ENERGY SAVING BENEFITS OF SHADE TREES IN RELATION TO WATER USE

Stephen Livesley, Monash Sustainability Institute,
School of Geography and Environmental Science, Monash University

Project overview

Increasing the number of urban trees, and appropriately positioning those trees in relation to buildings and impervious surfaces, is a simple but effective strategy for urban climate change adaptation / mitigation to reduce energy consumption, greenhouse gas emissions whilst improving human thermal comfort and reducing human heat stress. To better communicate and value the importance of urban trees it is necessary to directly quantify these benefits, and to understand the properties and processes that influence the magnitude of these benefits. This project is being funded by Nursery and Garden Industry Australia (NGIA) providing support for two Masters students to:

• directly quantify the dollar benefit deciduous and evergreen trees provide through reduced energy used for heating-cooling buildings.
• directly quantify tree water use to provide an indication of possible water costs.
• raise awareness of the thermal and energy benefits urban trees provide in the built environment through an educational installation at Melbourne Flower and Garden 2011.

Background

Urban trees provide many important social and environmental services such as: shading and cooling of walkways and buildings, mental health and well-being, biodiversity habitat and conservation, carbon sequestration, and atmospheric pollution filter (Bolund and Hunhammar, 1999; Nowak and Crane, 2002; Pataki et al., 2006). Only a few desktop studies have assessed the environmental benefits of urban trees in Australia (Brack, 2002; Brindal and Stringer, 2009; Killicoat et al., 2002; Moore, 2007; Moore, 2009; Plant, 2006). Better protection, planning and management of urban trees is necessary to ensure that Australian cities are best able to adapt to climate change whilst enhancing their livability (Beeton et al., 2006; Committee for Melbourne, 2008). To communicate the importance of urban trees in Australian urban centres, it is necessary to quantify some of the more important ecosystem services they provide, and to understand the ‘properties and processes’ involved. This will provide city managers, urban planners and developers throughout Australia with a scientific basis to maintain and enhance the urban forest.

Internationally, there has been great progress in the measurement and modeling of the carbon, climate and energy benefits of urban trees in the last 10 years (Akbari, 2002; Ca et al., 1998; McPherson et al., 1998; 2005; Nowak and Crane, 2002). Akbari et al. (1997) directly measured a 30% saving in summertime cooling energy use in a Sacramento, California home. Simpson and McPherson (1996) through simulations estimated that tree plantings around Californian homes could reduce energy used in summer cooling by up to 50%. Last year, Akbari (2009) released a “how-to” guide on planting trees in the US for energy use reduction, with expected energy saving benefits between 15 and 35%. If those same energy saving benefits could be demonstrated in Australia, urban vegetation would be taken seriously as a climate change adaptation initiative.

Fisher (2007), drawing upon international studies, estimated that shade trees in Australian cities such as Melbourne and Adelaide could save ~30 kWh per tree, therefore Melbourne’s 2.4 million shade trees could save 72 million kWh per year, avoiding 86,000 t CO₂ emissions per year from coal powered electricity. Estimating the benefit of Australia’s urban trees from international data is problematic. The actual benefits will vary greatly according to the climatic conditions in that Australian city, the type of tree shade (e.g. eucalypts) and the properties of the building being shaded.

Fisher (2007), drawing upon international studies, estimated that shade trees in Australian cities such as Melbourne and Adelaide could save ~30 kWh per tree, therefore Melbourne’s 2.4 million shade trees could save 72 million kWh per year, avoiding 86,000 t CO₂ emissions per year from coal powered electricity. Estimating the benefit of Australia’s urban trees from international data is problematic. The actual benefits will vary greatly according to the climatic conditions in that Australian city, the type of tree shade (e.g. eucalypts) and the properties of the building being shaded.

This project will quantify the energy saving benefits of trees in a built environment whilst concurrently quantifying tree water use, thereby providing a data platform to convert both costs and benefits into a dollar value.

• Energy used for cooling or heating a building has a dollar value, and an indirect carbon cost, so this project will directly quantify the energy savings that urban trees can provide to the built environment through modified micro-climate conditions.
• Water to keep those trees alive also has a dollar value according to urban water rates and irrigation infrastructure.

This collaborative research project is funded by NGIA and involves the Melbourne School of Land and Environment, the Faculty of Engineering and the Faculty of Architecture Building and Planning at the University of Melbourne and the Monash Sustainability Institute, School of Geography and Environmental Science.

