Floods and heavy rain – consequences for the health of urban trees

How Heavy Rain Can Damage Trees

Trees need rainfall because they provide water. This water supply is essential and is used directly by the tree or stored in its root system for later use. However, the excessive rain we have experienced in recent summers can also cause significant damage to our trees - sometimes even emergency tree removal is necessary. Since heavy rain and flooding occur again and again in the summer and, according to climate researchers, will also increase in the future, you can find some information here about how extreme weather events can damage trees and what you should look out for as a tree rescuer.

Weakened roots

In Europe, extreme rainstorms tend to occur in summer and autumn. They are often accompanied by strong winds that can cause further damage to trees. These storms tend to cause the most damage to trees when the soil is already saturated with water and the root system is weakened. This increases the risk of the roots becoming loose in strong winds and the tree falling over. The longer trees are exposed to strong winds and heavy rain, the more likely we are to have problems with trees falling over.

Soil oversaturation due to excessive rainfall

Heavy rain can cause problems by oversaturating the soil water content (for example when the soil cavities are completely filled with water). Soil that is oversaturated with water impairs the nutrient uptake of tree roots. Trees absorb oxygen from the air pores in the soil not only through their foliage but also partly through their roots, i.e. they absorb oxygen and release carbon dioxide through pores in the soil. During heavy rainfall, these pores can fill with water, allowing the trees to absorb oxygen through the soil. If the soil is oversaturated with water, it can prevent oxygen from reaching the root system. After a certain time, photosynthesis can come to a halt and the tree begins to live off the stored starch. Root death can be the result. 

This situation can lead to a number of problems and some trees are more susceptible to damage from heavy rainfall. Trees with compacted soil can fall over more easily when the ground is softened. When soil is compacted, air space is reduced and roots grow closer to the surface and have less purchase in the earth. Trees with large canopies are also more susceptible to falling over when the soil is oversaturated. Trees growing in a confined space, such as between a walkway - primarily urban trees - are also more affected. When a tree's roots do not have room to grow and expand, uneven weight distribution occurs, compromising the tree's structure. Finally, certain types of trees are more susceptible to damage from extreme weather conditions in summer.

Too much moisture promotes fungal infestation

Too much rain can lead to an increase in fungal infections such as root rot and anthracnose. Fungi thrive in the wet and humid conditions we experience during this warm, humid season and can affect trees in a variety of ways, including dropping leaves, lesions, weakened roots and discolored foliage.

What can be done about it?

There are different approaches to solving the problem at different locations. A distinction can be made between young trees, city trees and forest trees. For newly planted young trees, it would be a good idea to install erosion protection mats or grids in the ground to prevent soil erosion during flooding and heavy rain. These help to anchor the roots of the young trees better in the soil surface. The forestry and green space departments of the cities and municipalities must take this into account in their urban planning and act with foresight. The same applies to city trees, which can offer little root space for city trees due to the sealing of the soil surfaces or due to old town centers that are already underpinned (as underground installations and underpinnings exist). In self-regulating forests, clear-cutting of forest trees should be avoided.

As a private individual with your own garden or as a tree sponsor, you can also react after such incidents: Always go outside after heavy rains and inspect your trees for signs of root or soil damage. Look for exposed roots and excess soil. Also look for new cracks in the trunk of the tree and whether it has leaned to one side. After inspecting your trees, you should contact a certified arborist immediately if you notice any damage. The arborist can inspect your trees and perform any necessary tests to assess the health of the trees. In some cases, they can save a damaged tree. In other cases, it will need to be removed.

You can go a long way by taking good care of your trees year-round. Properly pruned and maintained trees are more likely to withstand the effects of excessive rain and storms. They are also less prone to spreading disease to other trees.

digression: Why have extreme weather events occurred so frequently in recent years?

Because warmer air can carry more moisture, storms that form over warmer-than-average oceans tend to bring a greater amount of rain. The most extreme example was Hurricane Harvey in 2017, which formed over unusually warm waters in the Gulf of Mexico.
Warm waters also increase the energy and wind speed of hurricanes. Researchers have found that storms are generally getting stronger and strong tropical hurricanes are becoming more common.
A third way that climate change is intensifying storms is that underlying sea level rise increases storm surge heights and pushes seawater further inland.

What may be confusing for non-meteorologists: Tropical storms are called hurricanes when they form over the North Atlantic and Northeast Pacific, cyclones when they form over the South Pacific and Indian Ocean, and typhoons when they form over the Northwest Pacific.

Not only do tropical storms bring extreme rainfall and flooding, but floods in Western Europe in recent summers have also become more frequent due to climate change. Recent research suggests that climate change is increasing the likelihood of slow-intensity severe weather events in Europe, which have a greater potential to cause severe flooding and heavy rainfall.

Atmospheric rivers are currents of air that carry huge amounts of water vapor and are typically 400 to 600 km wide. When an atmospheric river hits land, especially mountainous terrain, it releases much of that water vapor in the form of rain or snow and can cause severe flooding. These storm systems are common - on average, there are about 11 of them on Earth at any given time. They are predicted to increase in intensity as the atmosphere warms.