1 Introduction

In many parts of the world, deforestation has reached critical proportions. Africa, Asia, and Latin America have vast areas of once-forested land that is now denuded. Many have been left largely unplanted. Removal of tree cover threatens to destroy already fragile environments. Without trees and their root systems, heavy rains on the hillslopes cause rushing water that erodes the land and produces devastating floods that despoil highways, dams, bridges, towns, villages, and farm lands.

There is no end in sight to the destruction caused by unregulated deforestation: ever-swelling numbers of local farmers cut and burn the forest for land to grow crops; more and more villagers and entrepreneurs harvest trees for fuelwood and charcoal as escalating prices preclude the use of kerosene for cooking and heating; increasingly, pastoralists clear forests and shrubland for grazing; and timber remains the fourth- or fifth-largest export from the Third World to industrialized countries.

Some countries-Haiti, for instance-have essentially eliminated their forests, while many others could reach total deforestation before 1990 (see Table 1). The deforested land is often lost to coarse, almost useless grasses (such as Imperata cylindrica) or to a wilderness of shrubs and scrubby trees with little or no value to commerce or to villagers. Some 25 million hectares of such marginal land already exist in Thailand, the Philippines, and Indonesia alone.

These enormous areas of virtually unproductive land are increasing. Traditional revegetation methods should be applied more extensively, but the time also seems right for examining alternative methods.

Dropping seed from planes or helicopters is a well-known and well-established technique for sowing pastures as well as agricultural crops such as soybeans, wheat, and rice. Forests have also been established in this way. However, aerial seeding of forests is largely unappreciated, even by most foresters; it now seems worthy of increased testing and research.

When conditions and species are right, and seed supplies sufficient, aerial seeding could be an important technique for reforesting large areas. It is easy to organize and seems well suited for reforesting sites that have rough terrain, debris, or difficult access. If it can be developed for sites and objectives in developing countries, aerial seeding could offer opportunities for vastly accelerating their reforestation programs.

TABLE 1. Areas of Rapid and Widespread Deforestation (In these areas, the present rate of deforestation is so great that within about 10 years most virgin forests will be extinct. Source: Myers, 1980, pp. 169-170)

Region	Environment
Australia	lowland tropical forests in the north
Bangladesh	lowland and upland rainforests
India	predominantly seasonal forests, mainly upland
Indonesia	Sumatra and Sulawesi lowland, predominantly rainforests
Malaysia	lowland, mainly rainforests
Melanesia	mainly rainforests
Philippines	lowland, mainly rainforests
Sri Lanka	rainforests, mainly upland
Thailand	seasonal forests, both upland and lowland
Vietnam	seasonal forests, both upland and lowland
Brazil	eastern and southern Amazonia, Atlantic coast
Central America	rainforests, both upland and lowland
Colombia	lowland rainforests, Caqueta and Putumayo regions
Ecuador	Pacific coast forests, lowland and upland
Madagascar	rainforests, lowland and upland
East Africa	relict montane forests, especially in northern Tanzania
West Africa	seasonal forests

Aerial reforestation is not a replacement for planting seedlings by traditional methods. It is best considered as a potential complement to conventional planting and to natural seeding, an additional tool for foresters to use when the needs, sites, and species are appropriate.

Sowing tree seed directly in the field is an old technique but it was little used until the development of repellents to protect seed from insects, rodents, and birds. In the United States, the initial breakthrough came in 1953 when several chemical seed coatings proved effective in repelling birds. Then in 1956 it was learned that an additional coating of commercial insecticide would guard the seed against insects and rodents. These findings signalled the beginning of large-scale aerial seeding of forests, especially in Louisiana.

Today aerial seeding is already regarded as a practical reforestation technique in a few countries. There it is fully operational. More than a million hectares of well-stocked forests in the United States, Canada, Australia, and New Zealand demonstrate its success. Some of these forests have been established despite seemingly adverse conditions-for example, on steep slopes and on overburden from strip mines.

Although aerial seeding technology has been used mainly in industrialized countries in temperate areas, it would seem the techniques could be modified for use elsewhere. Whether it will prove widely applicable in the arid tropics is still unknown. The uncertainties regarding its application in new regions lie mostly in whether the native animals and plants, as well as local climatic conditions, will permit its success. Nonetheless, sufficient knowledge has been accumulated in large-scale operations in North America and Australasia to justify wide-ranging trials in developing countries.

