Engineered Spaces for Trees in Dubbo Regional Council: Modified Stockholm tree planting method

Abstract

As Australia’s climate warms our cities and towns, councils face an increasing array of challenges in mitigating urban heat and its impact on communities and biodiversity. Local governments are under increasing pressure to increase tree canopy cover to help improve resilience against projected increases in temperature.  While councils understand the need for more trees, increasing densification of urban centres through smaller lot sizes and infill development counters their ability to make significant gains.  Large residences on these smaller allotments reduce tree planting opportunities and the ability of the private urban forest to contribute to canopy cover. In many new subdivisions the width of road reserves has been reduced, including the verge that previously allowed large trees to be planted. Underground utilities and services further reduce planting opportunities.  Where majestic trees once lined and shaded streets, councils are now forced to plant smaller and narrower species which reduces shading and results in more heat being absorbed. Not only does this reduce the comfort level of residents but it has a financial impact on councils through reduced life of asphaltic road seals. Loss of tree-lined streets also impacts habitat and biodiversity links needed to support wildlife. These challenges place more pressure on the public urban forest to achieve the desired greener, shadier and cooler urban centres. To meet these challenges Dubbo Regional Council has constructed a variety of tree planting pits, including root vaults designed to allow growth under roads and footpaths. This paper reports tree growth in vaults constructed using structural cells and a modified Stockholm Method. A street beautification project completed in the early 1990s is also presented as a case study. In this project trees were planted into pits with vastly different volumes of soil. Thirty years on, the growth and the development of the trees serves to reinforce the need for adequate soil volume to be provided for urban trees.

Introduction

Dubbo Regional Council is located in central New South Wales, covers 7,563 km² and its population of 51,050 is based largely in the City of Dubbo (42,500) and Wellington (4,500).  Dubbo’s climate is characterised by hot summers and cool winters, with winter night temperatures dropping to well below freezing.  While Dubbo’s reported mean maximum temperature for January is 33°C, consecutive days over 35°C and over 40°C are increasingly common.  AdaptNSW (2023) identified that the region experiences 20 – 30 hot days (>35°C) every year, but in 2018 temperatures exceeded 35°C and 40°C on 52 and 9 days respectively.  2019 temperatures exceeded 35°C and 40°C on 67 and 16 times respectively.  In contrast, 2020, 2021, and 2022 saw a significant reduction of days over 35°C and 40°C as a result of the prolonged La Nina weather pattern.  Annual rainfall between 2018 and 2022 inclusive was 311, 211, 573, 564 and 863mm. While the La Nina provided some respite in recent years it is predicted that global temperature records will be broken within the next five years (WMO 2023). A warming trend is consistent with climate change projections from the NSW and ACT Regional Climate Modelling (NARCliM) that predicts that temperatures will rise by 0.7°C between 2020 and 2039) and by 2.1°C by 2079 compared with 1990 – 2009 average (NSW Govt. 2023). 

Projected global mean surface temperature has been predicted to rise this century due to carbon emissions by between 2°C and more than 5°C under various emission scenarios (Arias et al. 2021). In these scenarios the frequency and intensity of heat waves and drought conditions are anticipated to increase. These climatic changes and particularly the increased frequency of extremes add to the challenges of selecting and establishing tree species in urban environments. As an example, Red ironbark (Eucalyptus sideroxylon) occurs naturally in the Brigalow Belt South Bioregion which includes Dubbo, and it is considered to be extremely drought and heat tolerant. The Climate Assessment Tool (v1), an assessment tool developed by Botanic Gardens Conservation International (Climate Change Alliance of Botanic Gardens, 2023), is one tool available to help forecast species’ likely suitability for a given location under changing climate scenarios.

Under Dubbo’s existing climatic conditions with a mean temperature of 17°C, E. sideroxylon thrives within its natural distribution. Due to its local suitability, Eucalyptus sideroxylon has been widely planted in urban settings in Dubbo with a high degree of confidence that they will establish well and thrive. The climate assessment tool indicates that an increase in mean temperature of 2°C will make conditions at Dubbo’s location similar to warmer botanic gardens in which it currently grows. The tool does show that the species is known to occur in urban situations at this temperature. However, under this scenario an urban tree manager may begin to consider whether to use the species in street plantings or to investigate alternatives, but may be more likely to continue to plant it in parkland situations. A 5°C increase in mean temperature would place Dubbo at the upper limit of temperature known for the species, including in urban environments.  A tree manager at Dubbo may, therefore, consider planting E. sideroxylon to be unwise in traditional street and park planting approaches under a 5°C mean temperature increase scenario and may look for alternative species or cultural techniques. At higher temperatures E. sideroxylon might be unlikely to survive without significant intervention through urban design or support.    

