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Tree rings and forest management in Europe
Dendrochronologia 20/1-2 (2002) 191±202 ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/dendro
Tree rings and forest management in Europe Heinrich Spiecker Institut fuÈr Waldwachstum, Albert-Ludwigs-UniversitaÈt Freiburg, Germany
Summary Information about forest growth is essential for sustainable forest management. Not only site conditions, species composition and age structure have been modified by human activities but also economic, political and social conditions and aims have changed. Updated site ± and species related growth information is needed for efficient management. Information on forest growth does not only describe the volume production potential of forests and the dimension and quality of the produced wood but also is a valuable base for understanding interactions between trees and their environment. Tree rings provide precise information on past growth reaction to environmental changes. They allow a better understanding of sensitivity of species on a given site to environmental changes and provide information for risk assessment. Tree rings support the choice of tree species composition and the analysis of age effects on forest growth. Effects of spacing are indicated by ring width. Tree rings analyses improve wood quality control and contribute to cost efficient forest management. Keywords: Tree rings, forest management, forest growth, environmental changes, site productivity.
Introduction Centuries of exploitation and devastation of forests resulted in a shortage of wood which stimulated forest research and created the idea of sustainable forest management. Information was needed on how to produce and harvest wood in a sustainable way. The traditional aim of forest growth studies was the development of growth models of trees and stands on different sites under different treatments. Long-term research plots have been established world wide starting in Europe about 150 years ago and forests
were managed based on inventory data and professional cutting plans. Repeated tree height and diameter measurements on various sites were used to
Address for correspondence: Heinrich Spiecker, Institut fuÈr Waldwachstum, Tennenbacherstr. 4, 79085 Freiburg, Germany Tel.: +49 76 12 03 37 37; Fax: +49 76 12 03 37 40 [email protected] http://www.forst.uni-freiburg.de/Waldwachstum/
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192 H. Spiecker determine growth of trees and of forest stands. Based on these data growth models such as yield tables have been established for many species which served as a widely used guide for management. Recently however it was observed that site productivity is not constant at a given location but is changing with time (Spiecker et al. 1996). Tree and stand growth frequently deviates from yield tables. There are several reasons for this deviation: · Yield tables are very simplified models and are not able to represent reality in detail. · Site conditions encompassing all growth relevant environmental factors are changing with time on a large scale. · Species composition and genetic structure of forests are subject to changes. · Age composition of forests are changing. As forest management has extremely long lasting consequences, updated information about the current and future conditions are especially important. For more reliable prediction of future forest resources development a better understanding of changes in ecological, economic and social environment is needed. Information about the causal relations between the changing environmental factors, forest growth changes and the risks involved are still lacking. Long-term empirical field data and results from laboratory experiments combined with modelling are a good basis for multidisciplinary research to fill these information gaps. Societies' needs with respect to forest ecosystem management change with time as well (Pelkonen et al. 1999). Forest ecosystem management has been affected by the relatively decreasing importance of commodity functions of forests: Rapid changes in energy sources, changes in production, harvesting, and transportation technologies, as well as changes in the use of wood for construction work. In addition, with rising living standards and urbanization, protective, environmental, social and cultural functions became more important. The discussion about the future of the forest sector has never been more intensive and politically-orientated than it is today. Principles of sustainable forest management, forest laws, directions and regulations are being reviewed. Ecosys-
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tems with rich biodiversity are recognized to be essential goals. It is recommended to reserve parts of the forests for different kinds of conservation purposes; in addition to these special conservation areas, where any forestry activity is prohibited, there may exist some small ecologically valuable areas in managed forests worth conservation. With regard to global change the potential of carbon sequestration in forests and renewable wood products are considered important. Forest managers have to reduce risks, to increase wood quality and improve cost efficiency. They have to adapt to changing needs of forest owners, wood industry, nature protection groups and other users of forest goods and services. Changed objectives and restrictions require different management information. Forest growth information is the base for predicting not only sustainable wood production but also wood quality and other aspects like carbon sequestration, biodiversity and risk assessment. Updated information on tree and stand growth is relevant for ecological, economic and social aspects of forest management. As forests cover large ecological ranges and forest management differs as result of centuries of human activities this information needs to be site and species related. This means that a sound understanding of tree growth and growth reactions for various sites, tree species, provenances and management regimes is needed. Information about the effect of management should not only describe total volume growth but also tree dimension, branchiness, tree ring structure, technical properties of wood as well as forest growth dynamics, mortality and risk.
Tree rings: an important data source for management information Most tree species produce at least in the temperate and boreal zone annual rings which allow the reconstruction of radial increment and height increment from stem analysis (see fig. 1). Tree rings reflect environmental conditions and their changes and store the reaction pattern over time which can be later used as archive. They are a unique data source as they cover a wide range in
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· statistical tools for analysis of the environmentgrowth relation have improved · statistical tools for spatial variation based on GIS provide better information and better spatial description of growth pattern and environmental conditions · there is fast progress in related research fields such as soil science, meteorology, tree physiology, biochemistry, genetics, forest growth science.
Figure 1. Stem analysis for reconstructing annual radial increment and height increment. At various heights stem disks are sampled and annual radial increment is measured in predefined directions on the cross sections. In addition annual height shoot increment is measured. By comparing the number of annual shoots with the number of tree rings at a given stem height, quality of the measurements is controlled (Gerecke K.-L.1988).
space and time. By selecting sites and sample trees that grew in a specific environment information for answering actual questions may be provided. Tree rings allow a detailed reconstruction of tree growth: trunk, branches and even needles (Jalkanen et al. 2000) and under certain conditions the roots (Krause and Eckstein 1993). The growth patterns over time can be used as data archive. These archives are available where trees form annual rings and where living trees or subfossil or fossil wood exist. In recent years tree rings as a data base for forest growth became even more attractive because: · analytical tools for exploiting the data archives in tree rings have improved · information about environment and environmental changes has widened
The huge potential of interdisciplinary and international collaboration in the analysis of different environmental conditions and their effects on tree growth is not yet fully exploited. Progress in microsystem techniques, system analysis, physics, climatology, soil science, biochemistry, molecular biology, tree nutrition, tree physiology and genetics improve the research potential. The interpretation of tree ring analysis can further be improved by controlled experiments. Limitations caused by short observation periods of experiments may be compensated by long-term retrospective field observations based on tree rings which do allow investigations of long lasting effects.
Tree rings provide information on changes in environmental conditions Numerous long-term growth investigations indicate an increasing growth trend in European forests. An analysis of growth trends in European forests was carried out in a project of the European Forest Institute by 44 scientists from 12 countries (Spiecker et al. 1996). Some of the case studies cover rather small areas, others ± as for example some inventory data based studies ± refer to larger regions or nations. The time span of available data varies from a few decades to several centuries. The aim of the study was to analyze whether site productivity has changed. The term site productivity in this study is limited to the wood production potential of a site for a particular species, provenance or forest type. Biomass data were lacking and therefore could no be used for describing site productivity because only wood production data where collected by repeated forest inventories, by permanent plot observations, and stem analysis. Productivity of
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194 H. Spiecker forest sites in Europe has changed considerably in recent decades. Although the methods applied varied according to the data available, most studies showed the same general trend: site productivity has increased on many sites. The results derived from long-term observations on permanent plots and from tree analyses are supported by inventory results which are representative of large areas, but cover generally shorter observation periods. For the analysis of changes in site productivity height growth is a reliable indicator. When ring width is used changes in tree competition have to be taken into account (fig. 2). Site productivity increased on various sites in recent decades by up to 50 %, in some cases even more. Annual tree height increment increased by up to 5 cm and more, varying with species, site and age of the trees. Several other publications on changes in forest growth support the findings described by
Figure 2. Trend in ring width of silver fir (Abies alba Mill.) forests in the Vosges mountains. Above: average ring width over age; below: average ring width deviation from the expected ring width shown in the upper curve (Becker 1989).
