DRYLAND TREE PLANTING
Introduction - A technique of planting in areas with a summer drought period
Man has, through his drive to 'live' created an estimated million square kilometres of desert throughout the world. His clearing, grazing and cultivation practices, designed to ensure his continued existence, may well be leading to his extinction. The top soil, that 12 centimetres of the earth's mantle upon which we depend for our lives, is increasingly under siege - some, as you know, would claim that the siege is well-entrenched and impossible to lift. I do not believe that, for the signs of awareness are growing - evidenced locally by this seminar, and even on a world-wide scale, by such productions as the World Conservation Strategy.
We, in Australia, have our part to play, and I believe we are setting to it with a will. The role which vegetation plays in protecting our soil resources must be given increasing prominence. The soil which has taken over 1000 years to develop can be destroyed by a one-hour downpour, or a strong wind.
Australia
Approximately 70% of the area of Australia receives 500 mm or less of rainfall on an annual basis. This percentage amounts to a piece of country approximately 3000 kilometres E-W by 1800 kilometres N-S. It carries about 30% of our sheep and cattle and 3% of our population and now contains considerably less vegetation than it once did.
In Australia, our two most limiting resources are water (quantity and quality) and the soil (nutritional level). These two things alone will probably prevent Australia from becoming as great a power as the United States (even if we chose the same path) and restrict our national population capacity (C.S.I.R.O. projection) to about 66 million people.
The proper understanding of our resources to ensure long-term productivity is the obligation we have to future generations. We will not be viewed with affection by future generations should our irresponsible exploitive behaviour result in such defoliation as to make our 'planet earth' incapable of sustaining life.
Trees can provide a tool with which to 'husband' our resources and still allow increased productivity.
This paper simply describes one way of establishing and maintaining trees in the drier parts (less than 500 mm per annum) of this country.
Principle factors in vegetation replacement
There are two main factors which play a significant role in determining the success of any tree-planting programme. They are species selection and planting techniques.
Species selection
It is essential to select the correct species to suit (1) the site (2), the purpose of the planting, and (3) the expected maintenance programme. For the planting situation described here, tree maintenance (watering etc.) was to be minimal, and consequently choice of species was closely related to climatic factors preferred by the tree species. The factors which affect this choice relate to both the macro- and micro-climates which prevail. Not only are major matters such as preferred rainfall (total and pattern) and soil reaction important, but also site-specific issues, for example, gully or ridge, deep or shallow soil preferences, and elevation and aspect.
Planting technique
Effectiveness of the planting is evidenced by tree survival and tree growth rate commensurate with the species. To maximise both these factors is a good aim.
The capacity of a tree seedling to become established successfully in a planting situation can reflect how well the planters understand the tree's preferred planting environment. Some matters which were seen as being critical are described below.
Root growth
Rapid maximum exploration of the soil profile by the root system is one of the ways a young seedling counters our harsh summer weather. By nature, roots of Australian plants are able to quickly move to lower parts of a soil profile to secure adequate moisture. A method of ripping up to 0.75 m deep was employed to encourage this ability. Some root growth performances measured indicate a root penetration to the bottom of the rip line up to 0.7 m within three months of planting.
Planting times
A good root growth rate however is more closely related to the time of planting. In some recent experiments, the time of planting was related to roots and shoot growth. The principal conclusion was that for South Australian conditions, an early autumn planting leads to the development of a root system which supports the young seedling during the dry summers experienced. This has supported the approach of early planting, even attempting to anticipate the break in the season if planting took place in the middle of winter. The results suggested that a longer period of time (two months) elapsed before root growth began. Irrespective of the planting time during the year, there appears to be a period of up to a month before the plant initiates growth; a period of transplanting shock. Transplanting shock is an observed phenomenon (no objective measurement) related, we believe, to the change In environment at the time of transplanting.
Methods developed to overcome planting shock, particularly in managed, large, planting activities, will improve species' performance. (Although the use of paper tubes was unsuccessful, further experimentation may see these assisting the reduction of planting shock). We found that the correct understanding of these plant growth factors were of particular importance if the principle of broadacre, low maintenance, dry-land tree planting was to be a success.
Control of competition (weed growth)
There is no doubt that initial tilling of the soil has the effect of encouraging weed growth. The friable nature of the top layer of soil is often quickly exploited by weed seed roots when the conditions of temperature and moisture are just right. The presence of weeds and other plants in close proximity to a newly-established plant provides competition for available nutrients and water. This is not good. The rate of early seedling growth (after planting shock period) depends upon available growing materials, and the presence of vigorous grass competition reduces these.
