Direct sowing versus planting Experience with tropical afforestation and land rehabilitation, pros. and cons. Danish and Swedish field trials. Results until now.
Why are trees usually planted and not sown? Competitive advantage over weeds Raised before the rain season, prolongs the growth season To make land use more efficient Earlier use of thinning material Taungya system Natural rejuvenation Plant distribution
Biological aspects Benefits of direct sowing Natural root development 100 times larger root volume than in container plants (H:40 cm.) Potentially lesser root deformation Optimal root-soil contact Undisturbed taproot, anchoring, wind tolerance If tap root diameter is halved, the anchoring power is reduced to 0,5^4 = 0.0625 No transplanting stress No root pruning, some species are very sensitive to this Seedlings that originally have been directly sown may compensate for their less advanced development stage by the absence of planting- and field stress!
Benefits of direct sowing.. Reduced transport and labour costs No propagation No seed bed preparation No transplanting* No nursery costs! No plant transport to the field (no drying up of roots) No root pruning* No transportation (distance to the field and plant size) The justification of planting is the suspected higher survival rate. But that s only suspected, plantings can fail to!
Benefits of direct sowing.. Stand density The cost of the sowing operation is little affected by seed density. If seeds are available If the price is reasonable If the quality is good and of the right provenance The potential density for seedlings (after sowing) will be much higher than for plantations! This can lead to: - Higher chance to escape from pests and predation - Faster land cover and thus ousting of grass and weed - Chances of conducting a phenotypic thinning (important if cheap seeds)
Benefits of direct sowing.. It is easy to sow species mix Main tree species with pioneer tree species Or several main tree species to compensate for micro edaphic variations Multi species forest restoration If the primary aim is to establish woody vegetation cover: Wood quality is not the primary goal Genetic quality secondary then we sow a wide species diversity, that will benefit both the physical and the biological environment!
Limitations of direct sowing Mortality of tree seedlings normally declines with size and age. Germinants and very small seedlings are fare more vulnerable to: Shading (trees or weeds) Desiccation Anoxia (water logging) Washing away Predation Grazing Mechanical damage Fire
Biological Limitations of direct sowing.. Pre-germination predation (if not covered by soil) Vulnerable to desiccation after radicle penetration Vulnerable to excess moisture, water logging leads to anoxia Patchy mortality, not evenly distributed! Germination failure and early seedling mortality can be high Success rates can vary from 1-70 Seed size is correlated to the accepted establishing rate because of the number of seeds sown. Quercus: Relatively few seeds sown. Minimum 25-30% Birch: many seeds sown. Ok with 1-2 % Ensure to break seed dormancy!
Limitations of direct sowing Economic drawbacks Poor survival can undermine saved initial costs Labour cost for weeding can be high High density stands must be thinned! Seeds with high procurement costs are less attractive for sowing (rare, difficult to collect etc.) It is tempting to use cheap seeds, but improved seed orchard seeds have better vitality and genetic growth potential Use at least a mix of cheap and seed orchard seeds
Direct sowing versus planting + Initial costs Sowing - Success rate Planting + - Survival chance Nursery costs Root system - homogeneous Predation risk Initial costs Species mix Weed competition + homogeneous Root system Remote areas Mechanical weed control Risk of drying up before planted Plant density
Initial costs (from Denmark (1000 /ha) The present situation 8 Planting: Hardwood 6 Planting: Coniferous 4 2 Sowing: Existing methods 10 50 80 90 100 Success rate, %
Experiences of direct sowing Aerial sowing In large inaccessible areas China since 1950, 10 mill. ha. of hillsides India, eroded ravines Vietnam, hillsides Used species: Pines, Hippophae rhamnoides, Schima and Acacia species The terrain is mountainous and often grass covered It is necessary with controlled burnings before sowing Seeds are sown during the cool dry season Seed mixtures have been used sometimes with climax species under fast growing pioneers
Experiences of direct sowing.. Mine spoil rehabilitation, often done by drilling as soil preparation Used in Australia mainly with small seeded species (Myrtaceae, casuarinas and acacias) The areas are mostly devoid of any woody vegetation Mine spoils often have coarse grained structure, low on humus and nutrients Prone to erosion Toxic metal residues are left No microbial activity The trees shall stabilize the soil!
Experiences of direct sowing.. Small legume agroforestry species Legume woody species primarily used for soil improvement, food and fodder Direct sowing is applicable because the chosen species are fast growing and weeding is integrated in crop cultivation Fallow improvement in Thailand Tephrosia and sesbania species etc.
