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Urban trees play a vital role in reducing urban heat and improving climate resilience. They cool urban areas by providing shade and evaporating water from their leaves. In this context, the silver lime (Tilia tomentosa) has attracted attention due to its ability to turn the silvery underside of its leaves upwards during periods of intense sunlight. This leads to a change in the canopy albedo and a possible decrease in surface temperatures within the crown area. The aim of this study is to assess the potential microclimatic cooling effects of silver lime trees on their immediate environment and to derive evidence-based recommendations for promoting climate resilience in urban areas. In situ measurements are being taken at multiple sites in Kaiserslautern, Germany. The study considers variables such as tree species, developmental stage, and the availability of a directly comparable reference tree.
RPTU University Kaiserslautern-Landau, Standort Kaiserslautern, Germany
Sascha Henninger
*
RPTU University Kaiserslautern-Landau, Standort Kaiserslautern, Germany
*Address all correspondence to: sascha.henninger@ru.rptu.de
1. Introduction
The increasing heat stress in urban areas due to climate change poses a significant challenge with far-reaching implications for the health and quality of life of urban populations. Urban heat islands lead to elevated temperatures that pose serious health risks, particularly during heatwaves [1]. Urban heat islands are defined as urbanised areas that experience significantly higher temperatures than their rural surroundings. This phenomenon is primarily caused by human activities and the urban landscape [2–4]. It can have various environmental, energy consumption, public health, and economic repercussions [4, 5]. A comprehensive understanding of the causes, effects, and mitigation strategies of urban heat islands is essential to promote sustainable and resilient urban environments. Therefore, climate adaptation in urban areas is more urgent than ever [6, 7]. According to the IPCC Report, adaptation is defined as “the process of adjusting to actual or expected climate and its effects to moderate harm or take advantage of beneficial opportunities in human systems. In natural systems, it is defined as the process of adjustment to actual climate and its effects” [6].
There are several strategies that can help reduce the severity of the urban heat island effect. These include the implementation of green infrastructure, such as vegetation and green roofs, to enhance natural cooling processes [8]. Urban greening can affect temperatures through several processes. For example, urban trees can mitigate the effects of urban heat islands by blocking sunlight from reaching the ground, thereby reducing convection, which is a factor in heat island formation. Furthermore, tree canopies increase latent rather than sensible heat fluxes by absorbing sunlight and energy from the environment through evapotranspiration [9]. Additionally, the positive impact of nature on human health and well-being is widely recognised [10]. However, it is important to acknowledge that aesthetic preferences are subjective and can change over time. For example, shading or a dense canopy can be beneficial on hot summer days but less so in cool, humid summers [11].
In addition to their social importance, the demands placed on urban trees are increasing. Traditional species such as the summer lime tree (Tilia platyphyllos) and the sycamore maple (Acer pseudoplatanus) are increasingly suffering from stresses in urban environments [11–14]. Tree species from southeastern Europe, such as the manna ash (Fraxinus ornus) and silver lime (Tilia tomentosa “Brabant”), are better equipped to cope with higher levels of heat stress, solar radiation, and drought. This is achieved through lower soil and water potentials, prevention of exceeding the wilting point, increased water use efficiency, lower transpiration rates during the hot season, different leaf morphology, and more vigorous fine root growth [11, 15, 16].
The temperature of trees in urban areas can vary depending on several factors, including species, location, leaf size, and the morphological characteristics of the canopy. At air temperatures of 40°C, the maximum leaf temperature of certain species can increase exponentially by up to 10 K above ambient temperature without irrigation. Furthermore, species that are cooler at 25°C may not necessarily remain so at extreme temperatures. Additionally, species with smaller leaves tend to stay cooler when the ambient temperature is high [17]. Therefore, when selecting trees for urban areas, it is important to consider that not all species have the same cooling effect. Urban tree planting, therefore, needs to be planned carefully in the context of climate change in order to reduce the development of urban heat islands and avoid costs associated with inappropriate species selection.
Acknowledging the increasing role of urban vegetation in regulating climate, this study explores the cooling potential of deciduous tree species as a strategy for adapting to shifting climate conditions in cities. The research specifically examines variations in albedo and surface temperature of Tilia tomentosa canopies and evaluates their potential influence on the surrounding environment, with a particular focus on their use as roadside plantings.
