(This chapter is based largely on information supplied by Francis R. Moulds)
Aerial seeding of forest trees commenced in Australia on a trial basis some 20 years ago with a small number of eucalypts (Eucalyptus regnans, E. delegatensis, E. nitens, E. globulus, E. viminalis, and E. obliqua) and with Monterey pine (Pinus radiata). Before that, the technique of direct seeding (using hand broadcasting or hand-held spreaders such as the "pepperpot") had been used for spot sowing onto prepared seedbeds, normally following a hot burn of the slash left after logging the site. The direct hand-sowing method also had been used widely to establish windbreaks and shelterbelts (using, for example, E. cladocalyx, E. cornuta, and E. gamphocephala) on agricultural and pastoral land.
Today, foresters understand the behavior of the seed of Eucalyptus species in direct sowing. Details of stratification; prime planting time; controlling grasses and other competition; protecting the seed from insects, birds, and animals; and fencing the young seedlings, if necessary, against browsing or grazing animals are all well documented.
An important impetus for aerial seeding in Australia has been the movement of the timber industry into mountain forests, particularly in Victoria, New South Wales, and Tasmania. The valuable species in these forests, especially E. regnans and E. delegatensis, occur as pure stands. These species regenerate only in open sunlight; thus the forests are clear-cut.
Large areas must be regenerated each year. Hand methods are difficult or impossible to use, particularly because only a few days each year exhibit the combination of weather conditions in which it is both safe to burn the slash and satisfactory to sow the seed onto the ashes produced. Once these days are missed, a year is lost and the resultant growth of scrub species and the loss of slash by decay make it difficult to get the hot burn needed to provide a good seedbed. Aerial seeding provides the needed flexibility to take advantage of the few burning and sowing days, particularly when large areas are involved.
Each year in Australia, aerial seeding regenerates 8,000-12,000 hectares of cutover mountain eucalypt forests. The process is well established and faces little risk of failure, provided the standard operating instructions are followed. Lowland eucalypt species are largely managed on a group selection or shelterwood system with natural regeneration.
Monterey pine (Pinus radiata) and other exotic softwoods in Australia have been direct seeded experimentally to establish new stands or second rotation crops. But softwoods are not normally established by sowing because rodents eat the seed, seed-orchard stock is expensive, and row plantings enable thinning to be done more cheaply. Also, the sites chosen for softwoods are generally much easier to plant than the steep and remote mountain eucalypt sites.
Regenerating the mountain eucalypts is a relatively simple and straightforward procedure. It simulates natural processes that regenerate and perpetuate these high-quality timber species. For instance, periodic wildfires (caused by man or by lightning) are part of the natural environment, and such eucalypts regenerate themselves naturally in ash-strewn sites after a hot fire has caused the capsule (fruit) to open and release the seed.
The procedure involves:
· Removing the overhead canopy so that maximum light reaches the ground;
· Providing a favorable seedbed by burning the groundcover and the slash left after logging so as to expose the mineral soil; and
· Distributing an adequate quantity of viable seed onto this seedbed.
The seed is first tested for viability and coated with kaolin, insecticide, fungicide, and coloring matter (to make it easier for the pilot and ground crew to see how well the seed is being distributed).
The Forests Commission of Victoria publishes details of all operations, including recommended sowing rates and instructions governing aircraft calibration. (See Selected Readings) Great care is exercised at every step to ensure that the properly treated seed is sown on the properly prepared seedbed at the most appropriate time for germination and subsequent seedling development.