Jennifer Mullaney, Terry Lucke and Stephen J. Trueman
University of the Sunshine Coast, Sippy Downs, Queensland, Australia
Population centres are changing from rural to urbanised areas, with increasing development to create supportive infrastructure (North et al., 2015). Urbanisation is set to intensify with an estimated increase in global population of 44% by 2100 (United Nations, 2010). As populations continue to grow and urbanisation increases to support these communities, permeable land surfaces are being lost to urban development. The increase in impervious surfaces through the construction of roofs, pavements and roads creates an increase in urban runoff and increases the potential for downstream flooding and pollution. Urban areas also place increasing pressures on urban green spaces including street trees.
Trees are no longer viewed only for aesthetic purposes but are now increasingly being appreciated for the many environmental and economic benefits they provide. Trees are now considered valuable assets, with benefits including increased air quality (Tallis et al., 2011), stormwater attenuation (Seitz and Escobedo, 2011), noise reduction (Lohr et al., 2004), mitigation of the urban heat island effect (Gago et al., 2003), increased property values and energy savings (Donovan and Butry, 2010). Our towns and cities are more liveable through the shade provision, calming effects and aesthetics that trees provide. However, trees face increasing challenges within an urban environment.
Tree growth in urban areas is typically less than that of equivalent trees in a more natural environment (Buhler et al., 2007). Soils in urban areas can be highly altered from the natural greenfield state, largely as a result of human disturbance and activity (Watson et al., 2014). Street tree growth is constrained by the limited soil volume available for root growth and by reduced water and nutrients available under impervious urban surfaces (Mullaney et al., 2015a). Soil space in urban areas is limited by utilities and soil is often of poor quality and heavily compacted in the construction space, decreasing the amount for void space into which tree root tips can grow. Impervious surfaces surrounding tree pits affect street tree growth when water is restricted from infiltrating the soil where it can be accessed by tree roots. Impervious surfaces can also increase the soil temperature, particularly in the upper layers, which can reduce root growth and kill tree roots, particularly if temperatures exceed 40°C (Kozlowski, 1984; Ingram et al., 1989). Tree health and condition decrease as the distance between the tree trunk and the pavement decreases (North et al., 2015). When trees are less than 3 m from the pavement, the probability of conflict between the pavement and tree is increased (Sydnor et al., 2000). Trees planted in larger tree pits with a smaller area of impervious surface can be taller with a larger diameter at breast height (DBH) than trees planted at sites with a larger area of impervious surface (Close et al., 1996a, b; Grabosky and Gilman, 2004; Rahman et al., 2013).
A paradigm shift in the management of urban infrastructure is required to enable trees and urban areas to exist in harmony (North et al., 2015). The management challenge for street trees in urban areas is to provide an environment that functions like a natural environment, even though its appearance will be different (Watson et al., 2014). One approach to making the soil beneath impervious surfaces more conducive to street tree growth and survival, whilst maintaining the pavement structure and purpose, is the use of permeable pavements (Volder et al., 2009; Morgenroth and Visser, 2011; Mullaney et al., 2015a, b). Permeable pavements are commonly utilised to manage stormwater quality and restore infiltration rates but their ability to permit the exchange of water and oxygen into the soil can potentially also provide optimum growing conditions for street trees.
WSUD and Permeable Pavements
Urban stormwater discharge often contains high levels of pollutants such as sediment, nutrients, soil and heavy metals, which can have a negative impact upon downstream water quality and ecosystem health (Deletic and Orr, 2005; Shaffer et al., 2009). Appropriate stormwater management strategies are required to mitigate increased runoff volumes and the associated pollutants in urban areas. Many water management strategies are now being designed to comply with Water Sensitive Urban Design (WSUD) principles that embrace integrated water and land management practices and offer opportunities to mitigate adverse effects of urban stormwater runoff (Argue, 2004; Lucke, 2011; Mullaney and Lucke, 2014). WSUD management strategies include both vegetated measures such as swales, bioretention basins and wetlands, and non- vegetated measures such as permeable pavements, gross pollutants traps and settlement ponds. Highly urbanised areas generally have restricted space and so potential stormwater treatment systems should ideally fit within the urban area without an increase in land uptake. Permeable pavement systems have become a popular WSUD solution to reduce the burden of urban runoff and restore the infiltration and hydraulic functions of natural systems. Permeable pavements are specifically designed to promote the infiltration of stormwater through the paving structure, where it is filtered through the various pavement layers. The storage capacity of the permeable pavement structure can help to reinstate the infiltration capability and restore the natural hydrological cycle of urban areas where space for vegetation is limited or unavailable.