While this tool has been developed to assist botanic gardens’ managers to transition their living collections for resilience in the changing climate, it does provide some information to help urban tree managers to consider and when revising urban planting palettes in preparation for warmer and drier long-term conditions. On a broader landscape perspective (i.e. non-urban) it also identifies that there is potentially significant floristic and faunal changes that may result as temperatures increase, but the level at which the environment can buffer these predicted changes is not known. It does not, however, consider that plantings of tree species exist in urban centres in private gardens, streets and parks, which are beyond the climatic ranges represented by botanic gardens collections; these plantings demonstrate potentially greater suitability for urban applications than that indicated by botanic gardens data. 

Increasing shading over road surfaces by as little as 20% can effectively reduce ambient temperatures by 3 – 4°C (McPherson & Sacamano, 1993), and reduce the risk of UV radiation exposure by up to 75% (Parsons et.al. 1998). Increased tree canopies within the urban landscape can also provide significant infrastructure savings to councils and their communities, including reducing the number of reseals required within a given timeframe and extending the life expectancy of road pavements (McPherson & Muchnick 2005), and reducing and delaying stormwater peak flows and nutrients loading to water ways (Xiao et al. 1998). To achieve these benefits for residents, Dubbo Regional Council is investigating engineering approaches to better establish and nurture trees, to strengthen urban forest resilience in the region and provide a more comfortable and safer environment for the community in the changing climate.

Spaces for trees: comparing engineering approaches

Since 2011 Dubbo Regional Council has been trialling different approaches for constructing tree planting sites to identify cost-effective approaches suited to local social and environmental constraints and conditions. Initial approaches used up to 2011 were designed along traditional lines with the focus on excluding roots from soil that supported infrastructure, by containing them within a barrier. Limitations of this approach led to consideration of methods to increase soil volumes, which involved the testing of structural cells, the Stockholm Method (Figure 1).

The concrete ring design was used c2011 in response to concerns regarding roots potentially damaging road pavement. This low-cost approach allayed engineer’s concerns that roots might otherwise impact the road pavement and it provided a slight increase in soil volume and quality compared with the previous practice of planting directly into the road base. The benefits of this approach were limited by the small volume of quality soil, however, and it was only really suitable for small species as Crepe myrtle (Lagerstroemia indica).

In 2013 structural cells were installed under the road pavement to create root vaults as part of a major civil infrastructure project undertaken by Council. Series 5 and 6 Stratacell^TM have been used in various projects, with Series 5 shown in Figure 1. This approach has and continues to provide impressive results in terms of the rate of growth and the health of the trees.

In 2015 Council commenced trialling the Stockholm Method. This method was developed by Johan Östberg and utilises a rock matrix with biochar in the voids between the rocks. A 250 – 300 mm thick layer of rock is placed in an excavated pit and lightly compacted to ensure that the rock matrix is interlocked. Further layers are added to achieve the desired depth of the pit. Biochar is then washed into the voids and it is the biochar that forms the growing medium (Alvem et al., 2009). As the tree grows, new biomass is produced within the biochar through the growth and death of fine and very fine roots and associated micro-organisms, with the nutrients that are released then largely taken up to nourish further growth of the tree.

Figure 1. Tree planting methodologies used by Dubbo Regional Council have included a concrete ring tree pit design in 2011 (left), ‘Stratacell^TM’ structural cells in 2013 (centre) and the Stockholm structural soil method in 2015 (right).

In 2016 and after consulting with ENSPEC Environment and Risk a modification was made to the method: a soil and compost component was added to the mix. The revised blend was comprised of 50% rock (250 – 300 mm stones), 35% soil/compost and 15% biochar. This blend provides a high degree of structural integrity to the root vault, with the soil/compost and biochar providing a growing medium with a readily available nutrient bank from time of construction. As the tree develops its root structure, additional biomass and nutrients within the matrix are developed, turned over and renewed. A major benefit to Council of this modified method is its cost effectiveness, as it utilises rock which is a waste product from a council sub-division, compost from Council’s own composting system, and less time and labour are required for installation. Replacement of some of the biochar with the soil/compost mix delivers further significant savings. An updated version of the City of Stockholm’s manual similarly notes the use of a blend of nutritionally enriched biochar and compost amongst the stone (Embrén and Alvem, 2017).