2. Urban trees of the future
From 2010 to 2021, the “Urban Green 2021” research project examined 29 tree species to determine their suitability as climate-resilient urban trees. The project aimed to identify species that can withstand heat and drought stress, making them suitable for urban greening in a changing climate.
Southeast European species, such as the silver lime tree and the hop hornbeam (Ostrya carpinifolia), were found to control leaf temperatures significantly better than their native sister species, such as the winter lime tree and the hornbeam, during hot spells. They exceeded the critical threshold of 40°C less frequently and for shorter periods, indicating higher heat tolerance. This ability to regulate temperatures was also maintained during successive extreme summers, with the silver lime performing particularly well [18].
The “Urban Green 2021” project recommends the following tree species: silver lime (Tilia tomentosa), hop hornbeam (Ostrya carpinifolia), field maple (Acer campestre), Hungarian oak (Quercus frainetto), hazel (Corylus colurna), gleditsia (Gleditsia triacanthos), and oriental and hackberry trees (Celtis occidentalis and Celtis australis). It is important to note that the suitability of tree species varies depending on location and regional climatic conditions. The project’s recommendations are therefore regionally specific and take into account the municipalities’ local experience. However, the silver lime and the hop hornbeam are highlighted as particularly suitable across all locations [18].
2.1. Tilia tomentosa
Tilia tomentosa, commonly known as the silver lime in the UK and the silver linden in the USA, is a tree native to southeastern Europe. It is drought-resistant and thrives in warm conditions. It is naturally found in regions such as the Balkan Peninsula, north-western Turkey, Hungary, and Romania [19–21]. The climate in the silver lime’s natural habitat shows greater annual temperature variation than the current climate in Rhineland-Palatinate. Higher temperatures are expected in the future, particularly in the Upper Rhine region and river valleys. However, precipitation is expected to remain largely stable. Silver lime is currently considered suitable, but this suitability is expected to increase with rising temperatures by the middle of the century [21].
When exposed to high levels of sunlight, the silver lime tree exhibits unique behaviour by orienting the leaves at the top of its canopy outwards towards the sun. The silver colouring on the underside of the leaves (see Figure 1) reduces heat absorption and reflects incoming sunlight, which could help cool the tree – a crucial factor in hot weather [22].
Figure 1.
Photograph of the leaves of Tilia tomentosa, showing the different colours of the upper and lower sides of the leaves [Carl 2023].
The silver lime tree flowers from late July to early August, providing an important source of food for bees and other insects at a time when there is otherwise little available [23]. Therefore, the late flowering period makes the silver lime tree a useful resource for the local insect community. It was once thought that the nectar of the silver lime tree was toxic to insects; however, recent research has disproved this idea. The real reason for the widespread mortality of bumblebees under silver lime trees is food rivalry between insects [11, 24, 25].
3. Materials and methods
Field measurements are currently being conducted in Kaiserslautern (49° 26′ 36″N, 7° 46′08″E), a city in the state of Rhineland-Palatinate in southwestern Germany. Kaiserslautern is characterised as a university and industrial city. It covers an urban area of about 140 km2, 87 km2 of which is covered by forest [26]. The city’s average annual temperature is 10.2°C, with seasonal fluctuations ranging from an average of 1.9°C in January to 19.3°C in July [27]. Notably, the average annual temperature in Kaiserslautern has increased significantly by 2.0 K since the late nineteenth century, particularly in recent decades [28]. Regarding precipitation, the city receives an annual average of 764 mm, with the highest recorded amount being 69 mm in May [29]. These distinctive climatic parameters make Kaiserslautern an ideal location for studying the impact of changing environmental conditions on urban vegetation and microclimatic dynamics.
4. Sampling strategy
This study’s sampling strategy focuses on roadside greenery in the urban environment of Kaiserslautern. Streets were carefully selected based on specific criteria to ensure the reliability and representativeness of the data. The primary tree species under investigation are Tilia tomentosa and Tilia tomentosa “Brabant,” which were selected due to their distinctive characteristics and ecological importance in urban environments.