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Spiecker et al. (1996) (e. g. Schmidt 1969, Pretzsch 1985, Spiecker 1986, Gerecke E. 1988, Kenk and Spiecker 1988, Becker 1989, Kenk et al. 1991, Kauppi et al. 1992, Keller 1992, Eriksson and Johansson 1993, Becker et al. 1994, Kuusela 1994, Spelsberg 1994, Elfving and Tegnhammar 1996, Untheim 1996, RoÈhle 1999). These findings indicate changes in growth conditions. The observed trends are species specific, locally varying and modified by several years lasting, large growth variations (see e. g. fig. 3 and fig. 5). On a European scale, species and site specific quantitative information about the extent and spatial as well as temporal variation in growth acceleration is lacking (Spiecker 1999). By combining tree ring and shoot growth measurement additional information on changes in forest growth can be provided. Several causes for the observed changes in site productivity are discussed (Rehfuess et al. 1999): changes in land use history and forest management such as nutrient losses due to litter racking, changes in natural disturbances, in physical and chemical climate including changes in air temperature, in tropospheric CO2-, O3-, and SO2- concentration and changes in nitrogen deposition as well as in fertilizing and liming. The significance of each of these
Figure 3. Salvage cuttings of desiccated trees and trees killed by fungi and insects in % of allowable cut (bars) in the public forests of the Black Forest and annual variation of radial growth (ir, solid line) of Norway spruce (Picea abies [L.] Karst.) in the Black Forest region. Average annual radial increment of 45 Norway spruce trees (8 radii per tree at 1.3 m, tree age: 95 years in 1980) and mortality are correlated. The climatic water balance (ETP-index) derived from monthly air temperature and precipitation during previous five growing seasons is plotted as grey area. In warm and dry periods (grey area below zero) growth is reduced and mortality increased.
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possible causes possibly varies in space and time. Growth responses to the influencing factors are modified by species, provenance, site and stand conditions. It is difficult to draw general conclusions regarding their effects on forest growth. It is possible that one single factor or a factor combination, or even different factor combinations varying with location, influence growth. Several factors may influence forest growth simultaneously and their total synergistic effects may change the individual monofactorial effects. Environmental issues gained additional interest during the discussion about forest decline in recent decades. Site conditions are changing naturally but also an increasing impact of human on these site changes is observed. Increased site productivity is of direct relevance for forest management. Besides increasing the allowable cut as well as increasing tending and thinning intensity it may have an effect on species selection and risk management. Increased biological growth improves the economics of forestry. To assess the potential risk involved in these changes a better understanding of the causes is required. Tree rings are one source of data for improving this understanding (Schweingruber 1993).
Tree rings provide information for risk management Disturbances play a decisive role in forest development. They interrupt continuous succession and alter driving factors for forest growth, such as competition for light, nutrients and water. They may have an impact on selection processes and tree species composition. In central Europe the two most important disturbances factors are storm and snow. The annual variation of salvage cuttings is rather high. Changes in growing stock affect tree susceptibility to drought, frost, fungi and insect attacks as well as to other diseases. The risk of wind throw is increasing when trees get taller and when growing stock accumulates. Impacts of accelerating growth on forest management, markets and timber quality have been discussed by several authors (Karjalainen et al. 1999). Tree rings also allow a retrospective analysis of fires and their effect on forests (Berli and
Schweingruber 1992). Climatic changes as a result of the enhanced ªgreenhouse effectº are believed to be a global environmental threat. The exploitation of forests and other natural resources, the accumulation of anthropogenic emissions and the increasing amount of carbon dioxide and other greenhouse gases in the atmosphere might cause changes in air temperature, precipitation, frost events and wind speed. Environmental changes might damage the functioning of ecosystems and possibly modify biodiversity. Climate changes affect the competition between tree species. This may lead to major disturbances in natural as well as in managed ecosystems. Also, nutrient cycling will be largely affected by climate changes. Trends in forest productivity are generally associated with climatic variation, in the temperate zone particularly with variation in precipitation. High air temperature and low precipitation during growing season reduced growth even at higher altitudes of the Black Forest where average precipitation is high and average temperature is relatively low (Kahle and Spiecker 1996). Decreasing growth rate under certain conditions is accompanied by increased tree mortality (fig. 3). Suppressed trees in dense stands are especially susceptible to drought (Spiecker 1986). As compared to mortality due to storm, snow or fire the volume of desiccated trees and trees killed by insects and fungi is in managed forests of the boreal and temperate zone in Europe generally rather low. Trees are classified as `desiccated' when they are dead without showing visible damages. However impacts of climatic fluctuations on forests have often been underestimated in the past. It has been shown that growth rates and mortality are closely related to climatic fluctuations especially to drought conditions as for example in the 1940s, the 1970s or the 1990s (fig. 3, Spiecker 1995b). Growth response to climatic influences varies with species, provenance, competition status and site conditions. Changes in average climatic conditions such as air temperature affects the length of the growing season and influence site productivity (Fabian and Menzel 1999). A positive correlation between air temperature increase and plant growth seems likely
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196 H. Spiecker in the boreal and temperate climate zone. Extreme and unusual events such as late spring frosts, prolonged summer droughts, unusual cold and wet or hot and dry summers as well as extreme and abrupt temperature changes may reduce growth and increase mortality. Extreme climatic events not only have a direct effect on trees, but also on insects or microbial pathogens, on disturbances such as fire or storm throw, and on biological as well as chemical and physical processes in the soil. Correlations of these factors with growth have been reported (Nilsson et al. 1995). Droughts generally cause growth decrease: In Central Europe such a growth recession due to drought occurred for example in the late 1940s. A severe drought that attracted high tention occurred in the mid 1970s at the beginning of the discussion on defoliation and forest dieback (fig. 3). The long-lasting after-effects of such extreme events complicate the detection of possible causes. There are indications that in some regions Norway spruce (Picea abies [L.] Karst.) is showing an increased sensitivity to climate variation (Kahle, 1994). Changes in the frequency and intensity of extreme warm and dry climatic conditions during the last decades and effects of increased atmospheric depositions have been discussed as possible causes for this observation. Effects of insect attacks on tree growth can retrospectively be analyzed when the at-
Figure 4. Effect of defoliation by Lymantria dispar on growth of oak (Quercus sp.): After two consecutive years of Lymantria attacks growth of oak (grey line) has substantially decreased as compared to the oaks which have not been affected by this insect (black line) (Piper 1998, modified).
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tack has been registered carefully (fig. 4). Tree ring analysis allows a better insight in forest growth dynamics and their driving factors and therefore provide information for risk assessment. New planning and decision tools have to be applied to encompass uncertainty and risk in long-term forest planning. Traditional anticipation of goals and means based on past experience and current knowledge may not be adequate to scope with risk and uncertainty. Adaptive planning taking into account rapidly changing conditions on the other hand may lead to inefficient long-term management. Flexible planning which takes into account all conceivable scenarios and allows various options for future development may be best suited.
