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6 The Nature and Timing of the Deterioration of Red Spruce in the Northern Appalachian Mountains Arthur H. Johnson and Samuel B. McLaughlin INTRODUCTION Over the past decade, researchers have postulated a number of possible effects of environmental pollutants on forest ecosystems. Theoretical and experimental inves- tigations have shown that acidic substances might affect many aspects of plant and forest function. In addition, some researchers are now beginning to study how acidic substances interact with other pollutants and natural stresses to affect forest ecosystems. Thus, there is a reasonable amount of evidence suggesting how acid depo- sition might affect forests, but to date no studies have shown the postulated effects to be present in the field. The finding of widespread mortality of red spruce (Picea rubens Sarg.) in the northern Appalachians (Siccama et al. 1982, Johnson and Siccama 1983, Carey et al. 1984, Foster and Reiners 1983, Scott et al. 1984) sparked research and speculation about whether acid deposition is a cause. Because of the lack of documented mechanisms, only circumstantial evidence supports claims that acid rain has caused damage to spruce in eastern North America. The suspicion that atmospheric deposition may play a role in the decline of red spruce is supported mainly by the facts that large-scale changes in the forest have taken place in high-elevation areas receiving airborne heavy metals and acidic substances at rates that appear to be greater than those experienced by almost all other forested areas in North America and that no obvious natural cause has been documented. Red spruce have died in uncharacteristic numbers near the top of their elevational range where they are subject to extreme climatic conditions as well as to pollutant input. We find evidence that the current episode of 200
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201 mortality could have been caused by climatic changes and that there are other possible natural contributors that remain unevaluated at the present time. To some extent, understanding the nature and timing of this forest disturbance puts constraints on the range of natural and anthropogenic factors that might be involved, but there is no way now to determine rigorously whether the deposited materials are assimilated benignly by the forest or if the airborne pollutants have altered life- sustaining processes enough to cause significant mortality. HIGH-ELEVATION CONIFEROUS FORESTS OF THE APPALACHIANS Researchers have long studied the zones of vegetation types that exist on the mountains of the eastern United States where forest composition varies along an attitudinal gradient. Subalpine stands dominated by fir and/or spruce occur from tree line down to 750 m in the White Mountains (New Hampshire), 850 m in the Green Mountains (Vermont), and 900 m in the Adirondack Mountains (New York). To the south, spruce-fir forests continue as patches on the highest peaks of the Appalachians. The lower boundary varies from about 1060 m in the Catskill Mountains (New York) to 1525 m in the Great Smoky Mountains (Siccama 1974, Costing and Billings i951). Gradients in flora and environmental conditions in the eastern montane forests have been describer by numerous investigators (i.e., Foster and Reiners 1983, Siccama 1974, Myers and Bormann 1963, Harries 1966, Adams et al. 1920, Holway et al. 1969, Scott and Holway 1969, McIntosh and Hurley 1964, Whittaker 1956). Figure 6.1 summarizes attitudinal gradients on Camels Hump (Vermont), a site that has figured prominently in recent research. In the Green Mountains (Vermont), sugar maple is the most important canopy species (by density and basal area) below 750 m, and balsam fir is most important above 850 m. A rather narrow transition zone lies between. Red spruce is a minor species in the hardwood forest (<750 m) and above 1150 m. It is most important in stands of the transition and lower boreal zones (760 to 1150 m). Vegetation patterns are similar, but somewhat more complex, in the Catskill, Adirondack, and White mountains because they are irregular massifs rather than linear ridges like the Green Mountains (Siccama 1974, Holway et al. 1969, McIntosh and Hurley 1964).
