|Sowing Forests from the Air (BOSTID, 1981, 54 p.)|
(This chapter is based largely on information supplied by Alan H. Nordmeyer)
Aerial seeding of pasture grasses was developed in New Zealand in the mid 1940s, and today New Zealand farmers use aircraft extensively to seed crops and spread fertilizer, especially on hill-country farms. Indeed, this "aerial agriculture" has become a major industry and applies, for example, over 2 million tons of fertilizer each year. However, little has been done to apply the technique to sowing forests. It was not until the late 1 960s that the New Zealand Forest Service began testing the suitability of different aircraft for reforestation in remote areas. Nevertheless, aerial seeding has now developed to the stage where several types of fixed-wing aircraft and helicopters are used in reforestation, using various attachments for spreading tree seed or forest fertilizer.
Aerial seeding is not widely used in lowland forestry in New Zealand. It is used primarily in high-altitude protection forests that stabilize eroding mountain land. New Zealand's mountains are young, steep, and easily eroded. Much of their vegetative cover has been burned or severely browsed by large numbers of introduced animals, resulting in enormous losses of topsoil. Aerial seeding has been an effective tool for reestablishing vegetation in inaccessible, high-country sites.
Following initial trials, it was clear that of all the species tested, lodgepole pine was the most successful at altitudes up to 1,500 m elevation. But in the most eroded sites, the initial attempt failed because of extreme infertility and exceptionally harsh conditions. Few seedlings survived their first winter, all though good germination and reasonable establishment were achieved. In the cold, clear, mountain atmosphere, needles of ice as long as 25 cm formed in the soil and "heaved" the seedlings into the air, leaving them, with roots exposed, to die of desiccation in the spring. New Zealand foresters observed, however, that this did not occur when the soil was covered by a mulch of stones, litter, grass, or low-growing plants. Guided by this observation, they now spread a mixture of the seed of trees, grasses, and herbaceous legumes (legumes used successfully include trefoil (Lotus pedunculatus), white clover (Trifolium repens), and perennial lupin (Lupinus polyphyllus).
Grasses include Yorkshire fog Holcus lanatus), browntop (Agrostic tenuis), and fescue (Festuca rubra).
The "carpet" of vegetation that forms during the first season protects the young and vulnerable lodgepole pine seedlings by reducing the threat of frost-heaving from ice needles. Eventually, the kees appear through the "carpet" and benefit from the nitrogen fixed by the legumes, from the more equable ground temperatures that the mulch of vegetation provides, and from the erosion control induced by the grasses and legumes.
As a result, seedling survival is markedly increased by sowing tree seed simultaneously with legumes, grasses, and added fertilizer. The fertilizer (superphosphate) must be carefully regulated, because too little gives poor legume growth and the tree seedlings die from frost-heave; too much gives dense legume growth that suppresses small tree seedlings (the quantities of superphosphate applied vary with site but are in the range of 200600 kg per hectare. The legume fixes each year approximately 50-100 kg of nitrogen per hectare, which continuously increases soil fertility).
This interesting aerial seeding technique is probably applicable to problem sites elsewhere in the world (it is used, for example, for revegetating strip-mine spoils in much of the Appalachian region of the United States. There, black locust (Robinia pseudoacacia) is aerially seeded along with grasses and herbaceous legumes). However, it works particularly well where (as in New Zealand) precipitation is well distributed year-round. In areas with long dry seasons, grasses and legumes can prevent tree establishment by competing for the scarce soil moisture during dry periods.
In New Zealand high-altitude forest establishment using aerial seeding is best on a thin herbaceous turf where some topsoil is present; moderate on sites with a mulch of surface stones; and poorest on compact, eroded subsoils prone to frost-heave.