After trialling a number of approaches, Dubbo Regional Council has now settled on a preferred root vault design for planting trees in streetscapes using the Modified Stockholm Approach. The volume of the root vault is determined by the formula V= 𝜋𝑟² x 0.6, where r is the radius of the mature canopy cover of the tree species chosen. The 0.6 figure accounts for the fact that the majority of a tree’s roots typically occur in the top 0.6 m of soil, although deeper root penetration can be achieved through the addition of air vents through the rock matrix.

An open-ended concrete “box” is installed, which protects the road pavement and encourages the roots into the surrounding rock matrix.  The bottom of this box is installed within the rock matrix, with the matrix extending out beyond the box.  To limit the potential of root intrusion into the sub-base of the road a geo-fabric is installed at the interface between the rock matrix and the road. The box’s straight edges are favoured by engineers and construction personnel it makes compaction around the tree pit easier to achieve, thus reducing the potential for future settlement of the road pavement. 

Grates are made to fit the top of the tree ‘box’; a local metal fabricator cuts them from plate steel using a plasma cutter and then they are galvanised (Figure 2). The size of the box and grate can be varied to suit specific site and tree species requirements. The grate enables a degree of passive water capture of the road and further supplementary watering using a water truck as required. To assist air and water movement slotted PVC pipes are inserted into the rock matrix at varying depths.Drainage of the structural soil mix may also be required, depending on site characteristics such as catchment size and imperviousness, soil type, and surrounding land uses including vegetation.

Figure 2. A tree pit constructed using the Modified Stockholm Method, Dubbo Regional Council’s ‘box’ design and a galvanised steel grate.

To investigate the impact of the different planting methodologies Dubbo Regional Council began a trial in December 2016 in collaboration with Citygreen. The trial was established to examine root development and plant growth (height, calliper and condition) over a 10-year timeframe. As the trial requires access to the tree roots at the end of the trial the trees were planted in 6 skip bins of 9 m³ each (Figure 3). The bins were installed above ground, with soil mounded around them to shield the bins from excessive heat and so mimic more-natural growing conditions. Each bin contained a different soil treatment and one Silky oak (Grevillea robusta) tree was planted into each bin. The six soil treatments are:

• Control 1: natural soil
• Control 2: Australian Native Landscapes (ANL) native soil mix
• Stockholm Method
• Stockholm Method (Dubbo Regional Council)
• Stratacell Series 5 (ANL native soil mix)
• Stratacell Series 6 (ANL native soil mix)

Trees were planted from 200 litre containers on 2^nd December 2016. An irrigation system was installed to water the trees and no additional fertiliser has been applied. Tree height and diameter at breast height (DBH) were measured at the time of planting and on 21 December 2019 (36 months), 16^th April 2021 (52 months) and 16^th May 2023 (77 months) (Figures 4 and 5). During the 77 months of growth from 2 December 2016 the height of the trees increased by between 82% and 109% in control soils and treatments and the DBH increased by between 86% and 213% (Table 1). Initial measurements revealed greater height increase in the Stratcell treatments and controls than in the Stockholm method treatments, but the trees in the Stockholm treatments grew most rapidly in the last 24 months.

Figure 3. Installation of Stratacells in skip bins (left) prior to filling with soil mix (right), with Series 5 Stratacells (front) and Series 6 (rear).

After 77 months the DBH had increased more in both controls and the Dubbo-modified Stockholm treatment than in either of the Stratacell treatments. The density of the foliage of the trees appeared to vary when height and DBH measurements were taken in May 2023, with the tree in the Dubbo Regional Council-modified Stockholm treatment appearing to have the greatest foliage density (Figure 7). The health of the tree in the DRC-modified Stockholm treatment is considered best overall after 77 months in-situ growth. When measured in May 2023 all trees appeared in average to good health with no evidence of pests or disease. The trial still has another 43 months to run, after which the pits will be disassembled and the roots of each of the trees will be examined.

Figure 4. Grevillea robusta tree height increase in control and treatments over 72 months since planting date.

Figure 5. Grevillea robusta tree trunk diameter at breast height (DBH) increase in control and treatments over 72 months since planting date.

Table 1. Change in height and Diameter at Breast height (DBH) over 76 months since the commencement of the trial

Figure 7. Grevillea robusta trees in controls and each treatment photographed after 77 months growth.