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202 a 100 ~ J > UJ z 50 cr o cat 5 4 b j / \. 1-~ oL c ELEVATION (m) -? i_ 3 1 1 1 1 1 40 I 20 1 _ O _ I I I I I ~ 600 800 1000 1200 600 800 1000 1200 ELEVATION (m) d 103 60 1 o2 ~ - 1 ~ ~ 50 1 Do ~6 1 10-1 _ ,~. 600 800 1000 1200 600 800 1000 1200 ELEVATION (m) ELEVATION (m) FIGURE 6.1 Altitudinal gradients in tree species and environmental characteristics at Camels Hump, Vermont. Curves are based on measurements at 11 elevations (except Figure 6.1(d)) by Siccama (1974) in 1964-1966. (a) Species importance value (based on basal area, density, and frequency) for sugar maple (Acer saccharum Marsh) (SM); yellow birch (Betula alleghaniensis Britt.) (YB); beech (Fagus grandifolia Ehrh.) (B); red spruce (Picea rubens Sara.) (S.: balsam fir (Abies balsamea (L.) Mill) (F); white birch (Betula papyrifera var cordifolia (Marsh) Regel) (WB). (b) Basal area of all live woody species of >2-cm dbh. (c) Soil pH (H2O) and exchangeable calcium (NH4OAC extraction) in the B horizon. (d) Growing season throughfall collected by three to five rain gauges under the canopy at seven elevations. Amounts were measured weekly during the ice-free period during 1964-1966. (e) Maximum and minimum air temperature under the canopy. (f) Average number of frost-free days during 1964-1966.
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203 e 40 20 O to -20 ~0 maximum __ _ minimum , 1 1 1 1 1 ~ 600 800 1000 1200 E LEVATION (m ) FIGURE 6.1 (continued). 160 140 120 100 80 60 f - 1 1 1 - 600 800 1000 1200 ELEVATION (m) In addition to the elevational gradients, patterns of vegetation are influenced by natural disturbance and disturbance caused by human activity. Logging, fire, landslides, and windthrow have affected the composition and structure of the coniferous forests that have been studied in relation to red spruce decline. There is evidence that red spruce was a much more important component of the coniferous and hardwood forests of the northern Appalachians prior to the period of extensive timber harvesting in the late 1800s and early l900s (Siccama 1971). Extensive harvesting of red spruce coupled with unfavorable conditions for regen- eration after harvesting are generally thought to be the main reasons (Pielke 1979). DEPOSITION OF ACIDIC SUBSTANCES AND HEAVY METALS IN NORTHERN APPALACHIAN FORESTS The reasons that cause many to suspect pollutant stress as a factor contributing to mortality in red spruce are the following: (1) Pollutant deposition in subalpine forests appears to be very high relative to deposition in nearly all other extensive forested areas of North America. (2) The pollutants in sufficient doses are probably capable of altering many of the life-sustaining processes key to maintaining forests. (3) There have been no obvious natural causes offered to explain the observed mortality. m e following factors hinder a satisfactory scientific assessment of the role of atmospheric deposition: (1) The variety of pollutants and their delivery rates are not defined precisely. (2)
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204 Testing the impact of the array of known pollutants on the many life-sustaining processes is a complicated task. (3) Any change caused by pollutant substances is superimposed upon naturally occurring changes that alter forest composition, structure, and function, making it difficult to distinguish between natural and anthro- pogenic effects. The subalpine coniferous forests of the northern Appalachians are above cloud base for considerable portions of the year. The estimates of the duration of cloud cover vary from approximately 200 h/yr at 600 m in the Green Mountains to about 40 percent of the year at 1220 m in the White Mountains (Siccama 1974, Lovett et al. 1982). Cloud moisture tends to be much more acidic than precipitation, with average H+ concentration during the growing season reported to be 288 + 193 umol/L (Lovett et al. 1982). Based on a model of cloud droplet capture and on measurements of the chemistry of 10 cloud events, Lovett et al. (1982) estimated cloud water inputs of major ions to a subalpine balsam fir forest at 1220 m on Mount Moosilauke (New Hampshire) to be considerably greater than bulk precipitation inputs (Table 6.1). Excluding dry deposition, the deposition of acidic substances (H+, NH4, Sod , Nod) in the subalpine balsam fir forest is estimated to be three to six times greater than in the lower lying hardwood forest TABLE 6.1 Annual Deposition in Bulk Precipitation to a Northern Hardwood Forest at Hubbard Brook, New Hampshire, Compared with Estimated Annual Deposition by Bulk Precipitation and Cloud Droplet Capture in a Subalpine Balsam Fir Stand on Mt. Moosilauke, New Hampshire Hubbard Brook (Northern Hardwood Forest) ( 1963- 1974) (kg ha- ~ yr- l)a Mt. Moosilauke (Subalpine Balsam Fir) (1980-1981) (kg ha- ' yin ~ )b Bulk Bulk Cloud Ion Precipitation Precipitation Water Sum H+ 1.0 1.5 2.4 3.9 NH4+ 2.9 4.2 16.3 20.5 Na+ 1.6 1.7 5.8 7.5 K+ 0.9 2.1 3.3 5.4 SO2- 38.4 64.8 137.9 202.7 NO3 19.7 23.4 101.5 124.9 a Date from Likens et al. (1977). bOata from Lovett et al. (1982).