Therefore to ensure our objective of maximum survival, early growth and 'get-away' was attained, the weeds had to be controlled.
Ground preparation
In general, control of weeds and moisture go hand in hand, and although there is a gain in maintaining fallow ground during the spring and summer, the available 'lead' time did not often allow the application of this practice. Further, it was felt there were greater benefits to be obtained from the use of efforts nearer the newly-planted seedlings.
The provision of deep rip lines, normally prepared during the summer, allowed water from early rains and strong summer thunderstorms to penetrate well into the soil profile, often directly to the water-retentive clay layer. Another cultivation method, designed to allow good water penetration, was chisel ploughing. This was more effective in areas which had this clay layer close to the surface.
Often this rip line was composed of large clods and these were broken by disc plough. This cultivation increased the amount of fine soil material available to filter down into the gaps made during ripping. Under some conditions and soil types, the ripping left cavities in the ground which were not adequately filled by soil during the planting. This led to seedling death from low to zero moisture availability and the impact of large air spaces. In general, however, this combination of ground preparation procedures ensured a good moisture regime near the plants and effective early weed control - until the spring of the planting year.
Provision of mulch
We were still ambitious. In this low rainfall site (average approximately 350 mm), we wanted fast-growing trees with a low maintenance requirement. In particular, this last phase meant no watering, although we found the need to modify this stand as time passed. The desire to achieve this style of planting suggested the use of a mulch on a large scale. Black plastic was chosen because it was
(a) light and therefore easily carried in large numbers;
(b) easy to lay, although time consuming;
(c) an effective barrier to weed growth;
(d) an improved way of catching and directing rain to the tree roots;
(e) an effective barrier against soil moisture evaporation;
(f) we believe, effective in maintaining soil temperatures for longer periods in that range which is conducive to good root growth and subsequent crown vigour;
(g) a good container in which to put water should watering become necessary.
Other mulches such as straw and pine bark were tried, but they were not so effective.
Lead time
When planting begins, it indicates successful completion of the preparation phase. But realistically, it is but one deadline in a whole sequence of activities and deadlines, some of which, if not adequately completed, can result in unsatisfactory planting.
For the purposes of planning, the planting date determines the 'date position' of many of the other necessary activities. Invariably, lead times have been too short, and if only one pearl of wisdom comes to you from this paper, let it be this - make sure planting lead time allows preparatory work to be completed before planting day.
Work such as vermin control (rabbits and hares predominantly) and seed 'collection' (including purchase) takes place, at the very least, six months before the set planting date. On some occasions when either the vermin problem is large or species requirement is unusual, lead time to the planting date can be as much as eighteen months. Ground preparation often occurs during the summer prior to planting and, of course, must be well underway before planting can begin.
Some of the activities in the above schedule are described in more detail below.
Detailed Description Vermin Control
Since the introduction of rabbits and hares to Australia, they have grazed contentedly upon almost all forms of greenery growing in this land. Regeneration of native forest and woodland, particularly in the lower rainfall areas, crop yields, and even the survival of commercial Pinus radiata seedlings have all been threatened by these small animals.
The success of tree planting at Monarto was similarly jeopardised by populations of rabbits over the whole site. The employment of a trained vermin officer ensured that control programmes would be thorough and undertaken in a systematic manner.
His first task was to survey the land area to be planted, and plot assessed rabbit activity on cards. This information allowed specific decisions on the type of control measures which would be best. The most severely infested areas, which often were in locations almost inaccessible to machines, were poisoned using 1080A. Sodium fluoroacetate (1080A) on oats was fed following the laying of three 'free-feed' (non-poisoned) trails. A bait layer towed behind a four wheel drive vehicle was used to deliver a measured amount of grain per kilometre of trail. The most impact from this method is achieved when living conditions for the rabbit are harsh - late summer before the first rains encourage green feed. The method is frequently more than 90% effective and is an efficient way to use man power. Handled by trained officers, the side effects of using poison are minimal.
The poison programme is followed by ripping to destroy the warrens. The next step is to begin a six-week fumigation cycle using phostoxin placed in the few freshly-opened rabbit holes and killing the remaining population. The ripping of warrens and the subsequent gassing are used in lieu of poison in areas with moderate or low infestation levels. This programme must continue to prevent the rabbit population from building up, and although expensive for every rabbit killed, the per hectare costs gradually reduce because of the effectiveness of a systematic eradication programme.
