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DECLINE OF RED SPRUCE IN THE NORTHERN APPALACHIANS: DETERMINING IF AIR POLLUTION IS AN IMPORTANT FACTOR A.H. Johnson Department of Geology University of Pennsylvania Philadelphia, PA 19104 ABSTRACT Declines are multiple stress diseases in which combinations of abiotic and biotic stresses weaken trees, eventually causing death. Airborne chemicals could play a role in decline diseases by altering normal functions and enhancing natural stresses which initiate a decline, and by causing injury which competes for carbon with naturally-induced injuries, ultimately causing available carbon to drop below a level critical for maintaining vital functions. The decline of red spruce in mountain forests of the northern Appalachians appears to have been initiated by repeated winter damage to needles and buds from freezing or desiccation. The spatial pattern of decline severity is associated with age, site conditions, and the nature of the pathogens and insects which serve as contributing stresses. Emerging research results suggest that air pollution at ambient levels is capable of altering resistance to winter stress in high-elevation spruce. Additional research is needed to determine if ambient levels of airborne chemicals serve to reduce the carbon available for defense against and repair of the injuries caused by pathogens, drought and winter weather. INTRODUCTION Recently, the unusual mortality of red spruce in high-elevation forests of New York and New England has raised the possibility that air pollution is responsible, at least in part, for the decline of that species. Similarly, a recent episode of unusual mortality and symptoms in several species in central Europe has intensified the effort to determine if airborne chemicals are important stresses contributing to "forest decline." Some current forest problems in eastern North America and Europe are occurring in areas subject to relatively high doses of air pollutants, but many other declines (diseases characterized by a combination of biotic and abiotic stresses) have occurred in places where, and at times when, air pollution is almost certainly not a factor (e.g., Mueller-Dombois 1983~. One useful conceptual framework divides the stresses involved in declines into predisposing, inciting, and contributing (or secondary) stresses (Marion l 981~. Figure l puts this concept in a simple model showing how the resources a tree has available to repair injury and resist pathogens might change through a period of decline. During a "decline" some trees in a population will be depleted in usable reserves to the point that death, although possibly several years away, is inevitable (e.g., Waring 1987, 91

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92 McLaughlin 1987~. Other individuals may be stressed by the inciting factors, but recover due to lesser effects of the predisposing and secondary stresses. 1 E1 PREDISPOSING FACT - S ~ m INcl~lNG if STRESS Ul ~ z . I us E1 PREDISPOSING , | z al o AL IN J ID o RECOVERY 9 . ~ THRESHOLD FOR / SY~PTO - S OF STRESS RESHOLD | I pathogens ~tt~cti DEATH TINE ~ Figure 1. Schematic representation of energy available for defense and repair during various phases of a decline disease. Represented are, ( 1 ) predisposing factors that influence the vigor of an individual prior to injury, (2) a drop in reserves as repair begins after an initiating stress, and a period of recovery or (3) decline when pathogens or other stresses eventually become lethal. Using Figure 1 as a guide, I have summarized the natural factors that appear to be involved in the decline of high-elevation red spruce, suggested how air pollution might fit into the disease complex and reviewed recent evidence suggesting that air pollution at ambient levels may alter red spruce to a degree sufficient to promote a decline. FACTORS RELATED TO THE OCCURRENCE AND SEVERITY OF SPRUCE DECLINE The gradients in vegetation, soil and climate on the Adirondack, Green and White Mountains have been summarized by many authors, and reviewed in the context of the current decline by Johnson and McLaughlin ( 1986) and Johnson and Siccama ( 1983~. Exposure of trees to airborne chemicals in those mountain forests is now being characterized with considerable precision (e.g., Mohnen, this volume). Earlier estimates have all suggested relatively high deposition of acids and heavy metals, and prolonged exposure to acidic cloud water and elevated levels of ozone (e.g., Lovett et al. 1982, Burgess et al. 1984, J. Panek, unpublished data on hourly ozone measurements at 1026 m at Whiteface Mt., NY). In particular, the frequent occurrence of clouds above 1000 m and the lack of diurnal fluctuations in ozone concentrations above 1000 m make for prolonged exposure to airborne chemicals.

