NEWS RELEASE
August 12 2002
UniSA
researchers developing guidelines
for choosing street trees
You’ll find
them lining the footpaths of the most sought-after leafy suburbs. Street
trees improve the landscape, enhance the environment, increase land values
and extend the urban wildlife corridor by providing a valuable habitat for
fauna.
Street trees
also extract moisture from the soil, resulting in extra soil shrinkage
that can cause significant surface and sub-soil movements, leading to
damaged pavements, distortion of footings, cracking of buildings and
roads, and at worst structural damage.
So what trees
are suitable for your suburb? Researchers at the University of South
Australia have been conducting studies on tree selections, footing designs
and urban soil moisture changes to give local government a rational basis
for better selection of species of street trees. Their research will also
help engineers design footings that allow for extra soil movement based on
the water usage of different tree species.
The research has
been undertaken by Don Cameron, Director of UniSA’s Structural Materials
and Assemblies Group, and postgraduate student Aaron O’Malley from the
School of Geoscience, Minerals and Civil Engineering, with industry
partner, the City of Salisbury.
The project
involved a year-long study monitoring four streets with established tree
plantings of both Australian native and exotic species – SA Blue Gum (Eucalyptus leucoxylon), Red Ironbark (Eucalyptus sideroxylon), Smooth-bark apple (Angophor costata), Jacaranda (Jacaranda
mimosifolia) and Nettle (Celtis
australis).
Soil moisture
was monitored at three points, between the rows of trees, under the
pavements and away from the trees. Soil types were generally moderately to
highly reactive.
The Australian
standard for footing designs does not include guidelines for trees close
to buildings but engineers are being pressed to accommodate extra soil
drying that may arise from street trees, according to Cameron.
“With the
current trends towards smaller housing lots, prescribed safe distances for
tree plantings could lead to a relatively treeless streetscape, which
would be aesthetically unacceptable,” Cameron said.
While
Adelaide’s semi-arid climate dictates that house footing designs should
be based on an active soil depth of four metres, Cameron’s research
found that large mature trees can extract water from depths of up to four
metres and a row of trees competing for soil moisture could be active to
depths of six metres.
Knowing how soil
moisture changes around trees is essential to reliably estimate the ground
movement in expansive clay soils. It also avoids unnecessary over design
of footings or footing failures as trees reach maturity, according to
Aaron O’Malley.
“Once a
pattern of ground movement is established, footings can be structurally
designed to facilitate an acceptable performance of the structures that
they support,” he said.
“While there
is ample information on soil water extraction, evaporation from leaves and
the physiological behaviour of many plant species in agriculture and
forestry, similar information on ornamental plant species used as street
trees and in urban environments is very limited. Tree planting guidelines
and current footing designs are based on limited data and do not
appreciate the differences in tree species,” O’Malley said.
The study of
mature tree species was not conclusive as no one tree stood out as being
significantly better than the others, according to Cameron.
Now UniSA
researchers have begun a second study to determine the effects of immature
trees planted on streets at Walkley Heights, where soils are extremely
reactive, with up to 100 mm in movement. The trees planted include the
Chinese Elm, Bradford Pear, Coral Gum (the only native) and the Golden
Raintree.
With councils
now being held accountable for damage to property caused by street trees,
the study, which will be completed in 2003, should provide vital
information to councils on the species of trees best suited to the
differing soil types in their local government area.
Media
contact: Geraldine Hinter (08) 8302 0963 or 0417 861832
