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Hurricane Gilbert,_LSU Earthscan Lab Photo
Hurricane
Resistant Urban Forests
Prof. Buck Abbey, ASLA, pa AIA*
LSU Hurricane Center
Louisiana State University
Northern fronts begin to wane.
Soft southern winds blow out of the tropics.
Southern magnolia flowers have faded and fallen.
Those cool crisp blue-sky days of spring have given way to
popcorn like summer cumulus clouds floating in from the Gulf of Mexico.
1.
The gathering of these clouds, rising afternoon temperatures above 85 degrees and sudden rain bursts are the calling card of the summer hurricane season along the Louisiana coast. Indeed, hurricane season starts June 1 and goes late into the fall ending generally in November. Anytime during this season, the state can be visited by tropical disturbances, tropical storms or damaging hurricanes of high cyclonic wind. All of these storms can cause property damage and massive destruction of the urban forest.
High wind is a problem for community forests in Louisiana.
Wind storms can cause damage to canopy trees that grace our southern landscape. Trees along streets, within parklands, in shopping centers and in subdivisions can all be impacted by tropical weather. Storms can remove favorite trees from gardens, cause trees to fall on homes and autos and flatten tracts and swaths of forested land.
There have been several studies in recent years on the effects of coastal storms on vegetation. One study prepared by LSU following hurricane Andrew examined how property owners can prepare ahead of time to prevent wind damage. This study and several others indicate that windstorms and coastal flooding will remove weak, declining and unhealthy tree stock. These are the first to go. Healthy, well-maintained and younger trees have a better chance of surviving. Certain species will fare better in storms. A profile of the perfect wind resistant tree paints a picture of a tree that can withstand moderate hurricane winds and offer protection to our homes and gardens. (See sidebar) We know for example that after every windstorm, damage to trees can be expected. This damage may take the form of de-leafing, branch removal, severe structural damage or death of the tree.
De-leafing occurs where leaves are either blown away or desiccated by fast moving wind. Desiccation, the forced removal of oxygen from leaves will cause leaves that remain on the tree to wither and fall off within days. Normally within hours or days, the tree will send up a new generation of leaves to replace the old and carry on the business of capturing sunlight.
Branch breakage is caused in weak wooded trees. Stronger storms, those with winds below 100 mph may cause twig removal and branch breakage. This type of damage will litter the yard but will not cause major problems to the tree. Minor pruning can remove any hanging branches or branches that are holding dead leaf matter.
Severe storms like category two and three hurricanes, those with winds over 100 mph can cause major structural damage to trees often leading to disfigument.. Major branching, prime leaders and the trunk of the tree itself can be twisted, split or cracked. Professional arborists can treat this type of damage and insure the life of the tree. Arborists can remove broken limbs, reshape leaders, treat minor structural damage, lower the center of gravity of the tree and reshape the trees outline to improve it appearance. The tree will show scars but can function for years after the storm.
High winds, those coming out of category three and four storms or from tornados that are often spawned within hurricane fronts, cause major structural damage, severe wood splitting, trunk twisting, uplifting and bole snapping all of which can result in loss of the tree. In some instances if expert arboricultural practices are performed quickly and the tree is of a forgiving species, the tree can be pruned, anchored and severely trimmed to new form. Quick action will often save the tree but the scars of the storm will remain. Many examples of these survivor trees can be seen in Louisiana communities. They are part of our native landscape. Part of the character of our urban forest.
Very strong storms, those with wind speeds above 150 mph cause terminal damage to the urban forest. De-barking as a result of sand and stone throw will in itself strip the tree of its protective bark leading to tree termination. Structural breakdown, uprooting, over-throw, stem splitting and saturated soil failure also lead to the death of the trees. Trees are often overturned and uprooted during these storms and must be replaced and should be replaced with more hurricane resistant vegetation.
Landscape architects should be asked to offer recommendations for species with better wind resistant capability.
New studies of trees following hurricanes offer new knowledge into the way vegetation reacts to hurricane storms. We know from studies of storms like Camille 1969, Hugo 1989 and Andrew 1992 that the trees that come down during storms are always the weakest and most mis-shaped trees in the area. They are often non-native trees unsuited by nature to our exposed coastal landscape. Trees that are often snapped, broken or overturned are often found to be diseased, hollow and rotten in the core, old and in decline and severely imbalanced. Many of these trees have had severe root pruning from sidewalk construction or the installation of underground pipe, line and cable. Many trees that come down during storms show scars from mankind’s interference and carelessness during construction.
