Local Provenance and the Urban Tree
When bushland areas are revegetated, horticulturists and the community recognise the importance of planting new plants that are genetically similar to those that might have grown in the site previously. Revegetation programs almost always require use of “local provenance” indigenous plants. Why? Most agree that in revegetation projects we want to replace plants that can grow and reproduce in the same way as the plants that were lost. We are also trying to re-create some sort of wilderness that reflects what the landscape was like before human intervention.
In urban conditions, we most often plant trees to survive in areas that have been modified by human intervention; we have designed our cities to have roads, structures and services. These interventions make the revegetation approach inappropriate, since we can never re-create a pre-human or pre-urban wilderness. So what local provenance issues should urban horticulturists concern themselves with?
When managing reserves or revegetation plantings, horticulturists must assess the existing vegetation and understand the extent of the populations of plants that are present. Often, a species will occur in limited pockets where the plant has begun breeding within the pocket rather than outside. These pockets, or remnants, can become quite different from other plants of the species outside the remnant population. In some instances, it may be valuable to maintain the specialised genetics of a remnant; in others, by re-enforcing breeding only within the remnant, we may be magnifying the genetic characters that will lead to that species’ extinction. In the early 1990s, concerns about genetic restriction were examined, and the book Genetics and conservation of rare plants (Falk, D.A. & Holsinger, K.E., eds., Oxford University Press, 1991) addressed many of these issues. Horticulturists considering revegetation sites should be aware of the issues presented in chapters 1-3.
As eucalypts are one of the most widely distributed tree taxa in Australia , most questions revolve around their provenances. Through many years of significant research, many questions about eucalypts have been answered. We now know that most eucalypts will breed with genetically-different-others (outbreed) at a higher rate than they will breed with genetically-similar-others (inbreed). Still, as widely-distributed populations become remnants, research has shown that many eucalypts will become successful inbreeders. The consequence of this inbreeding may limit the vigour of seedlings, with those seedlings showing the most genetic variation (and typically the least inbreeding) succeeding at the highest rates.
Remember, there is no tree indigenous to a nature strip or a degraded landfill site. For any successful landscape, the plants must grow in the soils available, and with the resources that can be obtained. Also, there are other bottlenecks that will limit the benefits of using local provenance material. As an example, in providing food sources to local birds by planting indigenous trees, you still may not get any more local birds in the planted area. You may not get any more local birds because the Indian Mynahs may predate the local bird species eggs to such a high level that your indigenous tree planting does not give you the result you’re after.
Unlike mammals, tree species can frequently interbreed. Further, if trees of the same species from one provenance are planted close to those from another provenance the resultant seedlings will most likely be related to both provenances. The pollen from street trees can strongly affect any indigenous trees that are within a pollination distance. For eucalypts this pollination distance can be a few kms, for acacias, it tends to be smaller distance. Responsible streetscape and park managers will select tree taxa that do not have a chance of breeding with any of the endemic flora: e.g., it would not be responsible to plant Eucalyptus leucoxylon ssp. megalocarpa in areas where the local E. leucoxylon ssp. connata is endemic; nor would it be responsible to plant Acacia melanoxylon from Otways seed in the north-west area of Melbourne where there is a local provenance.
If there is an endemic species, sub-species or remnant, responsible environmental management indicates that the horticulturist streetscape manager chooses a tree variety where cross-breeding with these locals will not occur. There are enough tree choices available to make this possible and easy.
As a species or subspecies has evolved in a specific environment, the more successful offspring for that environment will thrive and reproduce. Hence, over many generations, a stable, specialised form, or ecotype, of that plant can emerge that shows tolerances different to that which is generally shown by that species. Frequently, these ecotypic differences will include:
These adaptations can be very useful for urban horticulture, as the sites in which most trees are planted are not ideal. So by choosing a provenance or ecotype with adaptations to harsh soil and climatic conditions, you can have a better chance that the tree will survive and grow well in harsh urban conditions.
Metropolitan Tree Growers Pty Ltd grows a number of provenances of trees indigenous to the Melbourne area. Also, we grow known seed sources of many “exotic” Australian trees that are useful in Melbourne . We grow either the Bodalla Forest or Mottle Ranges provenances of Corymbia maculata because these ecotypes have better tolerances for growing under Melbourne conditions.
For different areas of Melbourne , we can suggest the following indigenous selections:
Many local government areas include these soils, including parts of Darebin, Hobsons Bay, Hume, Maribyrnong, Melbourne, Melton, Moonee Vale, Moreland, Wyndham and Yarra, although there is local significant variation in soils in various areas of these municipalities.
This region has relatively rich soils is the result of many lava flows in the last million or so years from volcanoes to the north and west of Melbourne . These lava flows pushed various streams including the Plenty River, Darebin Creek, Yarra River up against the Silurian hills of northeast and east Melbourne that form this region’s eastern boundaries. The soils are heavy clays, easily waterlogged and often have many lava rocks on or close to the surface. These soil characteristics do not support trees well through extended drought periods because of their immense adsorptive capacity. 25% of the fully wetted soils can be water and when these soils dry out, air replaces the soil and major cracks form. Root tips in these cracks can dry out quickly.
Trees that occur on these basalt soils include:
The localities of Boorondara, Glen Eira, Maroondah, Nilumbik Stonnington, Whitehorse , as well as parts of Melbourne and Whittlesea are found in this region. Again there is local significant variation in soils in various areas of these municipalities.
This region is the rolling hills of north-eastern and eastern Melbourne . They are derived from ancient marine sediments that have been uplifted and twisted over 450 million years. The clay soils have been leached over time and are very poor in nutrients. In many areas they are not much more than “dust and gravel”. The boundaries of this region tend to be the basalt flows to the west, the higher mountains to the north and east and the sand belt to the southeast. Although the soil type is similar across a wide region the topography causes a large variation in soil conditions and the retention of rainfall. North-facing slopes are very dry, and south-facing slopes support species that grow better in slightly more water. Lower areas can become very waterlogged and be frost affected. This soil and climate variation makes tree selection a very site-specific choice.
Trees that occur in these soils include:
The cities of, Bayside, Frankston, Greater Dandenong, Kingston, Monash, Port Philip and parts of Glen Eira are found in this area, although there is local significant variation in soils in various areas of these municipalities.
This region is the open plain of south-eastern Melbourne . It is predominantly sand or alluvial silts. The sandy areas are a product of the dominant western winds over Port Phillip Bay and the rises and falls of sea level over the millennia. As sea levels changed, sand was deposited over different areas to the east of Port Phillip Bay . Sandy soils supported heath woodland. Heath woodlands were mostly a diverse scrubland and only had a few over storey trees. There are also extensive areas of old wetlands with alluvial soils, dominated by a Red Gum Woodland. These areas had an over storey of Red Gums and an open grassy aspect. These two dominant soil types are a complex mosaic across the region. It is one clear region but tree selection must be fairly site-specific.
Trees that occur in these tertiary sands include:
Many of the municipalities listed in other soil type have coastal areas where the soils become beach sands. Coastal areas have unique characteristics whatever their geological origins. The effects of salty winds and water influence all vegetation within a kilometre or more of the coasts. Much of the soils is sandy and requires very specific tree selections.
Trees endemic to these coastal areas include: