| About the Catchment Area
: Issues & Problems
The problem with decidous trees.
Below is a table that summarises some of the harmful
impacts that exotic trees can have on the natural environment.
| Attribute |
Environmental
Impacts |
| Deciduous |
Pronounced leaf fall in autumn provides
a discontinuous and foreign food source for native fauna,
especially macroinvertebrates. Phenolics and tannins present
in exotic leaves may be harmful to native insects, macroinvertebrates
and fish. A seasonal influx of leaves leads to high nutrient
pulses within the system. |
| One, short pronounced flowering event |
Exotic trees do not provide a sustainable
food resource for nectar feeding insects, birds and /or marsupials |
| Dense canopy |
Dense shade created by exotic tree canopies can decrease
light availability and river/stream temperatures (especially
during spring/summer).These modifications may cause a decline
in instream primary production: limited regeneration of
native flora and a decrease in dissolved oxygen concentrations.
|
| Shed few branches and bark across the year. |
Exotic trees tend to produce comparatively
smaller amounds of woody detritus than native trees. This
detritus breaks down rapidly reducing food resources for many
instream macroinvertebrates. The compacted bark produced by
many exotics also provides less habitat space for native insects
and spiders. |
| Softwood trees - produce rapidy decomposing snags
and few hollows |
Rapidy decomposing snags reduce habitat
space, food resources and spawning sites for instream fauna
such as macroinvertebrates, fish and tortoises. Softwood trees
also produce few hollows therefore reducing habitat space
for native birds and marsupials. |
| Dense shallow - root systems |
Large masses of fine lateral roots created
by some exotic trees (eg willow, alders) help to stabalise
riverbanks, but they also tend to trap sediment, which can
lead to channel contraction. Excessive instream growth by
exotic trees can exacerbate flooding as the water is forced
to move around dense stands. |
| Water use |
High water use requirements of exotic trees
compared to native trees may lead to the drawing down of water
tables or the drying out of creeks, streams and swamps. |
| Salinity |
Some exotics (eg Tamarisks) are highly salt tolerant because
they are able to store salt in their leaves. During autumn the
accumulated salt in their leaves may then become redistributed
to the upper profiles of the soil or directly/indirectly enter
rivers and streams. |
Due to the harmful nature of deciduous trees, the Onkaparinga Catchment
Water Management Board encourages the community to take the following
action:
- sweep up all deciduous leaf litter, especially those leaves
that have blown into street side gutters.
- for those planning a new garden, opt for local native trees
- consider removing exotic trees, particularly in they are near
a watercourse..
People interest in finding out more about local native plants suitable
for their area are encourage to contact Urban Forest Biodiversity
Program, Greening Australia or Trees for Life.
 Back
to Water Quality - Deciduous Leaves
|