Healthy and well-maintained trees often have a much better chance at surviving tropical winds.
What is the perfect wind resistant tree for Louisiana?
Native trees, particularly those with wide spreading branches, low centers of gravity, strong deep penetrating root systems, and small leaf size seem to hold up better in tropical storms! Especially so, if they are found growing in mixed groves of trees. Lone growing solitary specimens have less wind resistance than massed trees.. Trees like live oak, cypress, crape myrtle, winged elm, magnolia, tulip tree, red oak, American holly, sweetgum, sycamore and red cedar hold up to tropical storms better than most.
The cypress tree either Taxodium distichum or Taxodium acendens is Louisiana’s premier storm resistant tree! This is largely due to their deciduous nature, fine leaf pattern, well tapered and balanced central leader, minimal branch pattern, broadly balanced root system with their peculiar “knees,” and their native habitat along coastal margins.
What to do to prepare for that storm?
Remove weak and diseased trees as well as any large trees within one hundred feet of your home or car. Prune and thin trees to give them a lower center of gravity and to lessen leaf mass. Sculpting a tree will allow wind resistance to increase. Lightly fertilize annually and be sure that all trees are growing where their root zones are covered with a good organic mass of healthy forest floor material. Watering during the drought periods of summer will allow the roots of the plants to maintain their turgor and there fore their holding power. Plant trees in groves and add ample shrub mass to deflect wind upwards. This places what is called the point of overturning pressure farther from the root zone into the area of the tree with more flexibility. One further action to protect the urban forest, remove non-native over story vegetation, keeping in mind that canopy trees in any community are composed of native trees that have withstood coastal storms in the past and will do so in the future.
Vegetation in your community offers some protection to you and your property from damage caused by low category hurricanes and tropical storms. Maintain the urban forest in your community and around your home and your trees will protect you. Pay no attention to the forest in your community or garden and the next tropical storm that comes along will have its way. Healthy community forests forming an over story vegetation canopy and growing as a windbreak reflect, deflect and filter strong winds. Green communities will slow the wind offer some protection to your homes and will provide cover for urban wildlife that also must survive the storm.
Contact the LSU Hurricane Center at www.hurricane.lsu.edu for assistance with storm proofing your garden, or your community.
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1. Note 1, print lead-in to story as shown above
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Native species
Slow growing trees
Hard woods
Young to middle age tree
Very healthy and vigorous
Well maintained
Low center of gravity
Deep penetrating radial roots
Open branching character
Heavy stout leaders, flexible limbs and short leaf branching
Low open globular sillioute
Fine leaf texture
Deciduous leaves
Growing in a grove or windbreak
No damage to root zone
No construction scars
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Side Bar - Times Picayune
Top 10 Louisiana Survivor Trees
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Bald Cypress - Taxodium distichum
Live Oak - Quercus virginiana
Cabbage (or Sabal) Palm - Sabal palmetto
Windmill Palm - Trachycarpus fortunei
Mexican Washington Palm - Washington robusta
Black Gum - Nyssa sylvatica
Cow Oak - Quercus michauxii
Ironwood - Carpinus caroliniana
Shumard Oak - Quercus shumardii
Winged Elm - Ulmus alata
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The Perfect Wind Resistant Tree For Louisiana Communities
Side
Bar
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American Elm, hard to find
American Holly
American Hop Hornbeam
Ashleaf Maple
Bald Cypress
Black Gum
Black Walnut
Black Locust
Catalpa Tree
Cherrybark Oak
Cherry Laurel
Cow Oak
Crape Myrtle, should be staked, will roll over in saturated soils
Dahoon Holly
Ironwood
Green Ash
Hackberry
Live Oak
Long Leaf Pine
Mexican Fan (or Washington) Palm
Nutall Oak
Osage Orange
Pond Cypress
Red Cedar
Red Maple
River Birch
Sabal (or Cabbage) Palm
Savannah Holly
Shumard Oak
Siberian Elm
Southern Magnolia,
Sycamore
Sweetbay Magnolia
Sweet Gum,
Tulip Tree
Water Ash
Water Oak, young to middle age
Willow Oak
Windmill Palm
Winged Elm
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Wind Resistant Trees For Louisiana Communities,
Louisiana State University School of Landscape Architecture
Washingtonia
robusta, Mexican Fan Palm.
Designed
by nature to withstand wind.
Monrovia Nursery Photo
Taxodium
distichum, Bald Cypress.
Louisiana’s
most wind resistant tree.
Abbey
Associates Landscape Architects Photo