Trees: The new cool kid on the block

By Gabrielle Stannus

The current drought affecting areas of southern and eastern Australia is reminding us of the importance of water both in the production of greenlife, and also in the maintenance of our green spaces. Also of concern is how a changing climate may affect greenlife in the future due to water supply variability, especially in urban spaces where the urban heat island effect is well-documented. So what sort of greenlife do we grow if we want to cool down our cities?










Which Plant Where

The Which Plant Where program is a five-year series of research that will find out where current plant favourites are unlikely to thrive under the more extreme climates that Australian cities face. The program will learn from past successes, and stress-test major landscape species to find opportunities for new species and varieties to be planted.

Dr Linda Beaumont, at Macquarie University, says that some plants in urban areas will increasingly be faced with conditions that they are not used to. The Which Plant Where program seeks to assess which species are going to be the winners and losers in our changing climate.

Whilst still less than half way through, the Which Plant Where program has seen more than 13,000 individual plants and over 500 popularly planted tree and plant species put through rigorous testing and bioclimatic models to see how each would perform in the years 2030, 2050 and 2070.

Selected species are being tested to determine the influence of certain attributes and climatic tolerances on their ability to not only tolerate but thrive in a changing urban climate. These attributes include growth rate and form, height, canopy density, ground cover, longevity, seasonality, water quality, allergenicity, insect resistance, ornamental and amenity features, biodiversity impacts, as well as air and water quality influences and urban temperatures.

Evaporative cooling
One attribute that perhaps could be examined more closely is a plant’s ability to cool a local micro-climate through evaporative cooling. Traits associated with this attribute include large leaf surface area and overall leaf area index (total area of all leaves). Rough leaves have more surface area than smooth leaves, providing more space for evapotranspiration to occur across the leaf surface. This results in greater cooling benefits due to the high humidity surrounding trees with rough leaves 1 .

Trees are essentially large evaporative coolers, only much prettier in my opinion! Trees and other vegetation absorb energy, mainly heat, with only a small fraction used in photosynthesis. When the leaves transpire, they convert liquid to vapour, thereby using heat (latent heat flux-cooling). This process cools not only the leaf, but also the surrounding air. If there is no water in that setting, then a leaf can only cool by conduction and convection.

Numerous studies show that vegetated areas are cooler than their non-vegetated counterparts:

  • Air temperatures over irrigated agricultural fields 3ºC cooler than air over bare ground
  • Suburban areas with mature trees 2 to 3ºC cooler than new suburbs without trees
  • Temperatures over grass sports fields that are 1 to 2ºC cooler than over bordering areas 2

A large oak tree can transpire 40,000 gallons of water per year compared to one acre of corn which can transpire 3,000 to 4,000 gallons a day. So it seems that when looking for evaporative cooling benefits, trees may beat smaller plants hands down.

We all know the need to use water wisely in our production and retail nurseries. So what sort of plants will we be growing in the future that can help to cool our cities evaporatively without further contributing to the pressures facing our water supplies?

Water still needs to be a part of this equation otherwise we will not benefit from the cooling effects of evapotranspiration. So how do we do more with less?

Many urban areas experience low water availability, given the domination of impervious surfaces. These surfaces are designed to allow for rapid stormwater runoff to drains, i.e. moving water. Rainfall infiltration is low, even more so in drought conditions. These conditions lead to high water stress in urban trees, and potentially less evaporative cooling.

Dilemma?
If we plant drought tolerant, low water use species, then we provide less shade and transpiration cooling. If we plant exotic, deciduous species and supplement decreasing rainfall with irrigation, then we need greater irrigation water use efficiency and alternative water sources.

Through water sensitive urban design (WSUD), we are getting better at slowing down the flow of water in our cities, so that it is able to nourish urban vegetation. The associated evaporative cooling is just one benefit provided by this water in the environment.

Hopefully, through the Which Plant Where program and other future research, we will find out which trees are the winners in this context. Then we can all cool down.

References

1. Drago, S 2017, 'Leaf color and shape enhance cooling effect', viewed 19 September 2018, https://asknature.org/strategy/leaf-color-and-shape-enhance-cooling-effect/#.W6KDNHszbIU

2. U.S. Environmental Protection Agency 2008, ‘Trees and Vegetation’, Reducing Urban Heat Islands: Compendium of Strategies, Chapter 2, viewed 19 September 2018, https://www.epa.gov/sites/production/files/2017-05/documents/reducing_urban_heat_islands_ch_2.pdf