Abundant rainfall: the effect on trees

It will not have escaped anyone's notice that we are experiencing an abundance of rainfall in large parts of Europe this year. Last autumn was also extremely wet; a big contrast to previous years characterised by heat and drought. Now, high groundwater levels play a role and we see thoroughly saturated soils. Even on normally dry (sandy) soils, water stagnates. Due to the abundant rainfall, trees have to cope with too much water, which can lead to various health problems. In this article, we discuss both the symptoms and consequences of too much water for trees. We describe measures that can help keep trees healthy during long periods of heavy rainfall and flooding.

Trees need a number of essential elements to grow well, such as water, oxygen, light, nutrition and good soil life. For optimum growth, all these factors need to be in balance. Once one of these elements gets the upper or lower hand, it is detrimental to the tree. In the case of overwatering, this can lead to a number of crucial problems that can seriously affect the growth and overall health of a tree. This does not only apply to recent plantings, trees that are well established can also show problems.

Excessive water reduces oxygen in the soil, resulting in root rot. Wet conditions can also affect the overall structure of the soil. This is largely due to the lack of oxygen, mainly because the excessive water kills soil life.

Symptoms of overwatering

The first symptom of overwatering is discolouration in the leaves, which become dull. The follow-up symptoms are all very similar to desiccation, simply because with root rot, the tree can no longer absorb moisture and thus basically dries out. With drought, we see the damage from above in the outer crown parts turning inwards, whereas in an overly wet situation it is exactly the other way around. In that case, the damage spreads from the heart of the crown outwards and the tops of the branches are often still fresh green.

Trees in excessively wet sites show the following symptoms:

• Wilted leaves
  Despite sufficient moisture, the leaves may still droop limply. This is caused by a lack of oxygen at the roots and possibly even root rot, which has made moisture absorption impossible. This leads to wilting of the leaves.
• Leaf discolouration
  The tree shows brown leaf margins and yellowing. Overwatering can lead to nutrient deficiency, mainly because essential nutrients are washed away from the roots.
• Early leaf fall and withering
  A key feature of desiccation is death of the young shoots, including leaves. However, this can also be a feature of too much water. Both over- and under-watering can cause a stress reaction by the tree.
• Root rot
  Underlying almost all symptoms is a poor condition of the root zone, resulting in root rot. Carefully digging away the soil around the roots exposes them; black roots are a sign that they are affected by root rot. Fungi that have a negative impact on the roots and thrive under oxygen-deficient conditions can also often be found. Root rot can be recognised as a musty smell, sometimes with the characteristic stench of rotten eggs.

Ensure optimal growing site

It may seem obvious, but proper layout of the growing site is essential. For this, a number of things are important in and around the planting hole:

• Drainage
  Good drainage is an important part of this. One way to do this is by drilling vertical holes with a soil drill and filling them with lava substrate. Abundant drainage can also cause the tree to dry out. It can also help to apply better planting soil around the roots. This can be tree sand or a mycorrhiza-containing planting soil.

• Oxygenation rate
  Oxygenation around the root zone is essential for proper root system development. Especially when planting in petrified environments such as streets and squares, this plays a major role. Compaction of the soil around the root zone by traffic or machinery also reduces oxygen levels. An oxygen percentage of up to 12% causes rotting. Between 12 and 18%, root growth stagnates and at more than 18%, the root system grows.

• Moisture content
  When planting, it is important that the soil is moist, but no more than that. Planting a tree in a waterlogged planting hole is highly inadvisable. Tilling wet soils before planting is also not recommended, as this will compact the soil. Compaction in waterlogged soil may even allow ‘bad’ fungi - such as verticillium - to develop. For optimal planting results, it is advisable to always work in dry conditions.

Dealing with wet sites

In many places where trees do not survive in this wet year, it is because of the growing site conditions. This may be because work was done in wet conditions anyway, ruining the soil structure. It could also be that the tree was planted too deep in relation to ground level and/or groundwater. Due to the completely wet growing location and the lack of oxygen, the tree does not root from its root ball and produces too few or no root hair, which is crucial for absorbing moisture and nutrition. Also, a root ball consisting of a different soil composition than the surrounding soil can dry out because it does not absorb water. Wet locations are often very localised and the moisture percentage of the root ball should therefore always be considered separately from the surrounding soil.

The simplest way to take wet conditions into account is to plant the trees on a mound: make sure the bottom of the root ball is always planted above the highest groundwater level. This ensures that water always runs off the root ball and the roots always get enough oxygen when the groundwater level is high.

Choosing the right tree species

There are certain tree species (mostly with fleshy roots) that have a harder time with wet weather conditions and structureless soils. These are Fagus sylvatica (common beech), Sorbus (rowan), Prunus (cherry), Acer (maple) and Juglans (walnut). This does not mean they can no longer be used; however, it is wise to avoid wet situations. Extra attention to the growing place and soil types make it possible to continue applying these trees.

Some tree species are highly adaptive and can cope with flooding. Often these are only short-lived floods and trees are then fixed for longer periods. Real powerhouses that can better withstand long-term flooding are Salix (willow), Alnus (alder) Taxodium (swamp cypress), Populus (poplar) and Ulmus (elm). For situations where permanently wet conditions are expected, it is better to choose planting young trees as they can cope with wet conditions more easily.

In summary, excessive rainfall has a huge effect on trees. For all tree species, the damage from stagnant water can be seen after some time. Trees have huge reserves that they can use to get and keep leaves. Even on their way to death, trees can still look lively for quite a few years. It is possible that the overall damage may not be visible for several years. Creating the right growing conditions and meeting the essential needs of trees is crucial for their health and growth.