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205 TABLE 6.2 Mean (x), Standard Deviation (s), and Number (n) of Samples for ionic Concentrations in Bulk Precipitation at the Huntington Forest, Newcomb, New York Bulk Precipitation (1leq/L) Ion x s n NO3 31 22 67 SO2- 66 37 66 H+ 88 56 64 K+ 6 15 70 Ca2+ 13 19 70 Mg2+ 5 9 70 Na+ 4 8 70 NOTE: Elevation of forest approximately 500 m. SOURCE: Mollitor and Raynal (1983) in Burgess et al. (1984). at nearby Hubbard Brook, New Hampshire (Likens et al. 1977). Values for major ion deposition at high elevation in the northern Green Mountains estimated from rime ice chemistry, cloud chemistry, and droplet capture by artificial collectors (Scherbatskoy and Bliss 1984) are in rather close agreement with the estimates of Lovett et al. (1982). The estimates of acid deposition at high elevation exceed by severalfold the values calculated from low-elevation data collected by the major monitoring networks (see Chapter 5), but it is clear that the cloud-water input estimates are subject to uncertainty owing to the small number of samples, the variability in cloud chemistry, and the difficulties associated with determining cloud-water capture by the forest canopy. Roman and Raynal (1980) have reported recent alterations in patterns of tree rings of red spruce in mixed conifer/hardwood stands in the Huntington Forest (Adirondack Mountains). Precipitation in that area had a mean H+ ion concentration of 88 + 56 ~mol/L; overall precipitation chemistry is presented in Table 6.2. As in the case of lake sediments (see Chapter 9), lead has been shown to be a useful indicator of atmospheric deposition to forest soils (Reiners et al. 1975, Siccama et al. 1980, Johnson et al. 1982, Friedland et al. 1984a,b). High concentrations of lead have been observed in the subalpine forest floor in New England (Reiners et al. 1975, Johnson et al. 1982, Friedland et al. 1984 a,b, Hanson 1980). The amount of lead in the forest floor increases with elevation (Figure 6.2)--amounts at 1000 m
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206 _ 2.5 E - O 2.0 o J LL 'A 1 5 A: o I 1~ 1.0 Z 6 UJ ~ 0.5 t |Mean + S.E. (n=5) P< 0.01 0 500 1000 1500 ELEVATION (m) FIGURE 6.2 Altitudinal gradient of lead in the forest floor (Oi + Oe + Oa horizons) for 5 random samples collected at 27 sites in Massachusetts, Vermont, and New Hampshire. After Johnson et al. (1982). are approximately three to four times higher than those at S00 m. A similar elevational trend in the concentra- tions and amount of lead in the forest floor was observed at Camels Hump (Table 6.3), but no trends in the concen- trations of other metals were noted (Friedland et al. 1984a). Lead concentration in the forest floor at Camels Hump peaks in the Oe horizon (2 to 8 cm beneath the surface), reaching a mean of 302 + 14 agog (Friedland et al. 1984b). Between 1966 and 1981, lead concentrations in the forest floor at Camels Hump increased by 57 percent in the northern hardwood (548 to 731 m), 124 percent in the transition (762 to 853 m), and 95 percent in the boreal (883 to 158 m) zones (Friedland et al. 1984a). Copper and zinc concentrations also increased at somewhat lower rates during the 15-year period. Converting the 15-year increases in metal concentra- tions to annual rates, Friedland et al. (1984a) determined that the increased concentrations of lead, copper, and zinc in the forest floor were about equal to regional
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207 V o . ca O ~ _' o No ~ o Ct ._ V Ct I: Cal so 4~ O ~ o . O Ct ~ ._ ._ so C) O o V a_ _ U) ~ ~ o ~ o ~ _ o ~ U) o o U. I._ au is: o ._ o V Ct Ed Ct rig .= ca - , ~ ~ ~ ~ an REV of , 3 o o ·Z c) r. ~o4 - - - ~ ~ oo . . . ~i ~ ~ ~ oC ~: . . . - ~ ~ ~ - ~ - ~ ~ - . . . o~ o~ o - oN - - - ~ - ~ - ~ ~ oo o - o - o ~ . . . a~ ~ _ - ~ o - . . o o~o~ o oo ~ - . . . - ~ ~ ~ u~ ch o o o ~ a~ a~ 0 ~ ~ ~ oo ~oo mxo- ~ ~ ~ ~ ~ oo ~ ~ x ~ ~ ~ ~ =- - ~ , O ~n . . . ~ oo ~ oo ~ oo v) oo oo ~ oo oo ^ o · o v) ~ ~ ~ ~ ooo - ~ - v~ ~ ~ ~ ~ ~ - - - - - - oo - - ~ o o oo o oo ~ o - ~ ~ - ^ ~ - oo ' ~ - r~ - - o u~ o o ~ - ~ ~ - ~ o o 3 3 cĒ ~ ~ o ~ o ,~ - - c,) - - ~ ~ o o ~ o z ~ m z Et m C) ~0 C~ O :t 5 C~ Ct Ct ~ 00 U, ~ ~ ~L) :~S ~ C~ .= ~ O ~ ~ ~ ·~ 7 ~ ,_ . . _ ~ .. ,= ~ E~ Z ~ ~Q
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208 estimates of atmospheric deposition of those metals (Groet 1976, Siccama et al. 1980, Smith and Siccama 1981, Scherbatskoy and Bliss 1984). These data are consistent with the contention that subalpine coniferous forests receive considerably higher rates of atmospheric depo- sition than the adjacent lower-altitude hardwood forests. To date, the estimates of atmospheric inputs are imprecise because there are few data from short periods of record that show large variations in chemical compost tion. Largely because of the effect of cloud-droplet interception, the subalpine coniferous forests apparently receive acidic substances and heavy metals at rates that appear to be greater than the rates in neighboring low- elevation forests. With the exception of areas near large sources of sulfur or metal emissions no other forests in eastern North America receive trace metals and - acidic substances at rates as great as those estimated for subalpine coniferous forests of Vermont and New Hampshire. There is, however, little reliable evidence that we can use to assess whether we should expect chronic effects on biota resulting from the past and current atmospheric inputs of acidic substances. CHARACTERISTICS OF RED SPRUCE IN HIGH-ELEVATION CONIFEROUS FORESTS Red spruce are abundant in cool, soloist climates of the high elevations of the southern and northern Appalachians, in coastal regions from Maine to Nova Scotia, and in interior areas of southern Quebec, northern Vermont, northern New Hampshire, Maine, New Brunswick, and Nova Scotia; in the montane forests, sites with a high capacity to hold moisture are particularly favorable (McIntosh and Hurley 1964). Red spruce older than 300 years are common in the northern Appalachians (Siccama 1974, Burgess et al. 1984), and tree ring patterns indicate that some individual trees remain suppressed beneath the canopy for 80 years or more, attesting to the shade tolerance of the species. These characteristics allow it to be competitive in high-elevation forests with its more vigorous but shorter-lived competitors, balsam fir and white birch. At present, red spruce in the understory or in the canopy may show symptoms of disease or injury in the montane forests of New York, Vermont, and New Hampshire (Johnson and Siccama 1983, Burgess et al. 1984, Friedland et al. 1984c). One set of symptoms is common to all of
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209 the northern Appalachian sites discussed below. Trees show needle discoloration and death, crown thinning, an altered branching pattern, and the eventual death of branches and individuals. Friedland et al. (1984c) observed that in all size classes browning of the newest needles was widespread after the winters of 1980- 1981 and 1983-1984. They suggest that the most coupon observation of foliar loss--that it is most noticeable at the outer tips of the branches--might be accounted for by repeated winter damage to new foliage. Typically (but not uniformly) the crowns of mature, declining red spruce appear to die back from the top down and from the outside inward (Johnson and Siccama 1983), and severely declining trees generally have five or fewer year classes of needles. Studies in the northern Appalachians of root fungi indicate the presence of pathogens that play a secondary role. Early studies (Hadfield 1968, Mook and Eno 1956) found infection by Cytospora kunzii, Fomes pint, and Armillaria mellea. These studies also noted the absence of insects or pathogens that might be a primary cause of disease. A recent study of 288 red spruce in hardwood, transition, and boreal stands in the White, Green, Adirondack, and Catskill mountains showed that declining spruce were infected with Armillaria mellea (Carey et al 1984). Armillaria infected approximately 4 percent of the severely declining trees in the boreal zone, 25 - percent in the transition forests, and approximately 45 percent in the hardwood zone, indicating that Armillaria is an agent related to deteriorating red spruce. But owing to its absence in many declining trees it is not the main cause of deterioration. Decline, Disease, and Injury . Burgess et al. (1984) have offered an important perspective on the current status of red spruce in the northern Appalachians. Most commonly, "decline" has been the term used to describe the current deterioration of red spruce (e.g., Johnson and Siccama 1983, Carey et al. 1984, Siccama et al. 1982, Scott et al. 1984). Declines differ from most diseases in that they are not the result of a single causal agent. Manion (1981) has suggested a model for declines whereby three categories of stress may act to produce mortality. Environmental factors (e.g., poor site quality, airborne pollutants, nutrient defici-
