Trees face a constant barrage of natural threats. Death is rarely the result of a single event, but rather a consequence of multiple interacting stressors. This article explores the causes of tree mortality, including weather, disease, insects, and aging. Mechanisms of death range from acute, sudden events to chronic, slow-acting biological warfare.
Acute Environmental Stressors
Acute environmental stressors cause rapid tree death by interfering with the plant’s internal systems. Prolonged drought is a frequent killer, causing mortality through hydraulic failure. When water is not replenished, the water column in the xylem tissue comes under immense tension. This negative pressure causes air bubbles (cavitation) to block water flow, leading to desiccation.
Trees may also die from flooding and waterlogging, which lead to root suffocation, or anoxia. When soil pores fill with water, oxygen is rapidly depleted by soil microbes and root respiration. Without oxygen, the roots cannot perform aerobic respiration necessary for absorbing water and nutrients, causing the root tissue to die.
Severe weather events cause immediate mechanical failure and physiological damage. High winds can uproot an entire tree (windthrow) or snap the trunk and branches, causing structural failure. Ice storms coat branches in heavy ice, leading to catastrophic limb and crown breakage that reduces the tree’s ability to photosynthesize and survive secondary threats. Extreme heat stress increases the respiration rate, depleting stored energy reserves. This energy depletion, combined with difficulty cooling tissues, can lead to cellular damage and mortality, especially when paired with drought.
Pathogens and Fatal Diseases
Fungi and bacteria cause fatal tree diseases by disrupting internal transport and structural integrity. Vascular wilt diseases, like Dutch Elm Disease, attack the tree’s plumbing system. Pathogens invade the xylem vessels, but the tree’s defense response often causes harm by plugging the vessels with gums to contain the infection. This blockage prevents water movement to the leaves, leading to rapid wilting and canopy death.
Other pathogens cause localized tissue death that can spread to encompass the entire circumference of the trunk or branch. Cankers are sunken, necrotic areas of bark and underlying tissue that form after a fungus or bacterium enters through a wound. If a canker expands completely around the stem, the process is called girdling, which severs the phloem tissue that transports sugars down from the leaves.
Root rot diseases, often caused by fungi such as Armillaria or Phytophthora, work below ground to compromise both stability and function. These fungi destroy the root tissue, which hinders the tree’s ability to absorb water and nutrients from the soil. The resulting decay weakens the entire root system, making the tree highly susceptible to being blown over in even moderate winds.
Destructive Insect Infestations
Insects cause tree mortality through physical destruction, nutrient consumption, and the introduction of fungal partners. Bark beetles excavate galleries in the phloem, the layer beneath the bark that carries sugars. The larvae feed on this tissue, and a mass attack can effectively girdle the tree, interrupting the sugar supply.
Many bark beetles also carry symbiotic blue-stain fungi, such as Ophiostoma species, in specialized pockets on their bodies. When the beetle bores into the tree, it inoculates the sapwood with the fungus, which quickly colonizes the xylem vessels. This fungal growth blocks water transport, compounding the damage caused by the beetles’ physical tunneling.
Wood borers, including the larvae of certain beetles and moths, inflict deep structural damage by tunneling into the sapwood and heartwood. While feeding in the vascular tissue can cause girdling, boring into the heartwood compromises mechanical strength. This internal damage makes the tree vulnerable to snapping during storms and provides entry points for wood-rotting organisms. Defoliators, such as caterpillars, contribute to mortality by stripping the leaves, the tree’s photosynthetic organs. Repeated defoliation forces the tree to use stored energy reserves to produce new leaves, depleting these reserves entirely and leading to death by carbon starvation.
The Inevitability of Age and Competition
Beyond the acute and biological threats, trees ultimately succumb to the slow, intrinsic forces of age and constant struggle for resources. Senescence, the natural aging process, involves the gradual breakdown of internal repair mechanisms and the accumulation of damage over decades or centuries. As a tree ages, its vascular system becomes less efficient, and its capacity to compartmentalize decay and fight off minor infections diminishes.
Competition for resources exerts a continuous, chronic stress that often pre-disposes a tree to death by other causes. Trees growing in dense stands struggle for access to sunlight, water, and soil nutrients, which reduces their overall vigor. This chronic stress compromises the tree’s ability to produce chemical defenses, such as resin or sap flow, necessary to repel bark beetles or wall off canker-causing fungi. While a tree may appear to die from an insect infestation or a drought, the underlying cause is often the long-term weakening brought on by relentless competition and the limits of its own lifespan.