Mechanization of Short-rotation, Intensive-Culture Wood Crops

William B. Stuart, Professor, Industrial Forestry Operations, Department of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia

ABSTRACT

Three impediments to harvester development--cultural, operational, and economic--that have plagued the development of short-rotation harvesting equipment are discussed. Strategies for the future include concentrating on cropping strategies that (a) result in material that can be harvested by heavy-duty agricultural equipment (<3 mm in diameter, <10 cm tall), (b) result in material that can be handled by conventional forestry equipment (>10 mm DBH, >20 m tall), or defining a small subset of options between these limits and develop purpose-built equipment to suit. Demand/supply and cost/price relationships have to be more stringently defined before short- or long-line equipment manufacturers will enter this market.

INTRODUCTION

The need for harvesting equipment tailored to short-rotation, intensive-culture (SRIC) plantations of woody crops has been an issue and concern since the initiation of the program. The development has been slow, faltering, and expensive for a variety of reasons. Many of the historical impediments are still in place and frustrating current development efforts.

These impediments or barriers may be broken into three broad categories: cultural, operational, and economic. The associated controlling forces are the grower, the consumer, and the market place.

CULTURAL IMPEDIMENTS

An advantage of short-rotation, intensive culture is the plethora of permutations, species planting density, cultural intensity, and rotation ages that are available. While this offers considerable opportunity for finding the best or "ultimate" fit for a particular location or scenario, harvesting is reduced to shooting at a moving target, or encounter the nightmare of having to develop a different harvesting methodology for each of several broad categories of permutations.

Species

The species to be harvested is among the most critical concerns, for this defines six critical severance and materials handling parameters.

Stem form, whether deliquescent or excurrent, has a strong impact on the design of capture and handling mechanisms. When rotations are very short, deliquescent forms tend toward a situation where each plant is unique. Severance components must deal with branching or forking in the zone of severance as well as single, relatively clean stems. Capture after severance and handling demand a method that both holds the stem or stems and forces it into a standard shape or volume for subsequent processing. The severance problem becomes less severe as rotation age lengthens and the plant takes on a single-stem form, self-prunes at the base, and the main stem can be used to pull or force the top onto processing equipment.

Excurrent forms generally have a more predictable growth habit, a greater tendency toward single-stem development and regular branching habits. The problem of fitting the plant into a standard form or volume persists for very short rotations.

These branches tend to fold upward along the stem with less breakage and are easier to compress for processing or storage. Branch angles of 45 to 90 degrees require more energy and effort to fold, are more likely to break, and are more likely to resist compaction. Downward sloping branches are the most difficult to work with. Folded downward, the path of least resistance, they tend to interfere with severance devices on small trees and resist handling by the butt in larger stems. These limbs are more likely to break in linear feed or skidding operations as well as require more energy and do more damage when dragged, as in skidding.

Branch angle adds a further complication. Species with branch angles of 45 degrees or less from the vertical require less hardware and energy in handling than those with branch angles more nearly perpendicular to the stem. Branch size, both length and diameter, varies with species and within species. Small-diameter, flexible limbs are the easiest to handle, providing they can be kept from tangling around shafts and other rotating machine components. Branching

habit and rotation age combinations which allow branches from stems in adjoining rows to become entangled are especially troublesome; the harvest becomes one continuous tug of war.

Self-pruning, especially, near the point of severance, is especially desirable. Living or dead branches in this portion of the stem interfere with severance devices; become entangled in feed rolls, belts, or chains of continuous travel machines; and tend to harbor large amounts of grasses, vines, and forbs that interfere with machine operation.

Specific gravity determines the "energy density" or "fiber density" of the harvested material leaving the tract. Specific gravity among candidate species ranges from 0.30 to 0.355 for hybrid poplars and willows to 0.65 for black locust. Packing densities (volume of material per unit volume of occupied space) is roughly the same for both species, whether in roundwood or chip form. Species of higher specific gravity deliver twice as much product for each activity carried out.

Differences in coppice or sprouting habit also impact on harvester design. If the species chosen only has stump sprouts or has stump sprouts that quickly crowd out root sprouts, the row characteristic of the plantation is maintained and machine design held constant between harvests. Root sprouting, however, transforms the planting from what is referred to as a "row crop" in agriculture to a "field crop" after a few harvests, and harvesters much be changed accordingly.

Cultural intensity

Cultivation and irrigation can affect harvesting machine and system design, cultivation by loosening the soil between rows to the point that machine trafficability is reduced or by mounding soil around or near the stem or stool such that it interferes with severance. Irrigation lines must be removed prior to harvest and special care exercised to assure that no pipe, posts, or stakes are left on site, adding costs that are too often identified as harvesting, rather than cultural, in accounting systems.

Demands for special harvest treatments increase with the level of cultural intensity. Restricting harvesting to the dormant season to maximize coppice production and minimize nutrient losses is a fairly common requirement, and one that can be accommodated, providing weather during the dormant season is not so wet that access to the sites is limited, nor so cold that stools shatter during severance, nor snow so deep as to interfere with operations. Sufficient volume must be available during the operable periods of each year to amortize the investment in harvesting equipment. Seasonal operations also demand equipment that requires moderate to low operating skills.

