Concerning the choice of sites, the nutrients and water needs, species and clones, techniques and treatment required for intensive energy plantations.
Gustaf Sirén
GUSTAF SIRÉN; is associated with the Energy Forestry Project at the Swedish University of Agricultural Sciences, Uppsala.
There are two extremes of energy forestry and a variety of intermediate forms.
Intensive energy forestry is a land use form suited to conditions in which a future land shortage is anticipated. Annual production is, therefore, based on exceptionally high-yielding woody species usually grown in vegetatively regenerating stands or repeated short (1- to 10-year) rotations and subjected to intensive agricultural techniques and environment management, including recycling of nutrients. Establishment of large stands must take into consideration the probable dimensions of future tending and harvesting machines, since spacing-geometry has to be adapted to track widths and cutting devices. To achieve sufficient profitability at all levels, from farm forestry to large-scale projects, the average net energy harvested needs to be high per area and per time unit. Since labour costs may be formidable, productivity must be sufficiently high to make product prices competitive with oil or other commercial fuels.
Extensive energy forestry resembles conventional forestry in most respects, except that even existing rapidly growing low-quality coppice stands are exploited. They may also be grown intentionally for energy production purposes only. Rotation may vary between 10 and 30 years, depending on species and site conditions. Harvesting techniques would be more similar to those of conventional forestry.
The goal of energy forestry is to grow the raw material for specified energy production at the highest profitability with the lowest adverse environmental impact. In addition, ecologically well-balanced treatment may help to restore sites from earlier abuse. Except for farms and other independent households, the introduction of energy forestry - especially on a large scale - requires planning to provide the producer with guarantees concerning annual deliveries. In large-scale intensive energy forestry, the operational production lots should ideally be larger than 1-2 hectares. In extensive energy forestry, the lower limit for a farmer may be defined by a mere single thicket or a group of trees, if their energy production coincides with his average long-term demand. If charcoal making is the only energy-consuming activity in question, a few hundred square metres of intensively tended high-yielding stand may be sufficient to produce the energy output needed annually.
Among the basic needs of large scale energy forestry, land availability plays a decisive role. In many cases, energy forests will compete with agriculture or other land uses. The following types of land are likely to be most readily available for forestry because they are least in demand for agriculture: abandoned arable land; low grade semi- or unutilized coppice forest; sedge or cane-growing coastal or riverine areas; saline land; mountain slopes; dry areas; sludge deposits; other types of waste land. Few of these are suited to intensive short rotation forestry. Limiting factors may be of soil-chemical, topographical or climatic origin. In many cases, extensive energy forestry may offer a better prospect.
When the costs of alternative energy sources are unacceptably high and certain sites promise very high wood yields, there may be a strong case for establishing energy forests on these sites and relegating agriculture to other sites. The case is strengthened if energy forests can be concentrated in the vicinity of consumption areas, thereby considerably reducing transport costs. It is important, where land is in short supply, to ensure the maximum production of energy per unit of area and time, thus releasing land either for agriculture or for conservation of natural ecosystems.
Seasonally flooded alluvial land in river valleys constitutes a special case. The flooding may render the land unsuitable for short-statured herbaceous annuals but may facilitate very high yields from trees of genera such as Alnus, Salix, Populus and Platanus.
Another possibility is to combine trees for energy with agricultural crops as a special form of agro-silviculture. This could take the form of shelter-belts or windbreaks, or it could be more intimately mixed with agricultural crops, as commonly practiced with Populus in the Po valley of Italy. The decision on the type and location of energy forests must depend on a critical examination of the relative value of the energy output versus other uses of the land, and will vary in accordance with local conditions.
