Forest Soils Research and Prospects for Sustainable Forest Management
Department of Forest Resources and Environmental Sciences,
Korea University, Seoul 136-701, Korea
Forest trees depend directly upon the soil for physical support, temperature moderation, nutrition, and water but soils in forested areas contribute in many other ways to both the lives of trees, associated plants, microbes, animals and humans. However, the overwhelming importance of soils in the life and health of the forest has not been understood until recent years. Now everyone would agree that a major achievement of forest soils study has been to get the soil recognized as one of the major resources in forestry and one which must be carefully managed. Although significant progress has been made in our understanding of forest soils, more information on forest soils and related areas is needed to ensure sustainable forest management. Furthermore, we can expect a variety of new concerns in the future, as well as reappearance of some old concerns or problems. Many will be consequences of new demands or technologies, or of intensification of forest management in areas where relevant information is limited or ignored. In this paper, I firstly presented future trends and challenges in forest soils, and then described research directions and topics of the area especially for east Asian counties such as Korea and China. Lastly importance of education and research organization development was stressed.
Future Trends and Challenges in Forest Soils
It is important to consider major problems or challenges lie ahead for those forest soils area. Mainly these are based on forest management shaped largely by external factors. Trends and their likely consequences suggest priorities for forest soil related research (Son, 1999) (Table 1).
Table 1. Trends likely to affect forest management in the future and implications for forest soils research (modified from SSSA, 1987)
¡¤Increased use of chemicals
¡¤Long term impacts on
¡¤Soils and biota relationship
Rising energy costs
¡¤Afforesting marginal farmlands
through incentive programs
¡¤Poor sites managed less
¡¤Low cost reforestation
¡¤Site specific predictions
¡¤Long term impacts on site
Rising cost of skilled
¡¤Increased reliance on unskilled
¡¤Remote sensing of site and
¡¤Better nursery stock
Land use changes and
¡¤Special use zones
¡¤Effects of management
practices on water quality
¡¤Physical impacts on site
¡¤Rehabilitation, soil stabilization, Nitrogen-fixation
¡¤Uneven-age management on
¡¤Intensive management zones
¡¤Site specific prediction models
1). Nitrogen fixation in forest ecosystems
The topic is far from new; use of lupines, black locust, alders, and other legumes for amelioration of difficult planting sites is promising. Especially nitrogen-fixing species will be appropriate for plantation in marginal sites including deserts. Some biological nitrogen fixation occurs in all forest, but usually the rates are low relative to precipitation inputs and to tree requirements. Forests with species capable of symbiotic nitrogen fixation, however, may have nitrogen fixation rates that rival the annual uptake requirement for nitrogen. Some nitrogen-fixers, such as red alder and black locust, can be used directly for commercial products. In other cases, nitrogen-fixers may be used to increase the growth of interplanted crop trees. Crop trees mixed with nitrogen-fixing species experience increased nitrogen availability, but may suffer from competition for other site resources. The value of silvicultural systems with nitrogen-fixing species depends on the balance between enhanced nitrogen nutrition of the crop trees and increased competition for other resources. Biological nitrogen fixation can be a useful silvicultural tool, and is a potential alternative to nitrogen fertilization. As with all tools, however, it is not appropriate for all situations. The choice between nitrogen fixation and nitrogen fertilization requires an understanding of the ecological and economic effects of both sources (Fisher and Binkley, 2000).
Since the first attempts to increase forest growth using nitrogen fixation, biological nitrogen fixation has been intensively studied. Numerous studies focused on symbiotic nitrogen fixation and rates were well quantified (about 100kg N/ha/yr). However, the rates and importance of nonsymbiotic nitrogen fixation in forest ecosystems remains incomplete. The reported rates of nonsymbiotic nitrogen fixation in temperate forest ecosystems varied greatly, and ranged from <0.01 to 5kg N/ha/yr. However, an average input of 2-3kg N/ha/yr could be expected when all ecosystem components were included (Son, 2000a).
