Direct Seeding Literature Review
Direct Seeding Literature Review
The world’s total forest area is just over 4 billion hectares, which corresponds to an average of 0.6 ha per capita (Figure 1). The five most forest-rich countries (the Russian Federation, Brazil, Canada, the United States of America and China) account for more than half of the total forest area.
Deforestation – mainly the conversion of tropical forest to agricultural land – shows signs of decreasing in several countries. Around 13 million hectares of forest were converted to other uses or lost through natural causes each year in the last decade. At a regional level, South America suffered the largest net loss of forests between 2000 and 2010 – about 4.0 million hectares per year – followed by Africa, which lost3.4 million hectares annually (Figure 5). Oceania also reported a net loss of forest (about 700 000 ha per year over the period 2000–2010), mainly due to large losses of forests in Australia, where severe drought and forest fires have exacerbated the loss of forest since 2000. Asia, which had a net loss of forest of some 600 000 ha annually in the 1990s.
Need of Forest Restoration: Within the last ten years, tropical rainforests have been destroyed at an annual rate of 0.8% of area (Whitemore, 1997). So this things brings to human being to think about the forest restoration. Forest restoration is defined as “actions to re-instate ecological processes, which accelerate recovery of forest structure, ecological functioning and biodiversity levels towards those typical of climax forest” Various forest restoration methods have been developed, for instance, the accelerated natural regeneration ( ANR) (Jansen and Pfeifer , 1989), The Framework Species method(Goosem & Tucker, 1995) and The Maximum Species Diversity method. The ANR method is Effective where natural regeneration is already underway. Eliminates nursery and tree planting costs.
Relatively cheap – Capitalizes on “free” ecological services US$ 1,400/ha. The Framework Species method is suitable where native vegetation is close by. Its only need a single planting and it is self sustaining. In the Maximum Species Diversity method a large percentage of species are from the mature phase and primary promoters are avoided. The major disadvantage of this method slower growth rate and post planting management is required. Now a days there is some evidence that , reforestation plays a key role in the long term of restoration of landscape functioning, as well as social and economic development. Reforestation can catalyses and induce succession of forest ecosystems using native species (Parrotta, 2000). Direct Seeding:
An ancient method of establishing trees is by sowing seeds directly where the future trees are required. In many circumstances, this method is superior to the traditional method of planting nursery-raised plants. In several tropical countries, the demand for reforestation is often not met due to limited resources. Supplementing traditional planting with direct seeding can enable these countries to increase reforestation. Direct seeding is a regeneration method of sowing seeds directly where the future trees are required, whether it is for forest plantations, shelterbelts or in agroforestry. In this way the laborious task of raising nursery plants and transplanting them to the planting site is omitted. Direct seeding offers various interesting possibilities e.g. the ability to rapidly increase the area being forested or the ability to provide rural people with an inexpensive method to obtain benefits from trees. History of Direct Seeding:
Direct seeding has over the last 50 years gained in importance, especially in North America and China where large areas have been direct seeded from helicopters or aero planes. Direct seeding from the air has been widely used in China where more than 15 million ha have been reforested between 1956 and 1985 (Xinhua & Jingchun, 1988). In the developed countries more than 1⁄2 million ha were reported to be direct seeded in 1974 or about 20 % of the area being planted (Appleroth, 1975). Direct seeding in the tropics is less reported compared with the subtropics (Australia and USA) or the temperate regions (USA, Canada and Scandinavia). Trials have been carried out and reported from various tropical countries for example Indonesia, Philippines, India, Central America, Northern Australia and the Sahel in Africa. Direct seeding in the tropics has been carried out many times without the results ever being published. In the dry tropics traditional forestry is limited and most trees are raised in connection with agriculture.
Here, direct seeding already plays an important role, this could increase in the future by being implemented in afforestation and in establishment of shelterbelts. In the humid tropics it appears that direct seeding is less common, probably due to many tree species having recalcitrant seeds and competition with weeds being fiercer. Nevertheless direct seeding has the ability to contribute significantly to tropical forestry in the future, probably not as the main regeneration method but as a method that under certain circumstances is superior to other methods. The following will describe various scenarios where direct seeding is employed in the tropics. Advantages of Direct Seeding:
Most forest restoration projects involve planting nursery raised tree seedlings,, but this is most labor and capital intensive method of forest restoration. Seed collection, raising seedlings in a nursery, planting and maintaining planted saplings until they can establish and become independent all require substantial labor inputs (Hardwick et al., 2000). Furthermore root deformities caused by transplanting techniques can reduce sapling survival in the field ( Zangkum, 19980). The Potential advantages of direct seeding over other plantation establishment techniques include cost savings associated with nursery care and planting as well as the possibility that trees established by this means may develop more naturally or quickly, than would transplanted seedlings or cuttings( Engel & Parrotta,2001).
The Eden Project in Niger is recommending direct seeding as an appropriate method of establishing trees where water is scarce (Eden Foundation, 2000). It is claimed that nursery plant use precious irrigation water, whereas direct seeding carried out prior to or during the rainy season does not need irrigation. They further maintain that plantation established by direct seeding tend to produce extensive root system, whereas the above ground shoots grow more slowly.
In contrast seedlings in nurseries tend to produce large shoots and have to be irrigated or they they will suffer high mortality, whereas plants that established through direct seeding are more likely to be able to reach moisture remaining in the soil after rainy season.(Oschner,2001). This was quantified by Samba (1992) who sowed the seeds of Faidherbia albida at the same time as planting as planting nursery raised plants. Four months after sowing direct seeded plants were about twice as tall and had a mean dry root mass 25 times higher than that of the nursery raised plants. The disadvantages of Direct Seeding:
The significant disadvantages of direct seeding that can outweigh these advantages include low germination survival percentages, seedling mortality associated with weed competition and also poor weather condition (Evans, 1982). Steven stated that the results of direct seeding are affected by a number of factors including species, soil conditions, site preparation and techniques for seed germination. Garwood (1989) pointed out that seed destructed by animals also plays a key role in reducing seed germination. Direct Seeding in different purposes:
Direct deeding can used for many purposes like for plantation, restoration, in mine spoils rehabilitation, in agro forestry. Negreros et al., 2003 worked in Mexico to restore the area by Mahogany seeds. Because this is very valuable for timber. He found that Burying Mahogany seeds were clearly better than sowing them on the soil surface and it yields double comapared to sowing method. For restoration purposes by direct seeding method, Tunjai and Elliott (2011) found that seed trait is very important to achieve the goal of forest restoration by this method. They said Large or intermediate sized seeds, Round or oval seeds, Medium or low moisture content seeds showed good establishment when they sowed in the field for forest restoration purposes. For mine site rehabilitation Parrotta and Knowles(2001) found that In term of species richness, densities and life span over 40 years Direct seeding got the most priority.
They experimented with four forest restoration techniques and direct seeding showed the good result for rehabilitation mine site in Brazil. Factors effecting seed germination when applied by direct seeding: Seed germination is the activation of the metabolic machinery of the embryo, leading to the emergence of a new seedling ( Poulsen and Stubsgaard, 1995). Seeds of some species do not germinate due to hard seeed coats hindering intake of water ( Baskin & Baskin, 1998). Seed treatments can be applied to break dormancy and improve the seed coat permeability. For germination to be initiated three conditions must be fulfilled: first seeds must be viable that the embryo must be alive and capable of germination. Second, the seed must be subjected to a proper environmental conditions, available water, proper temperatures, a supply of oxygen and sometimes light. Third dormancy must be overcome. ( Poulsen and Stubsgaard, 1995).However seed treatments can be applied to break dormancy and improve seed coat permeability such as scarification, soaking in water or boiling in hot water or hot sand.( Hardwick & Elliott,1992).