Project approach

Nursery and Garden Industry Australia has provided funding to establish and instrument three weather-board, single-room, galvanized-iron roof buildings (3.2.x 3.7 x 3.3 m) at the Burnley campus of The University of Melbourne. Each building has been equipped with a reverse-cycle air conditioner attached to a continuously logging power meter to record energy consumption.

Along the northern and western walls of two buildings (A and C) trees have been placed in large (75 L) pots that receive potable irrigation twice a week through timed drippers. Building A is flanked by nine potted Eucalyptus sideroxylon (Ironbark) trees, whilst building C is flanked by nine Fraxinus excelsior aurea (Golden Ash). These trees are placed 1.5 m from the building. Building B act as a control with no trees, however, due to subtle differences among the three buildings each building acts as its ‘own control’ for one week in every 8 weeks by simply removing the trees.

The energy benefit of the evergreen eucalypt and the deciduous trees can be directly and simply estimated by differences in mW/h energy use in air conditioning, as each house is kept at a near constant temperature (21°C) through heating in winter, and cooling in summer. At the same time, heat flux sensors have been installed on the inside of each wall, ceiling and floor (under carpet). These are connected to a central logger in building B (DT85, Datataker™, Australia). Each heat flux sensor (Omega™ HSF-4, USA) records surface temperature (°C) and heat flux (W m^-2) every 30 seconds. The flux of heat into a building on a sunny day produces a positive reading, whilst heat flux out of the building is indicated by a negative reading. From this data and standard weather station information at the site (air temperature, relative humidity, rainfall, wind speed and direction, net radiation) it is possible to construct a heat balance for each building. Direct measures of building heat balance are being related to simulated estimates of building heat balances using Integrate Environmental solutions (IES™, Glasgow, Scotland) software based on input data of building properties, tree properties and climatic data.

At the same time, direct water use by these trees will be measured using two methods. By keeping the trees in large pots (75 L), water use can be measured from the change in the mass of the potted tree in between rainfall or irrigation events. One E. sideroxylon and one F. excelsior aurea tree are constantly seated on two 300 kg platform load cells linked to a laptop PC with mass logged every 60 seconds. A 3 cm layer of scoria gravel on the pot surface minimizes evaporation from the pot surface. As an alternative, leaf level measurements of stomatal leaf conductance (transpiration) are being made seasonally using an infra-red gas analyser (LICOR™ 6400, USA). Branch diameter to leaf area allometrics will be established through destructive harvests of two E. sideroxylon and two F. excelsior aurea trees in mid-summer and then related to repeated and detailed branch diameter measurements to accurately estimate the total leaf area (m²) of each tree over time. From leaf level transpiration measurements (mmol m^-2 s^-1) it will be possible to estimate transpiration in mm tree^-1 d^-1

Project future

This project commenced in July 2010 and has yet to provide sufficient temporal data from which to present initial results and draw any conclusions. The project will run for one year, but provides the basis and research infrastructure to investigate many research questions that relate to the use of urban green infrastructure. For example, in addition to quantifying the energy saving benefits of different urban shade trees, this research infrastructure can be equally applied to investigations of green roofs, green walls and even white roofs. Furthermore, this research framework can investigate issues of urban green infrastructure with regards to building energy use, thermal load, plant water use, micro-climate, rainfall retardation and run-off quality.

This study provides a powerful educational and awareness-raising resource that public and policy makers can easily understand because it provides strong visual, three-dimensional demonstration. At the same time, this research is advancing the scientific skills and capacity of urban tree researchers in Australia, by providing a multi-disciplinary and collaborative research program that should deliver more than the individual contributors could alone.

References

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• Akbari, H., 2009. Cooling our Communities. A Guidebook on Tree Planting and Light-Colored Surfacing. 0269-7491, Lawrence Berkeley National Laboratory, Berkeley.
• Akbari, H., Kurn, D.M., Bretz, S.E. and Hanford, J.W., 1997. Peak power and cooling energy savings of shade trees. Energy and Buildings, 25(2): 139-148.
• Beeton, R.J.S. et al., 2006. Australia: State of the Environment. DEH, Canberra. Bolund, P. and Hunhammar, S., 1999. Ecosystem services in urban areas. Ecological
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• Brack, C.L., 2002. Pollution mitigation and carbon sequestration by an urban forest. Environmental Pollution, 116: S195-S200.
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