Biomass and Bioenergy, Vol. 9, Nos I-5, pp. 161-179.1995 ELveviex science Ltd

09Gl-9534(95)uQo88-7 Printed in Great Bdtain 096L9534l95 S9.50 + 0.00

ENVIRONMENTAL CONSEQUENCES OF INTENSIVE HARVESTING

C T SMITH

New Zealand Forest Research Institute, Private Bag 3020, Rotorua, New Zealand

ABSTRACT

Sustainable deployment of bioenergy production systems requires that we achieve the ability to predict the impact of intensive harvesting on forest site productivity. During the period 1992-94, collaborators in International Energy Agency Bioenergy Agreement (IEA/BA) Task IX Activity “Environmental consequences of intensive harvesting” have refined protocols for conducting field and laboratory research designed to reduce uncertainty associated with predictive models; have published comprehensive reviews of the state of our knowledge related to long-term productivity in intensively managed forests; have sought to improve our understanding of recent research advances on theoretical and empirical levels in the areas of carbon cycling, sustainable forest management, and managing site fertility; and to direct this information to developing acceptable and efficient bioenergy production systems. This paper summarises the findings of these efforts, and indicates where future international collaboration is required to achieve the predictive ability required by the IEA/BA.

KEYWORDS

Forest harvesting, site productivity, bioenergy, environmental impact, sustainable forestry, soil carbon, carbon sequestration, nutrition management

BACKGROUND

International collaboration to evaluate the environmental consequences of intensive forest harvesting for energy production has been stimulated by the activities conducted within the International Energy Agency Bioenergy Agreement (IEA/BA) Task III “Nutritional Consequences of Intensive Forest Harvesting on Site Productivity” (1986-88); Task VI Activit? “Environmental Impacts of Harvesting” (1989-91), as described by Brown’, Dyck and Mees , and Dyck and Bow3; and the current Task IX Activity “Environmental Consequences of Intensive Harvesting” (1992-94). As stated by Dyck et a1.“, the common goal of collaborators over this period has been to “improve our ability to predict the potential impacts of intensive harvesting on the environment, and particularly on long-term site productivity”. To this end, collaborative effort has included reviews of models to predict the impacts of harvesting on site productivity at workshops in Sweden4 and New Zealand’; site classification as a prediction too16; alternative research strategies’; techniques for restoring and maintaining site productivi$; and co-ordinated development of protocols for field trials and laboratory resear$ to develop the required empirical knowledge to refine our theoretical conceptual models. ’ However, collaborative efforts to predict the effects of intensive harvesting on long-term forest

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productivity have been limited by a lack of appropriate research.2 Inadequate information has been available to describe the critical fundamental mechanisms driving the relationship between harvesting impacts and site quality and forest productivity over one to several rotations. If reliable predictive models were available for a variety of forest systems, economic and ecologically sustainable bioenergy technologies could be developed.

During the current Activity period, collaborators have sought to reduce uncertainty about key forest ecosystem processes, and improve our predictive ability, by directing the collaborative effort to those forest ecosystem processes that are currently least understood. Using this approach, the Activity hopes to contribute to scientific progress by increasing awareness of recent research findings and theoretical developments, and stimulating international collaboration to reduce the time required to refine and validate predictive models. The broad objective of the Activity for the period 1992-94 was to “determine the environmental impacts of intensive management and harvesting of biomass crops” by investigation of the impacts on soil fertility, soil physical structure, landscape and amenity, and water quality and quantity. Topics that were considered of highest priority for the past three year period, that fell within the scope of the Activity programme, and that would achieve the objectives of the current Task IX, were discussed in workshops with the following themes.

Carbon cycling at plant, ecosystem, and global levels of resolution:

?? At the plant level, “fine root turnover” was discussed, specifically, methodology for measuring fine root turnover; the status of knowledge on fine root turnover; and to recommend a direction for future fine root turnover research. ’ I

?? “Impacts of harvesting and site preparation on carbon cycling processes in forests” was the topic of a workshop, with papers related to three themes: the effects of forest management on plant-level carbon allocation in forest ecosystems; the effects of forest management on processes that regulate carbon cycling in the forest floor and mineral soil; and the potential for, and constraint on, sequestering carbon in forest ecosystems.‘2

Sustainable management for bioenergy production systems:

?? “Assessing the effects of silvicultural practices on sustained productivity” was the topic of a workshop involving collaborators across several IEA5A Activities, with papers related to three themes: interactions between tree species and site quality with regard to sustained productivity; the cumulative effects of silvicultural technology, including harvesting, site preparation, and weed and pest control, on site productivity and environmental quality; and emerging concepts in landscape level alternatives for forest management.13

Managing site fertility:

?? “Diagnosing nutrient deficiencies”, with the objectives of: presenting methods for diagnosing plant nutrient deficiencies in commercially important tree species; examining the status of information systems for diagnosing potential nutrient deficient sites in intensively managed forests; and assessing the possible implications of changing species or breeds on nutritional management strategies.

