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CARBON ALLOCATION PROCESSES AS INDICATORS OF POLLUTANT IMPACTS ON FOREST TREES McLaughlin, S. B. Environmental Sciences Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-603X. ABSTRACT The physiological processes linking carbon assimilation and net primary production in forest trees offer a broad spectrum of reference points for documenting, evaluating, and predicting the effects of atmospheric pollutants on forests. Measurements of photosynthesis, dark respiration, leaf maintenance costs, energy storage reserves, secondary metabolites associated with plant resistance to pathogens, dry matter partitioning, and patterns of annual radial growth of forest trees represent useful indicators of pollutant effects that encompass levels of detection ranging from short term mechanistic processes to longer term responses that integrate seasonal or multi-year effects. Productive utilization of measurements of these processes requires that particular emphasis be placed on (1) concurrent examination of multiple processes, (2) integration of information on these processes into a whole tree physiological context and (3) seasonal integration of temporal variations in the magnitude of measured responses. Collectively these processes can provide much-needed tools for evaluating qualitative and quantitative changes in growth and physiological resilience of forest trees in relationship to chronic air pollutant exposure regimes. The effects of atmospheric pollutants on forests have been documented at scales of resolution ranging from biochemical and cytological alterations to changes in community dynamics and structure (Mudd and Kozlowski, 1975~. Effects on carbon allocation are particularly important in understanding the causes and consequences of these effects because of the pivotal role that carbon plays not only in biomass accumulation, but also in nutrient and water use capacity of forest trees (McLaughlin, 1 98Sa). The balance of carbon assimilated from the atmosphere and distributed to the many sinks within forest trees plays the pivotal role not only in the amount of growth, but also in the many processes that determine the resistance and resilience of growth processes in the face of environmental stress (McLaughlin and Shriner, 1980~. . The carbon allocation pathways and processes that link gross primary production and net primary production offer many reference points for evaluating pollutant effects at scales of resolution ranging from mechanistic to community level (Figure 1~. Included among the effects of primary concern are reduced photosynthesis, increased respiration, ~ . 293

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PROCESSES PROPERDES 294 NCR"SINGLY MECHANIC INCREASINGLY INTEGRATIVE A~IMllA~ON . . ~ ~ MAINTENANCE ~ , RATE DURATION SENSmVIrY RESILIENCE ~~ ]~~r MARC ]lIlK~ ~ _~ _ _. _ STOI AGE _ DEFENSE l ~ L~EL RESPIRATION TURNOVER OQNL~WG 88M~1693 GROWTH _: _ e . _ CHEMISTRY DlSrRIBUTION AMOUNT DISTRIBUTION TIMING RESILIENCE MPUCA DONS REDUCED ASSIMILATE INCREASED DECREASED PRODUCTION SUPPLY SUSCEPTIBILITY ALTERED SUSCEPTIBIUIY TO BIOTIC AND ALTERED COMMUNITY ABIO~C CREPES DYNAMIC Figure 1. Some components of carbon allocation pathways that provide useful endpoints in evaluating mechanisms of action and implications for impacts of air pollutants on growth potential of forest trees (after McLaughlin, 1988a).l,2 reduced translocation, and changes in patterns of storage, mobilization, and utilization of energy storage reserves. These reference points collectively represent a powerful system 1 Research sponsored by the USDA, National Acid Deposition Assessment Program under Interagency Agreement 40-1647-45 with the U.S. Department of Energy under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. 2 Publication No. 312S, Environmental Sciences Division, ORNL.

