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As the floodwaters recede, mountains of debris are left behind – sheets of plaster, loose clothes, mattresses and, of course, trees. Some debris I’ve seen in floods includes massive tree trunks weighing 5 tonnes of more, bobbing along like corks in the rapidly flowing waters.
The trees that line our creeks, rivers and floodplains are on the front line when major flooding occurs, and bear the brunt of the flood’s mighty forces. However, they are often victims of floods. Trees are our greatest allies.
From stabilising river banks with the strong grip of their roots to changing the course of floodwater, here’s how trees influence floods – and how floods can kill them.
How trees can affect floods
The fine and large roots of trees such as river red gums. Bind and consolidatesoil, stabilizing river banks and decreasing erosion. This reduces sediment in waterways and prevents them from becoming muddied or clogged with silt.
Smaller plants, such as shrubs, can be protected by large trees. They act as a physical barrier and shield other vegetation from floodwater’s forceful momentum.
This is because the presence of trees slows the floodwaters’ speed, as their trunks, roots and branches block and deflect water, and change the direction of flow.
However, floodwaters that are slowing down can cause floodwaters to expand, inundating areas farther away from the river course. This is called a Important consideration when creek and river banks are being revegetated – we want to capture the benefits trees provide, but also ensure that if floodwaters slow down there’s no greater risk to property or life.
A different, but related role, is that trees can help prevent landslides. Indeed, Landslides have happenedFloods continue to threaten homes and people in flood-affected areas like Illawarra and Kangaroo Valley, NSW.
Tree root systems can be found on slopes consolidate soilsThey also help to prevent super saturated soil from moving downhill. It can cause problems when people remove trees from their homes and along roads as part of bushfire prevention programs. They don’t realize that cleared sites or roadside verges could be susceptible to landslides.
Sometimes a compromise can be a better way to manage trees. Instead of removing all trees from a slope or verge and planting trees that can slow down or resist fires, you might consider leaving some trees with large root systems. You might also consider planting species that will reseed after fires. For example, tree ferns, so their roots systems aren’t lost and the soil doesn’t erode.
Floods can also cause damage to trees
Some trees won’t survive major floods as the water’s brute force undermines their root systems, bringing them down.
Other times, the debris can act as a battering device on large trunks, including whole trees or large branches. Although most large trees will survive, some trees will be repeatedly battered until their trunks, major branches, or root systems are destroyed.
These become part of the debris that causes infrastructure damage, such as bridges or other trees downstream.
Floods are usually a temporary event that lasts for a few days or weeks. For most trees, however, they are not a serious problem. Many tree species are able to adapt to this situation. But what about those trees that might be inundated for several weeks or even months after floods?
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After flooding, soils can stay very wet for a while. Some trees, like river red gum, can remain very wet for a long time after flooding.Eucalyptus camaldulensis) and swamp gum (Eucalyptus ovalta), can tolerate inundation for many weeks. We’ve seen populations of River red gumFor example, they can cope with up to nine month of inundation.
Others may not fare as well if they are left under water for too long. This applies to older trees, certain fruit trees such as citrus and stone fruit species, and some conifers.
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Waterlogged soils have low oxygen levels, which makes it difficult for roots to maintain their normal metabolism, health, function, and health. This also affects the fungi responsible for healthy roots. The less favorable conditions will be the longer low oxygen levels continue to persist.
Low oxygen in soils lead to anaerobic respiration – when cells break down sugars to generate energy without oxygen, producing alcohol and lactic acid. Both alcohol and lactate are mild poisons. However, as they rise, root and fungal cell levels will increase. Can be killed.
Waterlogged soils can also lead to roots becoming starved and deprived of their energy sources. And once the roots start to go, there’s a rapid downward spiral in the tree’s condition.
Trees can quickly die, sometimes in a matter days. rapid deathsThese are more common in stressed trees. These conditions are difficult to manage.
After flooding, be careful around trees
Riverbanks can be ruined by trees that have survived floods. This can pose a danger to those who approach the riverbanks at the wrong moment.
The trees become less stable when the roots die. Winds can cause soil to become unstable, causing the damaged root system to be susceptible to damage. Windthrow is possibleIt is the place where whole trees are blown away.
This can happen after a flood and it can happen for weeks or even months.
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The major floods inundating NSW and Queensland are not the first – and will certainly not be the last – many of us will experience in our lifetimes.
For those of us who have been acutely aware of the prediction of major flooding events as part of climate change, these events haven’t come as a surprise. They were unavoidableJust as deadly bushfires and torrential storms are expected, so too are the fiercer ones.
Trees have always been a part of flood-prone ecosystems and this land of extremes. Yet, many waterways are seeing trees disappearing at an alarming pace.
Climate change poses new dangers to trees, but it also creates opportunities. We have new opportunitiesto work with trees as allies in tackling climate change and its effects. We cannot work against them.
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Here are 5 ways trees can help you survive climate change
