Research into farm forestry for salinity control
Introduction
In many areas where dryland salinity occurs, annual rainfall has been considered too low (600-700 mm/year) for commercial forestry. Tree growth rates are slow, so economic returns from plantations are poor.
There is now increasing interest in planting trees in these low rainfall areas for their ability to help control the spread of dryland salinity. However, because these aren't traditional forestry areas, little information is available about the best species to plant, where in the landscape they perform best, and how to go about planting (including spacing, species mixes, season and site preparation).
To help provide this information, State Forests has established a number of trial sites in the 600-700 mm/year rainfall zone in New South Wales.
Trial sites
Demonstration trials
Forty-seven demonstration trials of two hectares each have been established in salinity prone catchments across the Murray-Darling Basin. The trials have been established on sites with varying soil types and seasonal rainfall distribution.
The species planted are Murray River red gum (Eucalyptus camaldulensis), Queensland western white gum (E.argophloia), sugar gum (E.cladocalyx), Mugga ironbark (E.sideroxylon), spotted gum vars. (Corymbia variegata & C.maculata), maritime pine (Pinus pinaster), and black wattle (Acacia mearnsii). At each site, six of these eight species have been planted. At six of the sites some clonal material, E.camaldulensis x E.grandis, and E.camaldulensis x E.globulus was also planted.
Operational scale trial
Approximately 400 hectares of plantations have been established in the Liverpool Plains with funding from the NSW Salinity Strategy.
These plantations are spread across a number of properties on a variety of soil types and in different landscape positions. Some of these plantations have been deliberately designed to answer questions about planting density, mixing of species, plantation configuration, effects on salinity, and site preparation. The plantations vary in size from ten to 60 hectares.
Macquarie River trial
Two plantations of 50 hectares each have been established in salinity prone sub-catchments of the Upper Macquarie - one at Lue near Mudgee and the other at Baldry, near Wellington. Species planted are E. camaldulensis, C. maculata and E. crebra.
Purpose of the trials
Information gathered from these trials on tree survival, tree growth, tree water use and groundwater changes, as well as establishment and management costs will help us to determine:
- the best performing species in different soil types and landscape positions
- the optimum planting density
- the best planting configuration (eg. belt width and spacing)
- the optimum planting time (autumn or spring)
- the best establishment methods (eg. soil preparation techniques)
- the effect of mixed plantings on plantation productivity
- the effect of planting trees on watertable levels and creek salt loads.
Best performing species
The trials have been established on a number of different soil types (Sodosol, Kurosol, Dermosol, Ferrosol, Vertosol, Chromosol, Rudosol, Tenosol), in different positions in the landscape, (upper, mid andlower slopes, toe slopes, floodplains, ridge tops) and with a variety of species.
This will allow the growth of these species in the different landscape positions and on different soil types to be compared. For example it is anticipated that E.camaldulensis will perform better on floodplains, while E.sideroxylon will be better suited to upslope and ridge positions.
Planting density
In plantations for timber production, trees are closely spaced so that competition for light encourages them to grow upwards, producing straight stems with minimum lateral branching. However, the trees should not be so densely planted that competition for light, water and nutrients causes the growth of all stems to be stunted.
The optimum planting density will be lower in the 600-700mm rainfall zone than in traditional plantation regions with higher rainfall.
On one of the trial sites, four eucalypt species have been planted at three different densities: 625 stems per hectare, 1,250 stems per hectare and 1,750 stems per hectare.
Planting configuration
Rather than planting trees in blocks, many landholders prefer to plant belts of trees along existing paddock fence lines to provide shelter for stock and to reduce fencing costs. Current theory suggests that on hill slopes the best design to reduce recharge, whilst maximising plantation productivity, is to plant tree belts on contour strips to intercept surface and subsurface water flows from upslope. Key considerations in designing this type of planting are the position of the tree belts on the slope, the width of the belts and their distance apart.
One trial is designed to examine the relationship between belt width and the length of the unplanted upslope contributing area.
Planting time
Seedlings planted in spring are likely to suffer high mortality during hot, dry summer conditions. Autumn planting may improve survival, but will subject seedlings to risk of frost damage. Knowledge of the optimal planting season will maximise survival, and thereby minimise replanting costs.
On two of the trial sites, some of the trees will be planted in autumn 2002 and some in spring 2002 so that the survival rates and growth rates of the seedlings can be compared after the following summer season. One site is on basalt-derived soil, the other on sandstone-derived soil. Two species of eucalyptus and one pine species are to be planted.
Establishment methods
One of the major costs of plantation establishment is the soil preparation at the site. A balance needs to be found between undertaking intensive site preparation to enhance survival and growth of the seedlings, and minimising establishment costs through less intensive site preparation. At two of the trial sites in the Liverpool Plains, ripping, mounding, minimum till and the combination of ripping and mounding are being compared. One site has basalt-derived soils, the other sandstone-derived soils.
Mixed plantings
Acacias are capable of fixing atmospheric nitrogen, turning it into a form that can be utilised by plants near them. Interplanting acacias with eucalypts may enhance growth of the eucalypts on sites with low soil nitrogen. However, if lack of water is the major limitation to plant growth, there may be no benefit from interplanting. A trial has been established to see if planting acacias with eucalypts enhances the growth of the eucalypts. The species mixes being trialed are 100% eucalypt, 100% acacia, 50% eucalypt and 50% acacia, 75% eucalypt and 25% acacia, and 25% eucalypt and 75% acacia. The species planted are narrow-leaved ironbark (E.crebra), spotted gum (Corymbia maculata) and black wattle (Acacia mearnsii).
Effect of trees on watertable and creek salt loads
State Forests is using a decommissioned NSW Agriculture research site in the Liverpool Plains to study changes in soil moisture, soil salinity and watertable depth as a result of planting trees. This site is equipped with piezometers and neutron probe access tubes. It is currently under pasture, and will be planted with eucalypts in autumn 2002.
Piezometers and neutron probe access tubes will also be installed at one other site in the Liverpool Plains and at the two sites in the Macquarie catchment, subject to available funding. Funding is also being sought for monitoring changes in water quality.
Measurements of existing plantations
To support the above studies, existing plantations on farms in 600-700 mm rainfall zones are being sought for measurements of growth and for measurements of tree water use.
How will this help?
The information obtained from these trials will help State Forests and other authorities and landholders to develop productive farm forests at least cost and to understand the likely salinity benefits of plantations in various locations.
Further information
For further information about any of the research trials, please call:
Annette Cowie
Program Leader
Research and Development Division
State Forests of NSW
PO Box 100
Beecroft NSW 2119
Tel: 02 9872 0138 Fax: 02 9871 6941
Email: annettec@sf.nsw.gov.au
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