How Do Trees Die?
It’s tough out there for trees, especially when they are trying to survive in the “un-natural” urban forests where we live. Trees tend to grow well on more natural, undisturbed sites like arboreal areas, but there is nothing natural about the built environment. Trees planted in suburban and urban areas require supplementary care and maintenance to keep them thriving and not just surviving.

Photo 1: Trees with limited soil volume and supplemental irrigation will decline. All photos courtesy of the author.
While some tree species can live more than 300 years, 25 to 50 years is the average in our urbanized areas, according to recent research. If urban trees don’t receive adequate care, especially during the establishment period and stressful environmental episodes, they decline and die much too soon. (Photo 1)
Managing trees in urban forests
There are vital arboricultural science principles to be considered when it comes to managing trees in the urban forest. It starts with the understanding that tree growth and development vary widely from site to site and may fluctuate widely on any given site. In fact, the same species with similar growing conditions can respond differently to its environment, especially from below-
ground dynamics. Individual trees vary in their response to stress factors when there is a change in intensity or the supply of a tree’s basic needs. It’s a generally accepted consensus that trees growing in the built environment are likely stressed by factors not encountered by trees in undisturbed forests and must be managed differently.
There are many causes of tree death, including biotic (biological, living) pests, abiotic (physical, chemical, non-living) factors and combinations of both. Although insects and diseases are often the first considered suspects, it is more prevalent that the cause of early tree mortality is environmentally induced. This damaging situation is then often intensified by some opportunistic pest or secondary causal agent.
For example, an adverse environmental factor, such as drought, often is followed by lethal attacks from a secondary agent such as scale insects that may otherwise not have reached the action threshold or need for chemical intervention. Many practitioners of plant health care (PHC) often focus primarily on a pathogen, insect or some biotic factor, when often the issue warrants a much more complex diagnosis and prescription requiring examination of environment, host and causal agent.

Photo 1: Trees with limited soil volume and supplemental irrigation will decline. All photos courtesy of the author.
Tree mortality
Tree mortality is a progressive physiological process. (Photo 2) Despite decades of research on plant tolerance to environmental stressors, especially drought, the physiological mechanisms to which trees succumb are a continuous point of examination. It could be argued the two core causes of trees experiencing physiological death are starvation and dehydration, in other words, lack of nutrients and water. Actually, it’s more complicated than that from a tree’s perspective, but that is the simple statement for decline and eventual death.
The primary factors that can be attributed to tree death are carbon starvation and hydraulic failure. When energy resources in the form of complex carbon molecules become exhausted, the tree can no longer support respiration, which is the consumption of carbohydrates to create energy. This happens when the tree has depleted its resources and is unable to manufacture simple sugars or carbohydrates. Carbon is the currency for trees and all plants.
Typically, the decline process begins from concurrent stressful growing seasons leading to an “overdrawn food account.” Dieback and decline infer a reduction in green, leafy tissue that is responsible for photosynthesis, or manufacturing food for the tree. If the tree cannot manufacture food, then the overdraft protection disappears!
Photosynthesis
Food sources are created through photosynthesis. Photosynthesis is a process by which phototrophs (organisms) convert light energy into chemical energy, which is later used to fuel cellular activities. (Image 1) The chemical energy is stored in the form of sugars, which are created from the sun, water and carbon dioxide. The manufactured sugars are broken down through the process of respiration. (Image 2) This is another critical physiological plant process.
If the tree can’t break down the sugars for energy, the tree simply runs out of fuel and begins the decline spiral. It’s a sequential process in which photosynthesis creates the energy source and relies on respiration to oxidize the carbohydrates for food. Then, the tree relies on translocation, which requires water and nutrient movement through the plant’s vascular system, allocating these resources throughout the plant.
The process of respiration is constant, even in dormancy, and production of carbohydrates must exceed its energy requirements, especially during times of high biological activity (leaf emergence). Without a surplus or reserve of food resources, decline begins and death can follow unless conditions improve within an acceptable time. This includes proper maintenance and care in providing adequate moisture and nutrition to help maintain healthy growth and energy reserves.