Problems with weeds:
Studies conducted by Sun, Dickson and Bragg ( 1995) in Australia, found that the survival and early growth of Alphitonia petriei through direct seeding are largely affected by weed competition and site conditions. The death of germinated seedlings, a few weeks after germination in the plots where weeds remained undisturbed through out the experiment, was mot likely due to the effects of competition for light from the existing weeds. Results from both the glasshouse and field experiments also indicate that weed compition severely limited the early growth of Alphitonia. Frequent weeding is necessary during the early growth phase.(Engel & Parrotta 2001).
Problems with predation:
Seed predation can be severe and looses up to 100% have been reported. (Crawley, 1992). Seed predation has identified as one of the biotic barriers to natural forest regeneration in the abandoned Amazon pasture derived from rainforest ( Nepstad et al., 1991). Garwood (1989) pointed out that seed destructed by animals also play vital roles in reducing seed germination. However Crouch And Radwan (1975) recommending seed coating and pelleting against seed predators. But little is known about this technique in tropical situation. The study of Woods and Elliott (2004) was designed based on the premise that scarifying seeds before sowing them in the fields cleared of weeds would shorten seed dormancy to decrease the time available for seed predation to occur and that burial conceals seeds from potential predator. They found that ants were the only predator observed. Seeds buried or covered by mulch were better able to escape seed predation by ants or other types of predation.
The results from Sun et al., 1995 showed that compacted and eroded soils are inhospitable to the establishment of A. petriei seedlings. So,at this point site preparation is necessary to overcome this problem.
Points to achieve succession of using direct seeding:
Sun et al.1995 conclude that weed control and maintenance of nutrient levels are essential for success. Engel and Parrotta (2001) suggested that what is required is a more systematic screening of potential species and their response to direct seeding under field conditions and evaluation of coast associated with plantation establishment and aftercare. Seed quality is also other concern, using fresh seeds collected from several parent trees growing on sites where soil and climatic conditions are similar to those being reforested and planting should be carried on high rainfall. Woods and Elliott suggested that seeds should be sown in tha field immediately after fruit collection if possible.
In order to minimize seed storage, species selected for dirct seeding should fruit at the beginning of the rainy season when direct seeding is most likely to be successful. Areas where direct Seeding could get an importence: Many tropical species have recalcitrant seeds, e.g. most trees in the dipterocarp family. This means that the seeds deteriorate rapidly after collection and can only be stored for a short period. If transport and temporary storage can be reduced to a minimum by processing the seedlot at the collection site before direct seeding the seeds at the planned site, the whole operation may be done in fewer days compared to a solution with a central processing centre and a nursery. This solution should result in a seedlot with better germination and possibly a higher overall percentage of established plants.
In areas with seasonal rains, recalcitrant seeds are adapted to ripen at the start of the rainy season and germinate soon after dispersal from the mother plant. Time for direct seeding will thus naturally be at the beginning of the wet season, just after collection of the seeds, where the rains will provide a good start for the seedlings. In contrast, nursery stock has to be planted out a year after seed collection at the onset of the next rainy season or alternatively at a time where drought can hinder the growth and survival of the seedlings. Many species will reach a fair size after a year in the nursery, resulting in more costly transport and planting of nursery stock compared to direct seeding. In Southeast Asia large areas of former rain forest lands have been turned into imperata (Imperata cylindrica) grassland, due to failure of reforestation. In Indonesia alone the latest estimate of Imperata grasslands is 8,6 million ha (Garrity et al. 1997).
In these areas it is difficult for trees to establish naturally due to annual fires. Direct seeding in connection with constructing fire-belts could perhaps offer an alternative method to reforest these areas. In a trial in Jamaica a variety of trees was successfully established on grassland (Roshetko et al. 1991, 1991). It was concluded that trees can be established by direct seeding even with heavy grass competition, providing that monthly weeding is carried out until trees reach a competitive size. In a Nigerian trial Gliricidia sepium was both planted and direct seeded in Panicum maximum pastures after mechanical seedbed preparation (Cobbina 1994).
