Impact of Storms on Trees

    Hurricane Beryl hit Texas in July 2024 with only a few days warning of its final track. Based on data from previous hurricanes, a Category 1 hurricane is expected to have devastating effects on trees, particularly trees in urban areas. There were specific factors associated with Hurricane Beryl, such as the fact that Houston and parts of southeast Texas were on the “dirty” side of the storm resulting in excessive rainfall and wind damage, which exacerbated the tree damage associated with a hurricane of this magnitude. Wind gusts of over 90 mph were reported, making the windspeeds of Beryl comparable to Hurricane Ike.

    According to the Tree Risk Assessment Qualification Manual, 2nd Edition (International Society of Arboriculture), “When wind speed reaches hurricane force (greater than 73 mph or 117 km/h), failure of defect-free trees can be widespread”. According to the Saffir-Simpson Wind Scale, the National Hurricane Center defines a Category 1 hurricane as: “Very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.” (https://www.nhc.noaa.gov/aboutsshws.php).

  Tree windthrow (trees falling over) frequency increases with increasing wind speed. The effect of wind is determined by the nature of the soil and the architecture of the tree. Trees act like a sail on a ship. The larger and denser the canopy (sail), the more force the wind has on the tree. Tree trunks act like a mast. The taller the tree, the more leverage the wind has to push the tree over. If the force and leverage is great enough, trees will fail at the weakest point.

    Soil saturation from excessive rainfall was a major factor that greatly increased windthrow during Hurricane Beryl. Wet soils increase the likelihood of windthrow in a storm because soil shearing (landslip) is more likely to occur, and roots can be pulled out of soil more easily. Besides the amount of rain that Beryl dumped on parts of Texas, much of southeast Texas has experienced above average rainfall all year. Soil conditions were much drier when Hurricane Ike hit southeast Texas in September 2008.  It should also be noted that large areas of Texas have heavy clay soils. Because of these poorly-aerated and poorly-drained-soils, the vast majority of tree roots are typically found in the upper 12 inches of soil, especially in irrigated landscapes, because tree roots need air (oxygen). Shallow rooting increases windthrow in saturated soils.

     In addition, numerous large urban trees in Texas communities are growing in locations with constricted rooting areas surrounded by concrete (e.g., street trees), which inhibit the ability of trees to explore a large enough area of soil to properly anchor trees to the ground. It is common for these trees to blow over in less than tropical force winds.

     There are numerous tree species in southeast Texas that have been identified in previous research to have a higher likelihood of failing in a windstorm (low wind resistance). These include trees such as pecan, Bradford pear, sugarberry (hackberry), and water oak trees, to name a few. Although tree failure data is currently limited regarding Hurricane Beryl, anecdotal evidence reveals that most of the downed trees were in the group of trees exhibiting low wind resistance.

   Cultural practices are also contributing factors in windthrow of trees. Overirrigation, fill soil, soil compaction, and other practices that cause root suffocation and dieback reduce a tree’s ability to adequately anchor itself in the soil. These practices also increase the occurrence and severity of root diseases, which are common in Texas. Urban trees, especially those grown in tree nurseries, tend to have poor limb, canopy, and root architecture, making them more likely to fail in a windstorm.

    Predicting weather events and their impact on trees is very difficult. It seems that every year brings a new weather-related surprise to Texas, so it is anyone’s guess what we should expect next. This will certainly not be the last windstorm that impacts your trees, so here is additional information to help you prepare and assess your trees before and after the storm: · https://hort.ifas.ufl.edu/woody/documents/FR173.pdf · https://sfyl.ifas.ufl.edu/media/sfylifasufledu/miami-dade/documents/disaster-preparation/hurricane-and-disaster/Assessing-Damage-and-Restoring-Trees-After-a-Hurricane-Gilman.pdf

     It is prudent to have a Tree Risk Assessor, who is qualified by the International Society of Arboriculture (ISA), assess your trees. Qualified Tree Risk Assessors (TRAQ) are proficient in the fundamentals of limited-visual and basic tree risk assessment and understand the principles of advanced diagnostic techniques for assessing tree risk. Tree Preservation Group, LLC has three TRAQ-trained ISA Certified Arborists and Board-Certified Master Arborists on staff, or you can go here to find one in your area: https://www.treesaregood.org/findanarborist/findanarborist.