Trees and flooding: too much of a good thing?

Trees need water, but in excess, such as that found in flood conditions, water can be more damaging than a prolonged drought, especially in urban settings. 

I am going to be exploring: 1) how flooding damages trees; 2) how we can undo some of the damage (mitigation); and 3) some of the tools to futureproof our urban trees. 

I’ll now start with part one. Soil is naturally composed of a mix of larger and smaller gaps, called macro and micro pores. Only macro pores are big enough to hold water under gravity (the water droplets cling to the surface of soil particles). Air can only effectively circulate within the macro pores. There are fewer macro pores in compacted soil, and urban soil is often compact and more prone to flooding. Tree roots and the array of organisms found in soil, such as worms, beetles and fungi, are dependent on oxygen in the soil. If the oxygen level falls below 15%, tree roots stop functioning properly, including, ironically, the ability to absorb water. 

Affected leaves will turn yellow and wilt, dying prematurely and causing branch tip dieback. Soil without sufficient oxygen contains high levels of iron and magnesium, which can also hinder photosynthesis and growth. 

As part of the process of photosynthesis, plants lose water from their leaves, via cells called stomata. The water loss happens as the leaf releases oxygen, which is toxic to the plant, and absorbs carbon dioxide. If the flow of water to the leaf is reduced, then the stomata close; this stops water loss, but also stops the release of oxygen and the absorption of carbon dioxide. The process of photosynthesis is paused, and vital sugars, which sustain the plant, become depleted while toxins build up. 

Flood waters can also leave roots vulnerable to some fungal pathogen attacks. Armillaria, Phytophthora, and Pythium can all be carried in water. Phytophthora thrives when there are low levels of oxygen, and its spores can travel in water.Flood waters can allow such pathogens to access the root collar; a part of the tree which is particularly susceptible to infection. 

Factors influencing flood resistance: 

l The duration of flooding is important. Many trees can resist the impact of a temporary, but not extended, period of flooding. 

l Trees with more reserves of carbohydrates are best placed to respond well. 

l Coniferous trees seem to be more vulnerable compared to deciduous trees. 

During flooding, the raised water level can destabilise roots, leaving the tree vulnerable to being uprooted in windy conditions. 

Repeated flooding can affect trees in the longer-term; the flooding can reduce root growth leaving the tree vulnerable to drought stress, (larger root networks enable the tree to access more water when it is scarce…) 

In part two, we will look at how to treat trees following flooding 

By Dr Glynn Percival 

2560 1920 admin
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