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93 40 - 30 - 20- 1 0 - 0 (4~ ~-0~ < - = (325) (448) - - 1987 O 0 1 982 - (1 79) - ~ (242) 1 1 ' 1 900 m 900 1049 no >1049 m Elevation Figure 2. Changes in percent dead red spruce over 5 years on 56 100-m transects on Mt. Washington, NH, Mt. Mansfield, VT, and Whiteface Mt., NY. Differences are significant in all elevational bands at P<.05 (W.L. Silver, A.H. Johnson and T.G. Siccama, unpublished data). Johnson and McLaughlin (1986) summarized the data from several studies which showed that red spruce density and basal area decreased dramatically (40-80%) on five mountains in New York and New England between the mid-1960s and early 1980s. Surveys of mature spruce on Whiteface Mt. (NY), Mt. Mansfield (VT) and Mt. Washington (NH) showed unusually rapid mortality between 1982 and 1987, particularly at high elevation (Fig. 2~. The most prominent symptom in declining high-elevation spruce has been a deterioration of the crown due to death of twigs and branches, and recent detailed studies have shown many important insects and diseases. specific symptoms associated with locally In a study of 331 permanent plots on Whiteface Mt. (NY), Battles et al. (1987) showed the following with respect to spatial associations between forest characteristics and the density of dead spruce: 1. More than half of the red spruce in all size classes are standing-dead in the three 1 OO-m elevational bands above 1000 m, while about 259/0 are dead in all size classes in the three 100-m elevational bands below 1000 m. 2. The percent dead spruce is slightly greater in larger size classes than in smaller size classes.

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94 3. The percent dead spruce is greater in all size classes above 1000 m on the northwest exposure which experiences the most severe winter winds capable of defoliating conifers. Thus, it appears that age, elevation (representing poorer site conditions and shorter growing seasons) and unfavorable exposure at high elevation can be considered as conditions which predisposed the trees to greater damage from other environmental stresses. Friedland et al. (1984) suggested that damage to foliage and buds which became visible in early spring was probably an important ~ type of injury promoting the decline of red spruce. Johnson et al. (1986, 1988) suggested that several consecutive years of winter damage to the previous year's foliage and buds probably initiated and synchronized the decline across New York and New England. The winters of the 1960s were unusually cold (Namias 1970~. Figures 3-6 show that the several years of winter damage were accompanied and followed by regionwide reports of dead and dying spruce, a synchronized regional-scale decrease in ring width, and an abrupt change in the long-term relationship between climate and ring width. Substantial and/or repeated loss of new foliage from spruce growing near the top of their altitudinal range is likely to be significant because it reduces photosynthetic biomass, removes storage tissue and energy reserves, and judging by the production of adventitious shoots (P. Wargo, U.S. Forest Service, and personal observations), shifts carbon allocation to the production of new shoots and needles. A systematic study in the Adirondacks by Curry and Church (1952) and repeated observations in western Maine (Stark, 1962-1970) and in patches of dead and dying spruce in the White Mountains of New Hampshire (Tegethoff 1964, Wheeler 1965, Kelso 1965) indicate that severe episodes of winter injury can kill red spruce. Thus, the repeated and severe episodes of injury to foliage and buds two decades ago represent a likely stress which initiated and synchronized the spruce decline and later episodes of winter injury may have served to keep the decline going. REPORTS OF WINTER DAMAGE south ME east north ME west NH VT NY QUE . . . . . o o ~ o 0 0 o. . .. ~ o . -. . o ooo_ ~ oooo o .... & , . . . 1940 1950 1960 1970 1980 i Severe, Extensive Figure 3. Reports of "winter drying," "winter burn," "winterkill," etc. where red foliage was observed in spring. The tabulated incidents were generally widespread within the geographic area. Source: Reprinted with permission of Kluwer Academic Publishers. Copyright c 1986 by D. Reidel Publishing Company.

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95 RED SPRUCE MORIALIU NH V] 1800 1850 1900 1950 Confirmed O Uncertain Spruce blindworm Major windstorms Figure 4. Periods when extensive or severe red spruce mortality were recorded. Source: Reprinted with permission of Kluwer Academic Publishers. Copyright c 1986 by D. Reidel Publishing Company. 2.5 on In ,_ 1.5 - z 1.0 BY 0.5 0.O 25 RED SPRUCE MEAN RINGWIDTH CHRONOLOGIES (>800 METER ELEVATION) um ~ Lou 1 ~ 1 920 YEARS 1940 1960 1980 Figure 5. Red spruce tree ring chronologies at 25 high-elevation sites in the Catskill, Adirondack, Green and White Mountains. Each site is represented by two cores from 12-20 trees >10 cm dbh. Values are raw ring widths for each year averaged for each site, then normalized to the 131-year mean ring width. (After Johnson et al. 1988~.