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295 of indicators that can be used both analytically to determine whether forests have been impaired physiologically and diagnostically to determine the principal points of impact and likely causes of those impacts. In examining the effects of pollutants or stress in general on tree growth and physiology, it is important to view stress and response from a whole tree perspective (McLaughlin, l98Sb). The concepts of whole plant allocation are particularly important to understanding the effects of pollutants on tree physiology since both sources and sinks for carbon may be influenced by pollutant deposition processes. This alteration may take the form of either a decrease or an increase in the activity of sources or sinks within the tree. Such changes may alter the patterns of tree growth as well as altering tree responses to other environmental stresses. In general we know considerably more about the assimilation of carbon and associated energy and the net annual increment of biomass, which we measure as growth, than about the many important processes that link the two. As an example, the obvious pivitol role of photosynthesis as the source of carbon and energy for tree growth and maintenance processes is reflected in the numerous studies of tree photosynthesis (Shaedle, 1975~. However, relatively little emphasis has been directed toward understanding subsequent allocation processes, which, in forest trees, may consume from 30 to approximately 80% of the energy captured in gross photosynthate (Kira, 1975~. Thus, the processes of respiration, translocation, allocation, and biosynthesis may be equally as important as photosynthetic capacity in determining levels of productivity (Evans, 1975~. Where the vigor of forest trees is reduced by stress, relatively small changes in efficiency of carbon allocation may have major consequences for the physiological integrity of trees. In spite of the potential importance of these processes, to date they have played a relatively minor role in efforts to quantify, characterize and predict the effects of pollutants or other stresses on forest physiology and growth. This paper briefly describes the basis of interest and information needs on four pivotal processes in the carbon allocation pathways: carbon assimilation, dark respiration, translocation and partitioning, and mobilization. Carbon assimilation. The exchange of carbon, both photosynthetic uptake and respiratory losses, by foliage of forest trees has been an obvious focal point in many studies aimed at evaluating tree growth potential. With respect to air pollutant impacts, changes in photosynthesis particularly have figured prominently in efforts to understand the concentration threshold for physiological responses (Botkin et al., 1972), characterize differences in sensitivity among genotypes of the same species (Boyer et al., 1986, Eckert and Houston 1980) or evaluate comparative sensitivity across a variety of different species (Oleksyn and Bialobok, 1986 and Reich and Amundson, 1985~. There are many dimensions of net photosynthesis (Pn) that can provide important insights into the impacts of pollutants on photosynthate production. These include both maximum capacity under saturating radiation levels, the light response curves (including the compensation point), kinetics of response to both pollutants and changes in radiation and the patterns of change in response over diurnal and seasonal cycles, and the distribution of Pn capacity within tree crowns both as a function of foliage age and position within the crown (McLaughlin, 1 98Sc). In addition to measures of Pn capacity, Pn response surfaces to light ~ Hanson et al., 1987) and to C02 (Farquhar and Sharkey, 1982) offer possibilities for evaluating the efficiency of leaf photosynthetic processes. Measurements of Pn at any point in time, while they may provide important information on the integration of exposure effects to that time, may not adequately describe the past or future kinetics of the photosynthetic system. Boyer et al. ( 1986)

