Vegetation as a Tool for Enhancing Climatic Comfort in Urban Areas

From Green Capital to Concrete Jungle? The Future of Ljubljana

The vast majority of European cities have, in recent decades, faced the challenge of continuous population growth and the construction of new housing developments, shopping centers, and other infrastructure. This expansion has led to the persistent spread of built-up areas. The migration crisis in recent years, marked by the arrival of people from other parts of the world into European cities, has further intensified this trend.

It is evident that the steady expansion of urban development is pushing out vegetation from modern city centers – plants that, until recently, at least partially coexisted with us in urban environments. A similar pattern can also be observed in Slovenia. What is particularly concerning is that there are few signs of improvement; on the contrary, the situation appears to be worsening on a daily basis. Nearly every recent renovation of a park, road intersection, or similar urban space in Ljubljana, has resulted in a reduction of green areas compared to their pre-renovation state (e.g., Ambrož Square Park and the Bavarski dvor intersection).

The importance of plants in regulating climatic conditions and maintaining them within an optimal range is irreplaceable. Their fundamental and most significant role lies in regulating temperature and air humidity. Plants control both their internal water balance and temperature through transpiration—the process by which water evaporates from plant surfaces, regulated by the opening and closing of stomata. At higher temperatures, plants open their stomata, thereby increasing transpiration and evaporative cooling, which lowers the temperature in their immediate surroundings.

For example, an extensive German study conducted in the city of Hamburg calculated that a single 5-hectare green roof can release as much as 22,000 cubic meters of water into the atmosphere, potentially reducing the surrounding temperature by approximately 3°C. Such an effect is substantial and can significantly contribute to more tolerable temperatures, particularly during periods of high daytime and nighttime heat. Moreover, the water vapor released by plants not only lowers air temperature but also increases ambient humidity, which, together with cooling, creates a more comfortable microclimate – for humans, animals, and plants alike.

In addition to reducing air temperature and increasing humidity, plants provide several other crucial benefits for air quality. Carbon and sulfur oxides enter plants through their stomata. Once inside, these compounds may bind to water films, participate in the formation of weaker acids, or attach to internal leaf surfaces, where they become effectively neutralized. In this way, plants possess a direct capacity to purify the air of harmful greenhouse gases and other pollutants.

Furthermore, plant foliage, as well as the bark of trunks and branches, captures airborne particles, thereby removing them from the atmosphere. Trees, in particular, play a vital role in this process due to their size and the presence of bark resulting from woody tissue development. Particles that accumulate on plant surfaces are eventually washed off by rain or fall to the ground with leaf litter, where they become incorporated into soil processes and the decomposition of organic matter. American studies estimate that vegetation can remove up to 75,000 tons of various harmful substances from the air annually.

Plants also exert important indirect effects on improving the quality of urban living. Expanding vegetated areas (green spaces) simultaneously increases the proportion of permeable, unpaved surfaces. These permeable areas allow rainwater to infiltrate the soil, thereby reducing the burden on urban drainage systems. From the perspective of flood prevention, this function is particularly significant. In recent years, as cities increasingly face more frequent and intense storm and heavy rainfall events, the role of vegetation in mitigating surface runoff has become even more critical.

Urban greening is therefore one of the most effective strategies for regulating climatic conditions in modern cities. This concept, however, is not new. Medieval cities, often portrayed today as unsanitary places with open sewage systems and wastewater flowing through the streets (which is historically accurate), unintentionally maintained a certain degree of climatic regulation through the presence of water and vegetation. These elements automatically moderated temperature and humidity levels within the urban fabric.

Modern society has largely forgotten the climatic efficiency of plants and open water surfaces. In the pursuit of sanitation and the prevention of disease, the past century has seen the development of systems that conceal nearly all water surfaces—wastewater and natural water alike—beneath concrete and asphalt. Similarly, very limited open space is left for vegetation, as exposed soil is frequently perceived as untidy and therefore undesirable in the urban landscape.

As a result, contemporary cities are undeniably cleaner and more orderly. After rainfall, traces of mud are minimal, and any that does appear can be quickly removed due to the dominance of paved and sealed surfaces. Modern construction incorporates numerous advanced materials with high insulation capacities, designed to shield buildings from significant temperature fluctuations.

Yet despite their cleanliness and technical sophistication, such urban environments are increasingly proving to be unfriendly to both human and animal life. This has become especially evident in recent years, as extreme temperature fluctuations intensify – manifesting most strongly in cities. Paved surfaces absorb and retain large amounts of heat, while also reflecting substantial levels of solar radiation. Reflection is further amplified by the widespread use of light-colored building materials intended to brighten the urban environment. Paradoxically, this combination contributes to rising urban temperatures, particularly during prolonged summer heatwaves.

At the same time, increasingly frequent heavy rainfall events generate large volumes of water that cannot infiltrate the ground due to the scarcity of permeable surfaces. Instead, runoff is forced into drainage systems, placing significant strain on urban sewer infrastructure. The result is often severe flooding, highlighting the vulnerability of heavily sealed urban landscapes.

We have largely forgotten that the primary culprit behind increasingly harsh urban living conditions is the very concept of city planning itself – one that prioritizes the exclusion of green spaces and vegetation and the removal of water from urban centers. If we want to make cities more climatically friendly for humans and animals in the future, particularly in the context of the ongoing impacts of climate change, it is essential to revive a robust urban green policy.

This requires increasing the proportion of green spaces in cities and incorporating a greater diversity of plant species. The most effective approach combines both woody plants and perennials, as such botanical diversity ensures sufficient water release into the air through transpiration, thereby raising local humidity. This process is particularly effective in mitigating high urban temperatures.

Expanding green spaces also creates permeable areas where surface water from rainfall can infiltrate the soil, reducing the burden on urban drainage systems. At the same time, it is crucial to reduce the share of concrete and asphalt surfaces, as these materials retain water poorly, decrease ambient humidity, and heat up significantly, contributing to excessive temperature increases in their surroundings.

When paving urban pathways and roads, priority should be given to materials and designs with larger gaps or permeable sections, allowing soil retention and water accumulation. The subsequent evaporation of this water helps to cool the surrounding environment, improving thermal comfort and overall urban livability.

Ensuring a high-quality urban climate does not depend solely on incorporating new building materials into modern constructions; rather, it requires embracing the plants that surround us everywhere. We must once again recognize that humans are an integral part of nature, and that increasing the presence of vegetation in urban spaces is the most effective solution to excessive heat in cities, to reducing greenhouse gas and particulate matter concentrations in the air, and to mitigating the impact of large-scale floods, which have become increasingly frequent in recent years. Modern construction materials should serve only as a supplementary tool for creating more comfortable urban climates.

Literature sources used in this article are available from the author.

prof. dr. Gregor Osterc, University of Ljubljana, Biotechnical Faculty, Department of Agronomy,
Jamnikarjeva 101, 1000 Ljubljana
E-mail: gregor.osterc@bf.uni-lj.si

Cover photo: Slovenia.info