When sites and species are right, aerial seeding can be as successful as the more conventional process of planting seedlings. For example, pine forests have been well established in over 90 percent of the attempts in the southern United States.

Some species successfully sown from the air in various parts of the world are listed in Table 2.

TABLE 2. Some Species Succesfully Sown from the Air

Species	Common	NameLocation
Acacia auriculiformis		Indonesia
Betula allegheniensis	yellow birch	Canada
Calliandra calothyrsus	calliandra	Indonesia
Cecropia obtusifolia	trumpet tree	Hawii
Eucalyptus camaldulensis	river red gum	Australia
E. delegatensis	alpine ash	Australia
E. globulus	blue gum	Australia
E. grandis	flooded gum	Australia
E. nitens	shining gum	Australia
E. obliqua	messmate stringybark	Australia
E. regnans	mountain ash	Australia
E. viminalis	manna gum	Australia
Leucaena leucocephala	leucaena	Pacific Islands
Liquidambar styraciflua	sweet gum	Honduras
Liriodendron tulipifera	tulip poplar	United States
Melochia indica	melochia	Hawaii
Picea glauca	white spruce	United States
P. mariana	black spruce	Canada
Pinus banksiana	jack pine	Canada
P. contorta	lodgepole pine	United States, New Zealand
P. elliottii	slash pine	United States
P. nigra	black pine	New Zealand
P. mugo	Swiss mountain pine	New Zealand
P. palustris	longleaf pine	United States
P. ponderosa	ponderosa pine	United States, New Zealand
P. radiata	Monterey pine	New Zealand
P. resinosa	red pine	United States
P. rigida	pitch pine	United States
P. taeda	loblolly pine	United States
P. virginiana	Virginia pine	United States
Populus spp.	Cottonwood	United States
Pseudotsuga menziesii	Douglas fir	United States
Robinia pseudoacacia	black locust	United States
Sesbania grandiflora	turi	Indonesia
Spathodea campanulata	African tulin tree	Hawaii

Direct Seeding

Aerial seeding is just one example of the more general process of broadcast seeding by which the seed may also be sown from the ground using mechanical spreaders or by hand. Ground-seeding methods will be preferable to aerial seeding in many situations in developing countries. In such cases, the principles and requirements are similar to those discussed here.

Ground seeding is not the focus of this report, but some of its techniques are depicted in Appendix A. The advantage of the airplane is its ability to quickly seed large areas, even remote areas, when conditions for prompt germination and survival are best.

Aerial Seeding Sites

Aerial seeding is best suited to sites whose remoteness, ruggedness, inaccessibility, or sparse population make seedling planting difficult. It is particularly appropriate for "protection forests" because helicopters or planes can easily spread seed over steep slopes or remote watersheds and isolated upland areas. It seems well suited for use in areas where there may be a dearth of skilled laborers, supervisors, and funds for reforestation (Large tracts can be seeded so rapidly that supervising personnel are freed for other duties in a relatively short time. A ground crew of only three flagmen and two men to weigh and load seed are usually required). It has the potential to help increase production of tree crops for forage, food, and honey as well as wood for fuel, posts, lumber, and pulp.

Aerial seeding can often be quickly deployed because there is no wait for seedlings to grow in a nursery (which may take 3 months to 3 years). It may be used on areas denuded by clear cutting or shifting cultivation. Also, sites of catastrophes such as forest fires, hurricanes, insect devastation, battles, volcanic eruption, or landslides can be promptly reseeded with useful tree species if seed is available (sometimes a deliberate burn or other technique may be needed to remove weeds and complete preparation of the seedbed). For example, if a wildfire bares the soil on a remote watershed and makes it vulnerable to erosion, aerial seeding can be used on the ash-strewn seedbed before weedy species overrun the site. In some cases, a mixture of seed can be applied so that herbaceous plants such as mustard, grasses, or herbaceous legumes provide a quick ground cover that protects and "nurses" the young tree seedlings and suppresses undesirable weeds.

On the other hand, rapid deployment may not be practical in some cases because the site may require preparation or the season may be wrong.