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296 indicate that Pn of white pine recovered following exposure to ozone at 0.05 ppm (6 in/d) but decreased more rapidly on each successive day of exposure, thus suggesting progressive impairment of the photosynthetic system. The manner in which response and recovery systems operate over time to determine seasonal influences on carbon assimilation capacity is an important issue. As an indicator of pollutant stress Pn has two dimensions: ( 1 ) the initial basal rate may be directly related to sensitivity to uptake of gaseous pollutants and hence the potential for Pn reduction. and (2) as an indicator of longer term capacity for the integration of pollutant and other stresses over the life of the foliage, decreases in Pn reflect sensitivity to deterioration of the integrity of photosynthetic or systems or (less likely) reductions in demand for assimilates. It should be noted that compensatory factors may partially offset the effects of stress on photosynthetic systems. The capacity of foliage of some plants to respond to a decrease in source-to-sink ratio by increasing Pn efficiency may be an important characteristic determining tree resilience to foliar damage (McLaughlin and Shriner, 1980~. Replacement of damaged foliage is, of course, another possibility for indeterminate species (Coleman, 1986~. Because of such compensatory factors, reductions in growth can not be predicted! on a 1:1 basis from reductions in Pn. Reich and Amundson (1985), for example, found that while reductions in growth were linearly related to reductions in photosynthesis, compensatory processes resulted in final growth reductions of plants exposed under laboratory conditions that ranged from 20 % for tree seedlings to about 60 % for crops. In contrast to results from studies with OF (Reich, 1987), there is little indication to date that acid precipitation at ambient levels adversely affects carbon assimilation (Reich et al., 1986 and Hanson and McLaughlin, 1987~. By contrast, Taylor et al. (1987) found that acid mist at pH 3.0 stimulated overall Pn capacity of red spruce seedlings due to increased foliar area produced at higher acidity and associated nitrogen levels. Much additional work is needed to understand the likely effects of acid deposition on carbon assimilation processes. This task is made particularly complex and interesting by the likely influences on leaf physiology of nitrogen and other nutrients present in acid rain. The potential stimulation of foliage growth by deposition of nitrogen may increase plant sensitivity to moisture stress as observed with greenhouse grown red spruce seedlings by Norby et al. (1986~. Results such as these emphasize the advantages of examining process responses to pollutants such as acid deposition from a whole plant perspective. Dark respiration. To date relatively little emphasis has been placed on pollutant-induced effects on dark respiration (Rs). However, stimulation of dark respiration is an expected consequence of plant repair mechanisms (McLaughlin and Shriner, 1980) and may deplete as much or more carbon from available energy pools as reduced photosynthesis. Increased dark respiration may be particularly significant when coupled with reduced rates of Pn, and in fact, reduced Pn may be a consequence of increases in light respiration, a component of the assimilation process that has received little emphasis with respect to pollutant effects. Barnes (1972) detected a reduction of photosynthesis (-10 % average) and stimulation of dark respiration (+33% average) in seedlings of three species of southern pines exposed to O.l5ppm O3 under laboratory conditions. McLaughlin et al.(l982) found that Rs was stimulated approximately 15% while photosynthesis was reduced only 6% in mature field grown white pine trees, thus showing high apparent sensitivity to ambient levels of ozone in east Tennessee.

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297 Changes in the Pn:Rs over a range of temperature and light conditions could provide a more complete picture of alterations in assimilate supply. Obviously, increased emphasis on dark respiration is warranted in research aimed at accurately quantifying pollutant impacts on total assimilate available for export from foliage. Translocation. The transport of assimilates away from production centers to points of utilization within the tree may be examined either at the leaf level or at the whole plant level. At the whole plant level there is good evidence that air pollutants may exert significant effects on plant productivity by altering partitioning of dry matter between plant parts (Manning, 197S, Oshima et al., 1979, Tingey, 197S, and Tingey et al., 1976~. At the leaf level, shifts in translocation may occur either as a consequence of interference of pollutants with the loading of the phloem with assimilates or as a consequence of increased assimilate demand by foliage. Internal demand in turn may be enhanced by either altered foliar nutrition (nitrogen assimilation directly by foliage from the atmosphere) or as a damage response requiring increased internal maintenance and repair. A review of studies with several plant species indicates that internal costs of maintaining leaf functions are high (McLaughlin and Shriner, 1980). These maintenance costs would be expected to be enhanced by exposure to pollutants at levels high enough to cause metabolic or cytologic injury. Several studies under both laboratory (Jones