Fig. 2.1.
Annual production of wood and leaf biomass of natural forests across a
latitudinal gradient from the Boreal Zone to the Tropics.
Eucalyptus species constitute about 38% of all short-rotation plantations worldwide and hardwoods in general make up about 63% of all plantations. In temperate regions, poplar, willow, and black locust predominate. Ranney estimates that about 10 million ha could be classified as short-rotation plantations. 23 However, he notes that less than half of this planted area could be termed as successful or commercially viable. Ranney's technical criteria for successful plantations for energy use are described as follows (with slight modification):
This chapter discusses the factors to consider in site and species selection, plantation establishment, maintenance and protection as well as yield expectations to achieve the above measures of success. In addition, some of the distinctive aspects of producing trees on short-rotations versus practices used for producing timber in longer rotations will be addressed. More detailed technical information suitable for landowners or companies considering woody crop establishment may be located in some of the references provided in the notes to this chapter.
It is often assumed that tropical countries have a major advantage over temperate regions with regard to biomass production, but this is not necessarily true. While total annual biomass production does increase from higher latitudes towards the equator, the increase is primarily in leaf production rather than wood production
Fig. 2.1.
Annual production of wood and leaf biomass of natural forests across a
latitudinal gradient from the Boreal Zone to the Tropics.
In general, the annual wood production of native tropical forests is slightly less than that of high latitude native forests. Reasons for the lower production rates of lowland tropical forests include acidic, highly leached soils and warm temperatures that increase respiration rates and burn off carbon that would otherwise be used to produce wood. Fortunately, well-managed plantations on good sites in tropical and temperate zones can achieve yields 2 to 10 times higher than natural forests. Examples of plantation yield levels currently being achieved around the world are summarized in Table 1.
Table 2.1. Plantation biomass annual production rates from around the world
| Species | High Yield | Average Yield | Production Region | References26 |
| dry tonnes/ha | ||||
| Poplar | 43 | 9-20 | Eastern US & Pacific Northwest | Wright, 1994 |
| Eucalyptus | 30 21 |
13-15 5-8 12.5 |
NW Spain SW Spain Spain |
San Miguel, 1988 |
| Eucalyptus | 9-26a | Mid Brazil | Hall et al, 1993 | |
| Eucalyptus | 27 | 13-27 | Hawaii | Whitesell et al, 1992 |
| Willow | 24 | 13-24b | Northeast US | White, 1995 |
| Willow | 14 | 8-14 | Sweden | Willebrand et.al 1993 |
| Willow | 23 | 13-23 | Sweden | Christinson, 1987 |
| Eucalyptus | 28 | 3-21 | NE Brazil | Carpentieri et al 1993 |
At a global scale, soil fertility generally decreases toward the equator.
At a regional scale there is variation in soil properties relevant to tree
growth at scales ranging from 10's of meters to entire continents. Any farmer
or forester is familiar with variations in soil fertility that produce great
differences in crop production across areas as small as a hectare. Factors such
as soil depth, water availability, pH, texture and slope are all extremely
important to crop production potential. While naturally fertile soils are most
desirable, there are instances where poorer soils can be managed.
One of the first challenges for any commercial activity requiring short-rotation plantations is to determine where suitable and available lands are located. A favorable site may allow a project to survive initial mistakes or miscalculations, while an unfavorable site requires great technical expertise, and even simple errors can result in major setbacks or failure. In site selection, there is no substitute for test plots of the potential tree species on the range of soils present on the prospective sites. A potentially disastrous consequence of over-estimation of yield is the under-estimation of planted area needed to support a power plant. With the increased plantation area needed on unfavorable, low productivity sites comes increasing expenses for road and harvesting infrastructure, increasing haul distances, less efficient harvesting, and greater potential impacts on the environment, society, and biodiversity.
Site selection must consider and balance a wide range of biological,
economic, and societal factors. The biomass decision tree shown in Fig. 2.2
summarizes the information required and decisions that need to be made to
determine whether a biomass plantation may be feasible. Site selection and
planning at the national, regional, and local level requires geographically
located information on soils and geology, natural vegetation, current land
uses, topography, watershed boundaries, stream/river systems, roads, local
political jurisdictions, land ownership and tenure information, location of
cultural and historical resources, location of nature preserves and rare
habitats and species. It is also very valuable to have site-specific research
data on the yields that can be expected from the preferred species. The
information is particularly helpful if it can be conveniently summarized,
compared, and presented on maps using computer-based geographical information
systems (GIS). The HNRIS database developed for the state of Hawaii is an
excellent example of the type of data and GIS systems that is very valuable for
making decisions regarding the best locations for biomass plantations.
Fig. 2.2. Biomass project decision tree.
