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Deaths from Wind-Related Tree Failures
By Thomas W. Schmidlin, Ph.D.
We prune for aesthetics, we prune for tree health, and new research confirms the need to prune for safety.
Recent research conducted at Kent State University examines the hazard posed by trees to human health and life during severe weather. Data maintained by the National Climatic Data Center tracks information on storm fatalities in the United States. Examination of this data revealed that fallen trees accounted for 407 deaths between 1995 and 2007.
Fallen trees or limbs caused 53 percent of all deaths from thunderstorms in this period, 46 percent of the deaths from nonconvective high winds, 31 percent of all tropical cyclone deaths, and 4 percent of the deaths from tornadoes. Nonconvective winds are those high winds associated with large pressure gradients in mid-latitude, low-pressure areas, and also winds of dust storms and down slope and gap winds, as may occur in mountains.
Summarizing deaths from wind-related tree failures in the United States allows arborists to place this hazard in the context of other severe weather hazards in the United States. It forms a basis for comparisons with similar hazards elsewhere, and provides guidance for improved preparedness and safety recommendations.
Tree species vary in their resistance to breakage or being uprooted. Vulnerability for failure is further affected by tree age, size and health, and local site conditions of exposure, slope, soil type, soil moisture and adjacent trees. A tree’s resistance to stem breakage under load can also be affected by temperature. Additional information on the resistance of trees to breakage or being uprooted can be found in “Trees and Wind: A Bibliography for Tree Care Professionals” by Scott Cullen (Journal of Arboriculture, January 2002).
The goal of the Kent State research was not to assess wind speeds associated with deaths from fallen trees, but some estimates are provided from the literature. The Enhanced Fujita Scale for ranking intensity of tornadoes includes estimates of the three-second-gust speeds necessary for damage to trees (www.spc.noaa.gov/efscale/ef-scale.html).
For hardwood trees, such as oak, maple, birch and ash, a three-second gust of 74 mph will break large (greater than 1 inch) branches, 91 mph will uproot trees, and 110 mph will snap tree trunks. For softwood trees such as pine, spruce, fir and hemlock, a three-second gust of 75 mph will break large branches, 87 mph will uproot trees, and 104 mph will snap tree trunks. These are not absolute numbers but a value near the middle of the range of minimum wind speeds expected to cause the damage.
Wind speeds of these magnitudes occur with weak tropical cyclones, weak tornadoes, thunderstorm downbursts and winds associated with mid-latitude cyclones, among other meteorological events. In addition, heavy accumulations of snow or ice may cause trees to fail even with lighter wind speeds.
The online resource Storm Data (www4.ncdc.noaa.gov/cgi-win/wwcgi.dll?wwEvent~Storms) was searched for events for all 50 states with at least one death under the event categories of tornadoes, hurricanes and tropical storms, thunderstorms and high winds, or snow and ice. Storm Data contains the best national information available for these hazards and is the primary source used to examine past severe weather events, in spite of some omissions and inconsistencies in the data. In addition, the Tornado Project Web site (www.tornadoproject.com) was examined for deaths from tornadoes, and the annual review of the Atlantic hurricane season as published in Monthly Weather Review was consulted for additional details on deaths during tropical cyclones.
The National Weather Service’s Service Assessments published after severe weather events were also consulted. Summaries of deaths associated with Hurricanes Marilyn, Opal and Katrina were obtained from Morbidity and Mortality Weekly Report. Deaths from Hurricane Katrina in Mississippi and Louisiana are not included here due to uncertainty in the data, although indications are that at least 15 deaths were caused across inland Mississippi due to fallen trees.
The sex and age of the victims were recorded, along with the state, county and weather event type. If a fallen tree or limb was given as the cause of death, it was considered a death from a wind-related tree failure. The location where the person was when struck by the fallen tree, such as outdoors, in a vehicle, in a mobile home or in a frame house was noted. Deaths in campers, tents, on motorcycles or on all-terrain vehicles were considered “outdoors.”
Deaths in vehicles struck by a fallen tree and in vehicles that struck a fallen tree on the road were considered to have been caused by the fallen tree. Deaths from vehicles veering off the road and into a tree were not considered as a tree related death. Deaths from falling trees or limbs a few hours after the trees  were damaged by wind were counted as fallen tree deaths. Deaths from trees being cut or trimmed before the storm or during tree removal after the storm were not included. Although not always associated with high winds, deaths caused by trees or limbs that fell under the weight of snow or ice were included in this database as trees that failed as a result of severe weather.
