Why are urban trees dying?

The mass die-off of trees is no longer the sad fate of a few exceptional specimens – it’s becoming an increasingly common phenomenon in cities. Many trees die young, and even those that once tolerated urban stress well can no longer survive under current conditions. But what is behind this trend? The answer is complex – and that’s precisely the point. We are already experiencing the effects of climate change on a daily basis: prolonged dry spells, increasingly frequent heatwaves, unexpected spring frosts, and altered precipitation patterns are presenting unprecedented challenges for plants. And the urban environment makes things even worse. Trees are not only battling the consequences of climate change. The urban setting itself often imposes burdens – in the form of root damage, compacted soil, paved surfaces, inappropriate species selection, or improper pruning. What might be survivable in a natural environment can become a deadly combination in the city. The decline of urban trees is almost always the result of multiple factors acting together. In this summary, we present the most common causes – both those brought about by climate change and those caused by human mistakes. Because if we understand how climate and urban life interact, we won’t just mourn the loss of trees – we can also take action to save them.

1. Heat Stress
One of the most relentless consequences of climate change is the increasing frequency and duration of heatwaves. While trees do have natural cooling mechanisms – such as regulating their temperature through transpiration – these only work if they have access to sufficient water and if their surroundings don’t become excessively hot. Urban environments, however, significantly amplify heat stress. Paved and asphalt-covered surfaces absorb and radiate much more heat than natural ground. Tree roots often grow in compacted, airless soil, from which it’s much harder to absorb water. As a result, the tree struggles more and more to cool itself, and if the roots have been damaged or can’t access enough water, heat stress can quickly become fatal. And of course, it’s not just about trees: mowing the lawn down to bare ground during a heatwave doesn’t just strip the soil of shade, it also deprives the grass of its evaporative surface, its ability to collect dew in the early morning, and countless pollinators of their daily food. A single poorly timed mowing can doom your lawn to burn out for the rest of the summer.

2. Water Shortage
An adequate supply of water is essential for trees – not only for their survival but also for transpiration, cooling, nutrient transport, and metabolic processes. Due to climate change, increasingly long dry periods are becoming more common. Often, it’s not the total annual rainfall that decreases significantly, but its distribution becomes more extreme: long droughts may be broken by sudden, intense downpours. However, this water often cannot infiltrate the soil and quickly runs off or evaporates. Urban environments make this situation even worse. Tree root zones are often located in compacted soil layers where water penetration is difficult. Moreover, paved surfaces prevent rainwater from reaching the soil, directing it into drainage systems instead of to the tree roots. When a tree doesn’t receive enough water, its leaves begin to wilt, and entire branches may dry out. During prolonged drought, active roots can begin to die off, weakening the tree and making it more vulnerable to other stress factors – such as pests.

3. Root Damage
A tree’s root system is just as extensive and sensitive as its canopy – often even requiring more space than the crown. Roots are responsible not only for absorbing water and nutrients, but also for anchoring the tree, communicating with soil-dwelling organisms, and responding to environmental changes. Yet this is the part of the tree most frequently damaged – often without immediate visible signs – because roots and infrastructure are constantly competing for space underground. Utility installations, road construction, and pavement replacement all frequently disturb the soil. Construction machinery often ignores the root zone, cutting through even major roots essential for the tree’s survival. The mass damage of fine, hair-like roots can also have severe consequences: the tree can no longer absorb enough water and nutrients, effectively leaving it dehydrated and malnourished. Although public construction work is increasingly subject to strict regulations, such damage can still occur unintentionally – especially during earthworks on private properties. Root damage doesn’t always produce immediate visible symptoms. A tree may continue trying to sustain itself for years, even as it weakens internally. Trees with damaged roots are more prone to toppling, are more vulnerable to drought, and are far less capable of coping with additional stressors like heatwaves or pests.

4. Compacted Soil
Tree roots need not only water and nutrients, but also oxygen – because root respiration is essential for a tree’s survival. In urban environments, however, soil structure often becomes completely unsuitable for healthy root function. Heavy vehicles, constant foot traffic, and construction machinery all compress the upper layers of soil, making them airless and dense. Roots are unable to penetrate these compacted layers, and the microscopic organisms that support nutrient uptake also disappear from this environment. Healthy soil has a crumbly structure, is well-aerated, and can both absorb and retain water effectively. In contrast, compacted, hardened urban soil struggles to absorb water, and even when it does, the water rarely penetrates deep enough. As a result, roots suffer from both water and oxygen deprivation. Soil compaction is especially dangerous for young trees that haven’t yet developed deep, stable root systems. Mature trees are gradually weakened by compaction, becoming less resilient to heat stress, disease, and drought.

5. Suffocated Root Zone
Many people believe that placing paving stones or decorative coverage around a tree creates a neat and aesthetically pleasing environment. In reality, however, this practice can be life-threatening for the tree. A tree’s root zone requires access to rainwater, air, and a living soil ecosystem. When the surface around the roots is sealed off with hard paving, water cannot seep into the ground, oxygen cannot reach the roots, and soil life dies off. As a result, the tree becomes isolated from its environment – it can’t even absorb rainwater when it does fall. What’s more, paved surfaces heat up significantly, increasing the environmental heat load while also preventing natural evaporation. This kind of “suffocation” intensifies the heat and water stress already worsened by climate change. Over time, the root system begins to deteriorate, and the tree’s canopy shrinks. While “uniform paving” might appear visually appealing – especially in places like parking lots – if enough open space isn’t left around the trees, it can amount to a slow death sentence for them. The solution: leave the root zone open, covered with loose mulch that allows water to infiltrate and the soil to breathe. Without this, a tree cannot provide the ecological benefits it otherwise would be capable of.

