The death of a tree is rarely a simple function of old age. Instead, it represents a complex physiological shutdown triggered by external forces like disease, environmental extremes, or physical damage. Trees possess a remarkable capacity for longevity, often succumbing only when their interconnected life-support systems are overwhelmed. Understanding tree mortality requires looking beyond the visible decline to the internal mechanisms that govern the flow of water and energy, which ultimately prevents the tree from sustaining its basic metabolic functions.
Physiological Collapse: Failure of Transport Systems
The immediate cause of tree death is the failure of its internal transport systems: the xylem and phloem. The xylem moves water and dissolved minerals from the roots to the canopy. The phloem transports sugars and energy produced during photosynthesis from the leaves to the roots and growing tissues. When either system is significantly disrupted, the tree starves or dehydrates.
A common failure in the water-conducting xylem is cavitation, where severe water stress causes air bubbles (embolisms) to form within the water columns. These blockages stop water flow, leading to a rapid loss of hydraulic conductivity. For some conifers, mortality occurs when the tree loses about 50% of its water-carrying capacity, though broadleaf trees can tolerate a higher percentage of embolism before catastrophic failure.
The phloem system fails when sugar transport to the roots is interrupted, a process known as girdling. This can happen physically when an organism chews a ring around the trunk, or physiologically when a pathogen blocks the sieve tubes. Without this vital energy supply, the roots cannot respire, grow, or maintain their defenses, leading to carbon starvation and the eventual death of the entire root system.
Mortality by Pathogens and Pests
Biological agents often trigger physiological collapse by physically destroying transport tissues or introducing systemic diseases. Insects and pests typically cause mortality through physical damage, especially those that bore into the cambium layer beneath the bark. The Emerald Ash Borer, for example, is a notorious pest whose larvae tunnel through the phloem, effectively girdling the tree and causing carbon starvation in the roots.
Other insect pests, such as bark beetles, introduce pathogens while burrowing or disrupt nutrient flow in the cambium, which weakens the tree. Defoliators like the Gypsy Moth remove the canopy’s ability to photosynthesize. This depletes the tree’s stored energy reserves, leaving it too weak to survive subsequent stresses.
Diseases and pathogens cause tree death through infection and systemic disruption, often leveraging the tree’s vascular system to spread. Fungi are a frequent cause, leading to vascular wilts that grow into the xylem, physically blocking water transport and mimicking drought-induced embolism. Fungal infections can also create cankers, which are necrotic areas that expand and eventually girdle the trunk or branches, cutting off the phloem supply. Root diseases, like those caused by the water mold Phytophthora, attack the root system, preventing the uptake of water and nutrients and causing a slow decline.
Death Caused by Environmental Stress
Non-biological environmental factors are powerful mortality agents, causing rapid death or predisposing the tree to biological attack. Drought is a major factor, leading to hydraulic failure when the tree’s internal water potential drops too low. The tree closes the stomata on its leaves to conserve water, which halts photosynthesis and can lead to carbon starvation if the drought is prolonged.
If the tree attempts to continue water uptake despite the stress, excessive tension in the xylem pulls air into the water columns, resulting in widespread cavitation and immediate hydraulic failure. Conversely, flooding and waterlogging cause a paradoxical “drowning” and “thirst.” Water-saturated soil displaces the air, and because oxygen diffusion is much slower in water, the roots quickly suffer from anoxia, or a lack of oxygen.
Without oxygen, roots cannot respire efficiently to generate the energy needed to absorb water and nutrients. This lack of oxygen can also lead to the buildup of toxic compounds like ethanol and acetaldehyde. The tree dies of thirst because its oxygen-deprived roots cannot function to take up the surrounding water. Temperature extremes also compound the effects of drought by increasing water loss and accelerating the metabolic demand for stored carbohydrates, speeding up carbon starvation.
Immediate Physical Trauma and Human Impact
Sudden, large-scale trauma often bypasses slower physiological decline and causes rapid tree death. Physical trauma from severe weather, such as hurricanes or ice storms, can lead to complete windthrow (uprooting) or cause the trunk to split, immediately severing the vascular connection between the roots and the canopy. Fire directly kills a tree by heating the cambium layer beneath the bark to lethal temperatures, causing immediate and irreversible damage to the phloem and xylem.
Human activities frequently result in a slower, chronic form of trauma that mirrors natural stresses. Soil compaction, often caused by heavy construction equipment or excessive foot traffic, crushes the pore spaces in the soil. This physically restricts root growth and prevents the necessary flow of oxygen and water into the root zone, creating conditions similar to waterlogging and inducing root hypoxia. Other forms of damage, such as severing major roots during construction or poor pruning practices, create wounds that provide entry points for pathogens and compromise the tree’s transport capacity.