Once a site is chosen, the next key to the success of the plantation is the selection of genetically superior tree species, varieties or clones suitable for the climate, soils, and desired products of the plantation. Most species achieve their best growth under a fairly narrow range of climate and soil conditions, and failure to match species to site has been the cause of the failure of many plantation projects. Even within a species that is suited to a particular site, there can be great variation in the growth and wood characteristics from different seed sources or regions within the species range." The largest, cheapest, and fastest gains in most tree improvement programs can be made by assuring the use of the proper species and seed sources within species."29 Many of the problems of plantation health, productivity, tree quality, and the economic viability of early plantation programs were the result of use of inappropriate species or clones and suboptimal plant materials. For example, Jari Cellulose S/A of Brazil is currently replacing all of the pines and gmelina planted on over 90,000 ha due to the poor growth.
Although indigenous species would be preferable from an environmental perspective, a large number of the plantations being successfully established in tropical regions are using exotics such as E. grandis and P. caribaea. Several acacia species, which are native to many tropical areas are showing good performance. In the U.S. and Europe, hybrid poplars and willows are, in most cases, outperforming native poplar species, although fast-growing native poplar clones have also been identified. The best hardwood species and clones for lumber and veneer are likely to be different than those that are best for energy or hardwood pulp.
In some temperate agricultural regions, herbaceous energy crops are being
investigated. One advantage of such crops is that they can be planted and
harvested using standard agricultural equipment rather than specialized
forestry machinery. Perennial herbaceous crops, such as certain native prairie
grasses, have the advantages of providing quick erosion control and wildlife
habitat, quickly reaching full production, and providing continuous harvests
without replanting. The best tropical analogy is sugar cane, although other
tropical grasses or canes might have potential as well.
Normal variation in topography, soils, and moisture conditions dictates that more than one species may be desirable to optimize sustainable production. While it may be desirable to use both grasses and trees as feedstock options from an environmental standpoint. Use of mixed tree species plantings would also provide more desirable habitat for wildlife than single species plantings. Where the conversion technology or product depends on feedstock uniformity, site differences can be partially compensated for by using many clones of the preferred species. Pulp companies such as Aracruz Celulose S/A in Brazil have developed a working set of 40 to 80 clones for taking advantage of small site differences, and attaining functional diversity for pest management and risk spreading
Nitrogen-fixing species, or fast-growing nurse species could be beneficial components of the overall species mix. Such species are likely to be particularly important where plantations are being established on degraded or abandoned agricultural lands though plantings would likely need to be subsidized since returns to the land owner may be limited due to low yields. In such cases, the initial species mix used to reclaim the site is likely to be different than the preferred species mix once the site has been improved by the initial rotations. Conversion technologies which can adjust to changing species composition over time will offer the greatest opportunity for producing environmentally sound, low-cost feedstocks.
Species and clonal selections cannot be made exclusively from a study of literature. Unless a research program exists in the vicinity of a desired project, some trials of promising species will have to be part of the project development process. As described by Evans, these trials will normally be of four different types:31
This sequence takes time and often decisions are made after only step 1. Steps 2 and 3 may be carried out while a project is being established and result in changes along the way. Step 4 is an ongoing commitment that is required once significant investment in a project has been made.
Plantation establishment is a critical phase of a large-scale biomass energy project, since the timing of wood supply must be coordinated with the construction of a complex and expensive power plant. Mistakes or miscalculations at any one of many stages can result in expensive delays in power generation. Issues that must be addressed include: site preparation, seedling or cutting supply and quality, availability of required soil microorganisms, nursery operation, spacing of plantings, fertilization, watering, weed control, road construction, protection of nursery stock and plantings from insect pests, disease, animals, humans, fire, and other threats.
The particular properties of each plantation site and the species selected will affect how each of the above issues should be addressed.
Site preparation is the key phase of establishing a short-rotation woody crop plantation. Site preparation on land not already used for cropping includes demarcation of boundaries, planning and construction of access roads, and installation of erosion control measures. While alteration of drainage to improve conditions for tree growth and minimize negative effects on the quality of water leaving the plantation may be needed, care should be taken that wetlands and or water quality regulations are properly addressed. The primary goal of site preparation is to create the best possible conditions for the growth of the tree species that will be planted. Site preparation requirements vary greatly among tree species and with variation in climate and soils. Most current recommendations suggest using land already cleared or planted in tree crops both for economic and environemental reasons. The most suitable sites for biomass plantations and the least expensive to prepare are lands recently or currently being used for agriculture. On many agricultural sites, ripping may be necessary to break-up hard pans that have devoloped over many years.