Nitrogen fixation has received much less attention than fertilization in forest nutrition management. At present, stand prescriptions for the use of nitrogen fixing plants are only guesses. Ideal prescriptions would balance nitrogen fixation rate (and subsequent effects on nitrogen cycling) against competition with crop trees for other resources. Nitrogen fixing trees are currently used in forestry in a few situations on temperate areas and commonly in tropical areas. About 0.5 million hectares of black locust plantations are found in the former Austro-Hungarian Empire, and black locust plantations exceed all other species in Hungry. The use of nitrogen fixation in silvicultural systems is more limited in other temperate areas, but it is increasing. However, in comparison with the application of industrially fixed nitrogen, enhancement of biological nitrogen fixation is; (1) currently limited by knowledge of nitrogen fixing species biology and silviculture, (2) likely to be more expensive on a unit applied nitrogen basis, (3) slower in producing fertility increases, (4) a possible source of vegetative competition for other crop species, and (5) likely to produce a managerially more complex ecosystems. These problems would be topics for the future to use nitrogen-fixing species (Fisher and Binkley, 2000).
2). Tree roots and microbes
Like almost all plants, forest trees draw water and nutrients from the soil by means of fungi associated with the fine roots. The biology and function of tree roots is likewise scarcely novel but will be examined far more comprehensively over the next few decades. Forests are vast reservoirs of biological and environmental variability. Among the least understood and exploited components of that variation are those underground, "out of sight, out of mine". The greatest challenge is to understand and use existing rhizospheric variation, and the two principal information needs to do this are; (1) an understanding of the control of carbon allocation and its relationship to carbon fixation and (2) a detailed understanding of the control of the interactions between microbes and roots, coupled with knowledge of the true range of existing variation. This approach would allow systems to be designed for specific purposes, and would direct us to critical genetic engineering tasks and give us estimates of the amount of improvement possible.
The rhizosphere is biologically more active than bulk soil because it is eutrophic relative to bulk soil. The basis for the increased activity, and the significance to productivity of the rhizosphere lies ultimately in the quantity and kind of carbon-based compounds translocated from the shoots to the roots. If the objective is to proliferate symbiotic tissue, or to make effectively by increasing its supply of energy, or to supply additional nutrition to free-living organisms in the rhizosphere, either a reallocation of carbon within the tree, or an increase in net photosynthesis is necessary. Some questions related to these objectives are; (1) how much carbon reaches the rhizosphere? (2) what compounds are released? (3) what controls quantity and kind? (4) how can quantity and kind be maintained through biotechnology?
The simplest approach to protection in the rhizosphere is to select organisms or combinations of them that are resistant to specific pathogens. Selection and breeding of protective or immune organisms is, however, limited by the problem of competition from indigenous organisms. Protective organisms must not only protect, they must persist in a highly competitive environment, and long term trials will be needed to verify their ability to do so. For pollution resistance, two approaches need to be simultaneously pursued; (1) understanding effects of pollutants on the rhizosphere and (2) understanding direct effects of pollutants on the rhizosphere chemistry, organisms, and symbioses (SSSA, 1987).
3). Long term productivity
The most important feature of forest soils management is long term sustainability of productivity. The productivity may be defined in terms of ability to support the rapid growth of trees. Millions of hectares of the region's forests managed as plantations include a wide range of soil and climatic conditions and a variety of tree species. Plantation establishment following clearcutting of the same species is standard practice in many areas where intensive silviculture is practiced. However, intensive practices may influence long term productivity. Plantations are literally man-made forests in the sense that they are established and maintained as the result of site manipulation. Such efforts to improve the site and increase tree survival and growth may have profound influences on certain soil properties. Whole tree harvesting, very short rotation, and drastic site preparation all increase nutrient losses associated with harvest. Especially the latter raise questions about the adequacy of long term soil supplies for future stands, and hence about productivity declines. At this time these questions can not be answered by means of soil analysis as now used except for the obviously infertile and highly fertile soils, where it is scarcely needed (Fisher and Binkley, 2000; Son, 2000b).
Alternative approaches for the future may include estimates of long term mineralization rates, availability, forest floor characteristics, and root uptake capabilities and modeling, and soil biology appropriate to the kind of soil and long times considered. The other important aspect of long term productivity is nutrient management on forest land. For many areas, researchers have developed nutrient cycles, which include the sizes of discrete forms or locations of nutrients and the relative movement of nutrients from one pool to another. In addition, the application of municipal waste to forest land is becoming more important in terms of fertility, recycling and environmental protection. However, we do not have enough information on nutrient distribution and cycling for major vegetations and the effects of biosolids on productivity and environments in this region. More efforts on these topics are needed. Also it should be emphasized that the political and policy ramifications of the maintenance of forest productivity will be with us for a long time, even though we believe that we scientifically understand the basis of forest productivity (IUFRO, 1997; SSSA, 1987).