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96 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.] O .1 .2 .3 .4 .5 ~,6 .7 .8 .9 .... ... ........ 1944 1954 1964 ~ 1974 O Aug-Dec Model ~ Nov-July Model Figure 6. Seven-year running correlation coefficients for actual vs. climate-predicted, standardized ring 'widths. Open circles reflect the predictive ability of a regression model which uses average monthly temperatures of the prior August and prior December. This combination of temperatures was significantly correlated with ring width from 1856- 1960. Prior August ceases to be related to ring width from 1960 to 1981. Instead, a model using prior November and current July is related to ring width. Methods are explained by Johnson et al. (1988~. Johnson et al. (1988), McLaughlin et al. (1987) and Cook et al. (1987) used tree-ring analysis to show that radial growth in high-elevation red spruce across New York and New England responded unfavorably to warm Augusts (year prior to ring formation) and cold Decembers and Januarys (winter prior to ring formation) for the 100 years prior to 1960 when the relationship between climate and ring width changed abruptly. While the physiological bases for those relationships are unknown, it is interesting to note that the most severe recorded periods of red spruce mortality occurred during periods of unfavorable summer and winter temperatures (Johnson et al. 1988, Fig. 7).

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97 AUGUST (SOLID) AND DECEMBER (=D AR (3) STREW INDICES 4 . 3 . 2 x 1 O -1 -2 3 4 it, A ~ ,1~-- ..! ~ ~ ~ 1 ~ ~ I ~ 1820 1~ 1~ 1~ 1~ 1~0 19" 19" 1980 YEARS Figure 7. Years when unusually warm Augusts and/or unusually cold Decembers occurred. Only monthly average temperatures that have a probability of <.05 are included. Positive index values represent periods of stress, negative values represent favorable periods. Solid areas are unusually cold Decembers. Methods are explained by Johnson et al. (1988~. Particularly notable was the spruce decline of 1871 - 1885 (Hopkins 1901 ~ which killed an estimated 50% of the spruce in the Adirondacks, and coincided well with the very unusual years 1871 and 1876 which had both especially warm Augusts and especially cold December temperatures. The most recent period of spruce decline apparently started at the end of a period of warm summers in the 1950s and at the start of a period of unusually cold winters (see Fig. 7 and Namias 1970). From those findings, it seems plausible that warm summer temperatures may have served as a predisposing factor. Several researchers have noted the occurrence of severe drought in the mid-1960s which could serve as an initiating stress. Johnson and McLaughlin (1986) and Johnson et al. (1988) discussed the conflicting evidence related to the possible importance of the mid- 1960s drought. Although regionwide red spruce mortality was noted prior to severe drought, large negative residuals from the temperature-predicted ring widths during 1965-67 suggest at least a contributing and, possibly in some areas, an initiating role. Many of the insects and diseases which have affected red spruce historically are present in dead and dying trees, but their occurrence varies spatially. The spruce beetle (Dendroctonus rufiDennis Kirby) has been observed in many areas (McCreery et al. 1987) but is rare in others, such as at Whiteface Mt. (T.C. Weidensaul, personal communication). Armillaria mellea, the shoestring root rot fungus is present in declining

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98 spruce at lower elevation, but rare at higher elevations (Carey et al. 1984~. Other fungi (Fomes nini and CYtosoora canker) have been identified in several places over the last two decades (e.g., Hadfield 1968~. At lower elevations, dwarf mistletoe (Arceuthobium ousillum Peck) is associated with growth loss and mortality (McCreery et al. 1987~. The presence of many more pests and disease organisms is expected to be confirmed by ongoing work. Early observers of spruce mortality at high elevation in New Hampshire and western Maine (Kelso 1965, Wheeler 1965, Tegethoff 1964, Stark 1962) looked for, but found no evidence of insects or fungal diseases as primary causes of the spruce mortality they studied through repeated visits in 1962-64. They attributed the mortality and visible decline of spruce to severe winter conditions. As in most declines, insects and diseases appear to be serving as contributing or secondary stresses in the recent episode of spruce decline. To date, many of the naturally-occurring factors that have been implicated in other declines have been shown to be temporally or spatially associated with the red spruce decline. These are summarized in table 1. TABLE 1. Stresses temporally or spatially associated with the occurrence or severity of the recent red spruce decline in the northern Appalachians and Adirondacks. PREDISPOSING INCITING CONTRIBUTING age elevation (site) winter damage insects to foliage and buds fungal diseases severe exposure (drought?) winter injury to winter winds (warm summers?) THE POSSIBILITY OF AIR POLLUTION INVOLVEMENT The presence of the factors listed in Table 1 does not rule out air pollution as a contributor to spruce decline. Figure ~ shows how air pollution could serve as a predisposing or contributing factor by reducing reserves available to repair injuries caused by the winter damage, drought etc. Possibly, air pollution could make trees more susceptible to the conditions that cause the winter injury to foliage and buds. Emerging findings (e.g., Alscher, this volume) suggest that those topics deserve attention.