There were 407 deaths caused by wind related tree failures in the United States during 1995-2007, an average of 31 per year. These occurred in 41 states and the District of Columbia. Of the 407 known deaths caused by wind-related tree failure during 1995-2007, the location of the victim (such as in vehicle, outdoors, in house) was known for all 407 deaths, age was known for 392 (96 percent), and sex was known for 391 (96 percent). Most (62 percent) of the victims were male with a median age of 44 years. Forty-four percent were struck by a fallen tree or limb while in a vehicle, 38 percent were struck outdoors, and 18 percent were struck while in their home – half in a mobile home and half in a frame house. Table 1 contains a summary of this information.
The regional distributions of deaths are shown on the map (Fig. 1, next page). Most incidents involved a single death, however, two fallen trees caused four deaths each. A falling tree struck a school bus carrying 10 children during a nonconvective high wind event in Queens, New York City, on March 6, 1997, killing four and injuring six. A tree fell on a vehicle in Yakima County, Washington, on August 26, 1997, killing all four occupants. This event is listed as a “high wind” in Storm Data, although other damaging events that day are listed as “thunderstorms.” The discussion below examines deaths based on the type of severe weather that caused the fallen trees.
Thunderstorm winds
There were 165 deaths from fallen trees caused by thunderstorm winds. About 40 percent of these thunderstorms are expected to have been associated with derechos. Derechos are widespread, long-lived windstorms that are associated with bands of rapidly moving showers or thunderstorms. Most (58 percent) of the deaths were male, the median age was 39 years (range 1-89 years), and deaths occurred primarily to persons in vehicles (47 percent) and outdoors (40 percent).
Deaths from fallen trees in thunderstorms occurred almost entirely (96 percent) east of the Great Plains (100oW longitude). This is perhaps due to lower population density and fewer trees to the west on the Great Plains or possibly the result of fewer severe thunderstorms farther west. A concentration of deaths occurred in the Megalopolis region from southern New England to northern Virginia, where population density is greatest. Most (78 percent) deaths occurred during May through August, when thunderstorms are most frequent in the eastern United States.
Nonconvective high winds
There were 143 deaths from fallen trees caused by nonconvective high-wind events. Most (61 percent) of the deaths were males, the median age was 45 years (range 1-90 years), and deaths occurred most commonly to persons in vehicles (50 percent) and outdoors (38 percent) (Table 1). Deaths from fallen trees during nonconvective wind events occurred most frequently in the Pacific Northwest and the mid-Atlantic coastal region followed by California and the southern Appalachians. There were three deaths in Hawaii.
Most (88 percent) deaths occurred during October through April, when mid-latitude cyclones are strongest. Although it occurred prior to the period studied here, one of the deadliest nonconvective windstorms in history killed more than 40 people in the Pacific Northwest in October 1962, many due to fallen trees.
Tropical cyclones
There were 57 deaths from fallen trees caused by 15 tropical cyclones during 1995-2007, not including deaths from Hurricane Katrina in Louisiana and Mississippi. Two-thirds (68 percent) of the deaths were male, the median age was 45 years (range 3-87 years), and deaths occurred in nearly equal numbers to people in homes, in vehicles or outdoors (Table 1).
The deaths occurred in 11 states from Texas to Vermont and all occurred within 200 miles of the coast. North Carolina had the most deaths with 14. Deaths occurred only in the months July through October, with a peak (58 percent) in September, corresponding with the tropical cyclone season in the North Atlantic. Trees may be especially vulnerable to uprooting or breakage during the winds of tropical cyclones due to the heavy rains that may weaken the soil strength. An additional factor is the existence of dense tree canopies on deciduous trees during the tropical cyclone season. The wind force placed on the canopies increase the likelihood of tree failure.
Tornadoes
There were 28 deaths that were directly the result of fallen trees during tornadoes. Of the 28 fatalities from tornado-fallen trees, 69 percent were males and the median age was 52 years (range 13-91 years). The most common location of a person killed by a fallen tree during a tornado was in a home (42 percent), followed by in a vehicle (32 percent) and outdoors (25 percent).