Successful energy forestry depends on maximizing yields without damaging the long-term productivity of the site. The basic questions which affect biomass production in all cases are identical. What are the sub-optimal factors? Which improvement measures will give the best results? Which type of crop will respond with the highest dry matter yield? Will an input-output analysis provide economically acceptable results? For biomass production in field conditions to approach the theoretical maximum governed by solar radiation, many climatic and edaphic preconditions have to be met. Very little can be done in the field of climate improvement. Irrigation is the one viable means of ameliorating the effect of climate, but it is generally expensive. By choosing favourable sites, some climatic disadvantages can be partly neutralized.
To establish good growing conditions, the main elements - soil, minerals, organic material, soil water and soil gases - need to be present in optimal proportions and quantities in relation to the demands of the crop. To maintain high site quality in the long run, the dynamic nature of the soil-crop system must be understood. Nutrient losses caused by harvesting must be compensated for, and the complex process of recycling the nutrients promoted by such means as adjusting the pH of the soil close to optimum for the crops in question.
Acidity. Optimizing pH is a categorical imperative in the field of intensive energy forestry. An unusually suitable range varies between 5.5-6.0 for fast growing broad-leaved tree species. There are, however, species that are growing fairly well at 4.5-5.5. Unsuitable pH conditions will dwarf the roots and decrease the uptake of otherwise available ions.
Fertilizing with phosphorus (P) on highly acid soils is almost totally useless because of its precipitation affinity with iron (Fe) and aluminum (Al). Leaching of potassium (K) and magnesium (Mg) is accentuated in acid conditions and decomposition of organic material impeded. Poisonous manganese (Mn) quantities are easily formed in low pH ranges. On the other hand, too high pH conditions also reduce availability of P; in this case, the precipitation goes with calcium (Ca). Shortage of boron (B). Mn and Mg occur always when the pH is too high.
Nutrient status and fertilization. Fertilization practice in forestry so far has generally been based upon a presumed or predicted shortage of NPK in mineral soils and PK plus some micro nutrients in peatlands. Dosage has been based upon experience from trial-and-error experiments. Losses from leaching generally exceed 50 percent. An analysis of nutrient availability in the soil and a corresponding adjustment of component relations and dosage is still considered, in forestry, a scientific extravagance. Modern agriculture, on the contrary, is already working with concepts such as standard nutrient analysis, optimum fertilization and dosage, nitrogen and carbon cycles, and nutrient recirculation.
In intensive energy forestry, the risk of nutrient losses has to be minimized without endangering the continuous uptake of essential nutrients. The best guarantee for this is an adequate but not excessive nutrient supply. To ensure adequate nutrients, knowledge is needed of the long-term availability of each nutrient element as well as of the specific demand of the species in question, when growing at optimum rate. The fertilizers to be used have to be composed to suit local soil conditions which influence the availability of nutrients. Since root absorption of nutrient ions directly from the soil solution is the most efficient way of supplying nutrients to the plant, the use of water as a low concentration fertilizer carrier should be seriously considered, at least where fresh (or brackish) water is available. Depending on soil properties and local climate, the application interval may vary between I and 4 weeks. The costs, of course, need to be covered by the higher harvestable production. If the water regime of the soil is satisfactory without any irrigation measures, a few top-dressings with adequate granulates corresponding to the total annual nutrient demand seems a considerably cheaper alternative.
The nutrient recycling system in a well-functioning energy forest deserves special attention. The loss of nutrients through the harvest itself as well as other known losses from leaching and animal consumption have to be counter-balanced. Losses caused by harvests are best covered by returning the ash (if this is an available conversion product) and by adding the difference - mainly nitrogen and potassium - to the first-year fertilization after harvest. A well-balanced and well-timed fertilization will also promote the decomposition (i.e., mineralization) of the litter which forms an essential link in the recycling system. How to control the decomposition rate so as to avoid too rapid mobilization and hence leaching is one of the problems still to be solved.
Nitrogen supply. Nitrogen is an essential macronutrient in the most vital organs of plants. A severe shortage of nitrogen always results in reduced dry matter production. Yet, too heavy dosages often cause costly damage, both to the environment through leaching into the ground water and to the crop itself. Therefore, nitrogen as a fertilizer must be handled with great care.
From the short rotation and energy forestry point of view, nitrogen is not only the biologically most important nutrient element, it is also the most expensive and the most energy-consuming in its production process. If the nitrogen needed in a well-functioning, high-yielding short-rotation forest could be provided by biological fixation, the input of energy (other than the sun's) would decrease, thereby increasing the net output considerably. Pending the development of operational techniques for biological fixation, fertilizers have to he used.
Since the uptake of nitrogen by roots is extremely efficient, the dosages should be low and should be well distributed over the first part of the growing season. The total uptake of the crop can be calculated with the aid of data on biomass production and nitrogen content percentage for the main components of the trees. For example, the average content of nitrogen in the leaves, bark, roots and wood in high-yielding stands of Salix is respectively of the magnitude of 3-4 percent, 1-2 percent, 1-3 percent and 0.4-0.6 percent. Decomposition of litter, and especially symbiotic nitrogen fixation processes, will reduce the annual demand for additional nitrogen in the form of NO-3 to 80-200 kg/ha-1. In Sweden, field experiments with Salix fertilization on four occasions, both at the beginning of and midway through the growth season, have resulted in three to four times higher harvestable dry matter production of stem wood compared to untreated plots, an increase which should more than cover the costs of application,
The goal in the application of other nutrients should be to achieve an optimum nutrient solution. Application of calcium (and magnesium) to acid soils for improving the pH level simplifies the fertilization procedure.
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Soil moisture. In dry areas such as the Mediterranean, water may often be the critical growth-restricting factor. Where available, intermittent irrigation, programmed so that stomata functions are not disturbed owing to a shortage of water, should be effective. Combining it with water-borne nutrition would most probably lead to maximum production figures.
Salinity and soil toxicity. Saline soils may be available as a potential niche for energy crops. They frequently have the advantage of flat topography and easy clearing. However, the concentration of certain ions in them frequently reaches such high values as to be toxic to the majority of higher plants. Salinity impedes the uptake of water and of nutrients other than those present in excess. Methods for improving yields from saline soils have been obtained in Israel.
Mining and sludge deposits provide another type of site which may be available for afforestation for energy production, but they are extremely difficult to adapt. Much research is necessary on the possibilities of neutralizing toxicity by chemical treatment on tree species tolerant to heavy metals and on the use of metallophytic herbs, either in combination with or in preparation for tree crops.
For high-yielding energy forests, intensive cultural treatments are required in the establishment phase. These treatments themselves consume much energy, but it should be emphasized that they are needed only in the first few years. There should be no need for any tillage or weed control in the later stages, provided that a vigorous stand has been established from the very beginning. If the crops can be regenerated from coppice, very little cultivation should be required in establishing the new crop for the second and later rotations.
Ploughing and tilling. Soil cultivation may benefit the crop and its utilization in several ways. The advantages of an even surface for harvesting machines are obvious. Cultivation may also improve the site ecologically.
For instance, an increase in oxygen supply to the roots on compacted or waterlogged soils has substantial yield-increasing effects. On the other hand, the well-known advantages of no-tillage agriculture may be applicable during the rest of the rotation (20-30 years) if the establishment stage is successful in all respects. The costs of a series of site-improving measures have to be compared with the potential value of the increase in future yields, at an appropriate discount rate.
The most visible advantage of ploughing and tillage is the reduction of weeds, especially deep-rooted ones. Tillage improves, however, more than that; it also enhances microbiological activity by providing better access of oxygen. Percolation of surface water and rain to deeper soil strata is promoted, although the total water regime may in some cases be impaired by increased evaporation. Here, experience on a case-by-case basis is necessary. Cultivation also deteriorates the nitrogen content, but the loss is a one-time event that occurs only during the establishment year. In the long total rotation, the no-tillage application implies a good chance of efficient use of the nitrogen.
An appropriate soil treatment regime in the initial stage of stand establishment offers the grower the possibility of improving the following:
· Soil aeration and soil biochemical activity.
· Percolation of rain to deeper soil layers.
· pH regulation and, in some cases, nutrient application.
· Planting conditions, especially for advanced equipment.
· Surface evenness, which will reduce damage to the stools during harvesting operations.
Most important of all is the fact that cultivation provides an efficient method of destroying weeds, both deep in the soil and at the surface.
The rooting characteristics of the crop determine the depth of tillage. The timing depends on the precipitation regime and the time required for restoration of soil capillarity.
Weed control. There are several methods of weed control available: site cultivation (following, etc.); mulching; cover cropping; soil fumigation; biological control; shade (with the aid of spacing); herbicides. Local conditions will determine the choice. If environmentally safe herbicides are available, they can result in low weed control costs during the establishment stage, especially if combined with following after the first application and a post-emergent herbicide treatment 1-2 weeks before stand establishment if the tree crop is sensitive to direct contact with the weedkiller. The use of chemical herbicides demands trained and skilled staff in the field. The chemical used must be selected with great care and application has to be timed in accordance with weather conditions and weed development - and with crop development if applied after crop establishment.
Of the other alternatives, cover cropping has been used with success (e.g., rape in the United Kingdom). Experiments with legumes seem promising in Sweden. Biological control is a specialized method still applicable to only a couple of plant species. Fire or flooding does not, for different reasons, seem applicable.
Mulching is a technique which leaves dead weeds in place to shade the soil and gradually decompose and integrate into the nutrient cycle of the site. Organic mulches add both nutrients and physical matter to the soil. Especially in semi-arid conditions, the most important effects of mulching are water retention, evaporation decrease and the prevention of crusting of the soil surface when subjected to heavy rainfall. Erosion risk will decrease. The root penetration of the upper soil strata improves due to better moisture conditions close to the surface. A multitude of combined mulching methods are available nowadays. In semi-arid conditions, water saving techniques deserve special attention. The minimum tillage-vertical mulch concept developed in Texas and Colorado (United States) may be of interest in this context. Here, mulch is concentrated in slots at the bottom of micro-watersheds conforming to an appropriate spacing of row crops. Increased yields support the efficiency of this water-saving method.
After successful establishment, the shading effect of the dense canopy - which depends on spacing - will reduce the survival of weeds considerably. Heavy litter fall adds to the nutrient cycle, weed control and moisture retention. Finally, effective site preparation to destroy weeds before the stand establishment is the easiest way to save costs and increase yields of future crops.
Rotation and spacing. Energy cropping with coppicing species depends upon an appropriate range of rotation ages and spacings to meet the requirements of optimal biomass productivity and maximum economic profitability. The best combination of spacing and rotation will depend upon a variety of ecological, genetical and other factors, such as response to changes in water and nutrient availability, coppicing capacity, age-dependent wood density, stability of ageing stools and the degree of urgency in efforts to reduce oil dependency.
Spacing of medium to long rotation species should not differ much from that of conventional forestry; improved ecophysical conditions and/or an intensive thinning programme may, however, justify denser spacing than normal - at least in regions with short transport distances.
Typical short-rotation species and clones can be grown at varying intensities. Extremely short rotations (2-3 years) are possible, but are sensitive to postponement of planned harvesting if spacing is too close. The number of stools in operational stands (e.g., Salix) may vary between 5000 and 20000 in the most extreme cases.
The geometry of spacing must be accommodated to the machines being used for planting, weeding, fertilization and harvesting to ensure efficient operation without causing damage to the stools or shoots. Close (70- to 100-cm) double rows with safety margins in the space between the double rows corresponding to the width of the tracks seem promising regarding both biomass productivity and machine operation.
The mixing of clones to avoid biological hazards of large monocultures deserves special attention, although practical evidence is limited so far.