To germinate successfully, seeds usually must fall directly onto mineral soil rather than onto established vegetation or undecomposed organic matter. Where organic matter has accumulated thickly, the site must normally be burned, furrowed, or disked. The soil disturbance left after logging is often sufficient. Rough terrain is especially amenable to broadcast seeding.

On certain sites ground preparation may be necessary. Site preparation and the seeding operation must be well coordinated to meet the biological requirements for prompt seed germination and seedling survival. Dry sites may have to be specially ridged or disked so as to optimize the rainfall that reaches the seed. Excessively wet sites may need to be ridged or drained.

The degree of slope is not critical as long as seeds find a receptive seedbed. Steep watersheds, eroding mountain slopes, bare hillsides, and spoil-banks where vegetation is sparse are often suitable for aerial seeding (however, on some steep slopes with smooth, bare soil, rain may wash the seeds away too easily for successful seeding). On steep strip-mine spoils in West Virginia and in Indonesia slopes of more than 30° (about 70 percent slope) have been successfully revegetated from the air.

Arid and savanna lands (for example, those where annual rainfall is under 500 mm) are most in need of reforestation. These are regions where aerial seeding in principle has exceptional potential. They include vast tracts of unused or poorly used land that has sparse tree cover and that is not confined to private land holdings, so it is generally accessible to aircraft. The native trees (such as species of Acacia, Prosopis, and other genera) in these areas are generally well adapted for survival under difficult field conditions. These are not species for timber as much as for firewood, forage, fruit, gum, erosion control, and other such uses. On dry sites, the amount of successful establishment is increased if the seed can be covered with a thin soil layer.

Species

As a prerequisite to any method of reforestation, the species selected must be adapted to the temperature, length of growing season, rainfall, humidity, photoperiod, and other environmental features of the area. Ideally, before aerial seeding takes place trial plots should be established to test those species most likely to germinate and grow successfully on the chosen sites. Even when one species has the right characteristics, it may be prudent to test seed of different provenances to fnd those best suited to the site.

Aerial seeding has been used mostly with conifers and eucalypts (see Table 2), although other species that reseed themselves successfully in a given region could also be aerially seeded with reasonable probability of success. However, in nature seed germinates over a relatively long period, and though environmental factors may be hostile at one time, they usually prove favorable at another. With broadcast seeding, only one or two applications are made, the seeds germinate together, and if timing is off, the results will be poor.

Characteristics that make a particular species appropriate for aerial seeding include:

. Small or medium-sized seed,
· Frequent and prolific seed availability;
· Ability of the seed to germinate on the soil surface;
· Fast germination and rapid seedling growth,
· Ability to withstand temperature extremes and prolonged dry periods,
· Ability to tolerate a wide range of soil conditions;
· High light tolerance;
· Seed that is easy to collect in large quantities and to store for long periods;
· Suitability of seed for handling with mechanical seeding devices; and
· Rapid development of a deep taproot by seedlings to enable them to withstand adverse
climatic conditions in the period following germination.

Species with highly palatable seeds have little prospect of success because wildlife eat the seed before it has a chance to germinate. Also, small seeds and lightweight, chaffy seeds are more likely to drift in the wind, so they are harder to target during the drop. (This can be compensated for by adding a thick coating to the seed.) Small seeds, however, fall into crevices and are then more likely to get covered with soil, thereby enhancing their chances of survival.

Aerial seeding may prove to work best with "pioneer" species, which germinate rapidly on open sites, are adapted for growth on bare or disturbed areas, and grow well in direct sunlight.

A list of species that may prove amenable to aerial seeding is given in Table 3.

Equipment

Aerial seeding is done by fixed-wing aircraft or helicopters, usually flying at 15-25 m altitude.

Airplanes already in use in pest control or aerial surveying can easily be adapted for use in aerial seeding by slightly modifying the hopper gate to control the flow of seed into the seed-distributing device (see below). Further, they can be used in association with their normal agricultural program, provided they can be utilized at short notice when the conditions for aerial seeding become suitable. Such aircraft are highly maneuverable and can operate from simple, improvised airstrips such as logging roads. Appropriate types are already available in most, perhaps all, developing countries (for a detailed study of the use of aircraft in agriculture see Akesson and Yates, 1974).