On recently abandoned agricultural lands, the main problem is often competition between planted seedlings or cuttings and pre-existing vegetation. Consequently, the goal of site preparation is to kill or remove as much as possible of the vegetation without degrading the site and creating erosion. For species that are extremely sensitive to competition from grasses, including most fast-growing eucalypts, poplars and willows, complete kill of the competing vegetation is essential. Figure 2.3 indicates the strategies that may be required depending on existing vegetation.
Fig. 2.3. Steps in a recommended site preparation strategy.32
Both herbicides and cultivation may be required if mowing is not sufficiant. This is particularly important in areas that have been invaded by aggressive grasses, such as Imperata cylindrica. Other species, including Acacia mangium, alder, and many pines, are tolerant of some competition from grass, and do not require such intensive site preparation, but also usually fail to produce high yields at a young age.
Experience in the U.S. suggests that site preparation should begin at least one full year in advance of planting short-rotation crops, if cultivation is required. Soil cultivation aids in reducing weeds, but also can be used to improve soil structure for root penetration in areas with compacted soil, impervious clay layers, or poor drainage. No-till site preparation may be appropriate if herbicides can be effectively used to achieve total grass and weed kill in strips where trees are to be planted. In some situations, clearing and burning may be appropriate, although this is usually ineffective for grasses because they have adapted to fire. Inadequate site preparation is the most common mistake made by companies and researchers initiating a new short-rotation woody crop project.
On severely degraded sites, competition with grass and weeds is less an issue than is the nutrient content and available moisture in the soil. Appropriate establishment methods may include leaving weeds, planting grasses in strips, applying organic matter, establishing a cover crop, and/or building berms or other structures to concentrate water.
Many first time plantings fail because high quality plant materials are scarce and poor quality planting materials are used. The materials may be too small, too big, diseased or too weak. Guidelines for optimal seedling and cutting size and handling requirements are available for the most commonly planted species. A common problem is that insufficient quantity of select material is available so the tendency is to use the poor quality material just to meet planting goals. Obtaining rapid growth early is so important for short-rotation systems that time and money should not be wasted on planting poor quality or unhealthy material.
Large centralized plant propagation and nursery operations that are owned by the power companies (or their subsidiaries) may be required for large-scale biomass power projects. Nursery operations are a standard component of most pulp and paper operations which depend on plantation grown wood. For smaller power project operations, it may be easier and more cost-effective to purchase seedlings or cuttings from commercial nurseries.
If plants are to be established from seed, the issues of seed availability, seed quality, seed storage, and germination requirements must be fully resolved in the project planning stage. The quality of seeds is particularly important, and success may depend on having a good supply not just of the desired species, but of the specific variety or provenance of that species that is best suited for the plantation site. To assure continuing supplies of improved high quality seeds, some investment in seed orchards and long-term breeding programs is necessary.
Most hardwood species receiving serious consideration as biomass plantations species can be propagated without seeds, from cuttings of twigs and branches that can be induced to grow roots, from root sections or branches that can be placed directly into the ground, or from tissue culture and micropropagation methods. A major advantage of vegetative propagation is that the individual trees that have the most desirable properties, disease resistance or rapid growth under specific local site conditions, can be rapidly multiplied as part of a tree improvement or clonal expansion program. Species vary greatly in their suitability for vegetative propagation, with some easily sprouting roots and leaves, and others only surviving under the most favorable conditions. Species that sprout and grow readily include: many Eucalyptus and Populus species, Acacia cyanophylla, Azadiracta indica, Cassia siamea, Chlorophora excelsa, Cordia alliodora, Dalbergia sissoo, Gmelina arborea, Leucaena leucocephala, Nauclea diderichii, Paraserianthes falcataria, Pterocarpus spp., Triplochiton scleroxylon, and Tectona grandis.
Depending on whether the species being planted are most effectively established as bareroot seedlings, containerized stock, or hardwood cuttings, propagation and nursery procedures will be quite different. Larger-seeded species can be sown in large beds and root- pruned to produce bare-root stock for planting. Trees being clonally propagated require stool beds of clearly identified clones for generating cuttings. Sprouts species such as poplars and willows are then harvested from the stool beds, sectioned, graded for size, packed, and kept in cool temperatures until planting season. Eucalypts which are produced from small seeds or small cuttings and species produced through micro-propagation, require well-run seed or propagule handling, containerization, and hardening operations to produce adequate sized planting stock. In general, container grown plants have a higher survival rate, than bareroot seedlings or cuttings, particularly in drier areas. However, container grown plants may cost from 25% to 250% more to produce than bare-root seedlings or hardwood cuttings. In all nursery operations, particular attention must be paid to fertilization and the prevention and control of fungal diseases and insect pests, as well as plant size and root quality/quantity.
