How to Grow Large Trees In Urban Environments

To grow large trees in urban environments presents unique challenges and considerations. Urban settings often have limited space, compacted soil, and various infrastructure constraints that hinder tree growth. However, finding effective methods for nurturing substantial trees in cities is crucial for enhancing green spaces, improving air quality, and promoting overall health and well-being for urban dwellers.

Why We Need Larger Trees in Urban Environments

From combating air pollution and mitigating the urban heat island effect to fostering biodiversity and improving physical and mental well-being, larger trees play a crucial role in enhancing the overall quality of life. 

Large Trees Mitigate the Urban Heat Island Effect

Strategic placement of trees in urban areas has been proven to have a remarkable cooling effect on the shaded regions, with temperature reductions ranging between 2ºC and 8ºC. This cooling phenomenon is a result of multiple interconnected factors.

Firstly, trees provide shade through their expansive canopies, which obstruct direct sunlight from reaching the ground. By blocking the sun’s rays, trees create a cooler microclimate beneath their canopy, shielding the area from excessive heat. This is especially beneficial in densely built urban environments, where concrete and asphalt tend to absorb and radiate heat, contributing to the urban heat island effect.

Secondly, through a process called transpiration, trees release water vapor into the atmosphere. This evapotranspiration process helps dissipate heat and lowers the air temperature, further contributing to the cooling effect in the shaded areas. In addition, as water vapor is released, it can also contribute to increasing local humidity, which enhances the comfort of the surroundings.

Large Trees are Great Urban Air Filters

Large urban trees play a pivotal role in mitigating environmental pollution and enhancing air quality in urban areas. As natural filters, they possess the remarkable capacity to absorb significant amounts of air and soil pollutants, including up to 150kg of carbon dioxide (CO2), along with fine particulates

The absorption of carbon dioxide by urban trees is a crucial process in combating the escalating issue of climate change. 
This carbon sequestration not only helps mitigate climate change but also aids in offsetting the carbon footprint of urbanized regions, contributing to broader efforts in achieving carbon neutrality and sustainability.

The effectiveness of urban trees as pollutant filter is influenced by their proximity to emission sources. Studies have shown that trees in close proximity to pollutant-emitting activities absorb up to nine times more pollutants than trees situated farther away. So, strategic tree planting near busy roads, industrial areas, and other pollution hotspots is essential to maximize their air purification capabilities and create healthier living spaces.

Related: Best Tree Species for Reducing Air Pollution

Large Trees Improve Water Quality

The presence of large trees in urban landscapes has far-reaching effects on water bodies and the ecosystems they support.

One of the key contributions of large trees in urban settings lies in their capacity to manage and clean stormwater runoff. Impermeable surfaces like roads, pavements, and rooftops prevent rainwater from naturally infiltrating the soil. As a result, large volumes of stormwater rush over these surfaces, picking up pollutants such as oil, heavy metals, fertilizers, and other contaminants.

See how Citygreen integrate trees into the Stormwater Management process with our Strataflow System.

Related Stormwater Case Study: 105th Street Edmonton, Canada

Large Trees Improve Physical and Mental Health

Large trees provide a host of physical and mental health benefits. Their presence in urban environments not only fosters cleaner air but also encourages higher levels of outdoor physical activity among nearby residents, as they create a cooler and more inviting environment for exercise

On the mental health front, spending time in the presence of large trees has been linked to reduced stress and anxiety levels. Being in nature and experiencing green spaces have a calming effect on the mind, promoting relaxation and mental rejuvenation. Moreover, large trees create aesthetically pleasing environments that positively influence mood and emotional states. 

Larger Trees Improve Property Values

Larger trees significantly contribute to improving property values and are regarded as valuable assets in real estate and urban planning. The presence of well-established trees in residential neighborhoods and commercial areas adds a touch of natural beauty, reduce noise pollution, decrease crime rates, and reduce power usage due to temperate reductions. Another study also showed that, for every dollar invested in planting, cities see an average US$2.25 return on their investment each year.

Related: Soil Vaults Unlock Massive ROI For Your Trees

So, the faster tree canopies grow, the sooner the benefits can be enjoyed by cities, stakeholders and communities.

How to Plant Large Trees in an Urban Environment

Planting large trees in an urban environment requires careful planning and the implementation of effective strategies to ensure the successful establishment and long-term growth of the tree. Utilizing advanced urban forestry practices and harnessing the potential of technology like Citygreen’s Stratavault soil vault system can significantly enhance the viability and sustainability of urban tree planting.

Another crucial aspect is the preparation of the planting site. Urban soils are often highly compacted due to the need to support surround infrastructure such as pavement, roads and building foundation. Highly compacted soil is not conducive to tree growth as the soil is and lacking in nutrients, oxygen and space for growth which hinders root development . 

When growing large trees in urban environments, Citygreen’s Stratavault soil vault product proves to be a game-changer when it comes to urban tree planting. Its modular design allows for efficient and scalable tree planting in confined urban spaces. The stratavault system creates an ample underground soil volume, providing ample room for root expansion and nutrient uptake while ensuring the roots are contained within the matrix and never impact surrounding infrastructure like roads pavements, and underground utilities. This innovative technology also facilitates stormwater management, reducing runoff, stormwater capture and storage while enhancing the tree’s access to water, ensuring its resilience during dry periods.