Although planting showed better results, direct seeding could still offer a worthwhile alternative if expenses are taken into account. In order to compete with the imperata grass it would be sensible to start the reforestation with fast growing exotic tree species for example Acacia spp., Leucaena, Cajanus cajan or Sesbania spp. In order to obtain more valuable forest products planting or direct seeding of native primary forest species such as dipterocarps could be carried out at a later stage when the grasses have been outcompeted. Otsamo (2000) reported that native trees can be establish in the understorey of fast growing plantation species; he found 63 tree species in plantations on Borneo, which was established on former Imperata grasslands.
Costs and benefits:
Angel and Parrotta (2001) reported that the establishment and maintenance cost of direct seeding of five species in Brazil during the 2 years study period averaged 747-912 US $ per hectare. This cost compare favorably with those for plantation establishment and maintenance at this site using nursery grown seedlings of native tree species in other plantation treatments include in same project , which averaged1200- 1500 US $.(Unpublished data).Therefore direct seeding in this project can reduce about 63% compared with the plantation using nursery grown seedlings.
Appelroth, S.E. (1975). Work study aspects of planting and direct seeding in
forestry. In Symposium Stand Establishment; Proceedings, p. 202-269. Engel, V. L. and J. Parrotta, 2001. An evaluation of direct seeding for reforestation of degraded lands in central Sao Paulo state, Brazil. Forest Ecology an Management 152:169-181.
Garwood, N.C.1989. Tropical soil seed banks. In: M.A. Leck, V.T. Parker and R.L. Simpson (Eds.). Ecology of soil seed banks. Academic press Newyork, 149-210. Goosem, S. and N.I.I. Tucker.1995. Repairing the rainforest. Theory and practice of rainforest re establishment. Wet tropics management authority. Cairns, Queensland, Australia.72 pp. Hardwick K (1999) Tree colonization of abandoned agricultural clearings in seasonal tropical montane forest in nothern Thailand. In, vol. Doctor of Philosophy. University of Wales, Bangor,p 168. Jansen, C.L. and S. Pfeifer.1989. Assisted natural regeneration: a new reforestation approach. USAID report prepared for the development of environment and natural resources. Government of the Philippines 22pp.
Negreros CP, Hall RB (1996) First-year results of partial overstory removal and direct seeding of mahogany (Swietenia macrophylla) in Quintana Roo, Mexico. Journal of Sustainable Forestry 3:65-76 Negreros, C. P. and R. B. Hall, 1996. First-year results of partial overstory removal and direct seeding of mahogany (Swietenia macrophylla) in Quintana Roo, Mexico. Journal of Sustainable Forestry 3:65-76 Oschner, P. 2001. Direct seeding in the tropics. IUFRO Joint symposium on tree seed technology. Physiology and tropical Silviculture, College of forestry & Natural Resources, University of the Philippines, Los banos. Parrotta JA, Knowles OH (2001) Restoring tropical forests on lands mined for bauxite: Examples from the Brazilian Amazon. Ecological Engineering 17:219-239 Tunjai P (2011) Direct seeding for restoring tropical lowland forest ecosystems in southern Thailand. In, vol. Doctor of Philosophy. Walailak University, Nakhon Si Thammarat, p 192 Woods K, Elliott S (2004) Direct seeding for forest restoration on abandoned agricultural land in northern Thailand. Whitmore, T. C. 1997. Tropical rainforest disturbance disapperance and species loss.In: W.F. Laurance And R.O. bierregaard Jr. (eds.0 . Tropical forest remmnants:Ecology management and Conservation of fragmant
communities.(pp.3-12). Journal of Tropical Forest Science 16:248-259.
Xinhua, C. and Z. Jingchun. 1988. Sowing from the air: China’s experience. Tigerpaper vol. XV: No. 1. FAO forest news.
Zangkum, S. 1998. Growing tree seedlings to restore forests: effects of container type and media on seedling growth and morphology. MSc thesis, Chiang Mai Univerity, Chiang Mai.
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 23 December 2016
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