For over 20 years in the southern United States, helicopters have been employed to seed forests. Generally, they are more maneuverable than fixedwing aircraft and give better placement of seed; for example, along the edges and in the corners of irregular tracts of land, on steep mountainsides, and in areas as small as 0.5 hectares. Moreover, helicopters can land on site and reload more quickly than fixed-wing planes and the helicopter pilot can check results and discuss plans more readily with ground staff.

Fixed-wing aircraft, on the other hand, are less expensive to operate, and at least in Ontario, Canada, are considered as efficient as helicopters.

TABLE 3 Possible Candidates for Aerial Seeding in Developing Countries (for information on many of these species see companion reports Tropical Legumes Resources for the Future ans Firewood Crops: Shrub and Tree species for Energy Production)

Humid Tropics	Semiarid Areas	Tropical Highlands
Acacia auriculiformis	Acacia albida	Acacia mearnsii
Other Acacia spp.	Acacia nilotica	Alnus acuminata
Albizia falcataria	Acacia saligna	Alnus nepalensis
Albizia lebbek	Acacia senegal	Alnus rubra
OtherAlbizia spp.	Anacardium occidentale	Callitris spp.
Anthocephalus chinensis	Azadirachta indica	Eucalyptus globulus
Avicennia spp. and some	Colophospermum mopane	Grevillea robusta
other mangroves	Eucalyptus citriodora	Inga spp.
Calliandra calothyrsus	Eucalyptus tereticornis	Mimosa scabrella
Cassia siamea	Haloxylon aphyllum	Pinus oocarpa
Other Cassia spp.	Haloxylon persicum	Robinia pseudoacacia
Casuarina spp.	Pinus halepensis
Cecropia spp.	Prosopis spp.
Croton spp.	Zizyphus mauritiana
Derris indica (Pongamia	Zizyphus spina-christi
glabra)
Eucalyptus deglupta
Other Eucalyptus spp.
Ficus spp.
Flindersia brayleyana
Gliricidia sepium
Gmelina arborea
Leucaena leucocephala
Macaranga spp.
Maesopsis eminii
Melaleuca spp.
Melia azedarach
Melochia indica
Muntingia calabura
Musanga spp.
Neoboutnoia spp.
Pinus caribaea
Pinus kesiya
Sesbania grandiflora
Spathodea campanulata
Syzygium cumini
Tenninalia catappa
Trema spp.

Seed containers (hoppers) and distributors that attach to aircraft are commercially available, well developed, and reliable. Most commercially available seeding devices have power-driven augurs to control the rate of seed flow and either venturi or powered slinger-type applicators for spreading the seed (fixed-wing aircraft usually use gravity flow of seed and an unpowered venturi-type distributor. Helicopters must use a power-driven slinger because their speed is inadequate for venturi action). Most units are also capable of sowing seeds in a range of sizes.

At normal altitudes and flying speeds the distributors on ftxed-wing aircraft throw seed over a swath about 20 m wide. The power slingers attached to helicopters throw seed over a swath about 30 m wide. In good weather an aircraft in the United States routinely sows 1,200-1,500 hectares daily. Under favorable conditions (e.g., with landing sites close by) as much as 3,000 hectares have been seeded in a day.

Ideally, before aerially seeding 10 hectares or more, the prospective site is delineated on the ground and the pilot is provided with maps or aerial photos. Arrangements are made for radio communication between the aircraft and ground crew (although desirable, this is not essential). A system for measuring the seed distribution on the site is worked out in advance and, if possible, flagmen are positioned to guide the aircraft so as to avoid overlapping runs (techniques newly developed in the United States include the use of aluminum flags (dropped from the plane) as well as electronic field triangulation, both of which guide the planes without using flagmen).

Pest Control

The major problem in aerial seeding is not one of engineering or plant science per se; it is the problem of seed predators. Almost any small animal (bird, insect, mouse, shrew, chipmunk, squirrel) will eat or remove seeds lying on the soil surface. It is essential to know the composition and population of seed-eating animals before aerially seeding an area. Mice and birds are generally the most destructive.