Macquarie Street, a case study

Dubbo’s Macquarie Street serves as a practical, real-life example of the importance of providing urban trees with an adequate volume of quality soil. In the early 1990s Dubbo Regional Council undertook at a major beautification of Macquarie Street. Prior to this Macquarie Street was largely devoid of trees, with shop awnings providing shade on verges and footpaths.

Two tree species were chosen for this project: Hackberry (Celtis occidentalis) and European nettle tree (Celtis australis). Three different planting environments were created by the engineering design. Some trees were planted on traffic roundabouts and in relatively large garden beds on street corners near the roundabouts, others were planted in tree pits along footpaths on both sides of the street, and trees were planted in a narrow median island along the centre of the street. 

The trees in traffic roundabouts and garden beds grew in soil with mulched surface areas of 90 m² to 100 m²; these gardens had quality soil to approximately 1 m deep. The roundabouts are surrounded by compacted road base materials and sealed asphalt road surfaces. The trees planted on road verges grow in tree pits with a surface area of 1.5 m² surrounded by brick-paved footpaths laid on a concrete base. Footpaths contain a variable array of underground utilities mains and service connections. Commercial premises built to the adjacent property boundary are typically on substantial footings. The kerb and road adjacent to the footpath were built on compacted base and subgrade. Trees on the median island were planted in 0.8 m² tree pits with a maximum soil depth of 0.6 m. The median island is 0.7 m wide between sealed asphalt road surfaces. Trees in these different planting sites are visible in Figures 8 and 9.

Figure 8. Macquarie Street, Dubbo, looking South from Talbragar Street; trees growing in the median island appear consistently smaller than those growing in larger tree pits on the road verges.

Figure 9. Macquarie Street, Dubbo, looking North towards Talbragar Street where the broader growth of a tree planted on a traffic roundabout is apparent.

Street trees in central Dubbo were independently audited in 2012. In total 49 trees were assessed in Macquarie Street: 43 C. occidentalis and 6 C. australis. Diameter at breast height was measured and recorded during the assessment but tree height and structure were not, due to past pruning of the canopies for traffic and building clearances. A follow-up audit in 2023 measured DBH so the growth of trees in the different growing sites over the intervening 11 years could be compared.

Comparison of the increase in DBH of Celtis australis trees growing in larger soil volumes on roundabouts (n = 2) with those in the smaller soil volume in the median (n = 4) through a two-tailed t test revealed the difference was significant (p = 0.02)(Table 2).

Table 2. DBH increase of Celtis australis growing in road median islands between 2012 and 2023 was less than for trees on traffic roundabouts and garden beds with larger soil volume.

The two-tailed t-test was used to compare growth in DBH of Celtis occidentalis in the different sites. Trees growing in roundabouts and large garden beds (n = 7) were compared with those in medians (n = 5); trees growing in footpaths (n = 31) were compared with those in medians, and the increase in DBH of trees growing in roundabouts and large garden beds was compared with growth of the trees in footpaths. The difference between trees growing in medians and those in roundabouts and large beds was significant (p = 0.02). The difference in growth between trees in the footpath and those in the median was also significant (p = 0.04). The test revealed no difference between trees growing on footpath tree pits and those in the larger soil volumes (p = 0.21)(Table 3).

Table 3. Growth of Celtis occidentalis between 2012 and 2023 in traffic roundabouts and garden beds, footpaths, and medians.     

As expected the results show that the trees with access to a larger volume of quality soil performed better. The trees in the roundabouts and adjacent large garden beds had a greater volume to accommodate roots and a greater soil surface area to support rainfall infiltration and diffusion of gases. For the trees growing in the footpath, water infiltration and gas diffusion were restricted by the impermeable paved surface over its concrete base, but not as much as for the trees in the smaller pits in the medians. Root development in some median trees has been observed to be minimal, with several having been wind-thrown in recent years. It was surprising that the analysis did not reveal a difference of any significance in growth between the trees in the footpaths and roundabouts.  

Conclusion

Dubbo Regional Council remains proactive in designing, trialling and mainstreaming urban tree planting and establishment methodologies with the intent of increasing our urban forest’s resilience against the impacts of urbanization and climate change. It is important to trial new engineering approaches locally, to identify methods that work best for any given situation. Economic considerations often guide decision making on new projects and on investigations such as those described in this paper, and trials can be helpful in providing the necessary information to balance benefits and costs. The changing climate means the goal posts are constantly moving so there is always need for more trials and over longer timeframes, but the projects described in this paper show that it is possible to improve urban forest resilience through appropriate engineering design and construction for trees.