4). Land use change
In terrestrial ecosystems the amounts of carbon in soil is usually greater than the amount in the living vegetation, and soil carbon storage is dependent on environmental, biogeochemical and land management factors. Changes in land use can have a marked effect on soil carbon contents as a result of the interactions between changes in detrital inputs and subsequent immobilization mediated by soil microorganisms. Such changes are important from the viewpoint of soil fertility and long term sustainability and for their influence on atmospheric carbon dioxide concentrations and global warming. In general, the loss of soil carbon by conversion of natural vegetation to cultivated use is relatively well understood. In many parts of eastern Asia the original forest was cleared during the past several decades and converted to farmlands and urban areas. However, currently urbanization and industrialization are rapidly developing and populations are continuously moving from agricultural areas to urban areas. Therefore, the area of abandoned agricultural lands is increasing throughout the region. Generally, however, there have been few comparative studies of the influence of these land use change on soil biochemistry. Intensive work on effects of agricultural land conversion to natural vegetation or afforestation on soil and global carbon dynamics is needed (Adger and Brown, 1994; Kronert et al., 1999; Post and Kwon, 2000).
Education and Organization
1). Education in forest soils
The world is currently undergoing a science and engineering reform with massive influx of public and private monies to counter this educational void. Understanding the near-surface earth properties, process, and functionality is essential to global habitat sustainability and to long term forest soil productivity. Forest soil science provides the educational framework to integrate components of forest ecosystems, to understand the causes and consequences of forest management, and view dynamic processes impacting ecosystems in a holistic perspective. The future of forest sustainability is heavily dependent on our ability as educators and scientists to effectively communicate this message (Baveye et al., 1994).
The clients of forest soil science education have belonged three groups; undergraduate students, graduate students, and those clients reached through extension activities. Our clientele are diverse; they represent multiple occupations, backgrounds, value judgements, interests, and experiences. Their understanding of soils and forest resources may be limited. In addition, a number of directions seemed clearly dictated by recent trends and events. The need for a shift of emphasis from conventional forest soil to environmental soil related issues mandates drastic changes in forest soil science curriculum. The rapid pace of technological advances challenge the need for and the usefulness of an extension service in its current forms. Furthermore, universities must prepare their students for a life of continuous learning. Traditionally, preferred mode of transmission of knowledge has been via formal lectures in classroom settings. However, visual aids (videofilms and multimedia technologies) are currently evolving at a phenomenal pace. Forest soil science educators have to approach the advising undergraduate and graduate students with more information on these technologies. Also, field and laboratory experiences in the curriculum should be emphasized (Baveye et al., 1994).
It is useful to mention the importance of organizations and organizational structures which will aid development and achievements to understand the function and properties of forest soils. Commonly organizations provide a basis for transfer of information and research discussions in a specific arena. In North America, forest soil scientists were heavily dependent for technical and instructional material on associates who were primarily interested in using soils information to manage agricultural lands. However, at this time Soil Science Society of America has a separate division of forest and range soils in the structure. It would be similar to this region; forest soil scientists should find a proper way to secure their own area in societies and organizations. Therefore, it is necessary to encourage soil science related societies and organizations to understand the values of forest soils in their field. Sponsorship of forest soil symposium from those societies and incorporation of forest soils into that structure would be one practical way to begin. Also it is important to consider international soil science activities and their impact on the study of forest soils in eastern Asia. Another possible organization which will be very effective in promoting forest soils and disseminating information about current management of forest soils in the area would be east Asia forest soils meeting like North America Forest Soils Conference (Gessel and Harrison, 1997).
The sketchy description given above of some of the directions that forest soils research will take in the years ahead indicated the tremendous excitement in this research area. However, we shall face enormous technical and intellectual challenges in order to provide some degree of confidence in our predictions of the consequences of a continuation of current land use pattern. Soil properties do play a central role in forest productivity, but so do many other factors. Soils also vary greatly from place to place on the earth's surface and also locally. Information and knowledge from a specific place can not be equally applied to everywhere. In addition, it should be noted that the concept of forest soil as a component of a delicate forest ecosystem where constant linkages between all parts of the forest ecosystem are critical to long term forest health is important. Therefore, the challenges to future forest soil scientists include developing knowledge of forest soils both as an entity worthy of study in its own right, but particularly as a critical component with myriad linkages to the whole forest ecosystem. It is also clear that the primary importance of forest soil research in the future will be to assure and demonstrate that forest management be sustainable in the long term. In this point, more close collaboration between Korea and China in this area is desperately needed.
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