Individual study: Techniques to improve direct seeding of native trees and shrubs in low rainfall areas on non-wetting sands in South Australia
Knight A.J.P., Beale P.E. & Dalton G. S. (1998) Direct seeding of native trees and shrubs in low rainfall areas and on non-wetting sands in South Australia. Agroforestry Systems, 39, 225-239
In Australia, large areas of regions with Mediterranean type climates (cool wet winters and hot dry summers) denuded of natural vegetation could benefit from large scale re-establishment of native shrubs and trees. Direct seeding is a cheap method for establishing trees and shrubs on a large scale; a single operator can seed 20 to 30 km of trees in a day. However, direct seeding has often poor results in areas with less than 300 mm average annual rainfall and/or on soils prone to erosion, including non-wetting sands. Following preliminary trials, several treatments that might improve seedling survival on non-wetting sands were trialed to improve the success of direct seeding in such areas.
In 1994 and 1995 at 12 sites in the state of South Australia, seeds of common endemic species of Acacia spp., Callitris spp. (cypress-pines), Eucalyptus spp. and Melaleuca spp. (myrtles, paper-bark trees) were sown by direct seeding in low rainfall areas with non-wetting sands. In order to try and increase planting success, techniques tested were:
i) ripping to improve water penetration and to break up hard soil layers;
ii) use of a grader blade (80 cm wide V-blade or 25 cm wide disc) to remove any non-wetting topsoil, harvest water and provide a weed-free seed bed;
iii) time of sowing to determine the optimum sowing season;
iv) use of bitumen and polyvinyl alcohol (PVA) mulches to stabilise the soil and retain moisture (and for bitumen also to warm the soil);
v) fertilizer application to enhance early seedling growth.
Weather: Rainfall in 1994 was below average with 140 mm at the 'northern sites' (55% of the average) and 300 mm at the southern sites (74% of the average).
In 1994, only Acacia established (at least 3 seedlings/10 m at the end of spring at eight sites; 43% survival after the first summer). In 1995 (average rainfall) survival was much improved: at least 3 Acacia and 3 Eucalyptus seedlings/10 m at the end of spring at eight sites, at least 3 Melaleuca seedlings/10 m at six sites; and at least 3 Callitris seedlings/10 m at three sites. Melaleuca had the highest overall survival at the end of summer (52%), followed by Acacia (42%), Eucalyptus (41%) and Callitris (29%).
Sowing 2-3 weeks after the onset of rains (mostly May) with weed control gave the best results. Over-summer survival was enhanced by 40% with herbicide weed control. Neither ripping nor fertiliser improved survival. Ground preparation with an 80 cm wide V-blade generally produced better results than the 25 cm disc, except for Melaleuca. The effect of bitumen varied but appeared beneficial on wind eroded sites.
Direct seeding combined with techniques to improve seedling survival could also be used for large scale revegetation in other areas of the world with similar climate and soil types.
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