To capture the full spectrum of tree development and its impact on microclimatic effects, the study includes trees at different life stages: youth, maturity, and old age [30]. This approach enables a thorough analysis of how the cooling capacity of the trees may change throughout their lifespan.
A key feature of the sampling strategy is the presence of a direct comparison tree adjacent to each silver lime specimen. This arrangement enables precise comparisons under real-world conditions, as both trees experience similar environmental factors [31]. Such paired sampling enhances the study’s ability to isolate the specific effects of silver lime trees on the local microclimate.
The selection criteria also consider various physical characteristics of the trees, such as total height, diameter at breast height, the height of the crown base, and the crown radius. These measurements provide essential context for interpreting the thermal behaviour and cooling capacity of the trees in relation to their size and structure.
The study aims to generate robust data on the microclimatic effects of silver lime trees in urban environments by employing this sampling strategy, which could inform future urban climate resilience strategies.
5. Data collection protocol
This research project examines how trees influence the microclimate within street canyons, focusing particularly on warm summer conditions and extreme heat events, when the cooling effect of trees is most evident. To ensure the collection of relevant data, specific meteorological criteria have been established. Measurements will be conducted exclusively on summer days characterised by low wind speeds (≤1.5 m/s), clear to nearly clear skies (degree of cloud cover ≤1/8), and temperatures reaching a minimum threshold of ≥25°C [30, 31].
In order to fully assess the impact of solar radiation, it is necessary to measure tree canopy temperatures at various times throughout the day. This approach provides valuable insights into how building orientation and shading affect the thermal behaviour of trees. Data collection is scheduled at specific intervals, if possible.
Early morning (07:00–09:00 UTC + 1)
Midday/early afternoon (12:00–15:00 UTC + 1)
Late afternoon/early evening (17:00–19:00 UTC + 1).
These time slots have been chosen to coincide with the researcher’s presence, ensuring accurate data collection while avoiding continuous measurement periods that could lead to excessive heat exposure and compromise health and safety requirements [31].
The various measured variables (see next chapter: Equipment and Measurement Techniques) are recorded at 10-minute intervals within the aforementioned time slots.
The study employs a multifaceted approach to understand the thermal behaviour of trees in urban street canyons. Due to the unique characteristics of each tree and measurement site, as well as the variability of weather conditions, the research focuses primarily on intra-tree comparisons to elucidate thermal patterns throughout the day.
6. Equipment and measurement techniques
In situ measurements employ a range of specialised instruments to capture accurate microclimate data around the Tilia tomentosa tree and its comparator trees (See Figure 2 and 3). The primary equipment includes SwitchBot IP65 Outdoor Sensors to measure air temperature and humidity, a PASCO wireless weather sensor (PS-3209) with GPS to record environmental factors such as wind speed and air temperature, and a TOPDON TC002 thermal imaging camera to measure the surface temperature of tree canopies and surrounding structures.
Figure 2.
Outline of the experimental setup, modified from [30].
Figure 3.
Insights into the measurements in Buchenlochstraße, Kaiserslautern, on August 11, 2023 [30].
In this study, a TOPDON TC002 thermal imaging camera is employed to record surface temperatures of both the tree trunk and canopy from three specific angles: the side facing the building façade, as well as the eastern and western canopy exposures [30]. These orientations are strategically selected to detect any potential temperature variations influenced by solar exposure and dominant wind directions. By examining these perspectives, the research aims to identify potential relationships between the thermal properties of the tree canopy and the adjacent building façade.
Additionally, thermal images of nearby buildings are taken to assess whether the canopy’s emitted radiation has any effect on the façades of surrounding buildings. This aspect of the study is a key to evaluating how tree canopy heat dynamics may influence local architecture and contribute to microclimatic conditions. To maintain consistency, all images are captured from a fixed point perpendicular to the wall at each measurement location. Care is taken to prevent or minimise contact between the canopy foliage and the wall surface in order to avoid any potential interference with the temperature readings.