Tree rings support the choice of tree species composition Species composition has been historically subject to drastic changes. In central Europe for example broad-leaved species dominated forests by covering about two third of the area until this had been reversed by human activities starting already at the end of the thirteenth century. The area of European beech (Fagus sylvatica L.), oaks (Quercus sp.) and other broad-leaved species has been reduced while the area of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst.) has been increased substantially. By substituting faster growing species for less productive species not only roundwood production has increased, revenues have increased even more because of the higher softwood prices as compared to hardwood at that time (e. g. Landesforstverwaltung Baden-WuÈrttemberg 1953± 1998). The recent start of a reverse change in tree species composition is stimulated by various factors: hardwood price increased relative to softwood. The popularity of close to nature forestry has led in many cases to a shift towards more broad-leaved species. Volume growth has become less important while wood quality has got higher attention in countries where salaries are high. Higher ecological and economic value of slower growing species may under current conditions in various countries compensate
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for losses in volume growth. By adapting principles of natural processes cost-efficiency may be improved as well. Current management strategies indicate that former human induced changes are supposed to be at least partly reversed in the future. The impact of the planned changes will be visible in the overall species composition only after decades as production periods are long and the percentage of conifers in existing younger stands is relatively high in many countries naturally dominated by broad-leaved species. Only in the youngest age classes the change will be visible in the near future. The ongoing drastic changes in species composition will have longterm effects on the ecological conditions of forest ecosystems. Mixed stands have been found to be more resistant against various forms of damage, more diverse in their fauna and flora composition than pure, single-species stands. Broad-leaved trees are supposed to improve the conditions for growth by affecting the root systems, litter quality and nutrient and carbon storage and soil acidity (Kreutzer 1981, Liljelund et al. 1986, Mitchell and Kirby 1989, Feger 1993). The question of conversion of pure secondary coniferous forests on sites naturally dominated by broadleaves is essential for sustainable fulfillment of society's needs. A conversion of some of these forests to near-natural, site-adapted and often mixed forests has to be considered. For planning such a conversion, information about the technical alternatives of forest conversion and their ecological and economic impacts during and after the conversion process is needed on tree, stand, landscape and regional level. Only in recent times interdisciplinary research projects addressing the conversion topic have been launched in several countries. The outcome of this research may help stakeholders take into account the consequences of the forest conversion, already planned on large areas. Impacts are expected in the fields of forest nutrition, forest growth, biodiversity and other aspects of forest ecology as well as in forest management, silviculture, roundwood production, wood quality, timber markets, forest economics, forest operations, employment and others, making clear that forest conversion is not only of practical relevance but also of political
interest. A better understanding of natural processes and forest dynamics provides information to improve species composition by planting, tending and thinning. Criteria for defining conversion priorities may help to improve conversion strategies. For site adapted species selection and management sound site-, species- and stand specific information on growth and mortality is essential. There are only few long-term research plots which provide this information. Stem analysis based on measurements of tree rings can contribute substantially to a better understanding of differences in the growth dynamics of tree species and to the question of conversion, the long-term effects of conversion on tree growth, and so furnish information for defining priorities of conversion and conversion strategies. The analysis of past growth reaction may help to understand how adapted a tree species is to a site and to climatic changes. A conversion generally requires a sudden release of remaining trees by cutting some trees to provide space for a new forest generation. Therefore information on reaction of released trees is needed. The growth reaction of trees that have been exposed to sudden release e. g. by snow breakage at different stages of development can be analyzed retrospectively based on tree rings for providing this information.
Tree rings for investigation of age effects The age composition of forests varies considerably with species, stand and site conditions and historic events. While extended forest areas in various regions were destroyed world wide, in Europe the forest area as well as the average age of forests increased during recent decades. In Switzerland the rising age is especially pronounced (Schweizerisches Forstinventar 1988). In former West Germany the area of the youngest age class has decreased in recent years while the area of forests in all age classes older than 60 years has increased (BML 1993). It is well known that the growth potential of forests is age dependent. While researchers who are interested in climate-growth relations want to remove this `age
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198 H. Spiecker
Tree rings and tree spacing Growing stock is affected by forest management practices such as regeneration methods, initial spacing, tending, thinning and harvesting regimes (Mitscherlich 1978). In dense stands radial increment of individual trees is lower while radial increment of released trees is higher (fig. 6 and 7).
Figure 5. Net annual volume increment and age. The periodic net annual volume increment of two Norway spruce stands (Picea abies [L.] Karst., solid lines) on long-term permanent research plots in southwestern Germany is plotted over stand age (data: Forest Research Station Baden-WuÈrttemberg, Freiburg). The horizontal bars describe the length of each measurement interval and the level of the average annual volume increment within the interval. The dotted curve describes net annual volume increment as expected according to a yield table (Wiedemann 1936/42). Observed growth deviates considerably from the expectations based on the yield table while stand density (not shown here) followed the yield table.
trend', managers are interested in the effect of age on growth. The production period is influenced by site productivity because fast growing trees reach the desired dimensions earlier than before and the economically optimal rotation age may be lowered. According to Eriksson and Johansson (1993), an increasing growth trend may also influence the growth rhythm of trees. Trees may grow faster when they are young but may reach the culmination of current annual increment earlier than in former generations. This effect may not be visible in today's forests because increased site productivity also accelerates the growth of old trees (fig. 5). As age of forests in Europe has increased in recent decades in many regions knowledge about growth of older forests is rather limited and existing growth models often do not describe growth of older stands adequately. Additional research based on tree ring analysis of older trees is needed to quantify the effect of age on growth.
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Figure 6. Differences in radial growth trends of Norway spruce (Picea abies [L.] Karst.) in the Black Forest caused by past competition: 1: wide initial spacing, no thinning ± data base: 8 radii per tree at 1.3 m of 11 dominant trees, tree age: 96 years in 1980 (Kenk 1990) and 2: heavy release after growing in a rather dense forest ± data base: 8 radii per tree at 1.3 m of 5 dominant trees, tree age: 185 years in 1980 (Spiecker 1992).
Figure 7. Effect of thinning on radial increment of oak (Quercus sp.): The average annual radial increment at 1.3 m above ground of two in 1961 heavily released trees and two unreleased trees is plotted over time. Eight years after release the accelerated growth reached its maximum when also the unreleased trees showed a relatively high growth (Spiecker 1991).
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Dense stands consisting of shade tolerant tree species offer less space for light demanding tree species and ground vegetation to exist and therefore biodiversity will be reduced (Mitchell and Kirby 1989). A conversion to more light demanding species is in the near future only possible through active forest management. This includes the necessity to cut trees. Without intervention, forest health will decline (Spiecker 1995a). Tending of young stands allows the creation of a mixed stand structure. Growth of young stands may be strongly influenced by soil preparation, selection of species and provenances, quality of plant material and weed control. Competition control requires site adjusted tending and thinning methods because diverse species may react differently to site conditions. Furthermore, changes in site productivity may alter competition between trees. An adjusted thinning regime is also required to counterbalance accelerating growth rates, otherwise the increase in average growing stock will continue and the stability of forests will be reduced. Mixed stand structures can be maintained and improved by thinning. Thinning in particular increases the proportion of large-sized timber in total harvest volume. Wood resources will change with a shift in tree species and age composition as well as in site conditions. Not only growing stock is changing but also wood assortments. Rising average tree age increases presumably the share of wood of large-sized trees. Implications of high standing volume of mature trees provides not only an increased timber supply capacity but also causes a higher risk for tree health because the resistance of dense and over-mature forests against disturbances like drought, storms, snow or pests may be reduced. Little information is available on growth behavior of old forests. Retrospective stem analysis may be the only way to get this information.
Tree rings and wood quality control Ring width is associated with various other factors affecting wood quality such as wood density, fiber length and cell structure within tree rings (Nambiar 1995, Park 2000, Spiecker et al. 2000). Tree rings
also correlate with internal branchiness as larger rings are generally produced by trees showing wider crowns and lower crown bases. As average ring width can be to a large extend modified by forest management, ring width is an important indicator for controlling quality. However annual variation of ring width caused by climatic variation can not be eliminated by management even so the absolute variation can be reduced by increasing competition and reducing average ring width. Sometimes the effect of ring width on wood quality has been exaggerated (Schulz 1959). Ring width also determines the diameter reached in a given production period. The diameter is an important criterion for determining wood prices. Furthermore the effect of pruning on growth and wood structure can be analyzed based on tree ring analysis (Shigo 1989). By chemical analysis of tree rings and a more detailed analysis of anatomical features of ring structure such as late wood formation or compression wood formation further information on wood quality can be obtained (Z'Graggen 1992, Saû 1993, Vaganov 1996).