The removal of ground surface rubbish which provides shelter for rabbits which do not use a warren is necessary. This process is also expensive but is generally a 'once-only' activity.
On-going action involves a regular inspection and perhaps some spotlight shooting. Generally the reintroduction of an extensive programme is not necessary providing surveillance is adequate. An assessment of likely vermin problems and a well-founded control programme is imperative if a tree planting programme of this type is to be a success.
Ground preparation
The planting land was previously grazed and cropped. Consequently, without adequate ground cultivation, seedling root growth would be limited.
A bulldozer was used to rip planting lines 6 m apart on the contour to a depth of 500 mm. This ensured breaking of the subsoil clays to allow root penetration to a depth where soil moisture was more consistently available.
Summer ripping in heavier textured soils sometimes produced air pockets in the subsoil clays. The result was poor survival. This was overcome by ripping when the ground was moist (preferred) and also by increasing the proportion of fine soil particles in the upper profile by a second discing after the early winter rains (less effective). Unfortunately, the impact of fallow on plant survival and growth was not assessed.
The use of a chisel plough further increased the early penetration of moisture around the rip line and began weed control. It also facilitated the disc cultivation and consequent development of a tilth. Offset disc harrows produced an almost weed-free surface which assisted the planting and laying of the plastic mulch. The placing of plastic mulch around the tree required the formation of a bowl and this was partly prepared by the use of a furrow plough. The presence of the furrow provided two further benefits. The bottom of the furrow allowed a planting surface which was 50+ mm below the surrounding soil surface. This reduced the risk of seedling death caused by premature drying of the soil near the seedling. Also, the furrow removed a quantity of the weed seed in the soil from the previous growing season and thus reduced the weed competition.
Planting stock
The provision of hardy planting stock (not overfed with nutrients) just right for planting out is part of the art of being a good nurseryman. Experiences at Monarto have highlighted the importance of a good root:shoot balance to reduce the transplant shock and to achieve good 'get-away'.
Large plants often have root systems restricted by their container (i.e. a poor root:shoot ratio) and the change in growing environment (from nursery to the field) can cause desiccation of the plant. A sound practice is to achieve planting stock with about 150 mm of top growth and a root system which has not out-grown the container. The use of planting machines can dictate a preferred plant size.
Planting
The seedlings provided for field planting were contained in plastic tubes approximately 40 mm in diameter and 150 mm long. These were presented in boxes in a predetermined number and species mix for the particular planting site. The plastic tubes had been split during packing to allow more convenient handling for planting.
Hand-planting using a small border spade, and during later work, a special planting tube, proved to be effective. The use of planting machines was a logical move. A modified Lowther planting machine was effective providing the ground conditions were good; free of rocks and with good soil moisture levels. Later a Quickwood planter was used in the rougher country and for the shorter planting runs; a consequence of the land form and resultant plantation design.
In all circumstances, to avoid disturbance of the soil:root association in the seedling tubes, the plastic was left on the seedling during planting but removed soon after.
Watering
The total planting technique described here was developed to minimise the annual maintenance requirements. The selection of appropriate species and good planting technique effectively eliminated almost all long-term maintenance needs (fire protection excluded).
It was desirable for soil moisture level to be at field capacity on the day of planting. This occurred and was maintained during the first year, but not in subsequent years. Furthermore, the first day of planting each year was generally in early May, not only because late autumn planting allows the development of a good root system by the following summer, but also because the long planting programme would not allow delay until adequate soil moisture was reached (often unlikely in a rainfall area of 350 mm). Watering soon after planting was therefore essential.
The timing and necessity of any subsequent summer waterings depended upon the time of tree planting, soil type and seasonal conditions. A combination of late planting, light spring rains, and a light-textured soil would suggest the need for a summer watering. A watering programme for 100,000 seedlings took about six weeks. Consequently, to prevent stress and plant death, the skills of a clairvoyant would have been necessary to predict the timing of watering.
Survival counts taken in spring and autumn suggest most seedling deaths occurred before summer, and that if a plant survives to mid-summer, then there is a good chance that it will continue to the first winter rains.
Weed control
Prior to planting, the ground preparation methods previously described produced a weed free planting environment.
