Tree selection and performance in relation to sub-regional variation in climate

Dr. Peter May
May Horticulture Services

Summary

  • Cities are warmer, especially at night, presenting planting opportunities.
  • Exposed walls and paving can be very hot, this may cause heat damage or increased water use by trees.
  • Paving can change the amount of rainfall being intercepted for tree use, this will affect the taxon selection.
  • City development produces complex shade and wind environments for trees, plan for these effects carefully.
  • Roof gardens are windy and exposed, tree selection and planting must reflect this.

Introduction

As Dr David Symons has shown in the previous section of this Tree Handbook, regional climatic variation plays an enormously important role in tree selection. However, there may also be climatic variations at a sub-regional scale that can also be taken into account when deciding on the suitability of a particular tree for a project. These sub-regional variations can be caused by small-scale local variations in geography but they can also come about as a result of human modification of the landscape. This section concentrates on this latter category. In the gardening literature this small-scale variation is often called microclimatic variation but this is not technically correct. The microclimate is the climate within or very close to the plant canopy. For the variations dealt with in this section, mesoclimate is preferred.

The urban “heat island” effect

Concentrations of human activity result in environmental temperature increases. There are a number of causes of this but they include fuel consumption and the use of building materials with the ability to store heat (such as concrete and masonry). These temperature increases are primarily the result of increased night temperatures rather than day temperatures. The effect of these temperature increases on trees will not be great but one would expect that in highly urbanized areas there is a reduced risk of low temperature injury as compared to the suburbs or surrounding rural land. This may allow a slightly broader palette of trees to be cultivated. In the case of Melbourne, sub-tropical species may be more successful in the CBD than in the outskirts of the city.

Trees that will benefit from a heat island effect include:

Arbutus canariensis (Canary Islands Madrone)
Ficus microcarpa var. hillii (Hills Weeping Fig)
Magnolia grandiflora and cultivars (Bull Bay Magnolia)
Yucca elephantitis (Spineless Yucca)

Effects of paved surfaces on water availability

In built-up areas the use of impermeable paving surfaces is very common. This can result in changes to the water budget of trees planted in or near paving. In some cases paving will impede water infiltration into the soil, resulting in a climate that is, in effect, drier than the regional climate. When coupled with higher air temperatures from radiant heat and possibly lower relative humidity, this may require the selection of tree species that are suited to climates more arid than the one being considered.

Consider these trees that may manage paved surfaces and lower water availability:

Acacia pendula (Weeping Myall)
Brachychiton acerifolia and hybrids
Ceratonia siliqua (Carob)
Dracaena draco (Dragon’s Blood)
Geijera parviflora (Wilga)
Jacaranda mimosaefolia (Jacaranda)
Olea europaea ‘Tolley’s Upright’ and O. europaea ‘Swan Hill’ (Olive)
Ulmus parvifolia ‘Burnley Select’ and U. parvifolia ‘Yarralumla’ (Chinese Elm)

The opposite can also occur. In some planting situations, (roadsides for instance), rainfall is collected by the surface and directed to soil adjacent to the paved surface. This, in effect, results in an increased rainfall for that small zone. This would allow the culture of trees with higher moisture requirements. It may also necessitate the selection of trees with higher tolerance of reduced soil aeration. Trees that may manage sites of different levels of compaction and low soil oxygen:

Quercus bicolor (Swamp Oak)
Quercus palustris and Q. palustris. ‘Early Defoliant Form’ (PinOak)
Taxodium distichum (Swamp Cypress)
Tristaniopsia laurina (Water Gum)
Waterhousea floribunda (Weeping Lilly Pilly)

Effects of high-rise urban development

The profoundest effects of humans on small-scale climatic variation are found in the high-rise parts of cities. In these environments new climate zones are created by the construction of buildings that block sunlight and modify wind speed and direction. In the early 1980s Kjelgren and Clark coined the term “urban canyons” to describe these new environments. Their characteristics include the creation of a number of planting zones with variations in light intensity and duration. Depending on street orientation and building height, trees can experience from complete shade to rapid change from shade to full sun. Other variations can include changes in relative humidity, wind speed and direction, rainfall and even day length. Perhaps the most difficult environments found in the urban canyon will include zones of permanent shade and wind tunnel areas.

Shade effects on trees

The highest levels of shade will be found on the north-facing sides of east-west streets. In this case a tree may get no sunlight at all except perhaps in late afternoon in summer. The effects of shade on plants are complex. The first response will be reduced flowering. For street trees this may not be significant. The next level of response will be canopy quality. Shaded canopies will be less dense with thinner leaves. For deciduous trees this may not be critical but the appearance of evergreens may suffer. Conifers in particular often lose quality when shade grown.

Wind tunnels

While cities are generally less windy than outlying countryside, tall buildings will create very windy locations within cities by deflecting wind downwards or by concentrating it along particular streets. Such wind pockets will damage wind sensitive trees and can cause increased evapo-transpiration. In Melbourne, the Docklands and South bank precincts will become very windy as high-rise development continues and tree selection may have to change to reflect this.

Trees on buildings: effects of building height

In the broader sense, altitude effects on climate fall under the consideration of macroclimate. In the built environment however, altitude comes into play when trees are placed on top of buildings. The construction of roof gardens is becoming more common in our cities and many designers want to include woody vegetation in their planting palettes. How then does the climate on a roof vary from that at ground level? The major variation is in wind speed. As height increases, wind speeds typically increase (doubling height increases wind speeds by 10%). Trees with known sensitivity to wind damage (structural faults, poor branch attachment, large leaves, delicate leaves) should not be used in such applications. Trees with tough, small foliage will be more resistant, as will trees that grow in naturally windy locations such as cliff tops. Planting relatively small trees is important to reduce wind loads on canopy and rootsystem. The presence of neighboring tall buildings may add further to wind loads by the creation of wind tunnel effects.

Consider the use of the following trees for windy roof top situations:

Acacia boormanii (Snowy River Wattle)
Arbutus canariensis (Canary Islands Madrone)
Ceratonia siliqua (Carob)
Dracaena draco (Dragon’s Blood)
Geijera parviflora (Wilga)
Olea europaea ‘Tolley’s Upright’ and O. europaea ‘Swan Hill’ (Olive)
Ulmus parvifolia ‘Burnley Select’ and U. parvifolia ‘Yarralumla’ (Chinese Elm)

Coppiced or multi-stemmed specimens (or Mallee forms) could be considered:

Corymbia eximia (Yellow Bloodwood)
Eucalyptus kitsoniana (Gippsland Mallee)
Eucalyptus risdonii (Risdonvale Gum)
Lagerstroemia indica x fauerii cvs. (Crepemyrtle)
Tristaniopsis laurina (Watergum)

Please note that these trees would require a maintenance regime to remove extension growth on an annual or bi-annual basis.

 

Further reading

Craul, P. 1999 Urban Soils: Applications and practices, Wiley. Detailed treatment of heat loads on and evapotranspiration of urban trees. Also a good discussion of wind loads on trees.

Harris, R., J. Clark and N. Matheny. 2003 Arboriculture: Integrated Management of Trees, Shrubs and Vines (4th ed.) While North American in focus, a good discussion of climate effects on trees with some information on urban mesoclimates.