Rehabilitation of tropical grassland Deforestation has resulted in problems with erosion and watershed The areas often appear as grassland or shrub land The grass is a serious competitor but as the trees grow they can shade out the grass If fire can be avoided, broadleaved pioneer trees should be established cause they are more attractive to animal dispersers and makes a shortcut to a species rich forest. Relevant in some regions of Thailand.
An example from northern Thailand Taken from an article written by Kevin Woods Background Since 1950 Thailand lost nearly 60% of the forest area Some important effects: Lower Biodiversity Water retention problems Erosion problems Cheap forest rehabilitation could be done through means of direct sowing.
The field trial 4 treatments of the seeds Species 1. Scarification Sapindus 2. Burial Lithocarpus 3. Scarification and burial Spondias 4. Application of mulch Erythrina There were tested for 2 Parameters: 1. Germination % 2. MLD (median length of dormancy) And furthermore: Effect of taken action against predation (cages)
Results: Scarification: No significant effect Burial: Significant positive effect Burial and scarification:significant positive effect Mulching: Depends on the specie Protection against predation: No! But burial protected the seeds better With higher germination % and lower MLD, the predation period gets shorter.
General considerations: we need to develop better establishment techniques and cover seeds Seed viability should be higher! Collection time Storage problems Labour demanding if storage is nescessary We need more investigation about fungicides and repellants to ants and rodents
Experiences of direct sowing.. Forest restoration in Brazil on former rain forest areas Field trials on all degradation levels Best establishment on barren soil Survival was best correlated with seed size. Dry zone planting in Niger, 225mm. Annual rainfall Water is a scarce resource skip nursery work Direct sowing! Seeds are sown in holes just before the rainy season
Mangrove rehabilitation - No weed competition and desiccation stress - Seeds are sometimes washed away by the tide
Areas suitable for direct sowing The seeds must germinate and establish fast in situ and in competition with other plants Poor soil is a stress factor but also for the weeds Flat areas (like farmland) are perfect for direct sowing Easy to access Easy to do the soil preparation, sowing and weeding Can be mechanized to a large extent
Optimizing survival by direct sowing Optimizing survival by direct sowing Land and soil preparation Mineral soil and eventually weed removal Timing Sow when moisture is plentiful Seed technology Pretreatment: insecticides, pesticides, nutrients, moisture etc Break dormancy: hot water, scarification, acid etc. Reducing seed predation Seed species, knowledge about the specific locality etc. Covering seeds Sowing techniques From plane, by hand, mechanized and micro site sowing Field maintenance Weeding, mechanical versus impossible.
A few claims about sowing and foresters Sowing requires patience and smaller ambitions! Patience: It can take years before you see the result. What look unsuccessful the first 1-2 years can end up in a dense stand later on. Nice rows are not typically seen in sown stands Sowing requires a real expert species, local knowledge, soil factors, predation etc. Sowing requires that the forester is willing to take a risk because trial results implies difficulties Sowing can fail, but without dealing with the stress factors (controlling or eliminating), neither planting nor sowing will succeed!
Sowing is favored in the future if: Labour costs increases (nursery and planting) Development of better field equipment is done Improved weed control techniques are found The shift towards environmental plantings and land rehabilitations with multiple species More results from field trials
Initial costs (1000 /ha) The future of direct sowing 8 Planting: Hardwood 6 Planting: Coniferous 4 2 Sowing: Existing methods Sowing: Future methods 10 50 80 90 100 Success rate, %
A few examples from Denmark and Sweden Status in Denmark In 2000, Forest and Landscape made a commission, with the goal to develop methods that could reduce initial costs of forest plantations with minimum 20 %. The quality of the plantations should be of equal quality as before the cost reduction The commission concentrated on direct seeding and small container plants They do tests including nearly all Danish tree species on all soil types and with different soil treatment.
Results until know: Direct sowing on old farmland can be very successful Trolleholm in Sweden have used direct sowing of oak 170 ha. Since 1992 with success! Use of agricultural machines Big areas Loose soil free of weeds Treated seeds of good quality Fencing
St. Hjoellund plantage, plants that attract deer!! Primary goal: Hunting or traditional forestry? Depends on the earnings from the two alternatives! But are they necessarily alternatives? Can we combine the two? 1.No fencing 2.The main tree species were mixed with species that attracts deer 3.Sowing of more seeds No final results yet, but I looks promising!
The main problem in Denmark: Mice