?? “Understanding plant nutrient uptake and supply, and opportunities for managing site productivity” was the topic of a workshop with the objectives of improving our ability to manage site productivity through an improved understanding of factors controlling plant nutrient uptake and supply. Papers were presented around the themes of: reviewing knowledge in plant nutrient uptake, species demand, nutrient allocation, and nutrient use

Environmental consequences of intensive harvesting 163

efficiency; reviewing knowledge in nutrient supply mechanisms theorised in affecting plant nutrient uptake; and the application of theory in models and management strategies for controlling site productivity. I4

The objective of this paper is to review the relevant literature and status of our theoretical understanding of the environmental consequences of intensive harvesting, as related to the topics listed above.

CARBON CYCLING AT PLANT, LANDSCAPE, AND GLOBAL LEVELS

Plant carbon allocation.

Predicting the environmental consequences of intensive harvesting requires an understanding of above- and below-ground responses of plants to changing site factors. While above-ground plant dynamics are relatively easy to study, albeit complex to model, below-ground dynamics are difficult to study and relatively poorly understood. Standing stocks of root biomass may not reflect below-ground plant carbon allocation, due to transfers of below-ground carbon to root exudates, respiration, and mortality, which are difficult to measure. As a result, there is substantial scientific debate about the proportion of carbon that plants allocate between above- and below-ground components as site factors ~ary.*~“~ Knowledge of below-ground plant dynamics are required for a complete understanding of the ways in which species and site factors affect whole-plant productivity, carbon allocation to above- and below-ground components, soil carbon dynamics and storage, and global carbon budgets, and hence the sustainability of bioenergy production systems.

Two substantial reviews of the conceptual approaches to modellin were published by Agren and Wikstr6m’7 and Cannel1 and Dewar. ?8

carbon allocation in plants Both reviews emphasised

that our understanding of the mechanisms controlling plant carbon allocation is very poor, and that below-ground and whole-plant process modelling lags far behind canopy process models.r9**’ As a result, many carbon allocation models are based on empirical data that represent the end result of poorly understood processes related to transport, storage and utilisation.” Thus, plant carbon allocation models are not substantially more advanced beyond varying approaches to developing partitioning coefficients, which range from simple constants (e.g. McMurtrie et aL21) to more complex approaches incorporating information about plant nutrient status (e.g. Hirose 22; Beets and Whitehead23) or plant phenological status.r7

Given our relatively poor understanding of the processes governing plant carbon allocation, the literature does provide a limited basis for generalising about the impact of intensive harvesting on plant below-ground processes. Research summarised by Ericsson24 indicates that if harvesting reduces soil nutrient availability below some “optimum” level, plant carbon allocation to roots can be altered. For example, low levels of nitrogen, sulphur, phosphorus, and iron increase the weight of roots relative to shoots; while low levels of potassium, magnesium, and manganese have the opposite effect. The effect of nutrients on plant carbon allocation can be offset by other factors; although the potential interactions with other environmental factors, such as water, light, temperature, and CO2 are too variable to generalise. Most research indicates that increasing soil fertility with nitrogen fertiliser will generally increase whole-plant productivity, provided other site factors are not limiting, and that greater amounts of carbon will accumulate in fine and coarse roots as a result (e.g. Johnso$‘; Smith et 01.~~). Nitrogen fertilisation may also change carbon partitioning coefficients among stem, branch, and root components of trees,

164 C. T. SMITH

resulting in increased allocation to stems relative to roots, as observed for Pinus rudiatu. 23

Harvesting induced changes in site factors that reduce site productivity would be expected to reduce carbon accumulation rates below ground as a result of lower whole-plant productivity.

Soil carbon storage with intensive management.

