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Tree failures cause a significant number of fatalities and a substantial amount of property damage every year. In the United States between 1995 and 2007 there were 407 deaths caused by wind-related tree failure. These were spread across varied weather events: 41% were caused by thunderstorms, 35% by non-convective high winds, 14% from hurricanes, 7% from tornadoes, and 3% from snow and ice. Wind-related tree failures were responsible for approximately half of the deaths from thunderstorms and high winds and a third of the deaths from hurricanes. These figures do not include additional damage such as nonfatal injuries, property damage, indirect deaths (e.g. lack of access to medical services due to downed trees), or trees and limbs damaged by the storm that do not fall until later. Property value losses in the state of Georgia alone from wind-related tree failure alone are estimated to be over $10,000,000 per year.

The definition of tree failure given by the International Tree Failure Database User Manual is “A structural failure or physical breakage of the tree trunk, one or more branches, or one or more tree roots.” The vulnerability of a tree to failure is dependent on three factors: the structure and condition of the tree, the particulars of the site, and the force to which it is subjected. In this chapter we will examine each of these factors and hopefully begin to gain an understanding of the complex interactions that affect whether a tree stands or falls. All the information covered in previous chapters has described many of these various elements; in this chapter we will focus on their integration into the relationship between cause and effect.