To ensure accuracy, all instruments undergo calibration procedures before each field session. Additionally, the placement of sensors is crucial for capturing the microclimatic effects of the tree. Air temperature measurements are taken at multiple points. The first temperature component is measured just beneath the tree canopy, at a height of 1.5 metres above ground level. This height is chosen as it better reflects conditions experienced at the human level compared to the conventional measurement height of 2 metres [32]. To obtain reliable temperature data, sensors are positioned in shaded locations and protected from direct exposure to sunlight. Additional air temperature measurements are taken from the tree trunk towards the house wall in order to analyse the microclimatic effects in this specific area, particularly with regard to the building façade.
7. Limitations and uncertainties
When conducting research into the impact of silver lime trees on urban microclimates, it is important to recognise and address the limitations and uncertainties of our methodology. These factors influence both the data collection process and the interpretation and generalisability of the findings.
Access and spatial constraints:
One of the primary challenges is restricted access to certain measurement sites. Private properties, the closure of business premises on weekends, and the varying distances from building façades can limit the ability to obtain a comprehensive dataset representing all canopy perspectives. These constraints may introduce a sampling bias, potentially resulting in the underrepresentation of certain urban configurations or tree-building interactions.
Weather variability:
The dynamic nature of urban microclimates makes it difficult to replicate precise weather conditions across different measurement sites and time frames. Factors such as wind patterns, cloud cover, and anthropogenic heat sources can vary considerably, even within small urban areas.
To address this variability, our study primarily focuses on intra-tree comparisons, analysing thermal behaviour within individual trees throughout the day. This approach enables us to control for site-specific factors and isolate the effects of tree characteristics and immediate surroundings on microclimate modulation.
Scaling and representativeness:
While this study can provide valuable insights into the thermal behaviour of individual trees, it is challenging to scale these findings to larger urban areas. The heterogeneity of urban environments, including variations in building materials, street orientations, and tree species composition, may limit the direct applicability of our results in other contexts.
To address this, we will use a hierarchical modelling approach that incorporates both tree-level and neighbourhood-level variables. This multi-scale analysis will help us understand the effect of individual trees on the aggregate at larger spatial scales and in different urban configurations.
By explicitly acknowledging these limitations and uncertainties, we aim to provide a transparent and robust framework for interpreting our results. Furthermore, this approach enables us to identify areas for future research and methodological improvements, thereby contributing to the ongoing refinement of urban microclimate studies and their applications in urban planning and climate adaptation strategies.
8. Initial results and outlook
As the research project is still ongoing, no final results can be reported yet. However, the following initial trends are emerging: The effect of a potentially cooler leaf surface on the silver lime tree could not be clearly confirmed in the measurements taken so far. Some results indicated a cooler surface temperature compared to a reference tree, while others showed a higher surface temperature. For instance, on 11 August 2023, the silver lime tree had a surface temperature of 31.1°C, whereas the reference tree had a temperature of 30.7°C; however, the silver lime tree showed higher temperatures from 14:00 onwards (see Figure 4). On 9 July 2024, the silver lime tree started at 33.0°C, while the comparison tree started at 32.2°C; the temperatures then alternated in subsequent measurements (see Figure 5).
Figure 4.
Surface temperature of the canopy of the silver lime tree and a reference tree in Buchenlochstraße on 11 August 2023 [Carl 2024].
Figure 5.
Surface temperature of the canopy of the silver lime tree and a reference tree in Buchenlochstraße on 09 July 2024 [Carl 2024].
In addition, temperature measurements taken from the trunk of the silver lime tree revealed an air temperature that was approximately 0.4 K higher than that recorded for the reference tree. This difference could be attributed to various factors, including microclimate, environmental influences, and parked cars. An initial analysis of the building façades and surface temperatures of the respective tree trunks revealed that direct sunlight had a greater impact on surface temperature than the presence of a silver lime tree. This suggests that external factors such as sun exposure and environmental design play a key role in shaping the microclimate and, consequently, the temperature readings of the building façades.