Soil organic matter generally is positively related to soil health, due to positive effects on soil fertility, biology, and soil physical properties, such as bulk density and water relations. Simple measures of total soil carbon are not acceptable indices of site productivity, since organic matter turnover by biological mineralisation processes is of greater importance. Notable examples of systems with low productivity and high soil organic matter levels are bogs and high elevation or cold temperate and boreal sites. However, maintenance of soil organic matter is generally considered a criterion of sustainable forest management. A recent literature review by Johnson2’ showed that the majority of studies reported negligible losses of soil carbon after forest harvesting and reforestation. Substantial losses of soil carbon were reported for systems involving harvesting followed by intensive burning and/or mechanical site preparation, and in two cases in the tropics. These results may indicate that total soil carbon levels are either difficult to measure with high precision, or are difficult to change because of the recalcitrant nature of the soil humic fraction, and the buffering capacity of various ecosystem processes. Certainly the problems associated with gene&sing on the basis of studies that differ in sampling protocols, sample intensity, and time scale are not trivialz5 However, if the buffering capacity due to ecosystem detritus inputs with highly recalcitrant carbon fractions is substantial, we would predict that intensive harvesting removals of the above-ground tree components for bioenergy production would not cause soil carbon levels to decline in the short run, and be difficult to detect.

Intensive forest management has the potential to increase soil carbon levels through afforestation of barren lands (e.g. sand dune ecosystems studies by Dyck et ~1.~~ and Smith et LIZ.~~), reforestation of former agricultural lands,25 productivity, such as fertilisation.25

and management practices that increase site Cultivation, repeated litter removals, and intense fires

appear to decrease soil carbon levels substantially. Based on these considerations, the results of Johnson’s review,25 and the model simulations of Bengtsson and Wikstrom,28 one might hypothesise that intensive forest management that only utilises above-ground biomass, that minimises soil disturbance, and that seeks to maintain high levels of site productivity, will conserve soil organic matter. However, Carlyle2g showed that soil carbon in podsolised sands was very sensitive to forest management operations that affected carbon input rates and decomposition, indicating the need for refining our site-specific understanding. In addition, Carlyle’s work highlighted the need to conduct long-term studies to separate the dynamics of the short-term labile carbon fraction from the long-term dynamics of the recalcitrant fraction (see also, Agren and Bosatta3’).

Based on his literature review of the effects of forest management on carbon storage,25 Johnson31 suggested that a co-ordinated, multi-site study be established to determine the effects of harvesting and harvest residue management, and other management practices, on soil carbon. A co-ordinated research programme would be essential to ensure common protocols for sampling, and could lead to the development of new analytical and conceptual approaches to soil carbon research (e.g., Bengtsson and Wikstr6m28; Harrison and Harkness32; Preston33; Parton et a1.34; Pastor and Post35). A multi-site approach designed to determine how soil properties (e.g. texture) affect soil carbon dynamics, including decomposition and organo-mineral complex stabilisation, could lead to a site-specific understanding of how such management practices as

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cultivation, residue management, thinning, and weed control affect soil carbon storage, as discussed by Carlyle. Such a multi-site approach could lead to refinement of site classification systems for predicting how specific systems will respond to bioenergy production (e.g., Jones36). Uncertainty about which management practices would increase or decrease carbon suggests that the research programme include the effects of fertility amendments such as nitrogen fixers, fertilisers, and such waste materials as wood ash, wastewater effluents and sludge or biosolids. These topics are related to the brief of the Task XII Activity “Environmental Issues” for the period 199597.

Landscape and global carbon sequestration.

Recent estimates indicate that soil processes have the potential for a positive, neutral, or negative impact on global carbon budgets. For example, Johnson2’ notes that if soil carbon contents are not in steady state due to an imbalance between detritus inputs and decomposition, the magnitude of the imbalance may be adequate to equal or offset global fossil fuel emissions. Land use change appears to have a very significant effect on global carbon budgets, with conversion of forests to agriculture, and agriculture involving annual cultivation, generally estimated to contribute to a net loss of carbon stored in soils; and afforestation of barren lands or reforestation of agricultural lands thought to increase carbon sequestration in soils.

Estimates by Hollinger et ~1.~’ of carbon sequestration by the intensively managed plantation estate of Pinus radiatu in New Zealand indicate that the annual storage of carbon in 198889 was equal to 70% of New Zealand fossil fuel emissions. This large amount of carbon storage was related to the age class distribution of the plantation estate, due to extensive new plantings in the 1970s and 1980s. They note that without expansion of the forest estate, net sequestration will eventually drop to zero. For New Zealand, as elsewhere, highest benefits of forest plantations as a carbon sink will come from afforestation of land previously under other land uses. 38 However, their estimate did not take soil or wood products storage into consideration, so was probably an underestimate of net carbon storage by the forest estate. Forest ecosystems composed of species with high decay resistance are likely to have greater soil sequestration, especially when rotation lengths are long, or forests are not harvested, as in old-growth stands containing Thujaplicata in the Pacific Northwest.39

Bioenergy systems have the potential to reduce fossil fuel consumption through substitution.40 Whole system, life cycle approaches are required to accurately estimate the contribution of intensive forest management and bioenergy systems to global carbon balances.

SUSTAINABLE MANAGEMENT FOR BIOENERGY PRODUCTION SYSTEMS

Cumulative effects of silvicultural technology.

As stated by Burger4’ one criterion of developing bioenergy production systems, based on conventional forestry, is that such intensive management and harvesting must be consistent with sustainable forestry. Burger refined a concept presented by Switzer4* which shows stand production (biomass accumulation) as a function of time (Burger4’, cf. Figure 3). This figure shows a logistic growth curve, which is restricted on the left by the species biotic potential, and limited on the top by site carrying capacity. The rate at which a species reaches maximum production is limited by “environmental resistance” from such factors as water and nutrient

C. T. Wrm

Figure 1. Hypothetical production curves (after Burgea’).

Curve 1 represents a no-treatment control curve; curve 2 represents a stand that received a treatment that reallocated resources to desired species while maintaining site carrying capacity; curve 3 represents a stand that received treatment that reallocated resources to desired species but reduced site carrying capacity. R and R’ represent alternative rotation lengths.

stress. Intensive forest management has sought to increase species biotic potential (shift the biotic potential curve left) with tree improvement programmes; to reduce environmental resistance to growth (increase the slope of the logistic growth curve) with cultural practices such as irrigation and fertilisation; and to increase site carrying capacity (raise the level of maximum site productivity) by altering the site by drainage and other factors to alter site characteristics.

Burger used this graphical approach to depict hypothetical production curves indicating three ways that stand productivity might respond to changes in site factors (Figure 1). Curve 1, a no- treatment control curve, represents the growth curve of a stand that reaches the carrying capacity of the site, but without cultural treatments that increase the rate of growth. The curve 2 growth pattern could be achieved with cultural treatments such as weed control or nitrogen fertiliser. Curve 2 indicates a situation where the site carrying capacity was reached in a shorter time period than occurred with no cultural treatments (curve l), and indicates that the carrying capacity was not reduced by the cultural treatment. Curve 3 represents a stand that had fast early growth in response to cultural treatments; but indicates that the carrying capacity of the site was reduced by one or more factors. For example, this situation could occur with a site preparation operation that reduces weed competition and stimulates early growth, but that alters soil physical properties and restricts root development as the stand matures.

Environmental consequences of intensive harvesting 167

Long-term trials are required to detect whether early responses will not be sustained because of a reduction in site carrying capacity. For example, if the stand represented by curve 3 had been harvested at time R, it would have demonstrated the response to treatment that resulted in stimulated early growth; but would not have indicated that site carrying capacity was reduced. Alternatively, if the stand had been harvested at time R’, and the experimental design had included appropriate control plots, stand productivity would have indicated that the site carrying capacity was reduced by the cultural treatment.

These hypothetical production curves are presented here to illustrate there are many forest management activities, in addition to harvesting, that affect site carrying capacity. They illustrate the difficulties associated with selecting appropriate curves to define potential productivity of a given site, and to serve as suitable bases for comparing levels of stand productivity and identifying reductions in site carrying capacity. Figure 1 represents the type of empirically based analysis that is required to validate our theoretical conceptual models of the cumulative effects of intensive forest management on site productivity. Similar graphic analysis was presented by Morris and MilleP3 (p. 46, Fig. 3.1). In essence, this analysis defines the information that the IEA/BA requires for environmentally acceptable deployment of bioenergy production technologies. Comprehensive, definitive reviews of the experimental approaches that are required for obtaining the empirical information required to validate theoretical models have been written by IEA/BA collaborators, including Burger and Powers,44 Dyck and Cole,45 and Van Miegroet et a1.46

The status of our current information base, and experimental conditions required to produce evidence of long-term productivity changes due to forest management activities, were reviewed by Morris and Miller.43 They conclude that “few studies meet adequate experimental requirements and none have been carried long enough to provide long-term results”. At this point in time, we can only make generalisations about harvesting impacts on long-term productivity (e.g., Dutch47). These conclusions indicate the need for continued IEA/BA collaboration in this area of research.

In spite of the accuracy of conclusions made by Morris and Miller,43 we must not overlook the importance of growth trends reported from rigorously designed trials that indicate site productivity declines following whole-tree harvesting and litter removal. For example, trial results reported by Cole and Compton48 reported growth after “complete” removal was 26 percent less than growth after bole-only harvest for Pseudotsugu menziesii in Washington; and Smith et al? reported similar reductions in site productivity due to whole-tree harvest plus forest floor removal for Pinus rudiatu in New Zealand. Both studies indicated that growth reductions could be corrected by nitrogen fertiliser, emphasising the point made by Lundkvist49 regarding the value of “compensatory” measures such as wood ash recycling or biosolids applications for offsetting nutrient removals by intensive harvesting. The results of these studies suggest that preliminary guidelines for harvest residue management could be made for nutrient- limited sites in specific regions for the species studied. Long-term results will be required to validate preliminary recommendations.

Neary and Hornbeck” reviewed the impacts of forest harvesting on off-site environmental quality with respect to air quality, surface water quantity and quality, groundwater, habitat fragmentation and biological diversity, and cumulative effects and impact assessment. They concluded that “the environmental effects of harvesting tend to be small to moderate and are short-term, especially when compared to other land uses due to the infrequent nature of forest harvesting and the generally good stewardship practiced by forestry professionals”. Their conclusions agree with those reached by Binkley and MacDonald” in their review of forests as

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non-point sources of pollution and the effectiveness of Best Management Practices (BMPs). In general, water quality is high in managed forests with respect to sediment, nutrients, and pesticides; and BMPs are effective in preventing reductions in water quality due to harvesting and related activities, and at other times during the rotation. The effectiveness of BMPs can be attributed to the quality of the long-term research conducted in this area. However, both reviews point out that the general public continues to be very concerned about the effects of forest management activities on water quality. This indicates that current legislation and BMPs will be carefully scrutinised to identify needed improvements. Future research will need to concentrate on clarifying cumulative impacts of management activities; management of riparian zones; and in defining high risk landforms during infrequent, catastrophic events such as cyclonic storms (e.g. Marden and Romania).

Emerging concepts in forest management.

As stated earlier, the common goal of collaborators investigating the environmental consequences of intensive harvesting is to gain predictive ability. However, the end product of such an effort will also enable us to refine forest management practices to achieve sustainable management. The current debate on issues of sustainable forest management is wide ranging, and includes concepts that include ecology, green religion, and environmentalism, as described by Kimmins.53 Although the current debate is highly political, major regional, national, and international efforts are currently directed at developing workable definitions of sustainable forest management. For example, Canada has funded a Model Forest Program to develop such a definition,54 which will be based on research conducted at ten model forest sites across the country. The program will address issues of biodiversity; water quality and soil conservation; economic diversification; public education and worker training; cultural and spiritual renaissance; and scientific research and development of new technologies. Canada’s Model Forest Program is perhaps the most comprehensive, formally funded effort being conducted internationally. Most other regional or national efforts at refining concepts of sustainability typically are either not provided funds for comprehensive programmes, or address single sustainability issues (e.g., biodiversity and old-growth structure55). Programmes similar to the Model Forest Program are likely to help achieve the goals of the IEA/BA, especially as they contribute to the development of ecologically-based new technologies that have benefits for researchers, forest management, and the general public (e.g. see Kimmins53).

For new technologies to emerge for the sustainable deployment of bioenergy production systems, we envisage the need for the development of a comprehensive approach, involving four components: (I) a “code of

St: ractice” for forest management and bioenergy production systems

(e.g. LIR056; Beets et al. ; National Biofuels Roundtable58), based on a combination of theoretical (e.g. computer simulation) and validated concepts for sustainable forest management; (2) an ongoing research programme designed to test hypothetical concepts and provide forest- based validation for ecologically sound practices; (3) an ongoing monitoring programme to demonstrate compliance with the code of practice, and to provide evidence that ecosystem sustainability is being achieved, based on a system of permanent sample points (PSP); and (4) an information support system based on components of information storage and retrieval (e.g., Hypertext59; expert system6’), and geographic information system (GIS). Such an approach can be visualised as feedback loops between forest management practices and research (Figure 2; after Dyck6’). This approach is closely linked to the brief of the IEA/BA Task XII “Environmental Issues” activity over the next three years.

Environmental consequences of intensive harvesting 169

FEEDBACK BETWEEN RESEARCH AND PRACTICE

RESEARCH PRACTICE

New Knowledge

Conduct RrWlCh Knowledge Base

Evaluate & Monitor

Fmmul*te ..___-____--_-__--_-__ Idcjmy Gaps

Hypotheses in Knowledge

Figure 2. Feedback mechanisms between the application of Best Management Practices (BMPs) in forest management and research required to validate and improve BMPs (after Dyck61).

MANAGING SITE FERTILITY

Experience with intensive cropping regimes suggests that intensive, frequent removals of tree biomass for bioenergy production will lead to long-term reductions in soil fertility. Site productivity can only be maintained in the long-term by gaining an understanding of the factors controlling plant nutrient uptake and nutrient use efficiency; processes supplying tree nutrient demand; and management strategies for managing stand fertility. IEA/BA collaboration in this area has been essential for making progress in gaining predictive ability required for deployment of bioenergy production systems.