and Mansfield, 1982, Noyes, 1980, Teh and Swanson, 1982, and Tingey, 1978) and field conditions (McLaughlin et al., 1982) have indicated that carbohydrate translocation may be both sensitive to exposure to air pollutants and useful as a general indicator of pollution related stress. At present the level of understanding of the aboveground system is far beyond that for the belowground system and the reasons for this are obvious. Yet belowground processes including both maintenance and turnover of fine roots may play a major role in whole tree energy budgets (Harris et al., 1974 and Persson, 1980) and are likely to be particularly sensitive to pollutant impacts (McLaughlin, 1 98Sb). Measures of pollutant- induced impacts on transport of assimilates to support the belowground system are sorely needed as they relate directly to the functional integrity and to uptake of both water and nutrients by the root/rhizosphere system. Storage and Mobilization. With respect to evaluating impacts of air pollutants on tree production potential, the storage reserves provide a potentially useful and temporally integrative indicator of the carbohydrate economy of the tree and its capacity both to meet the energy demands of annual growth cycles and to resist insects and disease. Resistance to disease may be a particularly important characteristic because allocation of resources to formation of protective chemicals appears to be of relatively low priority when energy reserves are in low supply (Mooney and Chu, 1974~. Allocation of energy reserves to the production or maintenance of new foliage or roots represents a high priority that supersedes the demands for defensive strategies when trees are under stressed conditions (Waring and Pittman, 1985~. Plants have a wide variety of strategies for defense against diseases (Horsfall and Cowling, 1980), and many of those defenses could be weakened by the chronic exposure to atmospheric pollutants (Hain, 1987), particularly when alterations in carbon allocation occur. Phenolic compounds, tannins, and proteins are examples of metabolites that are considered important in host defenses against microbial attack (Schloesser 1980~. Perhaps the best example of the importance of secondary metabolites to disease resistance of forest trees under pollutant stress has come from research in the San Bernardino Forest where increased susceptibility of oxidant-stressed ponderosa pine to attack from bark beetles was noted (Stark et al., 1968~. Increased susceptibility to beetle attack was

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298 associated with both qualitative and quantitative changes in the oleoresin production of affected trees (Miller et al., 1968 and Cobb et al., 1968~. Additional potentially useful situations in which the energy reserve status of forest trees may be playing an important role in host-pathogen relations include the southern pine bark beetle and balsam fir/wooly aphid associations in the southeastern U.S. Both insect outbreaks have occurred in areas in which air pollutants are under investigation as contributing factors to forest growth decline. Since trees store energy in many different biochemical forms, the question of which constituent to examine as an overall indicator of tree energy reserves is of obvious relevance. Kramer and Kozlowski (1960, 1979) have grouped forest tree species into those that store food reserves primarily as fats (principally diffuse-porous species), starch (primarily ring-porous species), and a third group that utilizes both forms. Regardless of the primary storage form, starch appears to be a significant constituent of energy storage in roots of most tree species (Ziegler, 1964~. As an indicator of storage reserves, root starch offers apparent advantages over starch in foliage, which is more readily influenced by short-term climatic fluctuations (Adams et al., 1986~. Studies with defoliating insects (Gregory and Wargo, 1986 and Wargo, 1981 ) indicate that starch content of xylem and roots provides a useful index of whether stress has occurred and of the vulnerability of trees to additional stress effects from either insects or other natural factors that induce mortality. While starch may be a useful indicator of the vigor of root systems of pollution- stressed trees, it should be noted that interpretation of root starch data must be made within the context of the physiological status of the whole tree (Ericsson, 1980~. Starch depletion may reflect either stimulated utilization of storage reserves during rapid growth or diminished capacity of the tree to assimilate carbohydrates rapidly enough to replace those utilized in normal growth and maintenance demands. Similarly short-term increases in starch accumulation may occur when utilization of assimilates is slowed