TAFE, Wagga Wagga NSW
The strategies and methods mentioned are those that have evolved and been found by me to be successful in a drier area than the Riverina, and that is Broken Hill in the far west of NSW with an average annual rainfall of 230 mm and often dropping as low as 75 mm per year.
However, I believe there is room for these methods, and modifications of these, to be adopted in the Riverina where the normal plants have failed, and their loss could be attributed to water deficit, either through an abnormal drought or overplanting.
In the arid zones of Australia, naturally occurring tree densities are governed primarily by permanent water availability. The greatest tree biomass occurs in and along usually dry water courses and prior streams. The next most favourable area is where small catchment runoff is absorbed into favourable soil areas, such as at the base of large rock outcrops, or on rock outcrops where bedding or fault planes trap small amounts of soil or organic matter, enabling germination of tree seeds which can reach water deep in the rock crevices. This water can be very effectively used as it is protected from evaporation during dry periods.
The final and lowest density biomass is found where favourable deep soil types occur that allow total infiltration of the scant rainfall without surface runoff. In extreme arid areas such as the Simpson Desert, this sandy soil, if stable, may generally not be enough to support tree growth unless a supplement of local runoff from an adjacent clay pan occurs. There are occasional occurrences of isolated trees that can be attributed to this situation.
If using local species for planting, great emphasis on the study of naturally occurring tree densities must be made if long term survival is to be achieved. Of course, if water is available and a watering schedule can be maintained, then plantings can be made following the practices of the wetter regions of planting windbreaks and trees wherever one wishes. However, the long term survival can only be achieved by constant human effort, and water. The man hours (or should I say person hours now) and the water wasted on such a planting is better expended on different planting strategies. Many examples can be seen in the arid zone of dead mature trees around an abandoned homestead site.
What then are these strategies? I consider the first and most important is to totally abandon the concept of planting in lines, rows or windbreaks and plant only in those areas that can accumulate or provide sufficient water, and all the time paying great attention to planting density. Unfortunately the interaction between planting densities, site and species will be experimental, at least until after the first prolonged drought. It is prudent, I suggest, to tend towards wider spacing than to have to supply water later to enable trees to survive a drought.
The second strategy is the careful selection of the species that will be required to plant in the extended areas out from where the local species can survive. These species should not only be able to survive, but to grow well and resist insect attack.
These species will be from a more arid area than the area to be planted and preferably should be geographically from as close a location as possible to the planting site.
This is borne out by my experience with trial exotic arid species. The few that did grow well showed undesirable traits such as spines or thorns, salt exudates or, in the case of Prosopis species 'Mesquite', and Parkinsonia 'Palo Verde', the added risk of weed potential was present. The most successful were arid zone South Australian and West Australian eucalypts and acacias which proved to be more adaptable than any exotics, especially where heavy metal residues were present.
The third and last strategy consists of providing extended catchments for tree planting by the use of graded depressions which channel rainfall into a tree planting site. In totally flat areas a 360o turn of a tilted grader blade can be sufficient to permit establishment. These methods can only be used where soil infiltration rate is slow. On sandy soils the saucer formed can be lined with plastic sheeting held down with stones. Soil covering does not permit water accumulation in the case of a minimal rainfall.
The cost and labour involved in the above may seem excessive for tree establishment, compared with wetter region costs, but of necessity tree planting in dry areas must be carried out over a longer time scale, one that permits a thorough study of soils, and soil depths and where water ponds or soaks after rain and when annuals and perennials dry off after rain.
When it comes to planting out trees in the autumn break, which of course may come late in the winter or not at all, you must be ready to plant into already selected sites, which of course will mostly not line up like soldiers on parade. This I believe is a small price to pay for the establishment of trees in areas where they are of greatest value to man, birds and animals alike.
Finally, some of the species that were able to establish, survive and prosper in the far west with only a single establishment watering were: