Pavement lifting caused by tree roots, a universal problem

Functions of a tree

In our climate zone, a tree is largely evolutionarily developed in different vegetation types, for convenience sake 'in the forests'. A forest is a tree's frame of reference for optimal conditions. Everything is present here, a self-sustaining cycle in which everything that is shed from leaves, branches and trees becomes available again as nutrient for new growth. In fact, a tree is the most circular organism that exists and does not take in anything more than what it produces. Due to this circular process, the most important soil formation takes place in the upper layer of the forest. Soil composition, soil air, soil life, water and nutrients determine the extent to which roots develop. Related to this, most root formation takes place in the top layer of the soil and that is why you have to look down carefully when you walk in the forest, otherwise you will trip over a root, with your large walking shoes, where even in the forest they seem to have lost their way. Besides the fact that trees are patient organisms, it also seems as if they regularly want to wake us up in this way: Hello, I'm still here.

Need for roots

Trees have roots to fulfill two important self-serving functions, providing stability and allowing the absorption of water and nutrients. In the urban environment we still pay attention to what happens to trees above the ground, but we still have difficulty with what’s happening underground. Unfortunately, it is just as difficult as it is necessary. A tree cannot live without its roots, just as we humans cannot live without our food. Recognising that trees have a right to roots is a first step towards respect for the longest living organism on earth.

Now trees are incredible opportunists. Trees will seize any opportunities they find to develop. In the 1970s and 1980s we thought that a tree with a large wheelbarrow of soil in a planting pit would have enough space to develop. Remarkably, many trees have survived due to their enormous survival instinct and capacity to detect and colonise growth opportunities themselves, often with undesirable side effects such as pavement lifting.

Trees are opportunists who take every opportunity to survive

If we look at the development of the growing area a little later in time, we see that the intentions are often very good to give trees sufficient growing space in combination with the realisation of paving, as well as the willingness to invest in this. There are very good examples of this, but unfortunately at least as many bad examples. Especially if the civil contractor is responsible for the construction of a growing area, it is important to him that the pavement remains level and this will be ensured. Which is completely understandable. Despite the fact that all kinds of tree substrates have been chosen, we see that their composition is not the limitation, but aerating the soil and water access, which have become the limiting factors due to compaction and sealing. According to the book ‘Up by Roots’ by James Urban, the composition of a forest floor is 50% solid material (mineral parts), of which 2-5% organic matter and 50% open structure, which is filled in varying composition with half water and half air. If you assess the urban soil, it appears to consist of 70-80% solid material (mineral parts), of which 1% is organic matter, the remaining 20-30% remains for air and water. The poor organic matter content of the soil means that there is hardly any buffer effect of moisture and nutrition, but soil processes and growing conditions are also marginal for healthy growth. We are not yet talking about the self-sustaining leaf cycle of soil formation, which does not occur in a paved environment and the soil therefore only deteriorates further.

Causes of pavement lift

The presence of lifting is the most common cause of pavement marks. Now you can also talk about root pressure and then blame the tree, because if there were no tree then there would be no pavement that was pushed up. Trees have no voice and I worry more about trees than paving. Pavements are everywhere represented by man with his loud voice. I therefore want to use the power of our language and consciously do not talk about root imprint but about pavement imprint. In the consistent use of the term root imprint, I can already see the condemned tree hanging on the gallows.

The other cause may lie in the tree species, although I must say this with caution, there are a number of tree species that like to have very superficial roots. Consider poplar (Populus), false acacia (Robinia) and Wing Nut (Pterocarya). There are probably more species that form an intensive superficial root system. My caution in mentioning this, is because we quickly start a witch hunt and the place of growth has at least as much influence as the tree species.

High watertable is resulting in wide spreading rootplate

That is also an important factor, the place of growth. In the Netherlands we have the special situation that we have to deal with a high groundwater level, especially in the west of our country. The zone up to 60 cm often forms the most important root zone. Once in the fully saturated zone, a small number of tree species are capable to still develop roots. In the growing area, the design, composition and layout, amount of available air and water, as stated earlier, are decisive. The growing area is often so poorly designed that roots develop directly under the paving. With cobble stones, organic substances flow into the soil through the joints and condensation forms at the bottom of the paving. Both minimalist conditions that are optimally utilised by tree roots.