by growth inhibition at the site of utilization. The preceding discussion has focused on process level measurements as indicators of pollutant induced alteration of carbon allocation patterns. It should be noted in closing that useful evidence of altered carbon allocation patterns can also be derived from the observed patterns of growth itself (see Fig. 1~. The timing, duration, response and recovery cycles following natural or anthropogenic stresses, and density and quality of wood formed all provide potentially useful indicators of the nature and causes of impaired tree vigor (McLaughlin, l98Sb). Dendrochronology is an emerging discipline which has tremendous potential for examining the patterns and potential causes of annual growth patterns at annual and longer time scales (Cook, 1986~. Observed response patterns at the stem level can provide testable hypotheses about basic physiological processes strongly influencing growth of a species as well as likely causal factors leading to disruption of the growth process (McLaughlin et al., 1987~. Several summary points can be made regarding the use of carbon allocation pathways to identify and diagnose responses of trees to air pollutants: (1) There are many reference points within plant carbon allocation pathways that respond to air pollutants and offer potentially useful diagnostic criteria for detecting pollution-induced damage. These occur at many levels of organization within the tree; (2) It is essential to recognize that allocation processes are tied to both the supply and demand of carbon, water, and nutrient recourses and hence will be influenced by a wide variety of natural stresses. These stresses and the responses they induce must be considered concurrently with pollutant stresss. They may either be minimized by experimental design or used as modifiers to test the nature or consequences of pollutant-induced stresses; (3) The

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299 separation of natural phonological and diurnal patterns of response from those that may be induced by air pollutants requires that one consider these inherent response-recovery cycles in evaluating shifts in carbon allocation pathways; (4) Approaching the problems of detection and diagnosis of changes of carbon allocation processes can most effectively be accomplished from a whole-tree physiological perspective that recognizes and tests physiological interrelationships among both processes and plant parts. Within this context an analytical framework can be suggested that first documents the magnitude anti patterns of change in a biological indicator or indicator system and then tests both the physiological basis of the measured response as well as its consequences at successively higher levels of whole plant integration. The physiological basis must be understood to address adequately the causal relationships and the range of possible consequences, while the more integrative measures help document the range and probabilities of actual responses observed under field conditions. Consideration of both growth and process level responses within the carbon allocation system provides inferential cross references that can substantially improve our ability to address the inherent complexity and many uncertainties of forest decline issues. ACKNOWLEDGMENTS The author wishes to thank Drs. George Taylor, Chris Anderson, and Tim Tschaplinski of Oak Ridge National Laboratory for their review of this manuscript. REFERENCES Adams, M.B., H.L. Allen, and C.B. Davey.1986. Accumulation of starch in roots and foliage of loblolly pine (Pinus toeda L.~: effects of season, site and fertilization. Pp. 35-46 in R.J. Luxmoore, J.J. Landsburg, and M.R. Kaufmann, Coupling of Carbon, Water, and Nutrient Interactions in Woody Plant Soil Systems. Tree Physiology, Vol. 2. 467 pp. Barnes, R.L. 1972. Effects of chronic exposure to ozone on photosynthesis and respiration of pines. Environ. Pollut. 3: 133-138. Botkin, D.B., W.H. Smith, R.W. Carlson, and T.L. Smith. white pine saplings: Variation in inhibition photosynthesis. Environ. Pollut. 3: 273-289. 1972. Effects of ozone on and recovery of net Boyer, J.N., D.B. Houston, and K.F. Jensen. 1986. Impacts of chronic SO2, 03, and SO2 and Of exposures on photosynthesis of Pinus strobus Clones. Eur. J. For. Path. 16:293-299. Cobb, F.W., Jr., D.L. Wood, R.W. Stark, and J.R. Parmeter, Jr. 1968. Theory on the relationships between oxidant injury and bark beetle infestation. Hilgardia 39: 141. Coleman, J.S. 1986. Leaf development and leaf stress: increased susceptibility associated with sink-source transitions. Pp. 289-300 in R.J. Luxmoore, J.~. Landsberg, and M.R. Kaufmann, Coupling of Carbon, Water, and Nutrient Interactions in Woody Plant Soil Systems. Tree Physiology, Vol. 2. 467 pp.