Nutrient requirements of trees.

The topic of nutrient requirements of trees was comprehensively reviewed by Ericsson.24 In his review, Ericsson addressed important questions basic to understanding nutrient requirements of intensively managed plantations, such as: Do species require nutrients in different proportions for normal growth? Does age, nutritional- and water-status affect dry matter partitioning among tree roots, stem, and leaves? Do species require different amounts of nutrients for a given level of biomass production? In this paper, generalised answers to these questions will be presented. Additional details related to species or specific sites can be found in the papers cited.

There is general agreement about the elements that are considered essential for normal plant growth and development and reproduction. However, there is considerable discussion about the proportion, by weight, required by plants for normal growth. The work of Ingestad and associates, under laboratory conditions, suggests that species are very similar in their proportional requirements for nutrients (Table 1, Ericsson24). These results indicate that differences in relative nutrient contents observed under field conditions are due to differences in

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tree age, or soil and climatic factors that, in turn, alter plant nutrient supply, uptake, and allocation. In addition, these results also suggest that genetic differences in nutrient stress within a species (e.g. as described for the symptom of upper mid-crown yellowing in Pinus radiata by Beets and Jokela62) may be due to differences in nutrient acquisition ability, rather than in fundamental cellular differences in nutrient requirements. Because some plant species can adapt to a wide range of soil types and can tolerate nutrient stress, it is often difficult to determine plant requirements accurately. The Ingestad approach to understanding nutrient uptake requirements is useful for separating plant requirements from factors that determine plant uptake observed in the field.

Plant biomass accumulation (mean annual increment and cumulative) and carbon allocation patterns among roots, stem, and leaves are strongly influenced by age, species, and genotype, and a number of site factors, including light, temperature, water, nutrients, and atmospheric composition (e.g., C02). In addition, plant tissue nutrient concentrations are affected by numerous plant and site related factors. Thus, standing stocks of nutrients accumulated by plants at some point in a rotation are not independent indicators of plant nutrient requirements. Annual plant demand for nutrients can not be estimated by multiplying biomass accretion in root and shoot components times the concentration of each element contained in the biomass produced in the current year, since a substantial fraction of the nutrients accumulated by new growth is supplied by translocation within the plant. The fraction of new tissue nutrient accretion supplied by internal translocation differs by element, species, age, and site fertility. The greatest utility of information about standing stocks of nutrients at tree and stand levels, in the context of evaluating bioenergy production impacts, is for estimating nutrient removal rates associated with harvesting; estimating nutrient-use efficiency, or dry-matter production per unit of nutrient accumulated (under limited experimental conditions); and in determining nutrient accumulation and partitioning patterns under experimental conditions. It still remains problematic to develop nutrient management regimes for maintaining or increasing productivity from such information because of the complexity of soil factors affecting nutrient availability to trees.

Studies of internal translocation of nutrients and nutrient-use efficiency on low and high fertility sites indicate that nutrient poor sites are associated with a greater proportion of annual nutrient demand supplied by internal translocation; and greater nutrient-use efficiency.24,63 This mechanism would explain observations of greater nitrogen-use efficiency on nitrogen-poor sites, as compared with nitrogen-rich sites, made by Birk and Vitousek64 for Pinus taeda in the southeast USA, and Smith et a1.65 for Pinus radiata in New Zealand. Evergreen species generally have higher nutrient-use efficiency than deciduous species, especially for nitrogen,24 and nutrient-use efficiency generally increases with age, due to greater internal translocation from fully developed crown mass.

The studies cited by Ericsson24 provide the basis for making generalisations about tree nutrient requirements in bioenergy production systems. Developing the ability to predict nutrient demand by intensively managed forests will require both an understanding of nutrient requirements, and an understanding of the relationship between soil nutrient supply capacity and plant uptake.

Nutrient uptake by roots.

Comerford ef a1.66 reviewed the processes involved in nutrient uptake by trees, with emphasis on nutrient absorbing surfaces and the active uptake process (i.e., requiring metabolic energy) as described by Michaelis-Menten kinetics; interactions between roots and the rhizosphere environment; and placed this information in the context of forest nutrient management

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strategies. Their review indicated that the relative importance of active and passive (i.e., movement through intracellular spaces down concentration gradients for nutrients) nutrient uptake mechanisms for various categories of root systems (e.g., white, brown, woody, mycorrhizal roots) is poorly understood. It is clear that nutrients are taken up at rates depending on, among other factors, the concentration of the nutrient in solution at root absorbing surfaces. 66 Similar theoretical approaches to modelling the kinetics of nutrient uptake are being conducted by Kelly et a1.67 with Pinus taeda, and Van Rees68 with Pinus elliottii, to understand the chemical, physical, and biological factors controlling plant nutrition.