6. Trunk Damage
Tree bark is not just a “shell” – it’s a protective armour beneath which lies a complex transport system that carries water and nutrients from the roots to the canopy. If the bark is damaged, one of the tree’s vital nutrient pathways can be disrupted. In urban environments, there are countless sources of such injuries: string trimmers, hedge cutters, bicycles, car doors when reversing, or even well-meaning human interactions, like nailing up a bird feeder or wrapping a decorative chain around the trunk. For a young tree with thin bark, even a single wound can halt the flow of sap, causing the branches above it to dry out in a short time. Open wounds also make it easier for fungal infections and other pathogens to enter the tree, further weakening it. This is especially troubling because trunk injuries are almost always preventable.

7. Improper Pruning
Tree pruning may be necessary – for traffic safety, distance from buildings, or to maintain the tree’s health. However, if done at the wrong time, with the wrong tools, or using incorrect techniques, pruning can do more harm than good. The most common mistake is excessive pruning or complete topping. This results in the loss of a significant portion of the tree’s foliage, leading to reduced photosynthesis, disrupted water balance, and depletion of internal energy reserves. The remaining stubs are unable to heal naturally and serve as open wounds for pathogens to enter. For this reason, it’s better to plant low-growing species under power lines, as taller trees will inevitably be topped to protect the electrical wires. This repeated topping weakens the tree over time, causes erratic shoot growth, and often leads to premature death. Such trees may live only a fraction of the lifespan they could have achieved if a well-chosen species had been planted in the right location. The consequences of improper pruning may not be immediately visible – but within a few years, it shortens the tree’s life and significantly reduces its aesthetic and ecological value.

8. Salt Damage
Although winter precipitation and frost are becoming increasingly rare in Szeged, when roads and sidewalks do become slippery, de-icing is necessary. The most common de-icing material, sodium chloride (table salt), is cheap and effective but even small amounts can harm plants. Salt doesn’t just damage leaves or bark – It primarily affects the greenery through the soil. Once it reaches the root zone, it alters the soil’s pH, inhibits nutrient uptake, and disrupts the tree’s water balance. The roots essentially “dry out” even if there is sufficient moisture in the soil because the presence of salt prevents them from absorbing it. In spring, affected trees struggle to sprout new growth; their leaves yellow and often partially or completely wither. The solution isn’t necessarily to stop de-icing altogether. Fortunately, there are alternative, plant-friendly materials – such as zeolite, calcium chloride, wood ash, or sawdust – that can also effectively address slippery conditions.

9. Poor Species or Variety Selection
Trees have different needs, tolerances, and growth forms – what is an ideal environment for one species can be almost unlivable for another. The urban environment is especially challenging: limited soil space, high temperatures, frequent pollution, winter soil salting, and often paved root zones. A species that thrives in its natural habitat may quickly weaken or die in such conditions. Climate change makes this even more difficult: warmer winters, longer dry periods, and new pests all influence how viable a species can be. A common mistake is planting tall-growing trees under power lines, which later require regular topping at high costs to control their growth. Another example is selecting a tree with a wide canopy for a narrow street, causing excessive shading of houses or obstructing traffic – like what happened on Gogol Street in Szeged fifty years ago. Good species and variety selection is based on considering local conditions, soil type, expected climate, intended use, and the tree’s biological characteristics. A well-chosen species or variety will not only survive but also beautify, shade, purify the air, and cool its environment for many decades.

10. Pests and Diseases
Trees naturally encounter pathogens in their native habitats, but when healthy, they can resist or repair damage. However, urban trees are often already weakened – due to water shortage, root damage, heat stress, or poor soil – and thus become much more vulnerable to diseases and pests. In recent years, more species of pests and pathogens, previously uncommon in urban environments, have appeared. These organisms multiply rapidly and particularly attack stressed trees. Climate change also contributes to their spread: warmer winters and fewer (or no) frosts allow many insect species to survive the winter and colonize new areas. Moreover, they remain active for longer periods annually, sometimes producing additional generations, which increases the damage. For example, the juniper bark beetle has long been dangerous to arborvitae but did not cause widespread mortality before. Hot summers with atmospheric drought have created a new situation: dry air prevents arborvitae from producing enough resin, leaving them defenceless against this pest. Diseases and pests rarely kill a completely healthy tree – but if other factors have already weakened it, they become the last straw. Often, a pathogen is merely a symptom of a deeper problem: the tree has not received the environmental support it needs to resist.

11. Careless Interventions
Urban trees usually live very close to residents, pedestrians, or business owners – so they are exposed to much more direct human impact than forest trees. Some of these interventions come from good intentions but can still cause harm. A common example is “planting out” rooted Christmas trees in public spaces. This may seem like a nice gesture at first, but most firs are not tolerant of urban conditions and often arrive with damaged roots or in too small planting holes, causing them to die quickly. Residents often prune trees themselves or “scalp” the crown to prevent branches from hanging over houses – but this is often unprofessional and damaging. Sometimes, people attach advertising boards, chains, garden hoses to tree trunks, or simply hammer in a nail – not realizing they open the door to diseases or injure the transport tissues under the bark. It also happens that someone parks too close to a tree, repeatedly bumps it with their car, or pours washing water or cleaning chemicals onto the roots. Good intentions matter – but just as important is proper knowledge. Trees are not only elements that beautify cities; they are living beings that require consistent and skilled care for a long life.