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300 Cook, E. R. 1986. The use and limitations of dendroecology in studying effects of air pollution on forests. Pp. 277-290 in Effects of Atmospheric Pollutants On Forests, Wetlands, and Agricultural Ecosystems.(T.C. Hutchinson and K.M. Meema, eds.~. Springer Veriag, New York. Eckert, R.T. and D.B. Houston. 1980. Photosynthesis and needle elongation responses of Pinus strobes clones to low level sulfur dioxide exposures. Can. J. For. Res. 10: 357-361. Ericsson, A. 1980. Some aspects of carbohydrate dynamics in sects pine trees (Pinus sylvestris L.) Department of Plant Physiology, University of Umea, s-901 87 Umea, Sweden. Evans, L. T. 1975. Beyond photosynthesis--the role of respiration, translocation, and growth potential in determining productivity. Pp. 695 in J.P. Cooper, (ed.~. Photosynthesis and productivity in different environments. Cambridge University Press, New York. Farquhar, G.D., and T.D. Sharkey. 1982. Stomatal conductance and photosynthesis. Ann. Rev. Plant Physiol. 33:317-345. Gregory, R.A., and P.M. Wargo. 1986. Timing of defoliation and its effect on bud development, starch reserves, and sap sugar concentration in sugar maple. Can. J. For. Res. 16:10-17. Hain, F.P. 1987. Interactions of insects, trees, and air pollutants. Tree Physiol. 3:93- 102. Hanson, P.J., R.E. McRoberts, J.G. Isebrands, and R.K. Dixon. 1987. An optimal sampling strategy for determining CO2 exchange rate as a function of photon flux density. Photosynthetica 21~1 ):98- 101. Hanson, P. J. and S. B. McLaughlin. 1987. CO2 exchange characteristics of Pinus taeda 1. shoots exposed to variable ozone levels and precipitation chemistries. Plant Physiol. Suppl. 83:81. Harris,W.F., Sollins, P., Edwards, N.T., Dinger, B.E., and Shugart, H.H. 1975. Analysis of carbon flow and productivity in a temperate forest ecosystem. Pp. 1 16- 122 in D.E. Reichle, J.F. Franklin, and D.W. Goodall (eds.), Productivity of World Ecosystems. National Academy of Sciences, Washington, D.C. Horsfall, J.G. and E.G. Cowling. 1 980.Plant Disease: An Advanced Treatise. Vol. 5 How Plants Defend Themselves. Academic Press, New York. Jones, T., and T. (4 Mansfield. 1982. Studies on dry matter partitioning and distribution of C-labelled assimilates in plants of Phieum pratense exposed to SO2 pollution. Environ. Pollut. 28: 199-207. Kira, T. 1975. Primary productivity of forests. Pp. 5-41 in J. P. Cooper, (ed.), Photosynthesis and productivity in different environments. Cambridge University Press, New York. Kramer, P.J., and T.T. Kozlowski. 1960. Physiology of forest trees. McGraw-Hill, New York.

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301 Kramer, P.J., and T.T. Kozlowski. 1979. Press, New York. Physiology of woody plants. Academic Manning, W.J. 1978. Chronic foliar ozone injury: Effects on plant root development and possible consequences. Calif. Air Environ. 7: 3-4. McLaughlin, S.B. 1 988a. Carbon allocation as an indicator of pollutant impacts on forest trees. In M. Cannell and D. Lavender, (eds.), Proc. IUFRO symposium Plant Growth in a Changing Chemical and Physical Environment. Vancouver, BC. July, 1987. Invited presentation and paper (in press). McLaughlin, S.B. 1 98Sb. Whole tree physiology and forest responses to air pollutants. In Proc. Commission of European Communities Workshop Interrelationships between Above and Below Ground Influences of Air Pollutants on Forest Trees. Gennep, the Netherlands. Dec. 1987. Invited presentation and paper (in press). McLaughlin, S.B. 1 98Sc. The use of branch level measurements in evaluating whole plant responses to air pollutants. Pp. 165-185 in Proc. EPA/USDA Workshop on Response of Trees To Air Pollutants: The Role of Branch Chambers (W.E. Winner and L.B. Phelps, eds.~. Boulder, CO. November, 1987. EPA Corvallis, OR. McLaughlin, S.B., and D.S. Shriner. 1980. Allocation of resources to defense and repair. Pp. 407-431 in J.B. Horsfall, E.B. Cowling, eds. Plant Disease, Vol. 5, Academic Press, New York. McLaughlin, S.B., R.K. McConathy, D. Duvick, and L.K. Mann. 1982. Effects of chronic air pollution stress on photosynthesis, carbon allocation, and growth of white pine trees. For. Sci. 28: 60-70. McLaughlin, S.B., D.J. Downing, T.~. Blasing, E.R. Cook, and H.S. Adams 1987. An analysis of climate and competition as contributors to decline of red spruce In high elevation Appalachian forests of the eastern United States. Oecologia 72:487-501. Miller, P.R., F.W. Cobb, Jr., and E. Zavarin. 1968. Effect of injury upon oleoresin composition, phloem carbohydrates, and phloem pH. Hilgardia 39:135. Mooney, H.A., and C. Chu. 1974. Seasonal carbon allocation in Heteromales aroutl~olla, a calll~ornia evergreen shrub. Oecologia 14: 295-306. Mudd, I.B. and T.T. Kozlowski. 1975. Responses of Plants To Air Pollution. Academic Press, New York. 383 pp. Norby R.J., G.E. Taylor, S.B. McLaughlin, C.A. Gunderson. 1986. Drought severity of red spruce seedlings affected by precipitation chemistry. Proc. Ninth North American Forest Biology Workshop, Stillwater, Oklahoma, June 15-1S,1986. Noyes, R.D. 1980. The comparative effects of sulfur dioxide on photosynthesis and translocation in bean. Physiol. Plant Pathol. 16: 73-79. Oleksyn, J., and S. Bialobok. 1986. Net photosynthesis, dark respiration and susceptibility to air pollution of 20 European provenances of scots pine Pinus sylvestris L. Env. Poll. (Series A ~ 40: 287 - 302. ~ ~ ~ ) ^44~

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302 Oshima, R.J., P.K. Braegelmann, R.B. Flagler, and R.R. Teso. 1979. The effects of ozone on the growth, yield, and partitioning of dry matter in cotton. J. Environ. Qual. S: 474-479. Persson, H. 1980. Spatial distribution of fine root growth, mortality and decomposition in young sects pine stands in Central Sweden. Oikos 34:77-87. Reich, P.B. 1987. Quantifying plant response to ozone: a unifying theory. Tree Physiol.3:63-92. Reich, P.B., and R.G. Amundson 1985. Ambient levels of ozone reduce net photosynthesis in tree and crop species. Science 230: 566-570. Reich, P.B., A.W. Schoettle, H.F. Stroo, and R.G. Amundson. 1986. Acid rain and ozone influence mycorrhizal infection in tree seedlings. I. Air Poll. Control Assoc. 36~6~: 724-726. Schloesser, E.W. 1980. Preformed chemical defenses. Pp. 161-174 in I.G. Horsfall and E.G. Cowling (eds.~. Plant Disease: An Advanced Treatise. Vol. 5 . How Plants Defend Themselves. Academic Press, New York. Shaedle, M. 1975. Tree photosynthesis. Ann. Rev. Plant Physiol. 26:101-115. Stark, R.W., P.P. Miller, F.W. Cobb, Ir., D.L. Wood, and J.R. Parmeter, Jr. 1968. Incidence of bark beetle infestation on injured trees. Italgardia 39: 121. Teh, K.H., and C.A. Swanson. 1982. Sulfur dioxide inhibition of translocation in bean plants. Plant Physiol. 69:~-92. Tingey, D.T. 1978. Effects of ozone on root processes. Calif. Air Environ. 7: 5. Tingey, D.T., R.G. Wilhour, and C. Standley. 1976. The effect of chronic ozone exposure on the metabolite content of ponderosa pine seedling. For. Sci. 22: 234-241. Wargo, P.M. 1981. Measuring response of trees to defoliation stress. Pp. 248-256 in The Gypsy Moth: Research Toward Integrated Pest Management. (C.C.Doane and M.L. McManus, (eds.~. U.S. Forest Service Technical Bulletin 1584. U.S.D.A. Washington, D.C. Waring, R.H., and G.B. Pitman. 1985. Modifying lodgepole pine stands to change in susceptibility to mountain pine bettle attack. Ecology 66:~89-897. Ziegler, H. 1964. Storage, mobilization and distribution of reserve material in trees. Pp. 304-320 in M.H. Zimmerman, ed. Formation of wood in forest trees. Academic Press, New York.