Direct Seeding

Some framework tree species can be established in the field directly from seed. Direct seeding involves:
·     collecting seeds from native trees in the target forest ecosystem and if necessary storing them until sowing;
·     sowing them in the restoration site, at the optimal time of year for seed germination and
·     manipulating field conditions to maximize germination.
Direct seeding is relatively inexpensive, since there are no nursery and planting costs (Doust et al., 2006; Engel and Parrotta 2001). Transporting seeds to the restoration site is obviously easier and cheaper than trucking in seedlings, so the method is particularly suitable for less accessible sites. Trees established by direct seeding usually have better root development and grow faster (Tunjai, 2011a) than nursery-raised saplings, since their roots are not constrained within a container. Direct seeding can be implemented in combination with ANR methods and conventional tree planting, to increase both the density and species richness of regenerants. In addition to establishing framework tree species, direct seeding may be used with the maximum diversity method and to establish “nurse tree” plantations, but it does not work with all tree species. Experiments are needed to determine which species can be established by direct seeding and which cannot.
What are the potential obstacles to direct seeding?
In nature, a very low percentage of dispersed tree seeds germinate and even fewer seedlings survive to become mature trees. The same is true of direct seeding (Bonilla-Moheno and Holl, 2010; Cole et al., 2011). The biggest threats to direct sown seeds and seedlings are: i) desiccation, ii) seed predation, particularly by ants and rodents (Hau, 1997) and iii) competition from herbaceous weeds (see 2.2). By counteracting these factors, it is possible to achieve higher rates of germination and seedling survival than would occur with naturally dispersed seeds.
The problem of desiccation can be overcome by selecting tree species with seeds that are tolerant or resistant to desiccation (i.e. those with thick seed coats) and by burying seeds or laying mulch over the seeding points (Woods and Elliott, 2004).
Burying can also reduce seed predation, by making the seeds more difficult to find. Pre-sowing seed treatments, to accelerate germination, reduce the time available for seed predators to find the seeds. Once germination commences the nutritional value of seeds and their attractiveness to predators rapidly decline. However sometimes, treatments that break the seed coat and expose the cotyledons may actually increase the risk of desiccation or make seeds more attractive to ants (Woods and Elliott, 2004). It may also be worth exploring the possibility of using chemicals to repel seed predators. Obviously, any carnivores that prey on rodents (e.g. raptors, wild cats etc.) should be regarded as valuable assets on ANR sites. Preventing the hunting of such animals can help control rodent populations and reduce seed predation.
Seedlings germinating from seeds are tiny, compared with planted, nursery-raised saplings, so weeding around the seeding points is especially important and it must be carried out with extra care. Such meticulous weeding can greatly increase the cost of direct seeding (Tunjai, 2011a).
What tree species are most suitable for establishment by direct seeding?
Species that tend to be successfully established by direct seeding are generally those with large (> 0.1 gm dry mass), spherical seeds with medium moisture content (36-70%) (Tunjai, 2011b). Large seeds have large food reserves, so they can survive longer than smaller seeds and produce more robust seedlings. Seed predators find it difficult to handle large, round or spherical seeds, especially if such seeds also have a tough and smooth seed coat.
Tree species in the family Leguminosae are most commonly reported as being suitable for direct seeding. Legume seeds typically have tough, smooth seed coats, making them resistant to desiccation and predation. The nitrogen-fixing capability of many legume species, can give them a competitive advantage over weeds. Tree species in many other families have also shown promise and are listed in Table 5.2 (Tunjai, 2011a).
Published accounts of direct seeding have tended to concentrate on pioneer tree species (Engel and Parrotta, 2001) because their seedlings grow rapidly; but climax forest tree species can also be successfully established by direct seeding. In fact, due to their generally larger seed sizes and energy reserves, climax forest tree seeds may be particularly suited to the method (Hardwick, 1999; Cole et al., 2011; Sansevero et al., 2011). With the disappearance of large, vertebrate seed-dispersers over much of their former ranges, direct seeding may be the only way that the large seeds of some climax tree species can reach restoration sites (effectively substituting human labour for the former roles played by such animals).

When is the best time to carry out direct seeding?
In ever-wet tropical regions, direct seeding can be implemented at any time (except during drought conditions). In seasonally dry areas, direct seeding should be carried out at the start of the rainy season (alongside conventional tree planting). This allows sufficient time for germinating seedlings to grow root systems capable of accessing enough soil moisture to enable the young plants to survive the first dry season after sowing. Unfortunately the rainy season is also the peak time of year for weed growth and breeding of rodent seed predators, so control of these two factors is particularly important. It has been suggested that direct seeding late in the rainy season would avoid these problems, but recent results have confirmed that early sowing, to achieve extensive root growth before the dry season, is the overriding consideration (Tunjai, 2011).

What if seeds of desired species are not available at the optimum time for direct seeding?
Seeds must be stored from the time of fruiting until the start of the rainy season. Many tropical tree species produce recalcitrant seeds, which lose viability rapidly during storage, but for direct seeding, the storage period required is less than 9 months. So storage may be possible. See 6.2 and 6.6 for more information on seed storage.

How to carry out direct seeding?
At the beginning of a rainy season, collect seeds of the desired tree species (or remove them from storage). If pre-sowing treatments to accelerate germination are known, then apply those treatments. Dig out weeds in “seeding spots”, approximately 30 cm across, spaced about 1.5-2 m apart (or the same distance from natural regenerants if present). Dig a small hole in the soil and loosely fill it with forest soil. This ensures that beneficial symbiotic micro-organisms (e.g. mycorrhizal fungi etc.) are present when the seed germinates. Press several seeds into each hole, to a depth of about twice the diameter of the seed and cover with more forest soil. Lay mulching material, such as the pulled weeds or cardboard, around the seeding spots to suppress further weed growth. During the first 2 rainy seasons after seeding, pull weeds by hand from the seeding spots as required. If multiple seedlings grow up at any seeding point, remove the smaller, weaker ones, so that they do not compete with the largest seedling. Carry out experiments to determine the most successful species and techniques for direct seeding at any particular site.