The plastic mulch used to control weed growth adjacent to the seedling increased plant vigour significantly and contributed slightly to increased survival. The mulch was a sheet 1 m square and 0.075 mm thick and it had a 50 mm hole punched in the centre by the manufacturer. It was laid after the tree was planted and formed into a saucer-shaped depression (generally about 150 mm deep) around the tree. The technique used for laying made use of the good friable soil conditions which followed ground preparation. After forming a bowl of about the correct size the plastic was laid around the tree, then by standing one foot each side of the small seedling with one's back to the wind (for it often blew at Monarto), a small border spade was used to press the plastic into the soft ground. Where stony ground conditions prevailed, care was necessary to prevent tearing. The mulched area thus resulting was approximately 650-750 mm diameter - this was certainly sufficient for the first two years - the most important period.
The life of the plastic mulch was determined by the amount of sunlight it received. Breakdown of the plastic film was frequently noticed at the beginning of the second year.
The efficiency of the mulch, particularly in retarding weed growth, was determined by the amount of carbon black in the plastic film and, to some extent, the thickness. Sunlight must be prevented from reaching the soil in sufficient quantities to stimulate weed seed germination.
A sheet thickness of no less than 0.050 mm but up to 0.075 mm was suitable. There were some weed species which were not readily restrained from growing, even under plastic film. Weeds with bulbs, such as sour-sob (Oxalis pes caprae), often grew vigorously, particularly if the carbon black level of the film dropped. In areas which contained sour-sob, the plastic mulch surrounding the plant was covered with soil. This proved to be an effective counter. Couch grass and planted perennials, such as lucerne, are not effectively contained by mulch, and it may be necessary to treat the area in some way. Lucerne, for example, was removed using a 'duck's foot' attachment for the chisel plough which sliced the tops of the plant a few centimetres below the crown. Something like 60%+ of the lucerne plants were removed in this manner. Since lucerne is heavily-rooted and a strong competitor for the soil moisture, every effort should be made to remove this type of competition. Spot spraying of individual lucerne plants is therefore often justifiable.
The effect of an impenetrable plastic film surrounding a plant on water percolation is a point worth noting. In some areas, planted trees 'drowned'. Using a system of a saucer-shaped depression surrounding the tree and plastic mulch covering, the 50 mm hole surrounding the plant was often not sufficient to allow water caught from rain to penetrate sufficiently quickly. This was principally a problem in areas which had significant clay subsoil. The clay formed a saturated 'plug' which blocked the hole. More holes were punched in the plastic to drain away excess water. The impact of prolonged soil saturation around plants caused severe growth retardation, if not death.
There is no doubt that the use of plastic mulch, in a location such as Monarto, to achieve the objective of high plant survival in low-maintenance, broad acre planting of native trees was a good practice. However, the costs of mulching contributes as much as one third to the field costs of planting a tree. New ways are being sought to achieve similar results.
Cultivation after planting, to control the spring growth of weeds, was a further method of reducing weed competition to the seedlings. It was achieved using two offset disc cultivators mounted on a single bar about 1.5 metres apart on a three-point linkage frame. The tractor and cultivator straddled the row and allowed the cultivation of both sides of the planting row in one pass.
Ongoing maintenance
The protection of plantings from fire required the provision of fire-breaks. This has been considered an important part of the long-term management activity. A watch has been kept for insect attack, but need for treatment would be carefully considered before any action is taken.
A continuing surveillance of the rabbit population has been necessary in areas previously treated. A replanting programme may be necessary when the level of survival is not acceptable in relation to the landscape concepts.
Conclusion
A broad-acre dryland plantation programme using Australian species with the emphasis on good survival, good vigour and low long-term maintenance has proved to be possible.
It is not recreated local vegetation, but in the strict sense is a plantation eco-system resulting from the use of local plant species and sympathetic planning.
Every effort should be made to develop a planting technique appropriate to the circumstances. No 'one' technique is best. In Australia's drylands, awareness of the climatic factors and the natural characteristics of Australian plant species is the basis for the development of a planting method suited to the planting purpose.
Many trees and shrubs remain to be planted. We must, as resource managers, explore as many planting methods as possible if we are to overcome the problems of ensuring survival of seedlings and maintaining a responsible establishment cost.
This article is reprinted from the proceedings of the seminar hosted by the S.A. Council of the Australian Institute of Horticulture during October 1981.
Australian Horticulture, November, 1982
DATE: March 1983
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