If the cooling potential of silver lime trees can be reliably quantified, it opens the possibility for simulation using ENVI-met. This software is a robust microclimate modelling tool designed for detailed analysis at the neighbourhood or street scale. It includes visualisation features that help interpret spatial temperature patterns and integrates seamlessly with AI and GIS technologies, allowing the fusion of simulation outputs with geographic data relevant to the study area [33]. The TreePass module within ENVI-met offers an in-depth evaluation of environmental conditions influencing tree growth and sustainability. It takes into account variables such as wind exposure, light availability, and mechanical stresses, delivering valuable insights for professionals such as urban planners, arborists, and landscape architects. Such data support evidence-based decisions related to species selection, tree placement, and maintenance practices, ultimately enhancing the resilience and health of urban greenery [34]. The software is capable of modelling scenarios where silver lime trees are introduced more widely or exclusively in urban streetscapes, estimating their impact on local microclimates. It can also be used to compare thermal effects between streets with newly planted silver limes and those with mature or ageing trees. A key goal is to determine whether younger silver lime trees, due to their leaf rotation mechanism, offer more effective cooling than older specimens. Furthermore, simulations can estimate the time required for newly planted silver limes to match the cooling performance of established street trees. ENVI-met’s capabilities offer a valuable toolset for exploring how silver lime trees influence urban climates and for guiding climate-responsive urban greening strategies.
Ongoing research on Tilia tomentosa in urban environments could significantly influence sustainable urban planning and design. This study aims to provide evidence-based recommendations for urban greening initiatives, focusing on enhancing public spaces and selecting suitable vegetation.
Key areas for future investigation include evaluating the local climate impacts of silver lime trees in order to determine the most suitable locations for them in urban areas. Understanding how silver lime trees interact with green–blue infrastructure could improve urban ecosystem services and resilience to climate change. Developing strategies for the gradual replacement of trees and the integration of young saplings can help to ensure urban canopy coverage. Furthermore, it is essential to evaluate the costs, benefits, and social implications of integrating silver lime trees into urban landscapes.
It is also important to explore how silver lime trees can enhance urban resilience in response to changing environmental conditions. Addressing these research areas enables urban planners and landscape architects to make informed decisions that enhance the liveability, sustainability, and climate resilience of cities [31]. This comprehensive approach to studying silver lime trees in an urban context reflects the complex nature of urban ecosystems and the need for a holistic approach to urban planning.
9. Conclusion(s)
In the context of mounting climate challenges, urban areas have taken on a key role in leading the way with strategies to mitigate the effects of climate change. Urban planners and policymakers are spearheading the implementation of initiatives that enhance the resilience of current and future urban landscapes.
Municipal authorities are leading the way in implementing forward-thinking urban planning strategies to address climate adaptation. These strategies encompass updating building codes to enhance structural resilience against climate-related challenges, integrating green and blue infrastructure to reduce the impact of severe weather, and developing focused initiatives to safeguard at-risk communities and essential infrastructure. These actions are crucial for making cities more resilient and sustainable in the long term as they contend with the effects of a changing climate.
Although urban areas are particularly vulnerable to the effects of climate change, effective adaptation strategies are a cost-effective solution. Long-term adaptation efforts are more economically viable than taking no action at all.
Strategic urban greening is a vital component of climate adaptation in cities. This involves meticulous planning, species selection, and assessment of species-specific cooling effects and adaptability to local climate conditions, as well as consideration of long-term maintenance requirements. By carefully selecting climate-resilient tree species, cities can maximise the benefits of urban greening while enhancing their climate resilience. The benefits extend beyond mere adaptation to include improved microclimates, enhanced water management, better air quality, increased biodiversity, and greater urban vibrancy.
Effective climate adaptation requires existing policies to be aligned with climate-friendly objectives and climate-resilient investments to be ensured. This involves integrating adaptation measures into current policies to promote policy coherence and synergy while minimising counterproductive actions. At the municipal level, a comprehensive, multidisciplinary approach is essential. This includes fostering collaboration with neighbouring municipalities, regions, and countries to reinforce adaptation initiatives and pave the way for a more resilient urban future.
Funding
This research was funded by the Rheinland-Pfalz Kompetenzzentrum für Klimawandelfolgen, Forschungsanstalt für Waldökologie und Forstwirtschaft.
Data availability statement
The full datasets presented in this article are not readily available because the study is still ongoing.
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Written By
Judith Carl and Sascha Henninger
Submitted: 23 July 2025Reviewed: 26 August 2025Published: 25 February 2026
Open access peer-reviewed chapter - ONLINE FIRST