Street Trees and Permeable Pavements
The potential for permeable pavements to improve tree growth has recently been investigated (Volder et al., 2009; Morgenroth and Visser, 2011, Mullaney et al., 2015a, b). Permeable pavements may reduce the below- ground stress experienced by trees in urban areas by permitting water and oxygen to infiltrate into the sub- soil. The removal of an impermeable barrier, whilst maintaining structure, can potentially improve growing conditions for roots and reduce water and nutrient shortages for trees. Porous concrete pavement increased the height, DBH and root biomass of Platanus orientalis trees, when compared with an impervious concrete surface (Morgenroth, 2011; Morgenroth and Visser, 2011). In contrast, porous concrete pavements did not affect the DBH of Liquidambar styraciflua trees, when compared with impervious concrete pavement plots (Volder et al., 2009). A further approach to realise the benefits of permeable pavements is to change the moisture profile under the permeable pavement by installing an underlying base layer (Mullaney et al., 2015a). These layers may improve root growth by increasing the water storage capacity of the pavement system (Lucke and Beecham, 2011; Viswanathan et al., 2011). Stormwater can be effectively captured and stored within the base layer, allowing it to infiltrate into the sub-soil at a slower rate than natural conditions. The periodic drying of the base layer may also suppress shallow root growth, encouraging roots to grow deeper into the sub-soil where moisture and temperature levels are more stable (Mullaney et al., 2015a). This paper discusses the findings of a research project underway at the University of the Sunshine Coast (USC), Queensland, Australia, which aims to assess and quantify the long-term performance of permeable pavements with a base layer in improving street tree growth.
The aim of the research at USC was to investigate whether permeable pavements with varying depths (0, 100 or 300 mm) of underlying aggregate base layer (Figure 1) could improve soil conditions and increase growth of broad-leaf paperbark (Melaleuca quinquenervia) trees compared with trees growing in conventional asphalt pavements over two soil types (sand and clay). Thirty-two pavement research plots were installed in October 2012 at the University of the Sunshine Coast, Queensland, Australia (26°43’S, 153°04’E), as described by Mullaney et al. (2015a). The research site replicated an urban street environment with a car park on one side and grass edging on the other (Figure 2). Daily rainfall and daily mean air temperatures for the study period were recorded by a weather station at Sunshine Coast Airport, 13 km along the same coastal plain from the study site.
The research tested whether permeable pavements with different depths of underlying base layer had an effect on the following variables compared with a control asphalt pavement: (1) soil moisture and temperature at two depths within the soil; (2) tree growth, specifically tree height and tree trunk diameter; (3) leaf nutrient concentrations; (4) tree ecophysiological status, including photosynthesis (A1400), concentration of carbon dioxide at the carboxylation site in the leaf (Ci), stomatal conductance (gs), intrinsic water use efficiency (iWUE) and total nitrogen (TN), which provide an immediate picture of tree responses to nitrogen and water availability, and leaf and soil δ15N and δ13C, which provide a retrospective longer-term picture of nitrogen and water use by trees.
Four replicates of each of the four designs were constructed in both a sandy-loam soil (‘sand’) and a clay-loam soil (‘clay’) to give a total of 32 tree-plots. Each of the tree planting pits was 0.6 m × 0.6 m within the centre of each 3 m × 3 m paving plot. Thirty-two saplings of broad-leaf paperbark, Melaleuca quinquenervia (Myrtaceae), with mean (±S.E.) height of 1.88 ± 0.09 m, were randomly assigned to the plots and planted within the tree planting pits in October 2012, as described by Mullaney et al. (2015a). Tree growth (height and DBH) and leaf nutrients were measured at 6, 12 and 18 months after planting, using methodology as detailed in Mullaney et al. (2015a). Soil moisture and temperature was measured at two different depths within the soil, 50 mm and 500 mm below the pavement structure surface, using ECH2O 5TM probes (Decagon Devices, Inc.) (Mullaney et al., 2015a). Gas exchange measurements were conducted on all 32 trees at 6, 12 and 18 months after planting (April 2013, October 2013 and April 2014) (Mullaney et al., 2015b). Photosynthesis was measured using a portable photosynthesis system (Model LI-6400, LI-COR Biosciences, Lincoln, NE). Parameters obtained were leaf photosynthesis at 1,400 µmol m-2 s-1 PAR (A1400), CO2 concentration at the carboxylation site (Ci) and stomatal conductance (gs). Intrinsic water use efficiency (iWUE) at leaf level was determined as A1400/E (µmol mmol-1), where E was the transpiration rate (Farquhar and Richards, 1984). The samples used for photosynthesis were then assessed for total nitrogen concentration (TN) and N and C isotope composition (δ15N and δ13C) as described by Prasolova et al. (2000) and Xu et al. (2003).
