Why trees can dry out despite rain

It's raining. The meadows are turning green, puddles are forming on the paths, and the ground feels damp. Nevertheless, trees are showing signs of drought stress in summer: their leaves are turning brown, branches are dying off, or their crowns appear thinner than in previous years.

For many people, this is initially difficult to understand. If there is sufficient rainfall, why do trees suffer from a lack of water?

The answer usually doesn't lie solely in the amount of rainfall. What's crucial is how much water actually reaches the soil, is stored, and is available to the roots.

Key facts at a glance

• The amount of rainfall and the amount of water available to trees are not the same thing.
• Compacted soils can prevent water from penetrating deeper soil layers.
• Soils rich in humus store water significantly better than soils poor in humus.
• Heavy rain often runs off the surface before reaching the roots.
• Urban trees are particularly affected, but garden trees can also suffer from drought stress despite rain.
• Healthy soils help to better store precipitation and make it available to trees.

It did rain: why is the tree still suffering?

When people look at a tree, they usually only see the crown. However, the crucial part of the water supply takes place in the ground. For rain to help a tree, several things have to come together:

• The water must be able to seep into the ground.
• It must reach the root area.
• The soil must be able to store water.
  
• The roots must be healthy enough to absorb the water.

If one of these factors is missing, a tree can suffer from water stress despite regular rainfall. This phenomenon has become increasingly common in recent years, with long periods of drought alternating with short, heavy rain events. This poses a challenge for many trees.

How much water actually reaches the ground?

Not all rain has the same effect. Here's an overview:

Light rain

A light, steady rain lasting several hours is ideal for many soil types. The water can slowly seep in and gradually reach deeper soil layers. There, it is often available to the roots for an extended period.

Heavy rain

The situation is different during heavy rainfall: When large amounts of water fall within a short period of time, the ground often cannot absorb it all. Some of the water runs off the surface or collects in depressions.

Especially on compacted soils, surprisingly little water often reaches the deeper root areas.

Evaporation

Furthermore, some of the precipitation never reaches the ground. Leaves, grass, and warm surfaces retain water. Especially on hot days, some of it evaporates again shortly after the rain.

Measuring precipitation: How much rain actually fell?

If you want to know exactly how much rain has fallen, you can set up a simple rain gauge in your garden. These are available for just a few euros and provide much more reliable information than relying on guesswork. You can find more information in the article. Will there be enough rain for my trees??

Additionally, a look at the soil can be helpful. A small spade or hand trowel makes it easy to check whether the moisture has actually penetrated to deeper soil layers. Often, it turns out that while the surface appears damp, the soil just a few centimeters below is already dry again.

If the soil cannot absorb the water

The soil's ability to absorb water is crucial for a tree's water supply. Here are the key points:

soil compaction

Compacted soils These are among the most common causes of water problems. Frequent foot traffic, vehicles, or construction work compress the natural cavities in the ground. This makes it harder for water to seep into the ground. 
Instead of reaching the root area, it remains on the surface or flows away.

Surface runoff

Surface runoff occurs when water hits the ground faster than the ground can absorb it. This phenomenon is particularly common:

• during heavy rain
• on compacted soils
• on slopes
• on dry, hardened soils

Although the rain has fallen, it only partially reaches the tree.

Sealed surfaces

Roads, sidewalks, parking lots and other sealed surfaces further prevent rainwater from entering the ground.

Urban trees are particularly affected. They often only have a small area available through which water can seep into the ground.

If the soil cannot retain water

Even if rain seeps into the ground, this does not automatically mean that the water will remain available for a long time.

Humus-poor soils

Humus Humus acts like a sponge in the soil. It stores water and releases it gradually to the plant roots. Without humus, water often seeps away more quickly or is lost through evaporation. Therefore, soils low in humus dry out much faster.

Little organic matter

Leaves, compost, mulch, and other organic materials help improve water retention in the soil. If these substances are permanently absent, the soil gradually loses its ability to retain moisture.

Weakened soil life

The scope of MAP was expanded in two key areas: soil life Earthworms, fungi, and microorganisms play an important role. They improve soil structure and promote humus formation. This creates cavities that can store water.

A living soil can utilize rainfall much better than a biologically depleted soil.

Why city trees are particularly affected

Many people observe drought stress, especially in urban trees. There are several reasons for this.

• Small tree pits: City trees are often planted in small, open areas of ground surrounded by asphalt or paving. As a result, only a portion of the rainwater actually reaches the ground.
• Limited root space: Pipes, foundations, and roads often restrict the space in which roots can spread. As a result, trees can access less water.
• Urban heat islands: Cities store heat. Asphalt, concrete, and buildings heat up considerably and further increase evaporation. As a result, the water requirements of many urban trees rise significantly.

Why garden trees can also be affected

Drought stress despite rain is not solely a problem for urban trees. Similar causes also occur in gardens.

• Compacted soils: Frequently used lawns can become heavily compacted over the years. This impairs water absorption.
• Competition from other plants: Lawns, shrubs, and other plants compete with the tree for water. In dry summers, this can further complicate water supply.
• Shallow soils: In shallow locations, roots often have only a limited supply of water available. After just a few dry weeks, these reserves can already be exhausted.

What really helps trees during droughts?

When a tree is suffering from drought stress, simply adding more water is usually not enough. Often it's worthwhile to consider the soil as a whole.

• Building up humus: Humus improves water retention and soil structure. Compost, worm compost, bokashi, and mulch can help increase the humus content in the long term.
• Mulching: A layer of mulch protects the soil from direct sunlight and reduces evaporation. At the same time, it promotes soil life.
• Reduce soil compaction: Where possible, the root area should be protected from unnecessary stress. Loose soils can absorb and retain water much better.
• Slow irrigation: During prolonged dry periods, rain is often insufficient. In such cases, slow and deep irrigation is helpful. This allows the water to penetrate deeper soil layers and be available to the roots for a longer period.

Is your tree already showing signs of drought stress? Then this article will help you save your tree: How to recognize diseases, drought stress and nutrient deficiencies, and how to strengthen weakened trees.. 

Collect rainwater

Rainwater is especially valuable for trees. It is available free of charge, contains no lime, and is part of the natural water cycle. At the same time, collecting rainwater can help trees better survive dry periods.

Collecting rainwater from roofs in rain barrels or cisterns allows you to store larger quantities of water and later use it specifically for watering trees. This can be a valuable reserve, especially in regions with prolonged dry periods.

You can find more information in the article. Using rainwater for trees: Why every drop counts.