Normally, the seeds must be coated with chemicals to repel those animals most likely to eat them. Further, seeds should be sown, if possible, at a season when pest populations are low. Site preparation (i.e., burning or scarification mentioned above) not only prepares a good seedbed, but it often reduces predator populations. The likelihood of different predators stealing the seeds can be easily assessed by covering tiny sample areas with cages whose mesh sizes are selected to keep out different predators (mice, rats, birds, and ants, for example).

To avoid predation, the seed must be made to germinate as quickly as possible. Seed of some species, especially those with hard seed coats, must be stratified (subjected to moisture at low temperature to break dormancy) or scarified (treated with acid, hot or cold water, or mechanical abrasion) to increase seed-coat permeability and ensure rapid germination.

In the United States, the insecticide endrin (Hexachlorooctahydro-endo, endo-dimethanonaphthalene) is used to protect the seed from insects and rodents in the direct seeding of conifers. The chemical is added as a thin coating around each seed. Although it has been restricted for other uses, endrin is still allowed for direct seeding in most states because the amounts applied are small (about 5 grams per hectare), the chemical is bound to the seed with latex, and the treated seeds become so widely scattered that little lasting environmental hazard is likely (endrin is a toxic material that must be applied carefully, at low dosage, and always in combination with a bird repellent (to avoid harming birds). Seeding projects proposing to use endrin must be reviewed with health and wildlife officials to avoid affecting the nontargeted animals).

The common fungicide thiram (Tetramethylthiuramdisulphide) is generally applied as a seed coating to repel birds. It forms a hard coating, very effective in rendering seed unpalatable to birds. There are few restrictions on its use; however, if improperly used, it can be toxic to some plant species.

Latex is used as a binder to attach the coating to the seed. Aluminum powder is used as a lubricant to prevent the seeds from sticking as they pass through the machines. (It also indicates a warning to personnel handling them and helps repel some birds.)

Simple procedures are available for treating seed prior to aerial seeding. For example, U.S. Forest Service workers in Louisiana use mammal and bird repellents stirred together in a small pail with the latex binder blended in. The resulting mixture is poured over the seed in a small concrete mixer and tumbled for about 2 minutes. A small amount of aluminum powder is next spooned in and the mixture tumbled 1 minute more. By then, the seed is fully coated and is spread out to dry for several hours or bagged for drying in a low-temperature kiln. Using these simple procedures a three-man crew can treat a ton of pine seed a day (the procedures were worked out for pine seed but should he widely applicable to seed of other species. For detailed description see Derr and Mann, 1971).

Costs

On sites where it succeeds, aerial seeding can be less expensive than conventional methods of reforestation. In 1980 hand planting costs in New Zealand were about $250-300 per hectare; aerial seeding for protection forestry cost about $20-30 per hectare (information supplied by A. Nordmeyer).

With aerial seeding there are no large capital costs for establishing and staffing nurseries, transporting nursery stock, or buildings roads and camps to house the laborers. Further, there are no labor costs to outplant the seedlings.

On the other hand, there are extra capital costs for collecting and storing seed, and there are expenses for aircraft-support facilities, for seed-spreading devices, and for operating the aircraft. In most developing countries, planes are privately owned and it is sometimes difficult for a government to arrange and pay for their hire, relocation, and support. Helicopters are particularly expensive in developing nations.

Gogama District, Ontario. Helicopters require a power-driven centrifugal slinger for distributing the seed. The types in use in North America cover a swath 30 m wide. Flagmen are often used to guide helicopters and ensure that seed is spread evenly. (J. Scott)

Aerial application in the United States costs from $3.75 to $20.00 per hectare, depending on the size of the tract. Except for site preparation, the price of commercial seed is the biggest expense. The cost for pine seed is about $50.00 per hectare (spread at a rate of about 0.85 kilograms per hectare).

Aerial seeding is generally most economical for areas greater than 200 hectares. As a general rule, the most money is saved on sites where planting costs are highest. In the southern United States aerial seeding is reported to save an average of $7 per hectare on well-drained open land to $50 per hectare on rough sites where debris slows down planting crews (information supplied in 1979 by W. F. Mann, Jr.). Thinly stocked forests, or sites dominated by low-quality trees, are costly to plant but are easy and cheap to seed.