Contribution of tree ring analysis to cost efficient management Growth of individual trees can be reconstructed by tree ring measurements. By relating the growth of an individual tree to its growing space the space economy of the tree can be derived. By comparing growth rates of various species in mixed stands and growth rates of different social tree classes in pure stands growth dynamics, dynamics of self thinning and natural pruning can be analyzed (Spathelf 1999). Based on a better understanding of natural growth dynamics under untouched conditions, management can be more efficient. Unnecessary activities can be avoided by integrating natural processes into the management activities. By aim oriented concentration on essential activities at the right time and at the right location input in forest management can be drastically reduced without losing much output value. This implies a better understanding of natural processes. Tree rings can help to improve this understanding.
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200 H. Spiecker
Conclusions
Acknowledgment
Due to changing environmental conditions and changing forest management objectives, updated information about trees and their growth is needed. Tree rings are a valuable tool to provide information about tree growth and growth reactions. We can very well and in detail reconstruct growth of individual trees. Tree ring data can be potentially collected where ever trees form annual rings, growth patterns can be dated at least in an annual resolution by precise and more and more automatized measurement equipment. In addition statistical tools for time series analysis and geostatistical analysis have improved. However there are some limitations in respect to managers information needs. Tree analysis in general does not allow the reconstruction of the long-term growth of stands, not only because it would be very time consuming and the analysis of growth using wood samples is destructive, but also because some trees have formerly disappeared and may have shown different growth patterns as compared to the remaining trees. Another shortcoming is that former environmental conditions of the individual tree such as the competition with neighboring trees, former site conditions, insect attacks etc. may be difficult or impossible to be reconstructed. It is a challenge of tree ring research to overcome some of these difficulties by improving the methods for using tree rings as an archive of data about former environmental conditions. As environmental conditions do not only change in time but also in space, comparison of growth reactions in different parts of the world may serve as quasi-experiments. Here is a chance for future international cooperation. The amount and complexity of the scientific problems evolving from the observed situation show that solutions can only be developed by a multidisciplinary cooperation of scientists and decision makers at an international level. This cooperation will lead to a more comprehensive understanding and will provide a more realistic and reliable information basis for decision support. Changing social demands today require a widened scope of forest management. This congress here in Davos will help to improve this international cooperation.
During many years of teaching and research it was a pleasure to work together with Fritz Schweingruber. I just want to thank Fritz for his creative and stimulating input in this productive cooperation and I look forward for future joint activities.
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Tree rings and forest management in Europe 201 zwischen der Variation von klimatischen Elementen des Wasserhaushalts und dem Radialzuwachs von Fichten (Picea abies (L.) Karst.) aus Hochlagen des SuÈdschwarzwalds. Diss. Freiburg i. Br., 184 pp. Kahle HP, Spiecker H, 1996. Adaptability of radial growth of Norway spruce to climate variations: results of a site specific dendroecological study in high elevations of the Black Forest (Germany). In Dean JS, Meko DM, Swetnam TW (Eds): Tree Rings, Environment and Humanity: Proceedings of the International Conference: Tucson, Arizona, 17±21 May 1994. Radiocarbon: 785±801. Karjalainen T, Spiecker H, Laroussinie O (Eds), 1999. Causes and Consequences of Accelerating Tree Growth in Europe. Proceedings of the International Seminar held in Nancy, France 14±16. May 1998. European Forest Institute Proceedings 27, 285 pp. Kauppi PE, MielikaÈinen K, Kuusela K, 1992. Biomass and Carbon Budget of European Forests, 1971 to 1990. Science 256: 70±74. Keller W, 1992. BonitaÈt in Fichten-FolgebestaÈnden ehemaliger Fichten-VersuchsflaÈchen der WSL. Deutscher Verband Forstlicher Forschungsanstalten, Sektion Ertragskunde, Tagungsbeichte: 123±129. Kenk G, Spiecker H, 1988. Einige Ergebnisse zum aktuellen und fruÈheren Wachstum von Fichten. Kernforschungszentrum Karlsruhe Projekt EuropaÈisches Forschungszentrum fuÈr Maûnahmen zur Reinhaltung der Luft 35(1): 371±381. Kenk G, 1990. FichtenbestaÈnde aus WeitverbaÈnden. Entwicklungen u. Folgerungen. Forstwissenschaftliches Centralblatt 109: 86±100. Kenk G, Spiecker H, Diener G, 1991. Referenzdaten zum Waldwachstum. Kernforschungszentrum Karlsruhe Projekt EuropaÈisches Forschungszentrum fuÈr Maûnahmen zur Reinhaltung der Luft 82, 59 pp. Krause C, Eckstein D, 1993. Dendrochronology of roots. Dendrochronologia 11, 9±23. Kreutzer K, 1981. Die Sauerstoffbefrachtung des Sickerwassers in WaldbestaÈnden. Mitt. Dtsch. Bodenkundl. Ges. 32: 273±286. Kuusela K, 1994. Forest Resources in Europe. European Forest Institute, Research Report 1, 154 pp. Landesforstverwaltung Baden-WuÈrttemberg 1953±1998. Jahresberichte und Forststatistische JahrbuÈcher. Ministerium LaÈndlicher Raum Baden-WuÈrttemberg, Stuttgart. Liljelund LE, Nilsson I, Andersson I, 1986. TraÈdslagsvalets betydelse foÈr mark och vatten. En litteraturstudie med speziell referens till luftfoÈroreningar och foÈrsurning. ± SNV PM 3182, Statens NaturvaÊrdsverk, Solna. 47 pp. Mitchell PL, Kirby KJ, 1989. Ecological effects of forestry practices on long-established woodland and their implications for nature conservation. OFI Occasional papers no. 39, 172 pp. Mitscherlich G, 1978. Wald, Wachstum und Umwelt. J. D. SauerlaÈnder's Verlag, Frankfurt, M. second edition, 144 pp. Nambiar EKS, 1995. Relationships between water, nutrients
and productivity in Australian forests: Application to wood production and quality. Plant and Soil 168±169: 427±435. Nilsson LO, HuÈttl RF, Johansson UT, Jocheim H, 1995. Nutrient uptake and cycling in forest ecosystems ± present status and future research directions. Plant and Soil 168± 169: 5±13. Park YI, 2000. Zur Auswirkung von UmwelteinfluÈssen auf das Wachstum von Fichten (Picea abies (L.) Karst.) auf der Grundlage bildanalytischer Verfahren zur Quantifizierung der Zellstruktur an HolzquerschnittflaÈchen. Schriftenreihe Freiburger Forstliche Forschung Bd. 8, 193 pp. Pelkonen P, PitkaÈnen A, Schmidt P, Oesten G, Piussi P, Rojas E (Eds), 1999. Forestry in changing societies in Europe ± Information for teaching module ± Study book University Press, University of Joensuu, Finnland: Part I: Forestry in changing societies in Europe, 82 pp., Part II: Forestry in changing societies in Europe; country reports, 480 pp. Piper R, 1998. Auswirkungen eines Schwammspinnerkahlfraûes auf den Radialzuwachs bei Stieleichen. Allgemeine Forstzeitschrift 53: 54±55. Pretzsch H, 1985. Wachstumsmerkmale OberpfaÈlzer KiefernbestaÈnde in den letzten 30 Jahren. VitalitaÈtszustand-StrukturverhaÈltnisse-Zuwachsgang. Allgemeine Forstzeitschrift 40: 1122±1226. Ê gren GI, Andersson F, Cannell A, Friend A, Rehfuess KE, A Hunter I, Kahle H-P, Prietzel J, Spiecker H, 1999. Relationship Between Recent Changes of Growth and Nutrition of Norway Spruce, Scots Pine, and European Beech Forests in Europe ± RECOGNITION ±. European Forest Institute, Joensuu, Finland, Working Paper 19, 94 pp. RoÈhle H, 1999. Changed growth performance in Germany exemplified by spruce and conclusions for forestry. In Karjalainen T, Spiecker H, Laroussinie O (Eds), 1999. Causes and Consequences of Accelerating Tree Growth in Europe. Proceedings of the International Seminar held in Nancy, France 14±16. May 1998. European Forest Institute Proceedings 27: 217±227. Saû U, 1993. Die GefaÈûe der Buche als oÈkologische Variable: Bildanalytische Erfassung, dendroklimatologische PruÈfung, oÈkologische Bewertung. Dissertation Hamburg, 171 pp. Schmidt A, 1969. Der Verlauf des HoÈhenwachstums von Kiefern auf einigen Standorten der Oberpfalz. Forstwissenschaftliches Zentralblatt 88: 33±40. Schulz H, 1959. Untersuchungen uÈber Bewertung und GuÈtemerkmale des Eichenholzes aus verschiedenen Wuchsgebieten. Schriftenreihe der Forstlichen FakultaÈt GoÈttingen, Bd. 23, 90 pp. Schweingruber FH, 1993. Jahrringe und Umwelt ± DendrooÈkologie. Birmensdorf, WSL 474 pp. Schweizerisches Forstinventar, 1988. Ergebnisse der Erstaufnahme 1982±1986. Berichte Nr. 305 der Eidg. Anstalt fuÈr das forstlicheVersuchswesen. 375 pp.