Related Case Study: 10-year Urban Tree Growth Comparison

Another consideration when planting large trees in urban areas is the selection of suitable tree species. Opting for native or adaptive tree species that are well-suited to the local climate, soil conditions, and urban challenges is crucial. These species exhibit greater resilience to local environmental stressors, making them more likely to thrive in the often harsh urban settings.

Urban Tree Growth Case Study

At Citygreen, we love science. Since 2011, we’ve researched, developed and refined our range of sustainable urban landscape solutions to withstand rigorous testing – and produce outstanding results. So, when our Stratacell structural soil vault system became the subject of a study comparing the performance of soil treatments under concrete paving, we were keen to see the findings.

The study, entitled ‘Comparison of Soil Treatments Under Concrete Pavement’ was conducted by Thomas Smiley, James Urban and Kelby Fite at the Bartlett Tree Research Laboratory in Charlotte, North Carolina. It examined variations of the two main approaches that have been developed to provide rooting space for trees in urban areas – supported pavement and structural growing media.

Overall, the study found that structural https://citygreen.com/understanding-the-importance-of-tree-canopy/load bearing modules (like Citygreen Stratacell and Stratavault systems) grow the largest, healthiest trees in the fastest time. It also highlighted the superior all-round performance of these systems compared to others.

Study goals

The purpose of the study was to compare the growth of trees in different supported pavements and structural growing media. The aim was to determine which methodology would produce the largest, healthiest trees in the shortest time.

A key driver behind the research was canopy cover. Tree canopy provides a huge range of environmental, economic and health benefits, including:

• Combats climate change by reducing greenhouse gas emissions
• Provides natural shade and cooling
• Conserves energy and reduces power costs
• Enables water filtration and retention
• Provides habitats for wildlife
• Increases property and area values
• Promotes health and wellbeing
• Contributes to a sense of place
• Encourages community interactions

SUP Trial Plot Sept 2017

Methodology

To achieve the study goals, two plots were established at the Bartlett Tree Research Laboratory. The first plot was installed in 2004, exclusively for Study 1, which examined five variations of supported pavement systems. The second plot was established in 2014, specifically for Study 2, which examined six variations of structural media.

Citygreen’s Stratacell system was part of Study 2, which also included an open control, compacted control, sand based structural soil (SBSS), gravel based structural soil (GBSS) and another structural load bearing module, similar to Stratacell.

At the start of Study 2, on 19 August 2014, containerised 18mm caliper Liriodendron chinense trees were installed in the centre of each plot. A 5cm thick layer of concrete was poured over the plot, with a 20cm hole centred on each tree. A soil moisture sensor was also fitted.

Over the following three years, the trees were measured and assessed regularly against key performance indicators. On 23 October 2017, the trees were severed at the root for final measurements and findings. During the excavation, soil was removed from the roots, leaving the mass intact for examination.

Pressure wash and Hydrovac for removal of soil from the root systems

Airspade removing soil from around the roots

Weighing Process of all root systems

How to Grow Large Trees in Urban Environments Key Findings

Overall, the study found that soil treatments that provided a low density growing media aka uncompacted soil (such as Citygreen Stratacell and Stratavault) resulted in the largest, healthiest trees in the shortest time. These findings were consistent across Study 1 and Study 2. However, in Study 2, the tree growth differences between the systems were more pronounced than in Study 1.

The structural load bearing systems, including Stratacell, performed extremely well in Study 2. Compared to the compacted soil, SBSS and GBSS, these systems consistently achieved the highest scores across multiple measures of tree health and growth, including:

• Tree trunk diameter
• Tree height
• Foliar Colour
• Number of roots > 1.2cm diameter
• Maximum root spread
• Maximum root depth
• Weight of tree parts
• Moisture content of soil

Stratacell outperformed all other systems in terms of maximum root depth, moisture content and foliar colour. Like the other structural load bearing module, it also produced significantly larger trees with more large roots than other treatments, including compacted soil, SBSS and GBSS.

The final findings were consistent with observations throughout the study, which reported that the trees in structural load bearing modules began to diverge from other treatments by the end of 2015. Similarly, in 2016 and 2017, there were significantly larger than most other treatments.

Trees grown in structural load bearing modules

Tree roots developed in Stratacell and Silvacell vault systems

Root systems grown in gravel based Structural Soil

Conclusion

The study concluded that structural soil vault modules (such as Citygreen Stratacell and Stratavault) are superior for growing large, healthy trees in the fastest times when compared to other systems, such as compacted soil, SBSS and GBSS.

While the study did not point to a ‘best product’, it proved the methods that support the load on a pavement and keep that load off the growing media work better than those that don’t.

This is good news for urban planners, landscape gardeners, architects and developers. It means that, simply by choosing a structural load bearing soil system, they can achieve the canopy cover they require years sooner than they might with other systems.

Similarly, cities, communities and individuals can enjoy the environmental, economic and health benefits of tree canopies faster and for longer.

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Download the full report

Click here to download the full ‘Comparisons of Soil Treatments Under Concrete Pavement’ study.