Results and Discussion
Permeable pavements with underlying base layers increased moisture levels in drier sandy soil and decreased moisture levels in wetter clay soil (Figures 3 and 4). These moderating effects on soil moisture increased as the depth of the aggregate base layer increased. Moisture levels in sandy soil increased slightly beneath permeable pavements during and after rainfall events (Figure 3). The decline in soil moisture levels during the subsequent drying period was reduced by the inclusion of a base layer beneath the paving surface, suggesting that water was stored in the base layer before being released into the freely-draining sandy soil (Figure 3). Moisture levels in clay soil under asphaltic concrete pavements and permeable pavements with no base layer increased greatly after rainfall events and the soil became heavily waterlogged (Figure 4). Heavy waterlogging did not occur in clay soil when a base layer was installed under the permeable pavement. This suggested that water was retained for an extended period after rainfall within the base layer and it did not infiltrate rapidly into the poorly-draining clay soil. The results therefore confirmed that base layers can increase the water holding capacity of a permeable pavement system. However, it also showed that their effects on soil moisture levels were dependent on the drainage characteristics of the underlying soil (Lucke and Beecham, 2011; Viswanathan et al., 2011).
Permeable pavements decreased soil temperatures beneath the paving surfaces (Figures 5 and 6). The base layer appeared to provide a degree of insulation against high temperatures, with the insulation effect increasing as the depth of the base layer was increased. The provision of lower root-zone temperatures may decrease thermal damage to roots during summer.
Permeable pavements did not affect tree growth significantly in the freely-draining sandy soil, when compared with asphaltic concrete control pavements (Table 1). However, tree DBH in sandy soil was 65% greater using permeable pavements with no base layer than using permeable pavements with a shallow base layer (Mullaney et al., 2015a). Therefore, the inclusion of a base layer may not be optimal for tree growth when permeable pavements are installed over freely-draining soils.
Permeable pavements did affect growth greatly when trees were planted in the wetter clay soil (Table 2). In this soil type, permeable pavements with the deeper base layer increased DBH growth by between 55% and 73% compared with the other three treatments. Trees planted in permeable pavements without a base layer over clay soil had a reduced height increment of 37% to 38% compared with trees that were planted in permeable pavements with a base layer (Mullaney et al., 2015a). The base layer again acted as a water reservoir although it did not appear to release moisture into the clay soil after rainfall. Instead, it prevented waterlogging in this soil. These results demonstrate that inclusion of a base layer may be important for tree growth when permeable pavements are installed over poorly-draining soils.
The different effects of the pavement treatments on tree growth in sandy and clay soils, therefore, reflected their different effects on moisture levels in the two soil types. The study results supported those of Morgenroth and Visser (2011), who found that growth of Platanus orientalis trees in a well-drained sandy soil was not improved significantly with the use of porous concrete pavement with a 200 mm base layer. They also help to explain the results of Volder et al. (2009), who found that the use of porous concrete pavement without a base layer did not improve the growth of Liquidambar styraciflua trees planted in clay soil.