Roots are capable of affecting the concentration of nutrients at the root surface, so can, in turn, affect nutrient uptake rates. Roots also affect the rhizosphere by altering such properties as: nutrient concentrations, pH, redox potential, root exudates, and the activity of microbial populations,66 and have been shown to have a direct chemical effect on primary and secondary minerals through the dissolution action of organic acids associated with root exudates.69270

Root distribution in forest soils can be altered by harvesting operations, as well as by pedologic processes leading to changes in soil chemical and physical properties. Predicting nutrient uptake in managed stands will require information about the spatial distribution of roots, in addition to the absorption potential of various root categories. The theoretical conceptual models of nutrient uptake mechanisms described by Comerford et a1.66 can be used to design experiments that seek to refine fertility management systems for bioenergy production. In particular, nutrient management systems must increasingly be designed to closely match plant uptake requirements with nutrient supply from soils and amendments such as fertiliser, wood ash, and biosolids. Oversupplying nutrients above plant needs will be expensive and cause environmental pollution problems associated with nitrate and phosphate leaching to ground and surface water systems. These issues will be addressed in future collaboration in the IEA/BA Activity “Environmental Issues”.

Future intensively managed plantations will be established with genotypes selected for a variety of traits. Typically, tree improvement programmes have given highest priority to such traits as volume growth rate, tree form, wood quality, and disease resistance. Conclusions by Nambiar7’ (in Comerford et a1.66) suggest increased attention should be given to nutrient uptake in future breeding work.

Nutritional management for intensive forestry.

There seems to be general consensus that intensive harvesting will eventually result in declines in site fertility due to nutrient removals, and that the rate of decline will be faster on nutrient poor sites than on nutrient rich ones. However, we still lack the ability to precisely predict how fast, or where fertility declines will occur, and to estimate the magnitude (and probability) of productivity losses. Ballard72 suggested that fertilisers are an expensive means of replacing nutrients lost during harvesting, partly because of the low recovery rate of fertiliser nutrients by trees.65’7’ This review will identify approaches developed in New Zealand and Australia that may be useful for managing the fertility of intensively managed plantations in ways considered cost effective and environmentally acceptable.

The approach taken by New Zealand in developing a soil phosphorus maintenance model for maintaining the fertility of grazed pastures74 is conceptually very appealing. This model is based on the assumption that the size of the cycling pools of phosphorus should remain fairly constant from year to year. As a result, phosphate fertiliser is only added to replace phosphorus lost from

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the cycle by animal exports or excreta, or by incorporation into unavailable forms in the soil. Such a model is lacking for forestry applications; and although the principles are similar, less is known about basic components of the phosphorus cycle. For example, while organic matter mineralisation seems to play a large role in determining P availability to trees, little is known about phosphorus mineralisation rates in northern New Zealand forests.75 As a result, commonly used soil tests (e.g., Bray extract based tests) may over- and under-estimate tree requirements for phosphorus on different sites. Ongoing research collaboration will be required to achieve the type of management systems developed for pastoral systems.

Birk76 reviewed fertiliser use and approaches to nutrition management in intensively managed plantations of pines and eucalypts in Australia. Her findings will be reviewed here to identify generalisations that can be used to predict trends in approaches to nutrition management.

Nutritional problems in Pinus radiata plantations in Australia can be predicted if parent material is known. Similar preliminary soil interpretation information has been developed by Hunter et al.” for New Zealand. Major growth-limiting elements have been identified for soil mapping units, and estimates made for the growth responses expected following fertiliser application. In both countries, critical foliar concentrations for the major nutrients have been established, and are used to make fertiliser recommendations.

Fertilisers were considered a ‘luxury’ in the past; but are currently used as a basic management tool. Fertiliser applications have been integrated with other silvicultural operations (e.g., thinning and harvest residue management) to increase utilisation of native soil fertility and recycle nutrients in residues. Four themes characterise nutritional management and fertiliser use in Australia in the 1990~:‘~

?? site-specific management supported by GIS and regular data collection; ?? more intensive management to increase productivity per hectare, while taking wood quality

into consideration; ?? more efficient use and conservation of growth-limiting resources such as water, organic

matter, mineral soil fertility, and fertiliser; and 0 incorporating nutritional traits into tree improvement programmes.