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- z As In z IL c] ce lo IL CRITICAL T~RFCUOI n ; air pollution damage? 1 v THRESHOLD FOR SYMPTOMS Of STRESS 99 Winter injury 59 -65 v it_ Drought 65-66 or furtt~r winter injury ; '78, '81, '84 l v I _o, RECOVERY ~ 'a CAN AIR FOLIATION ENHANCE WINTER STRESS ? TIME - ~ DEATH Figure S. Events that contributed to the recent spruce decline in the northern Appalachians and possible ways in which air pollution might contribute. Many factors contribute to a tree's ability to resist damage in winter (e.g., Weiser 1970). Davison et al. (1987) and Alscher et al. (this volume) reviewed the types of damage plants are subject to in winter, and the ways in which air pollution might alter resistance to those types of stress. In winter, high elevation conifers must contend with photo-oxidation of chlorophyll (resulting in bleaching of the needles), desiccation (thought to be caused by water loss through cuticles on bright, sunny days when water in conducting tissues is frozen), and freezing injury (usually called "frost damage"). The latter types of injury cause the death of needles which turn red-brown in the spring. The exact mechanism that caused the winter damage observed in red spruce over the past two decades is unknown, and the climatic conditions under which the injuries occurred are, likewise, unknown. Freezing damage early in winter, desiccation on appropriate days in mid-winter, mid-winter thaws followed by very low temperatures, and spring frosts all have the potential to cause the damage observed. There is evidence that air pollution might affect resistance to winter stresses. Electron microscopy studies carried out in Finland along a gradient of air pollution exposure showed greater alterations of cells and greater winter damage in areas of higher pollutant exposure (Davison et al. 1987), but mechanisms or the causal agents are not known. Since several pollutants attack constituents of cell membranes, and since changes in cell membranes occur during the fall hardening period, Davison et al. (1987) suggested that there is a sound theoretical basis for suspecting that air pollution might interfere with the development of freezing resistance. Their experiments with

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100 ozone fumigations (120 ppb ozone, 6 h per day for 70 days) indicated that ozone at that dose rate was associated with increased cut~cular water loss from previous yearns needles in one of eight clones of Norway spruce tested. Four clones showed an increase in freezing-related damage when exposed to the ozone treatments. Weinstein et al. (1987) and Cumming et al. (1988) and Alscher et al. (this volume) exposed red spruce seedlings in open-top chambers to filtered air and to ozone at ambient and twice ambient levels at Ithaca, NY. Ozone at ambient and twice ambient levels was associated with several factors that indicated a delay in hardening. Ozone at ambient levels was associated with increased respiration, decreased pigment concentrations, delayed starch export or conversion, a delay in tannin accumulation, and unusually high photosynthesis rates during the hardening period. While there are obvious problems in extrapolating from seedlings grown under the climatic conditions in Ithaca to mature trees growing on the mountains, it is interesting to note that ozone exposure in the spruce-fir forest at Whiteface Mt. in the summer of 1987 was in the ambient to twice ambient range used in the experiments noted above. Hourly ozone concentrations averaged 47 ppb, day and night from June 1 through August 31. The maximum reading was 129 ppb, and 22% of the time, ozone concentrations exceeded 60 ppb (J. Panek, Atmospheric Sciences Research Center, SUNY-Albany, unpublished data). Long-term data from the summit of Whiteface (Burgess et al. 1984) indicate that those values are representative of normal conditions. As indicated in Figure 8, it is also important to try to determine if ambient levels of airborne chemicals cause a decrease in the carbon available for defense and repair. A clear record has been established with seedlings and saplings of several species that suggests ozone at ambient levels can reduce net photosynthesis and growth without visible symptoms (e.g., Wang et al. 1986, Skelly et al. 1983, Reich and Amundson 1985~. Thus, there is reason to suspect that air pollution might be capable of inhibiting repair of the winter damage sustained by red spruce during the decline. It would be useful to have experimental evidence from mature trees in the field for the best possible assessment. In this regard, Vann and Johnson (1988) have begun experiments using branch chambers for the exclusion of airborne chemicals from branches of mature red spruce. Their preliminary results suggest improved foliage with filtered air (Fig. 9), but the results are not sufficient at present to make judgments about productivity and carbon reserves. SUMMARY The recent mortality of red spruce has components which have been commonly associated with declines of other forest species in the northeastern U.S. Natural factors such as age, poor sites, exposure to winter winds, and possibly climate warming may act as predisposing factors. Repeated and severe winter damage to foliage and buds, and possibly drought, may act as inciting factors, and insects and pathogens appear to serve as contributing or secondary stresses. Air pollution stress might be a contributor by enhancing the effects of the conditions leading to winter injury, or by consuming energy reserves which might otherwise have been used to defend against pathogens or to repair damage from winter injury or drought. The information available at the present time suggests that ozone is the most likely pollutant for further study. The climatic conditions and mechanisms leading to winter injury in red spruce need to be understood in detail, and the effects of airborne chemicals on those mechanisms