Table 1 Height and DBH increment of Melaleuca quinquenervia trees in sandy soil
*Means with different letters within a time point are significantly different (ANOVA and l.s.d test, P<0.05, n=4)
Table 2 Height and DBH increment of Melaleuca quinquenervia trees in clay soil
|12 months||84a||8||81 a||6||93a||10||104a||6|
|18 months||29.2 b||3.5||26.5b||3.5||29.8b||5.8||45.0a||5.2|
*Means with different letters within a time point are significantly different (ANOVA and l.s.d test, P<0.05, n=4)
Leaf nutrient concentrations generally did not differ significantly between pavement treatments (Tables 3 and 4). Nutrient concentrations were generally high and commensurate with concentrations in nursery plants of other species from the same family (Cunha et al., 2009; Trueman et al., 2013). However, K concentrations were comparatively low, and some differences in the growth of Melaleuca quinquenervia trees among pavement treatments were reflected in differences in leaf nutrient concentrations (Tables 3 and 4). In particular, improved growth of trees in permeable pavements with increasing depths of base layer over clay soil was associated with increased leaf concentrations of three major cationic elements, K, Mg and Na (Table 3). Leaf K concentrations were often lower for trees growing in clay than in sandy soil, and the inclusion of a deep base layer above the clay increased leaf concentrations of K and Mg (Table 4). The use of a deep base layer above clay soil also increased leaf S concentrations (Table 4). Plants take up S as sulphates under aerobic conditions and waterlogging can cause the production of sulphides, making S less available to the plant (Maathuis, 2009). This again highlighted that inclusion of a base layer was important for preventing waterlogging, maintaining tree growth and optimising nutrient uptake when permeable pavements were installed over poorly-draining soils.
Table 3 Leaf nutrient concentrations of Melaleuca quinquenervia trees in sandy soil
*Means with different letters within a time point are significantly different (ANOVA and l.s.d test, P<0.05, n=4)
Table 4 Leaf nutrient concentrations of Melaleuca quinquenervia trees in clay soil
There had been little previous research on the ecophysiological responses of urban trees to the soil conditions provided by permeable pavements. This study identified that permeable pavements often did not affect the leaf gas-exchange and water use variables, A1400, Ci, gs and iWUE, of Melaleuca quinquenervia trees on individual days (Mullaney et al., 2015b). These variables did not correlate with tree growth during the first 12 months after planting. In sandy soil, growth varied little between pavement treatments during the initial 12 months, although permeable pavements with no base layer did provide greater trunk-diameter growth than permeable pavements with a base layer (Mullaney et al., 2015a).
The low levels of leaf δ15N of trees in permeable pavements with a shallow base layer above sand may explain their low initial growth. Leaf δ15N was correlated with tree growth in both soil types during the first 12 months of the study (Mullaney et al., 2015b). Leaf TN was also correlated with tree growth in clay soil. These results indicate that nitrogen cycling rates (and leaf nitrogen concentrations over clay soil) are important determinants of Melaleuca quinquenervia tree growth during establishment in pavement plots.
The relationships between the ecophysiological variables and tree growth differed during two different tree- growth phases. Tree growth was slow during tree establishment (up to 12 months after planting) compared with a subsequent period of more-rapid growth (12 months after planting). Tree growth was correlated to leaf δ15N in the first 12 months after planting. However, at 18 months after planting, tree growth in clay soil was positively correlated with A1400, Ci and gs and negatively correlated with leaf δ13C, but it was not correlated with leaf δ15N. Trees planted within permeable pavements with the deeper base layer had lower leaf δ13C at 18 months than trees within permeable pavements without a base layer, suggesting that they had preceding periods of higher gs. The study, therefore, identified that permeable pavements with different depths of base layer sometimes influenced the nitrogen and water relations of Melaleuca quinquenervia trees. The ecophysiological results (Mullaney et al., 2015b) supported the conclusions in Mullaney et al. (2015a) that the installation of base layers was required to prevent waterlogging and sustain tree growth when permeable pavements were installed over poorly-draining soils.
The benefits provided by urban street trees have been well documented in previous studies although urban trees often have higher mortality than trees in more natural environments (Buhler et al., 2007; Lawrence et al., 2012). Installation and maintenance of street trees can often be challenging due to the ever-increasing areas of impervious surface resulting from urbanisation. Tree growth in urban environments is often limited by the volume of penetrable soil available to meet the water and nutrient demands of the tree (Lindsey and Bassuk, 1992). Impervious surfaces, including block pavers, concrete or asphalt, also effectively seal the surface and prevent the infiltration of water, oxygen and nutrients into the soil.
The findings of this research study demonstrate that appropriately designed permeable pavements can improve tree growth in urban environments. Tree growth significantly increased with the inclusion of a base layer when trees were growing in clay soil. However, the inclusion of a base layer can cause sub-optimal tree growth in freely draining sandy soil. The research identified that pavement designs can improve soil conditions and increase tree growth in urban environments. Installing permeable pavements in an urban street, surrounding a tree, can reduce the costs associated with tree mortality and pavement maintenance, while the trees will continue to provide environmental, economic and social services. Urban landscape designers and engineers can employ these designs in urban street-scapes to effectively meet design criteria and improve natural streetscape aesthetics.
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