Found a new method for measuring the protective effect of forests against landslides

Roots as reinforcement

The doctoral thesis addresses a question that experts have long taken for granted: that forests stabilise steep slopes. DiBiagio explains that landslides typically begin when the slope gradient exceeds 30 degrees, or in some cases as low as 25 degrees. Norwegian guidelines for landslide mapping state that forests can help prevent landslides, but the effect has never been quantified. Nor has any method existed for doing so. DiBiagio wanted to change that.

To achieve this, DiBiagio investigated two mechanisms by which trees stabilise slopes. The first is mechanical and concerns how roots reinforce the soil:

"Soil can withstand a great deal of compression, but not tension. The same applies to concrete. The roots become the reinforcing steel in the concrete," she explains.

Concrete is strong under compression but cracks when stretched or bent. That is why concrete is reinforced with steel rods that absorb tensile forces. On a slope, the soil is subject to equivalent forces as gravity pulls material downwards.

"The roots do the same job as the reinforcing steel: they bind the soil together, absorb tensile forces and prevent the material from sliding apart. In addition, the roots can anchor the soil into cracks in the bedrock below. The older and larger the trees in the forest, the more extensive the root systems and the stronger the bonds," DiBiagio explains.

The second mechanism is hydrological. Tree canopies intercept some of the rainfall before it reaches the ground, and trees consume water through transpiration. This helps increase suction in the soil – the capillary forces that hold soil particles together and thereby strengthen stability.

"The hydrological effect is, on average, greater than the mechanical one. That was perhaps what surprised me most in working on the doctoral thesis," says DiBiagio.

The hydrological effect does, however, have a limit: during prolonged, heavy rainfall, the ground will become saturated regardless. But the measurements show that it takes significantly longer for soil in a forest to become saturated compared with open terrain.

A new tool: the forest factor

The main contribution of DiBiagio's doctoral thesis is a calculation tool she calls the “forest factor”. The method combines the two stabilising effects of forests into a single ratio: the stability of a forested slope divided by the stability of an equivalent open reference area. A forest factor of two means that the safety margin is twice as high.

"The tool is designed so that the factor from one study area can be transferred to similar slopes with the same type of forest. The opponent at the defence specifically highlighted this: that the methodology builds on calculation principles that consultants already use, and is therefore applicable in practice, not only in research," she says.

Another interesting finding concerns historical forestry practices and how these continue to affect slope stability. DiBiagio investigated specifically the effect of pollarding, an old tradition that was particularly common in western Norway until the Second World War, in which branches were cut from trees to use as winter fodder for livestock.

"We compared trees that had been pollarded with trees that had grown freely. We found that the pollarded trees grew much more slowly. This means that these trees provide less reinforcement of the soil, simply because their root systems develop more slowly than those of naturally growing trees," DiBiagio explains.

Clear-cutting creates a vulnerable window

The findings have direct implications for forest management. When a forest is felled, the hydrological protection disappears immediately. The trees are gone and can no longer absorb water. The mechanical effect of the roots persists for five to ten years before they decompose. But since new forest takes several decades to build up equivalent strength, what DiBiagio calls a vulnerability window of ten to twenty years arises, during which the slope is substantially more exposed than it was before felling and will be once new forest is established.

"I believe it is easier to make good and sustainable decisions if you have solid data that tells you how important the forest you might want to preserve actually is," DiBiagio notes.

DiBiagio's methodology may make it possible to document concretely which forested areas genuinely function as a landslide barrier, thereby giving decision-makers a scientific basis they have previously lacked.

To be tested further with spruce and pine

Through the research project FORTRESS, the methodology will now be tested on typical Norwegian tree species such as spruce and pine. The aim is to find out whether other root systems – such as the deeper roots of pine – produce different results than the deciduous forest DiBiagio has already studied.

"We will build further on the forest factor to test the effect of both spruce and pine forests. In an era of climate change and increasingly extreme weather, it is essential to know exactly which trees we can rely on to bind the soil together and secure loose material on steep slopes – and which forests we simply cannot afford to fell," concludes Amanda DiBiagio.