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101 need to be determined before a confident assessment can be made of the role played by air pollution. TOTAL CHLOROPHYLL CONTENT a-' 1 1 - ~2 - ~o At. 2~. ... I.. ..4 1.2 t.O / //: ~ J ~2 ~ t~: ~!~ : "1 S 7~3 7~16 7~28 COlLECrlON OA" "~ ~ 0~ ~7 acts F1L~ED 1987 NEEDY DRY "IGHTS HA l 2~: 2 ~ 1~- , t OA . /~ k;/ //,/r/ my' ~ 7/OS ?~1f 7~ ~7 / ~ CI3NTIIOL + Offt' lift ~ a/2' FILTERED Figure 9. Chlorophyll content and dry weight of red spruce foliage from a control chamber, a chamber receiving charcoal-filtered air, and an open branch. The experimental tree was approximately 125 years old (at dbh), and growing at 1170 m on Whiteface Mt., NY. Chambers were installed on July 3. Standard errors are calculated from replicate twigs from one branch. Methods and operating conditions in the chambers (temperature, PAR, relative humidity) are given by Vann and Johnson (1988~.

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102 REFERENCES Battles. I.. A.H. Johnson. T.G. Siccama. and W.L. Silver. 1987. Recent changes in spruce-fir forests of the New York, Vermont and New Hampshire. In: proc. of a U.S.-German Conference on forest decline. Burlington, VT. October 19-24, 1987. U.S. Forest Service, Broomall, PA. Burgess, R.L., M.B. David, P.D. Manion, M.J. Mitchell, V.A. Mohnen,~ D.J. Raynal, M. Schaedle, and E.H. White. 1984. Effects of acidic deposition on forest ecosystems in the northeastern United States: an evaluation of current evidence. New York State College of Environmental Science and Forestry, Syracuse, NY. Carey, A.C., E.A. Miller, G.T. Geballe, P.M. Wargo, W.H. Smith, and T.G. Siccama. 1984. Armillaria mellea and decline of red spruce. Plant Dis. 68:794-795. Cook, E.R., A.H. Johnson, and T.J. Blasing. 1987. Modelling the climate effect in tree rings for forest decline studies. Tree Phys. (3) 27-40. Cumming, I.R., R.G. Alscher, J. Chabot, and L.H. Weinstein. 1988. Effects of ozone on the physiology of red spruce seedlings. In: proc. of a U.S.- FRG conference on forest secline. Burlington, VT. Oct. 1987. U.S. Forest Service, Broomall, PA (in press). Curry, J.R., and T.W. Church. 1952. Winter drying of conifers in the Adirondacks. J. Forestry (50) 114- 116. Davison, A.W., J.D. Barnes, and C.J. Renner. 1987. Interactions between air pollution and cold stress. Proc. 2nd Internat. Symposium on Air Pollution and Plant Metabolism. Apr. 6-9, 1987. Neuhenburg, Fed. Repub. of Germany. Friedland, A.J., R.A. Gregory, L. Karenlampi, and A.H. Johnson. 1984. Winter damage to foliage as a factor in red spruce decline. Can. J. Forest Res. 14:963-965. Hadfield, J.S. 1968. Evaluation of diseases of red spruce on the Chamberlain Hill sale, Rochester Ranger District, Green Mt. Nat. Forest. File Report A-68-S 5230. Amherst, MA: USDA-Forest Service Northeastern Area, State and Private Forestry, Amherst FPC Field Office. Hopkins, A.D. 1901. Insect enemies of the spruce in the Northeast. - U.S. Dept. of Agriculture, Div. of Entomology Bull. No.2S, new series. Pp. 15-29. Johnson, A.H., and T.G. Siccama. 1983. Acid deposition and forest decline. Environ. Sci. Technology. 17:294a-305a. Johnson, A.H., and S.B. McLaughlin. 1986. The nature and timing of the deterioration of red spruce in the northern Appalachians. Report of the Committee on Monitoring and Trends in Acidic Deposition. National Research Council, National Academy Press, Washington D.C. Pp. 200-230. Johnson, A.H., A.~. Friedland, and J. Dushoff. 1986. Recent and ~ historic red spruce mortality: evidence of climatic influence. Water Air Soil Pollut. 30:319-330. Johnson, A.H., E.R. Cook, and T.G. Siccama. 1988. Relationshi~ps between climate and red spruce growth and decline. Proc. Nat. Academy Sci. (in press).

