A tree split is a major structural failure where wood fibers along the trunk or a branch separate along the grain. This failure is a serious safety concern, as the sudden separation of large limbs or main stems can cause catastrophic damage to property or pose a danger to people. A tree’s failure often results from a combination of a predispositional weakness and an environmental force. The causes of splitting are broadly categorized into inherent architectural defects, acute external forces, and chronic internal decay.
Structural Flaws from Tree Growth
Many splits originate from weaknesses that developed naturally as the tree grew, making the structure vulnerable to failure under stress. A primary architectural weakness is the presence of codominant stems, which are two or more main trunks of similar size that emerge from a single point. These stems grow side-by-side without forming the strong, interlocking wood structure found in a healthy branch union.
The most detrimental factor in these V-shaped unions is included bark, where the bark of the two stems becomes trapped between them as they expand in girth. This trapped bark prevents the stems from fusing with connective woody tissue. This creates a weak point held together only by a small amount of peripheral wood. Codominant stems with included bark are significantly weaker than those without it, making them highly prone to tearing apart under load.
The angle at which a branch attaches to the main trunk also determines its long-term strength. Wide, U-shaped branch angles allow for the formation of a strong branch collar, where the wood fibers of the branch and the trunk overlap and interlock securely. In contrast, narrow, upright branch angles create a tight V-crotch that often lacks the necessary connective wood tissue for a robust attachment.
Damage from External Environmental Forces
Acute external forces can overwhelm even healthy trees, causing immediate splitting, though structural flaws often exacerbate the damage. High winds from storms create immense torque and leverage on the tree’s canopy. This generates forces that can exceed the tensile strength of the wood or the integrity of a weak union. Trees with large, dense canopies or those in exposed locations are susceptible to this stress.
The weight of accumulated ice and snow is another common cause of sudden splitting, especially on broadleaf trees that retain their leaves late into the year. The frozen precipitation exerts massive downward and outward pressure on the branches and main stems, forcing them to separate at their weakest points. This load can cause a split failure even in the absence of high winds.
Lightning strikes cause a unique form of explosive splitting due to the rapid conversion of internal moisture into steam. When lightning, with temperatures up to 50,000 degrees Fahrenheit, passes through the tree, the sap and water instantly vaporize. This superheated steam expands violently within the tree’s structure, creating internal pressure that can cause the trunk to fracture, split, or even explode along the grain.
Internal Decay and Pathological Weakening
Splitting can also be the final result of a long-term decline in wood integrity caused by pathogens. Heart rot is a condition where fungal pathogens, such as white-rot or brown-rot fungi, colonize and degrade the non-living wood at the tree’s core. These fungi destroy the cellulose and lignin, the structural components of the wood, effectively hollowing out the trunk or a major limb.
Even a small loss of wood mass can lead to a disproportionately large reduction in the tree’s overall strength. For instance, a 10% weight loss from decay can result in a 70% to 90% loss in wood strength. This internal weakening means the tree can no longer support its own weight or withstand minor lateral forces, making it susceptible to splitting under normal wind or snow loads.
Extensive tunneling by boring insects, such as carpenter ants or certain beetle larvae, can further compromise the wood structure by creating voids and pathways for decay fungi. These pests contribute to the degradation of the trunk’s load-bearing capacity, accelerating the process of structural failure.
Another common defect is the frost crack, a vertical split in the trunk that occurs when rapid temperature fluctuations in winter cause the outer wood to contract faster than the inner wood. These cracks act as open wounds, providing easy entry points for fungal spores and insect pests. The entry of these pests and spores initiates internal decay, which leads to pathological weakening and eventual splitting.