(This chapter is based largely on information supplied by William F. Mann and Herschel G. Abbott.)
Direct seeding became an operational technique in the United States during the 1950s. Since then, over 1 million hectares have been sown, primarily from aircraft. Initially, most seeding was concentrated in the Northwest, using Douglas fir. Soon thereafter, seeding became widespread throughout the South following development of a repellent seed coating that sharply reduced depredations by birds, rodents, and insects. Pines such as loblolly (Pinus taeda), slash (P. elliottii), shortleaf (P. echinata), Virginia (P. virginiana), longleaf (P. palustris), and jack (P. banksiana) have been the primary species sown. Black locust (Robinia pseudoacacia) has been widely seeded on mine spoils. Numerous other species, mostly conifers, also have been sown successfully, although not on a large scale.
A broad array of sites has been aerially seeded, ranging from open level areas of the Atlantic and Gulf coastal plains to rugged mountainous ranges of the Pacific Coast. Some specific regions where aerial seeding has proved successful include:
· Former forest lands devastated by wildfires, insects,
floods, tornadoes, or hurricanes;
· Cutover forests or lands inadequately stocked with commercial tree species;
· Waterlogged sites where planting equipment could not be operated;
· Remote areas, and
· Rocky spoil and steep banks left after strip mining.
Some spoil banks where the terrain was too rocky for workers to plant seedlings have shown remarkable tree growth following aerial seeding. The stones formed a protective environment for seed dispersed over the area by helicopters. Weathering processes eventually decomposed this rocky, virtually sterile material and, together with organic matter from the trees, the quality of the "soil" was improved as the years passed. These spoil banks are among the easiest sites on which to sow forests because the freshly turned spoil harbors few animals to eat the seed and few plants to compete with the seedlings.
The vegetative cover on aerially seeded sites has ranged from dense native grasses to stands of worthless brush. Much of the successful seeding has been done immediately following timber hanesting or wildfires, although large brush-covered areas that had lain idle for up to 4 decades following logging of virgin stands have also been sown successfully.
While industrial landowners were the first to employ direct seeding in contemporary times, small private owners are now also using this method.
Although direct seeding has accounted for only 4-18 percent of the area reforested annually in the United States over the past 2 decades, its contribution ranges to over 50 percent in some states.
In the South, almost 1.2 million hectares have been successfully sown with pines since the mid 1950s. For example, in 1977 and 1978 41,000 and 32,000 hectares respectively were direct seeded, primarily by air, although hand seeders and tractor-drawn row seeders were also used. A sound base of research preceded commercial operations. Endrin has often been used to repel rodents and kill insects, and thiram or anthraquinone to repel birds. Seeds were deposited on various types of soils and sites: thin, rocky soils, silt-loam soils, sandy loam, and strip-mine spoils.
Grassy sites are most difficult to seed because dense grass blocks the seed from reaching the ground and its roots compete vigorously with tree seedlings for moisture and nutrients. Some hardwood-dominated sites are easy to sow because competing ground vegetation has been shaded out and the hardwoods themselves can be killed with herbicide if the species being sown demand strong light.
Direct seeding of southern pines is impractical in swampy sites where seed would be submerged in water for a week or more. These sites are sown only after they dry out or after they are disked to provide rows of seedbeds raised above the water level before being aerially seeded. Sandy soils are also to be avoided because their surface layers dry out rapidly, leaving too little moisture to support continuous seed germination. However, ground seeding, using equipment to bury the seed about 1 cm deep, usually assures satisfactory germination in both swampy and sandy soils.
Sowing time is important. Usually, seeding is done in the fall (mid November to mid December) or spring (mid February to mid March). The seed is stratified (if necessary) to obtain more rapid and complete germination.
Broadcast-seeding operations have been declining in the South during the past 10 years, mostly because large programs are nearing completion and because the forestry industry is turning to use of seed orchards, which have produced only limited amounts of seed, so far. Because of economic considerations during the past several years, however, small landowners have resumed direct seeding with the encouragement and support of federal agencies and corporate landowners.
Direct seeding in the North Central (lake states) region of the United States has lagged behind the Northwest and South. But in recent years, various interests have initiated large-scale operations with jack pine and in limited situations with black spruce (Picea manana). Overall success with other spruces and firs (Abies species) has been discouraging because their seedlings grow too slowly. Red pine (Pinus resinosa) has not been tested adequately, but it seems to show promise for direct seeding.
Soils in the North Central and Pacific Northwest regions usually contain a thick layer of partially decomposed organic material that poses a special problem in seeding operations. This layer dries out rapidly, and germinating, seeds often desiccate and die before the seedling root can penetrate to the moist soil below. To achieve consistent success, the organic material must be burned or mechanically scarified to expose the mineral soil beneath.
In western Oregon, summer soil temperatures (60° C) on newly burned sites proved too high for successful broadcast seeding. But by sowing mustard seed (Brassica juncea) at a low rate with the tree seed, success was achieved. The quick-sprouting mustard reduced both erosion and soil temperature and, in its cool shade, Douglas fir (Pseudotsuga menziesii) seedlings germinated and survived. The low density of seedlings that this allowed reduced the competition for moisture (information provided by C. M. McKell. see McKell and Finnis, 1957).