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103 Kelso, E.G. l 965. Memorandum 5220,2480, July 23, 1965. Service Northern FPC Zone. Amherst, Mass. U.S. Forest . , Lovett, G.M., W.A. Reiners, and R.K. Olson. 1982. Cloud droplet deposition in subalpine balsam fir forests: hydrological and chemical inputs. Science 218:1303-1304. Manion, P.D. 1981. Tree Disease Concepts. Englewood Cliffs, N.J. Prentice Hall. McCreery, L.R., M.M. Weeks, M.J. Weiss and I. Millers. 1987. Cooperative survey of red spruce and balsam fir decline and mortality in New York, Vermont and New Hampshire: a progress report. In: proc. Integrated Pest Management symposium for northern forests March 24-27, 1986. Cooperative Extension Service, University of Wisconsin, Madison. McLaughlin, S.B. 1987. Carbon allocation as an indicator of pollutant impact on forest trees. In Proc. of a IUFRO Conference on Woody Plant Growth In a Changing Chemical and Physical Environment. M. Cannell and D.L. Lavander (eds.~. Vancouver, B.C July 1987. (in press). McLaughlin, S.B., D.J. Downing, T.J. Blasing, E.R. Cook, and H.S. Adams. 1987. An analysis of climate and competition as contributors to the decline of red spruce in the high elevation Appalachian forests. Oecologia 72:487-501. Mueller-Dombois, D. 1983. Tree-group deaths in North American and Hawaiian forests: a pathological problem or a new problem for vegetation ecology? Phytocoenologia 11:117-137. Namias, ]. 1970. Climatic anomaly over the United States during the 1960's. Science 170:741 -743. Reich, P.B., and R.G. Amundson. 1985. Ambient levels of ozone reduce net photosynthesis in tree and crop species. Science 230:566-570. Skelly, J.M., Y. Yang, B.I. Chevone, S.~. Long, J.E. Nellessen, and W.E. Winner. 1983. Ozone concentrations and their influence on forest species in the Blue Ridge Mountains of Virginia. In: Air Pollution and Productivity of the Forest. Pp. 143-59. Izaak Walton League, Washington, D.C. Stark, D. Unpublished notes on red spruce disease, mortality and winter injury 1957- 1977. State of Maine Department of Conservation, Entomology Laboratory. Augusta. Tegethoff, A.C. 1964. High Elevation Spruce Mortality. Memorandum 5220, September 25, 1964. Amherst Mass.: U.S. Forest Service Northern FPC Zone. Vann, D.R., and A.H. Johnson. 1988. Design and testing of branch chambers for the exclusion of atmospheric pollutants. In Proc. of a U.S.- FRG Conference on Forest Decline. Burlington, VT, Oct. 1987. U.S. Forest Service, Broomall, PA (in press). Wang, D., D.F. Karnosky, and F.H. Bormann. 1986. Effects of ambient ozone on productivity of PODU1US tremuloides Michx. grown under field conditions. Can. J. Forest Research. 16:47-55.

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104 Waring, R.H. 1987. Characteristics of trees predisposed to die. BioScience 37: 569-575. Weinstein, L., R.J. Kohut, and Jay S. Jacobson. 1987. Research at Boyce Thompson Institute on the effects of ozone and acidic precipitation on red spruce. Proc. SOth Ann. Meeting, Air Pollut. Control Assn. June 21-26, 1987, New York, NY. Weiser, C.J. 1970. Cold resistance and injury in woody plants. Science 169:1269-1278. Wheeler, G.S. 1965. Memorandum 2400, 5100 July 1, 1965. Laconia, NH U.S. Forest Service Northern FPC Zone.