Successful energy forestry means maximum yields without site damage.
Neither edaphic factors nor cultural treatment can be considered in isolation from the choice of species or genotype to be used in energy forests. There is no such thing as a "universal species", but there are a number of desirable characteristics to be sought in selecting species for energy production. They may be considered under adaptability; ease of management; utilization; and environment.
Adaptability. A species should be able to survive and remain healthy under the given conditions of site and cultural treatment. It should be adaptable to the local climatic and soil conditions, including their deficiencies, and resistant to local pests, diseases and other sources of damage. It should be capable of high biomass production in short rotations. On harsh, dry sites, no species can be expected to produce yields as big as could be obtained in more equable conditions. There are, however, species with a remarkable capacity to adapt to droughty sites. They not only survive but use the available water with high efficiency per unit of organic material produced. Among trees, certain species of eucalyptus are well known for this property. Among other energy plants, the oil and rubber plants of semi-desert areas have considerable potential.
Ease of management. Plantation establishment, management and regeneration are greatly facilitated if large and regular supplies of seed are available locally, or if the species can be planted readily as cuttings, can be handled easily in the nursery and at planting, and can regenerate easily from coppice. It should also be easy to handle at harvesting, without defects such as excessive crookedness or thorniness.
Utilization. In energy forests, the heating value of the wood is one of the most important characteristics. A tree's specific gravity when over-dry and its moisture content when green have a big effect on heating value. The present custom is for production in energy forests to be expressed in tonnes dry weight/ha/year. Substantial variations in specific gravity may be associated with differences in site, treatment and age. When fuelwood is to be used for domestic purposes, the ease of splitting and the absence of sparking and obnoxious odours are desirable characteristics. Secondary uses such as leaves for fodder or flowers for honey may be possible in some species.
Environment. Conservation or amelioration of the land environment should be sought. Most species should be beneficial in providing shelter, shade and soil stabilization. Nitrogen fixing genera such as Alnus or Robinia can play a special role in improving soil fertility or in reducing the need for artificial fertilizers.
Many broad-leaved genera provide an excellent combination of adaptability, fast growth, high specific gravity and capacity for regeneration by coppice or cuttings. Further improvement in the energy input/output ratio can be obtained by selecting the best adapted provenances or clones within a species, but research into this possibility has barely begun in most Mediterranean and temperate broad leaved genera.
Research on the maintenance of site productivity has been taking place for more than a century and has established some principles:
1. Changes in soil characteristics and hence in soil productivity between the establishment phases of successive rotations may be caused by the stand itself, e.g., by the decomposition of litter and the effects of root formation' or by cultural treatments such as fertilization and the use of machinery. In selecting treatments, account must be taken of possible long-term effects on the soil. Nitrogen loss, for example, can be caused by frequent ploughing and harrowing.2. The delicate balance between water and oxygen in the pores of the soil is of major concern. Insufficient soil moisture will disturb the functioning of the stomata; too much may affect nutrient uptake by the roots. To avoid compaction of well-cultivated soils, the weight of the heaviest vehicle used should not exceed 200 g per square centimetre.
3. Understanding of both nitrogen and carbon cycles is important in managing soil fertility. Care must be taken to avoid leaching of excessive nitrogen application into the ground water. The growth in leaf area provides a valuable guide to correct dosages.
Biomass production from intensively managed energy forests will vary greatly according to species, clone, site and treatment. In Sweden, 12-15 tonnes/ha/year are considered the minimum to justify operational energy forests. Further south, in the Mediterranean area, the longer growing season should in theory allow higher productivity, but inadequate soil moisture during the hot dry summer limits yields unless irrigation is possible. So, in fact, there may not be much difference between these two areas. Appreciably higher production should be possible in places such as the Atlantic climate areas of northern Spain and Portugal. Intensive management of energy forests is still in its infancy and data on the yields to be expected are still all too sparse.
