In areas such as San Antonio, rainfall is generally seen as a good thing. Since this environment is prone to drought, small quantities of rain are welcomed as a way to provide plants and trees with the necessary water to thrive. Just like any other good thing, however, the excessive amount of rain in San Antonio has proven that Texas plants can be exposed to too much water. This has led to some trees becoming weaker or damaged and also increased the numbers of trees uprooted by natural causes.
Rainfall in Texas is generally fairly low, but May of 2015 saw enough rain to create the new state record. With an average of 8.81 inches across the state in May, Texas rainfall during that month beat the previous record (of 6.66 inches in June 2004) by a significant amount according to The Weather Channel1. This large quantity of rain gave San Antonio residents a glimpse of what happens in extremely wet weather, particularly to trees used to a dry, desert-like climate. Numerous trees were uprooted, but it is challenging to come up with a specific number of fallen trees due to the rainfall since it was accompanied by high winds and tornadoes, both of which also knocked over trees. In the case of the rainfall, experts may also be unsure whether a particular tree fell over due to the press of water from flooding or loosened roots in the soaked soil.
When the majority of people think about trees being uprooted or damaged during a storm, they will attribute this to lightning, high winds, or tornados. In the case of damage due to high wind, taller trees have a higher risk of windthrow, the technical name given to the tree trunk acting as a lever, causing the roots to experience more force.
In addition to wind and lightning, many trees also experience damage because the soil gets extremely wet. Given the same soil conditions over a wide area of land, some trees will experience issues while others don’t, and this typically is due to tree health. If a tree has wood decay from previous fungi, this can weaken a portion of the tree, although it won’t necessarily increase the tree’s chance of uprooting. Trees do tend to be more likely to be uprooted or fall over if they are large and located in a rocky environment or shallow soil. Trees that are top-heavy, with few branches on the lower limbs, also have an increased risk due to the uneven weight distribution and high center of gravity.
One of the biggest ways in that heavy rain impacts San Antonio trees is by causing damage to the roots, allowing them to come undone. There is no minimum amount of rainfall that will lead to this issue; instead, it depends on the drainage system of the soil. In areas such as Texas where wet weather is not typically a concern, landscaped areas aren’t necessarily designed with drainage in mind, leading to the accumulation of water in the soil. When the roots begin to fail due to wet soil, it is more likely that a tree will uproot than experience other damage.
To better understand the impact of wet soil on trees, you must first have base knowledge of tree structure, particularly the components below and above ground. If the roots are too shallow, the tree will not have as firm of a grip in the soil, increasing the risk of being uprooted. If, on the other hand, the roots are too deep, they may become stem-girdled, meaning the roots grow upward, wrapping around the tree’s base so the roots weaken and compress.
No matter the condition of the tree and its roots, when soil becomes wet, it becomes somewhat compacted. This allows for more room in the ground in which the roots may move more. Since this happens during heavy rainfall like that recently seen in San Antonio, it stands to reason that more trees have toppled or been damaged in the past month or two. It is also possible that completely saturated soil will make it impossible for oxygen to reach the tree’s root system, leading to root and branch die-back, and an increased risk of falling.
In addition to the higher number of San Antonio trees that became uprooted during the recent heavy rainfalls, this wet weather also had other negative impacts. It is common, for example, for an increase in foliar blights or fungi infestations to occur in heavy rain since the fungus involved thrive in these damp environments.
If you were lucky enough to avoid having your trees damaged or uprooted during the excess rainfall, you will still want to examine them to make sure they are not at risk if these storms are repeated. There are some obvious signs of risk that most people can notice by themselves, such as a significant lean of the trunk, particularly one that has begun recently, or obvious cracks in the tree’s stem.
You will also want to consult an arborist to help you evaluate other risk factors to your trees. They will look for mushrooms associated with wood decay that may indicate your tree has a weak spot. In some cases, an arborist may even be able to perform specific tests to check your tree’s risk, such as Sonic Tomography and a Resistograph Drill Test. They will also want to check for dieback in the crown and the root collar’s health.
After consulting an arborist, you will need to decide what to do with any damaged trees on your property. Uprooted and fallen ones will need to be removed from the property, but in some cases it is possible to save a damaged but not uprooted tree. An arborist may even be able to replant an uprooted tree in some cases, proved you act quickly. This is a more likely option in the case of smaller trees as they typically don’t undergo as much damage before falling over.