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202 H. Spiecker Shigo AL, 1989. Tree Pruning ± A worldwide photo guide for proper pruning of trees. Durham, New Hampshire, 186 pp. Spathelf P, 1999. Orientierungshilfe zur Prognose und Steuerung des Wachstums von Fichten (Picea abies (L.) Karst.) und Tannen (Abies alba Mill.) in ÛberfuÈhrungswaÈldern mit Hilfe der relativen KronenlaÈnge. Schriftenreihe Freiburger Forstliche Forschung Bd. 4, 192 pp. Spelsberg G, 1994. Zum HoÈhenwachstum der Fichte in Nordrhein-Westfalen. Allgemeine Forst- und Jagdzeitung 165(4): 77±80. Spiecker H, 1986. Das Wachstum von Tannen und Fichten auf PlenterwaldversuchsflaÈchen des Schwarzwaldes in der Zeit von 1950 bis 1984. Allgemeine Forst- und Jagdzeitung 157(8): 152±163. Spiecker H, 1991. Zur Steuerung des Dickenwachstums und der Astreiningung von Trauben- und Stieleichen (Quercus petraea (Matt.) Liebl. und Quercus robur L.). Schriftenreihe der Landesforstverwaltung Baden-WuÈrttemberg Bd. 72, 155 pp. Spiecker H, 1992. Which trees represent stand growth? In Bartholin T S, Berglund BE, Eckstein D, Schweingruber F (Eds), Tree Rings and Environment, Proceedings of the International Dendrochronological Symposium, Ystad, South Sweden, 3±9 September 1990, Lundqua Report 34: 308±312. Spiecker H, 1995a. Der Wald stirbt ± die HolzvorraÈte steigen? Forst und Holz, 50. Jg.: 53±54. Spiecker H, 1995b. Growth dynamics in a changing environment ± long-term observations. Plant and Soil 168±169: 555±561. Spiecker H, 1999. Growth trends in European forests, Do we have sufficient knowledge? In Karjalainen T, Spiecker H,
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Laroussinie O (Eds), 1999. Causes and consequences of accelerating tree growth in Europe. Proceedings of the international seminar held in Nancy, France 14±16. May 1998. European Forest Institute Proceedings No. 27: 157±169. Spiecker H, MielikaÈinen K, KoÈhl M, Skovsgaard JP (Eds), 1996. Growth trends in European forests. European Forest Institute Research Report No. 5, Springer-Verlag, 372 pp. Spiecker H, Schinker MG, Hansen J, Park Y-I, Ebding T, DoÈll W, 2000. Cell structure in tree-rings: novel methods for preparation and image analysis of large cross sections for statistical evaluation of cell parameters of various tree species. IAWA Journal, 21(3) 361±373. Untheim H, 1996. Zur VeraÈnderung der ProduktivitaÈt von Waldstandorten. Untersuchungen zum HoÈhen- und Volumenwachstum von Fichte (Picea abies [L.] Karst.) und Buche (Fagus sylvatica L.) auf Standorteinheiten der Ostalb und des FlaÈchenschwarzwaldes. Mitteilungen der Forstlichen Versuchs- und Forschungsanstalt Baden-WuÈrttemberg, Freiburg i. Br., 198, 239 pp. Vaganov EA, 1996. Analysis of seasonal tree-ring formation and modeling in dendrochronology. In Dean JS, Meko DM, Swetnam TW, (Eds): Tree Rings, Environment and Humanity: Proceedings of the International Conference: Tucson, Arizona, 17±21 May 1994. Radiocarbon: 73±87. Wiedemann E, 1936/42. Fichten-Ertragstafel. In Schober Ri (Ed.) 1975. Ertragstafeln wichtiger Baumarten. J. D. SauerlaÈnder's Verlag, 154 pp. Z'Graggen S, 1992. Dendrohistometrische-klimatologische Untersuchung an Buchen (Fagus silvatica L.). Dissertation Basel, 167 pp.
Tree rings and forest management in Europe Heinrich Spiecker Institut fuÈr Waldwachstum, Albert-Ludwigs-UniversitaÈt Freiburg, Germany
Summary Information about forest growth is essential for sustainable forest management. Not only site conditions, species composition and age structure have been modified by human activities but also economic, political and social conditions and aims have changed. Updated site ± and species related growth information is needed for efficient management. Information on forest growth does not only describe the volume production potential of forests and the dimension and quality of the produced wood but also is a valuable base for understanding interactions between trees and their environment. Tree rings provide precise information on past growth reaction to environmental changes. They allow a better understanding of sensitivity of species on a given site to environmental changes and provide information for risk assessment. Tree rings support the choice of tree species composition and the analysis of age effects on forest growth. Effects of spacing are indicated by ring width. Tree rings analyses improve wood quality control and contribute to cost efficient forest management. Keywords: Tree rings, forest management, forest growth, environmental changes, site productivity.