Image 2: Respiration is the process of breaking down the food into usable forms.
Transport systems
Trees have a vascular system with a remarkable transport capacity. The transport system can deliver water rapidly and preferentially to those parts of the crown that are most actively transpiring, or losing, water. This is called translocation, which is simply the movement of something from one place to another, in this case moving water and other soluble products. The transport system is subject to the impacts of environmental stress as well, especially temperature extremes and damaging pests. Damage from insects feeding on the cambium tissue or vascular diseases obstructing xylem is what can lead to hydraulic failure in the tree.
Hydraulic failure is the loss of conductivity or the inability to move water and solutes in the form of usable carbohydrates around the tree system. This means the tree cannot adequately translocate water and assimilates where needed. Water is an important component of many plant processes, and photosynthesis and respiration are no exception. This translocation of critical resources must occur, or the tree declines and can eventually die.
Why do trees die?
So why do trees die? The health and survival of trees depends on photosynthetic and respiration rates. Respiration rates are influenced by many internal and environmental factors. Age, health, available resources and tissue hydration are examples of internal factors. Available moisture and soil health are at the top of the list for environmental factors. It is well documented that trees can die of both hydraulic failure and carbon starvation, and during drought, the loss of fluid conductivity and lack of carbohydrate reserves also can co-occur.
Their death follows a reverse sequence of physiological processes. Trees die because respiration is terminated. Respiration ceases because carbohydrate production ceases and stored carbohydrates are exhausted. Carbohydrate production ceases because photosynthesis has stopped because of a lack of functional green, leafy tissue. Typically, these physiological factors have been interrupted because of anthropogenic (human-caused) impacts locally or negative environmental changes coupled with diseases and insect damage.

Image 3: Trees have basic needs in order to thrive in their environment.
Conclusion
The primary management focus for sustainable long-lived trees is to promote healthy growth that supports vigorous plant processes with adequate oxygen, water, soil health and nutrition in a suitable soil volume. Often, this is a seemingly impossible task to accomplish in the urban forest. Timing is everything for arboricultural practices, and abiding by the physiological demands of the tree is important for sustainable health and growth. Fulfilling the basic needs of trees, such as air, water, soil, nutrition and space, are critical for thriving trees. (Image 3)
Trees in our cities are essential to our quality of life. I consider professional arborists as stewards of one of the most important natural resources in our environment. As such, we must remain vigilant toward the many issues that imperil our community trees and respond with interventions that are ethical as well as current with research and best management practices.
References
Hirons, A. D. & Sjöman, H. (2018). Tree biology. Wiley-Blackwell.
Houston, D. R. 1981. Stress Triggered Tree Diseases: The Diebacks and Declines. Information Forestry NE-INF-41-81. Broomall, Pennsylvania: USDA Forest Service.
Koeser, A., Hauer, R., Norris, K., Krouse, R. 2013. Factors influencing long-term street tree survival in Milwaukee, WI, USA. Urban Forestry & Urban Greening.
Manion, P. D. 1991. Tree Disease Concepts, 2nd ed. Englewood Cliffs, New Jersey: Prentice-Hall.
Pallardy, S. G. (2008). Physiology of woody plants (3rd ed.). Academic Press.
Purcell, Lindsey. 2024. Arboricultural Practices; A science-based approach. Waveland Publishing.
Sinclair, W.A., Lyon, H. H. 2005. Diseases of Trees and Shrubs, 2nd ed. Ithaca, New York: Cornell University Press.
Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) Registered Consulting Arborist (RCA) and principal with Lp Consulting Group LLC in Bradenton, Florida. He spent many years as an urban-forestry specialist and teacher in the Department of Forestry and Natural Resources at Purdue University, and serves as the executive director of the Indiana Chapter of the International Society of Arboriculture.
This article is based on a presentation Purcell made on the same topic at TCI EXPO ’23 in St. Louis, Missouri. For a prerecorded video for that presentation click here.
So what can be done,if anything,to save the parking lot tree’s that we’ve all seen trying to survive in a small island midst a sea of black asphault.Then the landscape crews come in and remove the most recent dead branches/limbs till it’s nothing more than a dead snag,and they leave it at that.Eye opening article,thank you.
Something I think is important to add to the urban trees is how light pollution is a secondary stressor in interrupting dark cycle respiration.
Great question from Mr. Snyder… It is important to recognize the stressors experienced by trees in limited planting spaces. Primarily, it is lack of soil volume (rooting space) and moisture deficits. Since arborists and landscape managers inherit poor planting conditions, it is important to implement the appropriate cultural practices which improve sustainability. Mainly, this is about water, or lack of appropriate water supplies. Think of these planting spaces as a canteen with a limited supply of water. When the canteen (planting pit) is empty, there is no available water for the tree. The (canteen) planting pit needs refilled or supplemented as the tree loses water through evapotransiration and the soil dries from evaporation.
It has been estimated small trees can lose about 5 gals. of water daily. A 35’ tall tree with 2,000 SF of leaf surface can lose up to 100 gals. of water daily and a 60′ tall tree may lose up to 300 gals. daily. If a typical 4 x 4 x 3 tree pit with decent loam textured soil having average WHC of 12% and total volume of 48 CF could hold approx. 45 gals of water. The smaller size tree such as a 3-5″ caliper tree would lose all water supplies in just over a week.
So, if total water volumes, optimal or nearly so are hard to obtain in the current installation or planting plan, then additional maintenance inputs will be required, such as supplemental irrigation, regularly throughout the year.