The development costs to prepare a tract for intensive culture requires that maximum area be kept in production: "idle" areas in roads, landings, and on-site storage areas are minimized, further restricting operational flexibility.

Reproduction process

The reproduction method to be used is one of the major determinants of harvesting machine and system design. Reproduction from seed cuttings or seedlings at the start of each rotation allows the greatest flexibility. Each harvest is a new start; site preparation can remediate soil compaction or disturbance during the harvest. Tree size is likely to be consistent from one harvest to the next. Damage to the stump or stool is not a concern. Stem spacing can be maintained or adjusted with each planting and there is less tendency to leave irrigation systems or other cultural appliances in place between rotations, and harvests do not need to be tied to the dormant season.

Coppice regeneration reduces harvesting flexibility. The extent of the reduction is a function of the species being used and rotation length. Damage to the stool from the severance method used or as a result of machine movement across the site should be kept to a minimum. Stool size and character changes between coppice rotations and stem size decrease with each rotation. Machine traffic is usually limited to movement along, not across, rows, and dropping material on the site and then picking it up with grapples or forks increases the risk of stool damage. Machine designs that capture the material and carry it to row end are favored. Soil compaction and soil disturbance that may damage roots is to be minimized. Harvesting is generally restricted to the dormant season to maximize coppice vigor. Stem spacing and organization are fixed until the stools are removed and the stand re-established.

Coppice regeneration of species that root sprout readily, such as some Poplus species and Robinia, is most problematic, for the nature of the stand changes dramatically with coppice rotations. Root sprouts tend to spread between rows, masking any row organization of the stand. Root sprouting species are almost universally shallow-rooted, and many have a strong tendency for root grafting. Shallow rooting increases the probability of infection by disease fungi immediately after harvest; root grafts facilitate the movement of rot and disease organisms from one stool to the next.

Rotation length

The active question is, "how short is short?" Rotations which result in stems being harvested when they are between 2 and 3 cm at severance height and less than 4 to5 m tall can usually be harvested by heavy-duty agricultural (corn and sugar cane) equipment. Material over 15 cm on the stump can usually be harvested with conventional forestry equipment--feller-bunchers, skidders, and processors.

Stems between these bounds (>3, <15) have historically been the domain of land-clearing and right-of- way maintenance contractors. The equipment available (bush hogs, stirrup flails, chain flails) has been designed to destroy or reduce the material and leave it on site as mulch, with no consideration of capturing it as a marketable product. Survivability of stools and minimization of coppicing damage have not been design considerations. In fact, having few stems survive the bush hog, flail, or other treatment has been considered the mark of a good job.

Stems in the 3 to 15 cm DBH range do not fit easily with either area or single-stem acquisition technology. Area-based acquisition and severance equipment such as combine headers work best with a uniform distribution of material and a relatively homogeneous plant size. Single-stem technologies, which can be generalized to single-stool technologies, require a plant spacing open enough to get the implement on the stem (roughly 6,000 stems or less per ha) and stem or stool form that facilitates multi-stem accumulation.

These concerns are compounded when coppice reproduction is expected for several harvest cycles. Repetitive coppicing results in stool enlargement and a tendency toward an increased number of proportionally smaller stems. There is a point at which the change in material after repetitive coppicing is great enough to require a change in harvester or harvester design.

OPERATIONAL CONSTRAINTS

Transport and transport modes

Agricultural transportation modes parallel agricultural harvesting techniques for material less than 5 cm DBH. The three most common systems are: (1) reduce the material to chip or particle form in the field; (2) transport by off-road truck or wagon to site or bin storage; (3a) compact the material into a large bale or package during the harvest and transport these to a storage area along the edge of the field or concentration yard, or (3b) compact into smaller cubes which are transported to off-site covered storage. A fourth alternative, full-stem transport, is also available.

Chipping in conjunction with harvesting and small-bale strategies usually include transport in conjunction with processing. Harvesting must be restricted to those days that the material can be moved across and from the site with minimal damage to soils and growing stock, implying high-flotation tires or tracks with limited grouser depth. Vehicles which are unsuited to long distances over-the-road transport. Reloading at field edge for long-distance transport adds an additional cost center.

Large-bale technology offers greater economies for long-distance transport if bulk densities can be achieved that maximize legal loads. full-stem transport is to be used in some European operations, where full stems are transported by forwarders to field edge, piled, and covered. The processor (chipper) then moves along these piles, blowing the material directly to road transport equipment.

Material over 10 to 15 cm can be handled by conventional forestry equipment--feller-bunchers, grapple skidders and whole-tree chippers if the site and growing stock can withstand that level of disturbance.

Agricultural systems have evolved in a manner which minimizes grower or producer transport. Cropping strategies have developed that concentrate producers around markets and processors. Grain is moved to a local elevator; every small town in the Cotton Belt has a gin. Forage and cane crops are seldom transported long distances.