Introduction Centuries of exploitation and devastation of forests resulted in a shortage of wood which stimulated forest research and created the idea of sustainable forest management. Information was needed on how to produce and harvest wood in a sustainable way. The traditional aim of forest growth studies was the development of growth models of trees and stands on different sites under different treatments. Long-term research plots have been established world wide starting in Europe about 150 years ago and forests
were managed based on inventory data and professional cutting plans. Repeated tree height and diameter measurements on various sites were used to
Address for correspondence: Heinrich Spiecker, Institut fuÈr Waldwachstum, Tennenbacherstr. 4, 79085 Freiburg, Germany Tel.: +49 76 12 03 37 37; Fax: +49 76 12 03 37 40 [email protected] http://www.forst.uni-freiburg.de/Waldwachstum/
1125-7865/02/20/1-2-191 $ 15.00/0
192 H. Spiecker determine growth of trees and of forest stands. Based on these data growth models such as yield tables have been established for many species which served as a widely used guide for management. Recently however it was observed that site productivity is not constant at a given location but is changing with time (Spiecker et al. 1996). Tree and stand growth frequently deviates from yield tables. There are several reasons for this deviation: · Yield tables are very simplified models and are not able to represent reality in detail. · Site conditions encompassing all growth relevant environmental factors are changing with time on a large scale. · Species composition and genetic structure of forests are subject to changes. · Age composition of forests are changing. As forest management has extremely long lasting consequences, updated information about the current and future conditions are especially important. For more reliable prediction of future forest resources development a better understanding of changes in ecological, economic and social environment is needed. Information about the causal relations between the changing environmental factors, forest growth changes and the risks involved are still lacking. Long-term empirical field data and results from laboratory experiments combined with modelling are a good basis for multidisciplinary research to fill these information gaps. Societies' needs with respect to forest ecosystem management change with time as well (Pelkonen et al. 1999). Forest ecosystem management has been affected by the relatively decreasing importance of commodity functions of forests: Rapid changes in energy sources, changes in production, harvesting, and transportation technologies, as well as changes in the use of wood for construction work. In addition, with rising living standards and urbanization, protective, environmental, social and cultural functions became more important. The discussion about the future of the forest sector has never been more intensive and politically-orientated than it is today. Principles of sustainable forest management, forest laws, directions and regulations are being reviewed. Ecosys-
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tems with rich biodiversity are recognized to be essential goals. It is recommended to reserve parts of the forests for different kinds of conservation purposes; in addition to these special conservation areas, where any forestry activity is prohibited, there may exist some small ecologically valuable areas in managed forests worth conservation. With regard to global change the potential of carbon sequestration in forests and renewable wood products are considered important. Forest managers have to reduce risks, to increase wood quality and improve cost efficiency. They have to adapt to changing needs of forest owners, wood industry, nature protection groups and other users of forest goods and services. Changed objectives and restrictions require different management information. Forest growth information is the base for predicting not only sustainable wood production but also wood quality and other aspects like carbon sequestration, biodiversity and risk assessment. Updated information on tree and stand growth is relevant for ecological, economic and social aspects of forest management. As forests cover large ecological ranges and forest management differs as result of centuries of human activities this information needs to be site and species related. This means that a sound understanding of tree growth and growth reactions for various sites, tree species, provenances and management regimes is needed. Information about the effect of management should not only describe total volume growth but also tree dimension, branchiness, tree ring structure, technical properties of wood as well as forest growth dynamics, mortality and risk.
Tree rings: an important data source for management information Most tree species produce at least in the temperate and boreal zone annual rings which allow the reconstruction of radial increment and height increment from stem analysis (see fig. 1). Tree rings reflect environmental conditions and their changes and store the reaction pattern over time which can be later used as archive. They are a unique data source as they cover a wide range in
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· statistical tools for analysis of the environmentgrowth relation have improved · statistical tools for spatial variation based on GIS provide better information and better spatial description of growth pattern and environmental conditions · there is fast progress in related research fields such as soil science, meteorology, tree physiology, biochemistry, genetics, forest growth science.
Figure 1. Stem analysis for reconstructing annual radial increment and height increment. At various heights stem disks are sampled and annual radial increment is measured in predefined directions on the cross sections. In addition annual height shoot increment is measured. By comparing the number of annual shoots with the number of tree rings at a given stem height, quality of the measurements is controlled (Gerecke K.-L.1988).
space and time. By selecting sites and sample trees that grew in a specific environment information for answering actual questions may be provided. Tree rings allow a detailed reconstruction of tree growth: trunk, branches and even needles (Jalkanen et al. 2000) and under certain conditions the roots (Krause and Eckstein 1993). The growth patterns over time can be used as data archive. These archives are available where trees form annual rings and where living trees or subfossil or fossil wood exist. In recent years tree rings as a data base for forest growth became even more attractive because: · analytical tools for exploiting the data archives in tree rings have improved · information about environment and environmental changes has widened
The huge potential of interdisciplinary and international collaboration in the analysis of different environmental conditions and their effects on tree growth is not yet fully exploited. Progress in microsystem techniques, system analysis, physics, climatology, soil science, biochemistry, molecular biology, tree nutrition, tree physiology and genetics improve the research potential. The interpretation of tree ring analysis can further be improved by controlled experiments. Limitations caused by short observation periods of experiments may be compensated by long-term retrospective field observations based on tree rings which do allow investigations of long lasting effects.
Tree rings provide information on changes in environmental conditions Numerous long-term growth investigations indicate an increasing growth trend in European forests. An analysis of growth trends in European forests was carried out in a project of the European Forest Institute by 44 scientists from 12 countries (Spiecker et al. 1996). Some of the case studies cover rather small areas, others ± as for example some inventory data based studies ± refer to larger regions or nations. The time span of available data varies from a few decades to several centuries. The aim of the study was to analyze whether site productivity has changed. The term site productivity in this study is limited to the wood production potential of a site for a particular species, provenance or forest type. Biomass data were lacking and therefore could no be used for describing site productivity because only wood production data where collected by repeated forest inventories, by permanent plot observations, and stem analysis. Productivity of
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194 H. Spiecker forest sites in Europe has changed considerably in recent decades. Although the methods applied varied according to the data available, most studies showed the same general trend: site productivity has increased on many sites. The results derived from long-term observations on permanent plots and from tree analyses are supported by inventory results which are representative of large areas, but cover generally shorter observation periods. For the analysis of changes in site productivity height growth is a reliable indicator. When ring width is used changes in tree competition have to be taken into account (fig. 2). Site productivity increased on various sites in recent decades by up to 50 %, in some cases even more. Annual tree height increment increased by up to 5 cm and more, varying with species, site and age of the trees. Several other publications on changes in forest growth support the findings described by
Figure 2. Trend in ring width of silver fir (Abies alba Mill.) forests in the Vosges mountains. Above: average ring width over age; below: average ring width deviation from the expected ring width shown in the upper curve (Becker 1989).
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Spiecker et al. (1996) (e. g. Schmidt 1969, Pretzsch 1985, Spiecker 1986, Gerecke E. 1988, Kenk and Spiecker 1988, Becker 1989, Kenk et al. 1991, Kauppi et al. 1992, Keller 1992, Eriksson and Johansson 1993, Becker et al. 1994, Kuusela 1994, Spelsberg 1994, Elfving and Tegnhammar 1996, Untheim 1996, RoÈhle 1999). These findings indicate changes in growth conditions. The observed trends are species specific, locally varying and modified by several years lasting, large growth variations (see e. g. fig. 3 and fig. 5). On a European scale, species and site specific quantitative information about the extent and spatial as well as temporal variation in growth acceleration is lacking (Spiecker 1999). By combining tree ring and shoot growth measurement additional information on changes in forest growth can be provided. Several causes for the observed changes in site productivity are discussed (Rehfuess et al. 1999): changes in land use history and forest management such as nutrient losses due to litter racking, changes in natural disturbances, in physical and chemical climate including changes in air temperature, in tropospheric CO2-, O3-, and SO2- concentration and changes in nitrogen deposition as well as in fertilizing and liming. The significance of each of these
Figure 3. Salvage cuttings of desiccated trees and trees killed by fungi and insects in % of allowable cut (bars) in the public forests of the Black Forest and annual variation of radial growth (ir, solid line) of Norway spruce (Picea abies [L.] Karst.) in the Black Forest region. Average annual radial increment of 45 Norway spruce trees (8 radii per tree at 1.3 m, tree age: 95 years in 1980) and mortality are correlated. The climatic water balance (ETP-index) derived from monthly air temperature and precipitation during previous five growing seasons is plotted as grey area. In warm and dry periods (grey area below zero) growth is reduced and mortality increased.
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possible causes possibly varies in space and time. Growth responses to the influencing factors are modified by species, provenance, site and stand conditions. It is difficult to draw general conclusions regarding their effects on forest growth. It is possible that one single factor or a factor combination, or even different factor combinations varying with location, influence growth. Several factors may influence forest growth simultaneously and their total synergistic effects may change the individual monofactorial effects. Environmental issues gained additional interest during the discussion about forest decline in recent decades. Site conditions are changing naturally but also an increasing impact of human on these site changes is observed. Increased site productivity is of direct relevance for forest management. Besides increasing the allowable cut as well as increasing tending and thinning intensity it may have an effect on species selection and risk management. Increased biological growth improves the economics of forestry. To assess the potential risk involved in these changes a better understanding of the causes is required. Tree rings are one source of data for improving this understanding (Schweingruber 1993).
Tree rings provide information for risk management Disturbances play a decisive role in forest development. They interrupt continuous succession and alter driving factors for forest growth, such as competition for light, nutrients and water. They may have an impact on selection processes and tree species composition. In central Europe the two most important disturbances factors are storm and snow. The annual variation of salvage cuttings is rather high. Changes in growing stock affect tree susceptibility to drought, frost, fungi and insect attacks as well as to other diseases. The risk of wind throw is increasing when trees get taller and when growing stock accumulates. Impacts of accelerating growth on forest management, markets and timber quality have been discussed by several authors (Karjalainen et al. 1999). Tree rings also allow a retrospective analysis of fires and their effect on forests (Berli and
Schweingruber 1992). Climatic changes as a result of the enhanced ªgreenhouse effectº are believed to be a global environmental threat. The exploitation of forests and other natural resources, the accumulation of anthropogenic emissions and the increasing amount of carbon dioxide and other greenhouse gases in the atmosphere might cause changes in air temperature, precipitation, frost events and wind speed. Environmental changes might damage the functioning of ecosystems and possibly modify biodiversity. Climate changes affect the competition between tree species. This may lead to major disturbances in natural as well as in managed ecosystems. Also, nutrient cycling will be largely affected by climate changes. Trends in forest productivity are generally associated with climatic variation, in the temperate zone particularly with variation in precipitation. High air temperature and low precipitation during growing season reduced growth even at higher altitudes of the Black Forest where average precipitation is high and average temperature is relatively low (Kahle and Spiecker 1996). Decreasing growth rate under certain conditions is accompanied by increased tree mortality (fig. 3). Suppressed trees in dense stands are especially susceptible to drought (Spiecker 1986). As compared to mortality due to storm, snow or fire the volume of desiccated trees and trees killed by insects and fungi is in managed forests of the boreal and temperate zone in Europe generally rather low. Trees are classified as `desiccated' when they are dead without showing visible damages. However impacts of climatic fluctuations on forests have often been underestimated in the past. It has been shown that growth rates and mortality are closely related to climatic fluctuations especially to drought conditions as for example in the 1940s, the 1970s or the 1990s (fig. 3, Spiecker 1995b). Growth response to climatic influences varies with species, provenance, competition status and site conditions. Changes in average climatic conditions such as air temperature affects the length of the growing season and influence site productivity (Fabian and Menzel 1999). A positive correlation between air temperature increase and plant growth seems likely
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196 H. Spiecker in the boreal and temperate climate zone. Extreme and unusual events such as late spring frosts, prolonged summer droughts, unusual cold and wet or hot and dry summers as well as extreme and abrupt temperature changes may reduce growth and increase mortality. Extreme climatic events not only have a direct effect on trees, but also on insects or microbial pathogens, on disturbances such as fire or storm throw, and on biological as well as chemical and physical processes in the soil. Correlations of these factors with growth have been reported (Nilsson et al. 1995). Droughts generally cause growth decrease: In Central Europe such a growth recession due to drought occurred for example in the late 1940s. A severe drought that attracted high tention occurred in the mid 1970s at the beginning of the discussion on defoliation and forest dieback (fig. 3). The long-lasting after-effects of such extreme events complicate the detection of possible causes. There are indications that in some regions Norway spruce (Picea abies [L.] Karst.) is showing an increased sensitivity to climate variation (Kahle, 1994). Changes in the frequency and intensity of extreme warm and dry climatic conditions during the last decades and effects of increased atmospheric depositions have been discussed as possible causes for this observation. Effects of insect attacks on tree growth can retrospectively be analyzed when the at-
Figure 4. Effect of defoliation by Lymantria dispar on growth of oak (Quercus sp.): After two consecutive years of Lymantria attacks growth of oak (grey line) has substantially decreased as compared to the oaks which have not been affected by this insect (black line) (Piper 1998, modified).
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tack has been registered carefully (fig. 4). Tree ring analysis allows a better insight in forest growth dynamics and their driving factors and therefore provide information for risk assessment. New planning and decision tools have to be applied to encompass uncertainty and risk in long-term forest planning. Traditional anticipation of goals and means based on past experience and current knowledge may not be adequate to scope with risk and uncertainty. Adaptive planning taking into account rapidly changing conditions on the other hand may lead to inefficient long-term management. Flexible planning which takes into account all conceivable scenarios and allows various options for future development may be best suited.
Tree rings support the choice of tree species composition Species composition has been historically subject to drastic changes. In central Europe for example broad-leaved species dominated forests by covering about two third of the area until this had been reversed by human activities starting already at the end of the thirteenth century. The area of European beech (Fagus sylvatica L.), oaks (Quercus sp.) and other broad-leaved species has been reduced while the area of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies [L.] Karst.) has been increased substantially. By substituting faster growing species for less productive species not only roundwood production has increased, revenues have increased even more because of the higher softwood prices as compared to hardwood at that time (e. g. Landesforstverwaltung Baden-WuÈrttemberg 1953± 1998). The recent start of a reverse change in tree species composition is stimulated by various factors: hardwood price increased relative to softwood. The popularity of close to nature forestry has led in many cases to a shift towards more broad-leaved species. Volume growth has become less important while wood quality has got higher attention in countries where salaries are high. Higher ecological and economic value of slower growing species may under current conditions in various countries compensate
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for losses in volume growth. By adapting principles of natural processes cost-efficiency may be improved as well. Current management strategies indicate that former human induced changes are supposed to be at least partly reversed in the future. The impact of the planned changes will be visible in the overall species composition only after decades as production periods are long and the percentage of conifers in existing younger stands is relatively high in many countries naturally dominated by broad-leaved species. Only in the youngest age classes the change will be visible in the near future. The ongoing drastic changes in species composition will have longterm effects on the ecological conditions of forest ecosystems. Mixed stands have been found to be more resistant against various forms of damage, more diverse in their fauna and flora composition than pure, single-species stands. Broad-leaved trees are supposed to improve the conditions for growth by affecting the root systems, litter quality and nutrient and carbon storage and soil acidity (Kreutzer 1981, Liljelund et al. 1986, Mitchell and Kirby 1989, Feger 1993). The question of conversion of pure secondary coniferous forests on sites naturally dominated by broadleaves is essential for sustainable fulfillment of society's needs. A conversion of some of these forests to near-natural, site-adapted and often mixed forests has to be considered. For planning such a conversion, information about the technical alternatives of forest conversion and their ecological and economic impacts during and after the conversion process is needed on tree, stand, landscape and regional level. Only in recent times interdisciplinary research projects addressing the conversion topic have been launched in several countries. The outcome of this research may help stakeholders take into account the consequences of the forest conversion, already planned on large areas. Impacts are expected in the fields of forest nutrition, forest growth, biodiversity and other aspects of forest ecology as well as in forest management, silviculture, roundwood production, wood quality, timber markets, forest economics, forest operations, employment and others, making clear that forest conversion is not only of practical relevance but also of political
interest. A better understanding of natural processes and forest dynamics provides information to improve species composition by planting, tending and thinning. Criteria for defining conversion priorities may help to improve conversion strategies. For site adapted species selection and management sound site-, species- and stand specific information on growth and mortality is essential. There are only few long-term research plots which provide this information. Stem analysis based on measurements of tree rings can contribute substantially to a better understanding of differences in the growth dynamics of tree species and to the question of conversion, the long-term effects of conversion on tree growth, and so furnish information for defining priorities of conversion and conversion strategies. The analysis of past growth reaction may help to understand how adapted a tree species is to a site and to climatic changes. A conversion generally requires a sudden release of remaining trees by cutting some trees to provide space for a new forest generation. Therefore information on reaction of released trees is needed. The growth reaction of trees that have been exposed to sudden release e. g. by snow breakage at different stages of development can be analyzed retrospectively based on tree rings for providing this information.