Fatalities from fallen trees during a tornado occurred in 14 states, all in the eastern half of the United States, with Arkansas having the most (four) of any state. Although tornadoes are most common in spring and summer, deaths from fallen trees associated with tornadoes did not show this same strong seasonality. Deaths occurred in all months except July, and 32 percent of the total occurred in two months, April or November.
In two cases there were multiple fatalities. Two men died in Simsboro, La., when their truck was crushed by a tree, and two men seeking shelter in a ditch near Denmark, Ark., were killed by a tree. The descriptions in Storm Data for the tornadoes that caused one or more deaths from wind related tree failures typically showed a path length of 5 miles and path width of 200 yards.
More than one-third (38 percent) of the deaths from wind related tree failures were associated with tornadoes rated F0 or F1 on the Fujita Scale, yet only 7 percent of deaths from tornadoes during 1995-2007 were from tornadoes rated F0 or F1. This emphasizes that risk of death from wind related tree failures increases at wind speeds of 70-90 mph. This is a lower threshold than associated with risk for death from high winds due to destruction of houses or vehicles.
Snow and Ice
Fourteen people were killed by falling trees or limbs due to heavy accumulations of snow or ice. Most were males who were outdoors. Ice accumulations were the cause in 10 of the 14 deaths.
The median age was 69 years (29-78 years). These deaths were concentrated in the northeastern portion of the United States and in Washington state, a pattern that aligns with the regions experiencing the maximum number of hours with freezing rain. Somewhat surprisingly, there were two deaths in Louisiana following an ice storm. Deaths from fallen trees due to snow or ice occurred from October to April, with 50 percent in December.
Summary
Deaths from wind related tree failures occurred in 41 states and the District of Columbia. New York had the most (30), followed by Washington (28), Pennsylvania (27), Georgia (24), and North Carolina (24). A death rate was calc ulated for each state by dividing the total deaths from wind-related tree failures during 1995-2007 in each state by the 2000 state population in millions. Mississippi (5.3 deaths/million), New Hampshire (4.9 deaths/million), and Washington (4.8 deaths/million) were the three highest rates in the country. These 13 years of results are noteworthy, but the associated statistics could change significantly with the addition of a few deadly storm events in the future.
Persons killed by wind-related tree failures during tropical cyclones and tornadoes were more commonly at home (~40 percent) when struck than those killed by thunderstorm and nonconvective high winds (~12 percent at home). A tornado warning or hurricane warning causes people to seek shelter indoors. In contrast, driving and other ordinary daily activities continue during thunderstorms and nonconvective high winds, placing people who are in vehicles or outdoors at risk from fallen trees.
In addition to deaths caused directly by fallen trees, as summarized here, wind-related tree failures cause many non-fatal, but life-changing injuries. Indirect deaths and additional suffering are caused by trees that have fallen on roads and blocked rescue to injured persons. Additional indirect deaths and injuries occur when trees or limbs broken in a storm, known as “widow-makers,” fall later. Unsound or dead trees or large limbs also fall and cause deaths without the presence of severe weather, and are not included here.
People will always live and spend leisure time around trees and trees provide many benefits to our environment. Meanwhile, the legal liability of tree owners for damages caused by fallen trees has been rising in the United States. The risks from fallen trees described here emphasize the need to maintain trees in a healthy and structurally sound state in order to minimize risk.
Even healthy and sound trees may be broken or felled by strong winds or ice accumulations, so severe weather safety recommendations should continue to emphasize seeking shelter in sturdy buildings when any type of high winds are expected. Risks to people outdoors in tents or campers may be reduced if they are directed to seek shelter in a sturdy building or, if none is available, enter their vehicle and park in an open area away from trees for the duration of the high winds.
As a professional, the arborist needs to actively advocate for the selection and planting of those varieties of trees that are less susceptible to deadly breakage or uprooting. A continued effort should be exerted to educate property owners of the need for proper tree maintenance aimed at sustaining the health/structure of the tree and thereby minimizing harmful effects from storm-related stress. A tree risk assessment and tree risk management program should be in place, especially in public areas such as along roads and in parks.
Thomas W. Schmidlin, Ph.D., is a certified consulting meteorologist with the Department of Geography at Kent State University in Kent Ohio.
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