The normal trucking radius for conventional forest products is 160 km, with rail or barge transport used for long distances.

The market and supply have developed together in both instances, with production concentrated in those areas with a transportation advantage. In fact, both forestry and agriculture have seen an implosion of "procurement areas" over the last 20 years as rail transport has diminished. In the case of forestry, tract-to-plant transport has become fully integrated into the harvesting process.

ECONOMIC IMPEDIMENTS

Products

The intended market for the product is yet another delineation of harvesting systems and practices. The three major markets that have emerged are energy production by direct combustion, conversion to liquid or gaseous fuels, or pulp furnish. Each has its own set of requirements and a separate set of market prices.

Direct combustion purchasers prefer fuels of consistently low moisture content. Solid matter fuel losses during prolonged storage are of concern for those facilities requiring dormant season harvesting serving a facility with year-round demand. Storing the material in the round or field-drying the stems before processing can reduce the losses common to even the best-designed chip pile. Other product forms--chunks, billets, bundled, crushed, and baled, and other modes of storage--covered, enclosed, elevated, and packaged--have been tested experimentally, but the current level of demand has not justified major shifts in technology. Direct combustion facilities are usually fairly tolerant of a range of piece sizes and bark and minerals in the furnish.

Conversion to gaseous or liquid fuels through fermentation works best with feedstocks where the initial moisture content, free sugars, and starches have been maintained. Juvenile stems have a larger percentage of living cells per unit weight and a correspondingly higher loss to respiration after harvest. Ideally, the harvested material should be used fresh or kept very cold or dried after harvest to slow respiration.

Most pulping processes work best with fresh material as well. Demand is less seasonal than for energy, and most harvesting systems harvest with a week or two of conversion. Harvesting for pulp production tends to be hot (the wood arrives at the mill within a day or two or harvest), reducing the need for on-site storage. Direct combustion facilities are usually fairly tolerant of a range of piece sizes and bark and minerals in the furnish. Pulping processes are becoming increasingly restrictive concerning bark, fines, and dirt content of the furnish.

Markets are likely to become more demanding in their raw-material specifications as conversion technologies and environmental restrictions become more sophisticated. These demands will quite quickly translate into tightened harvesting requirements.

Cost/price

The value of these products cannot be established until the demand/supply situation is better understood, and this relationship is the function of issues well beyond the plantation or even the target industry. The market competition for SRIC material is from substitutes for biomass such as natural gas, petroleum, coal, and recycled materials, both petroleum- and biomass-based. The price of these alternatives will depend on international politics, national environmental and industrial policies, and the public's acceptance of individual products. Supply competition arises from natural timber stands, extensively managed plantations, agricultural crops, and residues and recyclables.

SRIC crops must carry high cultural costs. The key question is, "will the market price be sufficient to cover both these costs and the harvesting and transport costs of the material?" Traditionally, high "stumpage costs" as a result of cultural investment or market competition for the standing material have resulted in harvesting contract rates being suppressed to near the lowest level of economic activity for the harvesting firms, a level which cannot support the development of specialized machines or products.

Niche markets exist where these larger constraints do not obtain. Market prices may be levered upward in an attempt to save a plant or industry whose traditional raw material supply has been lost. Stumpage prices may be forced downward when the cultural costs are mitigated by, for example, farmers choosing to produce SRIC crops as a substitute for traditional agricultural crops which are no longer in demand, or for which their farm is a marginal producer. The up-front investment in land, mobile and cultural equipment is avoided, and the producer sees this cropping alternative as a means of salvaging his investment rather than an entrepreneurial effort.

These issues are far from resolved, and as a result, the contract rate per ton on million BTU's is still in conjecture. The era when a prospective equipment manufacturer could invest research and development funds in a project in the hope that the product would meet the desires of an undefined market is past. Supporting development with government funds allows mechanical, but not economic, development to go forward.

SUMMARY

Mechanization of short-rotation crops is a systems problem and involves the entire system from seed to feedstock. Both forestry and agriculture are rife with examples of the growing stock and cultural practices being modified to match harvesting requirements and harvesting being adapted to cultural requirements. The trade-offs have to be deliberated and the solutions defined.

The market for short-rotation harvesters is both limited and largely undefined. The nearly 25-year history of the SRIC concept has been one of shifting cultural and conversion scenarios and the search for the species/cultural combination that yielded the ultimate yield per unit area.

The current approach is restricting the development of short-rotation crops to those than can be harvested with agricultural equipment at one pole and those that can be captured with conventional forestry equipment at the other. It is unlikely that any major investment in engineering design or fabrication will occur until both ends of the system--cultural and conversion--have stabilized. Bringing a harvester or harvesting system to market is a long and expensive process. Local job shops may be enlisted to construct a single- purpose-built harvester for a specific application, but this level of development lacks the financial, sales and parts support and warranty backing provided by a short- or long-line manufacturer.

File posted on March 5, 1996; Date Modified: February 21, 1999