Tree rings for investigation of age effects The age composition of forests varies considerably with species, stand and site conditions and historic events. While extended forest areas in various regions were destroyed world wide, in Europe the forest area as well as the average age of forests increased during recent decades. In Switzerland the rising age is especially pronounced (Schweizerisches Forstinventar 1988). In former West Germany the area of the youngest age class has decreased in recent years while the area of forests in all age classes older than 60 years has increased (BML 1993). It is well known that the growth potential of forests is age dependent. While researchers who are interested in climate-growth relations want to remove this `age
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198 H. Spiecker
Tree rings and tree spacing Growing stock is affected by forest management practices such as regeneration methods, initial spacing, tending, thinning and harvesting regimes (Mitscherlich 1978). In dense stands radial increment of individual trees is lower while radial increment of released trees is higher (fig. 6 and 7).
Figure 5. Net annual volume increment and age. The periodic net annual volume increment of two Norway spruce stands (Picea abies [L.] Karst., solid lines) on long-term permanent research plots in southwestern Germany is plotted over stand age (data: Forest Research Station Baden-WuÈrttemberg, Freiburg). The horizontal bars describe the length of each measurement interval and the level of the average annual volume increment within the interval. The dotted curve describes net annual volume increment as expected according to a yield table (Wiedemann 1936/42). Observed growth deviates considerably from the expectations based on the yield table while stand density (not shown here) followed the yield table.
trend', managers are interested in the effect of age on growth. The production period is influenced by site productivity because fast growing trees reach the desired dimensions earlier than before and the economically optimal rotation age may be lowered. According to Eriksson and Johansson (1993), an increasing growth trend may also influence the growth rhythm of trees. Trees may grow faster when they are young but may reach the culmination of current annual increment earlier than in former generations. This effect may not be visible in today's forests because increased site productivity also accelerates the growth of old trees (fig. 5). As age of forests in Europe has increased in recent decades in many regions knowledge about growth of older forests is rather limited and existing growth models often do not describe growth of older stands adequately. Additional research based on tree ring analysis of older trees is needed to quantify the effect of age on growth.
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Figure 6. Differences in radial growth trends of Norway spruce (Picea abies [L.] Karst.) in the Black Forest caused by past competition: 1: wide initial spacing, no thinning ± data base: 8 radii per tree at 1.3 m of 11 dominant trees, tree age: 96 years in 1980 (Kenk 1990) and 2: heavy release after growing in a rather dense forest ± data base: 8 radii per tree at 1.3 m of 5 dominant trees, tree age: 185 years in 1980 (Spiecker 1992).
Figure 7. Effect of thinning on radial increment of oak (Quercus sp.): The average annual radial increment at 1.3 m above ground of two in 1961 heavily released trees and two unreleased trees is plotted over time. Eight years after release the accelerated growth reached its maximum when also the unreleased trees showed a relatively high growth (Spiecker 1991).
Tree rings and forest management in Europe 199
Dense stands consisting of shade tolerant tree species offer less space for light demanding tree species and ground vegetation to exist and therefore biodiversity will be reduced (Mitchell and Kirby 1989). A conversion to more light demanding species is in the near future only possible through active forest management. This includes the necessity to cut trees. Without intervention, forest health will decline (Spiecker 1995a). Tending of young stands allows the creation of a mixed stand structure. Growth of young stands may be strongly influenced by soil preparation, selection of species and provenances, quality of plant material and weed control. Competition control requires site adjusted tending and thinning methods because diverse species may react differently to site conditions. Furthermore, changes in site productivity may alter competition between trees. An adjusted thinning regime is also required to counterbalance accelerating growth rates, otherwise the increase in average growing stock will continue and the stability of forests will be reduced. Mixed stand structures can be maintained and improved by thinning. Thinning in particular increases the proportion of large-sized timber in total harvest volume. Wood resources will change with a shift in tree species and age composition as well as in site conditions. Not only growing stock is changing but also wood assortments. Rising average tree age increases presumably the share of wood of large-sized trees. Implications of high standing volume of mature trees provides not only an increased timber supply capacity but also causes a higher risk for tree health because the resistance of dense and over-mature forests against disturbances like drought, storms, snow or pests may be reduced. Little information is available on growth behavior of old forests. Retrospective stem analysis may be the only way to get this information.
Tree rings and wood quality control Ring width is associated with various other factors affecting wood quality such as wood density, fiber length and cell structure within tree rings (Nambiar 1995, Park 2000, Spiecker et al. 2000). Tree rings
also correlate with internal branchiness as larger rings are generally produced by trees showing wider crowns and lower crown bases. As average ring width can be to a large extend modified by forest management, ring width is an important indicator for controlling quality. However annual variation of ring width caused by climatic variation can not be eliminated by management even so the absolute variation can be reduced by increasing competition and reducing average ring width. Sometimes the effect of ring width on wood quality has been exaggerated (Schulz 1959). Ring width also determines the diameter reached in a given production period. The diameter is an important criterion for determining wood prices. Furthermore the effect of pruning on growth and wood structure can be analyzed based on tree ring analysis (Shigo 1989). By chemical analysis of tree rings and a more detailed analysis of anatomical features of ring structure such as late wood formation or compression wood formation further information on wood quality can be obtained (Z'Graggen 1992, Saû 1993, Vaganov 1996).
Contribution of tree ring analysis to cost efficient management Growth of individual trees can be reconstructed by tree ring measurements. By relating the growth of an individual tree to its growing space the space economy of the tree can be derived. By comparing growth rates of various species in mixed stands and growth rates of different social tree classes in pure stands growth dynamics, dynamics of self thinning and natural pruning can be analyzed (Spathelf 1999). Based on a better understanding of natural growth dynamics under untouched conditions, management can be more efficient. Unnecessary activities can be avoided by integrating natural processes into the management activities. By aim oriented concentration on essential activities at the right time and at the right location input in forest management can be drastically reduced without losing much output value. This implies a better understanding of natural processes. Tree rings can help to improve this understanding.
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Conclusions
Acknowledgment
Due to changing environmental conditions and changing forest management objectives, updated information about trees and their growth is needed. Tree rings are a valuable tool to provide information about tree growth and growth reactions. We can very well and in detail reconstruct growth of individual trees. Tree ring data can be potentially collected where ever trees form annual rings, growth patterns can be dated at least in an annual resolution by precise and more and more automatized measurement equipment. In addition statistical tools for time series analysis and geostatistical analysis have improved. However there are some limitations in respect to managers information needs. Tree analysis in general does not allow the reconstruction of the long-term growth of stands, not only because it would be very time consuming and the analysis of growth using wood samples is destructive, but also because some trees have formerly disappeared and may have shown different growth patterns as compared to the remaining trees. Another shortcoming is that former environmental conditions of the individual tree such as the competition with neighboring trees, former site conditions, insect attacks etc. may be difficult or impossible to be reconstructed. It is a challenge of tree ring research to overcome some of these difficulties by improving the methods for using tree rings as an archive of data about former environmental conditions. As environmental conditions do not only change in time but also in space, comparison of growth reactions in different parts of the world may serve as quasi-experiments. Here is a chance for future international cooperation. The amount and complexity of the scientific problems evolving from the observed situation show that solutions can only be developed by a multidisciplinary cooperation of scientists and decision makers at an international level. This cooperation will lead to a more comprehensive understanding and will provide a more realistic and reliable information basis for decision support. Changing social demands today require a widened scope of forest management. This congress here in Davos will help to improve this international cooperation.
During many years of teaching and research it was a pleasure to work together with Fritz Schweingruber. I just want to thank Fritz for his creative and stimulating input in this productive cooperation and I look forward for future joint activities.
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