Last but certainly not the only cause, the age of the tree also influences the extent to which lifting occurs. Especially as trees grow in age, and thus in trunk diameter, you often see the upward push accompanied by a sideways push. Trees appear to use their opportunistic properties and squeeze into every possible escape route. It's a bit like ‘Escape from Alcatraz’ where trees show almost human behaviour, only less self-destructive.

Consequences of lifting in practice

From an analysed municipal database of more than 90,000 trees, 27% of the trees are in ‘concrete’ environments. For trees of the biggest, first size, their condition begins to structurally decline in paved areas from the age of 30 compared to the same tree classes in open ground. With trees of the smallest, third size, this effect on the condition is much less evident, but it is clear that these tree sizes are mainly planted in ‘concrete’ environments and generally have a much shorter lifespan. In general you can say that paving is not positive for the healthy development of trees and limits life expectancy. The analysis reveals another interesting finding. Most reports of problems caused by root push-up are made in neighbourhoods that lack green spaces. Low green spaces are the neighbourhoods where there is less than 75m² of greenery per home. It is therefore not the leafy neighbourhoods with many trees in avenues that cause the majority of the reports, but rather the neighbourhoods that are cramped in design, contain hardly any trees in open ground and where the trees are mostly on hard surfaces.

Bad conditon caused by root reduction is extra poignant in low green areas

The financial consequences in this specific municipality woke me up. The financial consequences of paving have been perfectly mapped out. The municipal reporting system receives an average of 2,650 reports of problems per year caused by pavement lifting. On average, resolving each individual report costs €375.00 in external costs. This means that €993,750.00 is spent per year to resolve problems. The internal costs have not yet been included. This was a reason for the quality manager of this municipality to look at how this could be solved structurally. Because the quality manager realised that an enormous investment is being made, whereby the complaints continue to exist and the quality of the trees mainly deteriorates. Actually, you should not talk about an investment that is made, but money is pumped into a chronically ill management post where the problem is not structurally solved. The positive thing in this regard is that the reporting system works well. There is great awareness within the organisation that a lot of profit can be achieved by doing things better.

It can only get better

Where the tree can be opportunistic, we can also learn from this and seize the opportunities that present themselves. And that is necessary! Lifting marks caused by root growth are universal. In the Netherlands, this applies to every municipality because of our soil structure. There are also plenty of examples of lifting marks abroad. The first solution is to remove pavements. No paving, no paving marks, life can be that simple. But I am also no stranger to reality and I like to discuss it. How realistic is this? When I look at the great success of the many pavement slab lifting initiatives, it is good not to overlook this too quickly and to carefully assess whether paving is actually necessary. Simply going outside and not just judging from behind your computer helps with this. Expanding green zones and creating underplanting are the simplest and often relatively cheapest measures. The other solution, which is very much an extension of the previous solution, is to widen the space. As a starting point, 3x the trunk diameter of the trunk at its expected age is desirable.

Root reduction is the most common but least structural solution. Just by taking pavement away it can't be lifted anymore

Root reduction is the least structural solution. Regrowth can be expected very soon and the previous municipality has the experience of sometimes returning to the same streets every year. After all, root cutting is not structural, because the cause of the superficial root behaviour is not removed.

The other, possibly less desirable solutions, lie in installing root barriers or transplanting trees. Root screens are often overgrown or overgrown by roots as soon as they have an opportunity of less than a few millimeters. Root screens can be effective, but they are a huge installation challenge. Transplanting is an option if it turns out that it is the only solution left to save the tree. With a superficial root system, a growing area without pavement is necessary. And then finally, fell the tree, if the quality has deteriorated to such an extent that no viable future can be guaranteed, it can sometimes be just as wise to start all over again. Not as a rule, but only as a custom solution. In case of new construction, ensure that it is designed and constructed based on the tree. That's not two wheelbarrows of soil in the planting pit. Giving trees the space they need is the simplest measure to prevent problems in the future.

Malang, Indonesia. Lifting of pavement is a universal worldwide problem. It's very culturally determined how we handle it

If you came to earth as a tree and are unlucky enough to grow in the most tree-unfriendly environment called a city, as a human being you would only have to walk under a tree with your head bowed. This is out of respect for this opportunist who knows how to keep himself alive in a city.