Many species may have unique properties and problems at the nursery stage. Consequently, the experience that has been gained around the world with particular species under specific climate conditions should be utilized in nursery design. Local practices that work perfectly well for some species may result in total failure with different species. When Shell International chose to establish a eucalyptus plantations in Chile, local builders failed to follow exact specifications from Shell staff with previous experience in South Africa. This resulted in major problems and loss of much of the planting stock in the first year of operation. Once the buildings were reconstructed according to specifications, propagation was successful.33
A primary question during the planning phase is the spacing (or density). Planting density affects the total cost of planting stock, the rate at which the tree canopy closes, the growth form of the trees, the optimal rotation age and the size at optimal rotation or harvest age. While production of large saw logs is most effective at large spacings or in thinned stands, plantations for biomass or pulp generally use narrower spacings ranging from about 4 x 4 m down to about 1 x 0.5 m.34 The tightest spacings are characteristic of willow plantings, which are cut after 1 year to generate many sprouts, thereafter being harvested on a 2 to 3 year schedule. The widest spacings are more characteristic of plantings where the trees are expected to reach a 15 to 20 cm diameter (d.b.h.) by harvest age.
Within a certain range, maximum short-rotation plantation yields are not particularly sensitive to spacing, if weeds are effectively controlled and the stand is allowed to grow. The spacing does, however, affect weed competition. Tight spacings minimize weed problems by closing canopy within the first year. Wider spacings require weed control for 2 to 3 years, however, weeds can be effectively controlled by cultivation, and herbicides.
Planting techniques will vary depending on the type of material being planted and the terrain. Hand planting is probably the most common approach even in industrialized countries. Hand planting by an experienced crew can be nearly as fast as mechanized planting with fewer mistakes. Unrooted cuttings offer the greatest flexibility in choosing hand or machine planting methods. However, it is suggested that painting or marking tops of unrooted cuttings will ease planting efforts and ensure that the right side is planted up.
Weed competition can be severe in the establishment phase of plantations and most establishment methods involve some use of cultivation and herbicides.35 It is most important to eliminate weeds close to the trees. Cultivation is most effective if the spacing is square and wide enough to allow cultivation. Application of herbicides over the trees, prior to leaf out, may accomplish the control needed close to the trees. Mulching is another alternative. In countries where labor costs are low and chemical costs may be high, manual labor is likely to be used to reduce weed growth immediately around trees. Depending on the relative costs of labor, machinery, and chemicals, the most cost-effective weed control practices can differ greatly from one area to another.
Another weed control alternative is agroforestry in which manual labor is
used to tend food crops grown between rows of trees for one to two years.
Insect and fungal pests tend to be most serious when plants are stressed by poor soils, crowding, or other unfavorable environmental conditions. Pests can spread rapidly and cause extensive damage in single species plantations. The incidence of pest problems and thus the need for toxic chemicals could be reduced by use of multispecies plantings, either intermixed or blocked, by selection of resistant species and clones, and by carefully matching species and clones to site conditions to minimize stress. A combination of biological and chemical controls of pests using the principles of Integrated Pest Management (IPM) is usually much more effective and much less costly than large-scale pesticide application.
Fires are a major risk factor that must be considered in planning the layout of short-rotation woody crops. Separation of planting blocks with strips of natural vegetation may assist in reducing fire hazard as well as increase local biodiversity. The roads needed for harvest access and wood removal should be made wide enough to serve as fire breaks.
Large animals, such as deer and moose, can create problems by browsing the
tender shoots of young plants. While solar-powered electric fences could be
used for keeping out large animals, this would normally be too expensive for
large-scale operations. It may simply be necessary to accept some level of
damage and claim credit for game populations in the region. Hunting is
considered to be the most effective means of protecting plantations from
excessively large populations of browsing animals.
Rabbits and rodents can damage or kill trees by gnawing the bark and girdling the base of the trees. There is little protection for such animals in well managed plantations with low weed cover, thus providing another rationale for good weed control. There are no other suitable methods for protecting plantations against small rodents.40
The initial planting strategy should include a plan for stand renewal after harvest. Most short-rotation species being planted or under evaluation offer the option of stand renewal through resprouting or coppice growth. Although it was originally conceived that plantations would use the natural coppicing ability of hardwood trees, researchers growing poplar in the U.S. and eucalypts in Brazil are now suggesting that planting new and higher-yielding clones after each harvest will increase production and more than offset the increased costs associated with stump removal and site reestablishment. Continuous introduction of new clones also reduces risks associated with the buildup of pests and diseases.
As stated at the beginning of this section, the requirements for successful production of short-rotation woody crops are rigorous. Unless experienced personnel are available, a new project should include some learning time with smaller plantings before an aggressive planting schedule is initiated. The crop production techniques will likely vary at least slightly for every project and experimentation needs to be a part of any commercial activity. A new project must take into account harvest and handling, environmental, end-use and cost considerations, which are dicussed in the following chapters.
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