Hallie Dozier & Steven Wright

The tropical cyclone season is approaching for people living in the southeastern United States. Homeowners across the region are eyeing their trees and wondering how safely they will weather the season. Should the tree stay or should it go? For many homeowners, the visible and physical impact of the destruction of trees and property during storm events is enough to prompt them to engage in “chainsaw backlash,” or the unnecessary removal of perfectly good, sound trees. We have all seen it – a neighbor takes down a beautiful tree because he or she is afraid of what might happen when a storm strikes. And with the tree go all of the benefits that trees provide: cleaner air and water, soil conservation, climate moderation, higher property values, shade and beauty. In regards to trees, people seem to fall in with either tree cutters or tree huggers. The sad fact is that because of this dichotomy, many perfectly good trees are removed because of fear of what might happen while others of us hang on to our trees for dear life (“But it still has green leaves!”), unaware that perhaps some of these older, larger specimens may not be structurally sound. So we end up losing good, sound trees and hanging on to the rotten ones that will fail when the storm strikes.

So how do we choose wisely which trees to keep and which to remove? A lot of factors come into play when making the decision to remove or retain shade trees on your property. How old is the tree? How big is it? How much damage would it do if it came down during a storm? How much will it cost to have it removed? And how much will you miss the amenities it offers (beauty, shade, cooling, property value, etc.) if you remove it?

Although trees can cause considerable damage when they fall, in truth, most trees do not fall during storms and of those that do, only a small number strike a target. Moreover, just because a tree is big and old does not necessarily mean it is dangerous – but that is not something a typical homeowner has the experience to assess. At the same time, just because a tree is alive does not mean that it is sound and firm. So,before you get out your chain saw and start sawing away at your favorite tree (Figures 1), check out these results from a recently published study from researchers at the University of Florida. Over the last 15 years, Dr. Mary Duryea and her research team have surveyed over 18,000 trees in hurricane-stricken communities in the Southeast. They have scoured the scientific literature and interviewed dozens of tree experts to draw together some generalizations about southeastern city trees and how they fare during high wind events. In conjunction with the skilled advice of a licensed, certified professional arborist, homeowners can use these results to make informed decisions about removal of potentially hazardous trees and make wise selections of tree species to put into their landscapes.

Duryea’s on-the-ground surveys plus input from a variety of experts and the scientific literature reveal that tree susceptibility to hurricane damage depends first and foremost on wind speed. As wind speed increases, tree loss also increases. But wind speed aside, certain tree traits factor into how trees survive storms, such as wood density and flexibility, crown density, tree age and trunk diameter. Specifically, the study concludes that three characteristics give trees the ability to withstand hurricane strength winds: (1) defoliation during storms; (2) high elasticity of the wood; and (3) high modulus of rupture (how much the wood can bend before breaking) (Tables 1-3).

Table 1. Tree species with the highest and lowest leaf loss

The metabolic “cost” of growing wood is quite high for a tree, much higher than the metabolic cost of growing (or re-growing) leaves. So trees that shed their leaves easily in high winds actually exhibit a survival strategy that spares wood and avoid the high cost of replacing stems and branches. The end result is better survival, less branch loss and better recovery following the storm.

Table 2. Tree species with the highest and lowest elasticity

Tree species with wood that is flexible and has a high breaking point also fare better than trees with brittle, hard wood. If a tree can bend with the wind, it will not break, leading to better survival, less branch loss and better recovery. Size of the branches and trunk of the tree also comes into the picture. Older trees are larger and have many more layers (annual rings) of wood than younger trees. Like people, older trees tend to be stiffer, more brittle and therefore, more breakable than younger trees. And height of the tree also has an impact. Taller trees have their canopies high above the ground, where the wind speeds are highest. Smaller tree species that grow nearer the ground get more wind protection from nearby buildings and other trees and therefore tend to survive hurricanes better.

Table 3. Tree species with the highest and lowest modulus of rupture*

* High modulus of rupture means trees are not likely to break or fall.

Duryea also reported that native trees, such as live oaks, bald cypress and American holly, perform better during storms than non-native trees. Soil saturation and root depth were other important factors determining the likelihood of a tree failing. Shallow-rooted trees were more likely to fall than deeply rooted species, and they became more prone as soils became more saturated. Trees grown in clumps (5+ trees) also tend to perform better, probably because of the way they can protect each other from straightline winds.

To further validate these findings, Duryea headed a study that surveyed the damage of urban trees following each of the hurricanes Erin, Opal, Ivan and Dennis. This survey was carried out in 100 Florida neighborhoods and included more than 18,000 trees. From this research, they reported the percent survivability of twenty-six of the most common species. The table below lists the most wind-firm and the least wind-firm species.

Table 4. Highest and lowest survivability of tree species

Because these are urban trees, property damage is a concern and must be considered. Knowing that a tree is able to withstand and survive a hurricane does not necessarily mean it will not cause damage during a storm. With this in mind, they also reported the average branch loss of the same twenty-six species. The table below lists the species with the most broken branches and the fewest broken branches.

Table 5. Tree species exhibiting the most and fewest branches

A final step was to incorporate the tree survey data with the scientific literature and a survey of eighty-five urban forest professionals, arborists and scientists that asked them to rate the wind resistance of the southeastern urban trees based on their post-hurricane observations. The results are in Tables 6-9.

Table 6. Tree species showing the highest wind resistance (adapted from Duryea et al., 2007)

Table 7. Tree species showing medium-high wind resistance (adapted from Duryea et al., 2007)

Table 8. Tree species showing medium-low wind resistance (adapted from Duryea et al., 2007)

Table 9. Tree species showing the lowest wind resistance (adapted from Duryea et al., 2007)

*Invasive across the Gulf Coastal Plain. Not recommended by LSU AgCenter.

So how can a homeowner use this information to make wise tree removal and tree retention decisions before the storm? By combining these species lists with the expert advice of a Louisiana State-licensed or International Society of Arboriculture-certified arborist, a homeowner has the very best tools for decision making. By hiring an arborist the homeowner will have the best expertise for examining each tree for signs of decay or weakness and help assessing the overall health and vitality of the tree.

Sound, healthy trees of species that have proven to be resilient and consistently withstood hurricane wind damage are the best ones to keep. These include the hollies, live oaks, crape myrtle, palms, cypresses and Southern magnolia. Existing trees of other species – even species noted for being the least wind resistant (southern red cedar, spruce pine, sand pine and southern red oak) – should be assessed for overall health and structural integrity. If the tree is sound, there is no real reason to remove it, but because of demonstrated poor performance in high winds, these trees should be inspected on a regular basis so that problems can be corrected or avoided before damage occurs.

Enjoy your trees and benefit from the services they bring to you and your neighbors. Keep them as long as they do not pose a threat to people or property. Strong, healthy, vibrant, structurally sound trees of the right species that are planted in the right place should be a source of pleasure, not fear.

REFERENCES

Duryea, M.L., Kampf, E., & Littell, R.C. (2007) Hurricanes and the urban forest: I. Effects on Southeastern U.S. coastal plain species. Arboriculture and Urban Forestry, 33, 83-97.

Duryea, M.L. (1997) Wind and trees: Surveys of tree damage in the Florida Panhandle after Hurricanes Erin and Opal. Circular 1183 of the University of Florida Cooperative Extension Service. http://edis.ifas.ufl.edu/.

Duryea, M.L., Blakeslee, G.M., Hubbard, W.G., and Vasquez, R.A. (1996) Wind and trees: A survey of homeowners after Hurricane Andrew. Journal of Arboriculture 22: 44-50.