Current practices are based on empirical field trials. More intensive management requires development of process-based models to more thoroughly understand how to develop fertiliser applications that are appropriate across periods of low and high demand by the plantation. For example, Carlyle indicates there is scope for improving fertiliser effectiveness on the sandy, nitrogen-deficient soils of South Australia by varying timing of application according to season, thinning operations, and thinning residue management. Negligible leaching of nitrogen has been observed from thinning operations, and increased uptake by residual trees. However, nitrogen from thinning residue may not be adequate to improve tree growth on nitrogen-deficient sites. High leaching losses of nitrogen are typically observed following fertiliser application. Carlyle recommends varying fertiliser application to match stand uptake capacity and limit leaching losses. Spring, rather than winter, applications limit losses and increase uptake.

The approach suggested by Carlyle is consistent with that taken by Zabowski and Henry79 in their studies that seek to identify alternative fertiliser technologies for applying nitrogen. They have evaluated soil nitrogen levels, nitrogen leaching losses, foliar responses, and basal area growth to identify the fate of nitrogen in the forest ecosystem following fertiliser application. This rcscarch is essential to develop nutrition management systems that identify the balance point hctwccn economic benefits and environmental impacts associated with fertiliser use. This

Environmental consequences of intensive harvesting 173 approach is required to separate potential negative, positive, and neutral effects of biosolids use. These topics are related to the future direction of IEABA Activity “Environmental Issues”.

CONCLUSIONS

Sustainable deployment of bioenergy production systems requires developing site-specific ability to predict how intensive forest management and harvesting will affect long-term site productivity. To date, efforts to achieve predictive ability have been limited by inadequate empirical data bases that are required to validate and refine the existing conceptual models. The Activities conducted under IEABA Tasks III, VI and IX have made major progress by achieving the level of international collaboration required to develop reliable empirical information related to forest ecosystem processes, and to develop sustainable management systems.

Whole-plant carbon allocation models have not progressed far beyond a series of partitioning coefficients. Research is needed to develop a process-based understanding of plant carbon allocation in response to varying site factors and management activities.

Global carbon cycling models indicate that soil carbon processes that contribute to sequestration or net loss of soil carbon are relatively poorly understood. Additional work is required to develop site-specific understanding of the effects of forest management on soil carbon levels. Recent interest in taxing carbon emissions and requiring new tree planting to offset carbon emissions by various sectors of the economy require valid soil carbon models.

Recent reviews indicate that no long-term studies have been conducted with adequate experimental rigour to produce reliable evidence that intensive forest harvesting reduces long- term forest site productivity. IEA/BA activity has had a major contribution to initiating the required field trials, and must be continued in the future to realise the efforts of past decade of collaboration.

Comprehensive national and international research programmes are required to focus financial resources and long-term research teams on relevant problems. Developing sustainable bioenergy production systems will require a comprehensive management approach involving four components: (1) a “code of practice” for bioenergy production; (2) ongoing research designed to provide validation for ecologically sound “code of practice”; (3) ongoing monitoring based on PSPs and experimental plots, to demonstrate compliance with the code, and to monitor the health of the forest estate; and (4) development of information storage and retrieval systems (e.g., based on components of GIS and Hypertext).

Experience with intensive cropping systems suggests that bioenergy production systems will lead to declines in site fertility, and require development of nutrition management systems. Future IEA/BA Activity efforts must address development of nutrition management systems that satisfy bioenergy production goals by meeting plant nutrient demand, and that prevent excessive off-site movement of fertilisers. Development of such systems require development of process based models that link fertiliser additions to ecosystem sinks, sources, and fluxes of essential nutrients.

The current Activity conducted under IEABA Task XII will continue the level of collaboration required to develop preliminary guidelines for deployment of bioenergy production systems, and to develop reliable empirical information from well-designed experiments necessary for refining preliminary guidelines and management systems. J* *-l&L

174 C. T. !&rni

ACKNOWLEDGEMENTS

The International Energy Agency Bioenergy Agreement provided funds in support of the international collaboration associated with Task IX Activity 4 “Environmental Consequences of Intensive Harvesting” during the period 1992-94. The programme of work during this period was achieved as a result of the tremendous efforts and leadership of W.J. Dyck, leader of this activity through March 1994. In addition, we acknowledge the contributions of workshop technical chairs, national hosts, and editors and referees of workshop proceedings, including Mike Proe, Greg Ruark, Taumey Mahendrappa, Patricia Gaboury, Caroline Simpson, Alison Lowe, Jessica Hunter-Smith, Christine Bow, and Judy Griffith. Hamish Kimmins (Canada), Helene Lundkvist (Sweden), Mike Proe (UK), and Phil Pope (USA) contributed to the management and leadership of the Activity as representatives of their participating countries. We thank the numerous workshop participants, since the achievements of the IEABA is based on their enthusiasm and high standard of scientific endeavour.

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