it is important to use registered pesticide products and to read and fully understand the label instructions prior to application. Risks can be eliminated by refraining from using pesticides and considering alternative, low impact, pest management strategies.
Mechanical injury: Physical injuries can be caused by weather events, machinery, vandalism as well as other human activities. The frequency, intensity and extent of these injuries can vary and therefore can generate different levels of stress. Furthermore, specific physical injuries such as bark or branch removal can not only provoke defensive responses in trees but can also provide entry points for tree pests and pathogens.
Limited availability of mineral nutrients (macro and micronutrients required for tree growth and development): In addition to water and light, mineral nutrients directly influence tree growth and are necessary components for tree survival. Environmental factors such as the composition and pH of soil as well as cation exchange capacity (CEC) have direct effects on the availability of nutrients and a tree's ability to acquire them. Soil microorganisms also contribute to this process. Overall, 14 essential nutrients are directly involved in plant metabolism and physiological functions. Other marginal elements are also recognized as beneficial for plants.
Pollution: Although trees have the potential to reduce the formation of ozone O3 in the atmosphere, if they are exposed to high levels of O3, they can in turn generate other pollutive compounds. Elevated ozone can cause trees to produce more metabolic by-products, specifically reactive oxygen species ROS, a compound that can be harmful to tree cells in higher concentrations.
Self Check Quiz
Practice Quiz # 2
4 question practice quiz to be released prior to mid-term exam to reinforce learning objectives and to facilitate studying
Activities & Assignments
No Assignment this week. Kindly take the time to get into the readings.
Topic Self-review
The following questions are to help you reconsider the content shared in the readings, course Fundamental Concepts and discussion.
1- In what ways do urban environment help or hinder physiological processes in trees. Think about an example you experienced or an example you learned about in the readings.
2- Which environmental/abiotic factors impact trees the most?
3- Do you believe that a better understanding of tree physiology can promote smarter interventions in tree care?
4- What can tree physiology tell us about balance when incorporating and managing trees in urban settings? How is balance addressed in literature that we reviewed in 1.2 Tree Physiology?
UFOR 521, Hanna 2021
Requirements Changed
Topic 1.2: Tree Physiology Topic 1.4: Biotic Factors Influencing Tree Development and Condition