In countries with agricultural aviation services there need be no large capital outlays. Aircraft can be obtained under short-term contract when conditions for seeding are right. In some countries contractual services are also available to procure and treat the seed before sowing. Also, aerial seeding might fit developing countries having an air force that can be enlisted into civic-action programs.

Obtaining adequate amounts of seed may prove a major cost because aerial seeding often requires 10 times the amount of seed as reforestation with nursery grown seedlings. Ensuring adequate local seed supplies may require sectioning off part of a forest as a seed-production area. This is already standard procedure in many countries. Fortunately, if a storage facility is available, seed of many species can be stored for several years so that it is available for use when seasons and conditions are appropriate.

The emphasis on genetically improved trees in recent decades has depressed the amount of aerial seeding in the United States. Seed from hybrids and genetically selected specimens have been too scarce and expensive to allocate to aerial seeding. Today, aerial seeding is mostly done with seeds collected from wild stands and plantations of unimproved genetic strains.

Treatment of seed with endrin, thiram, and a coloring agent is only a minor cost in the operation.

Limitations

Aerial seeding is not a method to be applied casually. It is more subject to environmental uncertainties than is planting of nursery stock. Correctly used, it is cheap, fast, and reliable, but for maximum success with greatest economy the forest manager must learn where this method is better adapted than alternative techniques.

Aerial seeding is unlikely to be successful until a good working knowledge of seed and seedling behavior is obtained and satisfactory techniques of gathering and storing seed are developed.

As previously mentioned, when aerial seeding is used, seed-eating birds, animals, and insects must be prevented from destroying the seed and the sites must be suitable for promoting germination of the seed and survival of the young seedlings. As with conventional forestry, unchecked fire, grazing, and weeds will cause failures and they may be more troublesome at the remote sites where aerial sowing is likely to be employed.

Although aerial seeding is easier to organize and requires a smaller number of people than the planting of seedlings, it does require technicians trained in the storing, handling, and preparation of seed and it involves aircraft pilots, radio operators, and aircraft maintenance mechanics. Moreover, while aerial seeding appears to run counter to developing-country needs for labor-intensive technologies, it does require a lot of seed, which usually is collected by hand.

Only a few sites and species have been tested so far, and aerial seeding may eventually prove suitable only for certain sites and species. (For a list of species successfully seeded from the air see Table 2). It is already known, however, that not all tree species are suitable for aerial seeding. Infrequent seed production, the difficulty and high cost of collecting seed, and the sensitivity of the seed to long-term storage are all limiting factors.

A further impediment to aerial forestation is that it offers poor control over the density and spacing of the trees produced. It is most useful where the production of regularly spaced trees is of secondary importance. Forests resulting from aerial seeding are much like those produced by natural regeneration; they often become overstocked and require early thinning to promote growth and vigor. With some species (e.g., Pinus taeda) this is not a serious limitation; the stands thin themselves because individual trees dominate and suppress the less-vigorous ones. In the United States the cost of mechanically thinning the overstocked stands has been a deterrent to aerial seeding. But in those developing countries with serious firewood shortages, the excess of stems could be an advantage because wood removed as thinnings is likely to end up as fuel for family cooking and heating.

Despite the hope that aerial seeding could help reforest arid areas, dry sites represent some of the greatest challenges to the technology and it may prove only infrequently successful there. Nevertheless, several species of Acacia as well as neem and cashew are traditionally direct seeded (from the ground) in the Sudan and in neighboring Sahelian areas. This offers hope that aerial seeding also may prove successful with these species in similar dry areas. However, the only aerial seeding in a dry area that we know of is a trial involving seeding neem (Azadirachta indica) from an airplane in northern Nigeria in the early 1960s. The seeds germinated readily, but because the sowing season was wrong only those shaded by the brush vegetation on the site survived.

Other possible limitations on individual sites include lack of seed, lack of available aircraft, competition from weeds, or the inability to control wildfires. Also, aerial seeding requires relatively sophisticated logistical, administrative, and communications support, which may not be readily available, especially in some remote areas (Akesson and Yates, 1974).

As is true with seedling transplants, dry-season fires in the savanna regions and heavy weed growth in the humid tropics are likely to be major management problems following a successful sowing.