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210 encies) may predispose a tree to greater than normal damage from a short-term initiating stress (i.e., drought, frost damage, insect defoliation, severe air pollution event), which is followed by contributing stresses, which are often lethal attacks from biotic agents (e.g., root pathogens, bark beetles). Because there are several possible stress factors in each of the three categories, declines can be very difficult to resolve into their components. Historically, several tree declines dating back to the 1930s have been documented in North America (Marion 1981, Houston 1981, Mueller-Dombois 1983), often without satisfactory reso- lution of the causes. Many incidence s of decline show a strong relationship to drought conditions (Burgess et al. 1984, Houston 1981), with several other factors presumably determining the ultimate fate of the affected trees, since not all droughts trigger declines. It is also important to note that widespread and synchronized episodes of canopy diebacks are occurring and have occurred in areas not subject to high levels of air pollution (e.g., Hawaii, Mueller-Dombois 1983). The characteristic of slowly advancing dieback in red spruce (defined here as loss of foliage and branch death) and the presence of secondary pathogens are consistent with the concept of decline. On the other hand, declines are most commonly associated with mature trees (Marion 1981), and the observation of mortality and symptoms in smaller size classes is unlike most other known declines (Burgess et al. 1984). The observation of needle damage and dieback in red spruce in many size classes indicates that injury (short-term effects of a stress) may be involved at some stage in spruce deterioration. There is no evidence to date that red spruce are deteriorating solely because of a biotic disease (defined here as a long-term interaction of a host with a pathogen), but the understanding of biotic agents associated with deteri- orating red spruce is incomplete because no systematic studies of the occurrence of pathogens have been reported. Changes in Red Spruce Populations Johnson and Siccama (1983) reported on the status of red spruce crown dieback in northern hardwood, transition, and boreal stands in the Appalachians. Methods and data from that survey are given in Appendix A and Figure 6.3. The percentage of spruce that are dead or in an advanced
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220 Multiple-regression analyses of red spruce chronologies from old-growth, high-elevation stands in Maine (Conkey 1979) and in New York (Cook 1985) indicate that spruce ring width is related to temperature, particularly in the winter and spring. Cook analyzed an old-growth red spruce chronology from a stand at Lake Arnold in the High Peaks region of the Adirondacks. Models based on average monthly temperatures provided a good fit (adj r 2 = 0.436 to 0.481) to annual index values for the calibration period 1890-1950 (Figure 6.11). The models predicted index values reasonably well from 1951 to 1967. After 1967, ring width was considerably less than predicted by the models, suggesting that the trees were responding to factors other than, or in addition to, temperature (Figure 6.11). 1.6 1.4 1.0 0.e 0.6 1.2 ALN A IDA ~ ~ ~ ~ ~v~1 7vV~ ~ 7Y'V Actual Estimated 0.4 ~ . . 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 YEARS FIGURE 6.11 Actual and estimated red spruce tree ring indices at Lake Arnold in the Adirondack Mountains (elevation 1150 m) based on a temperature response model. The model was developed using mean daily temperatures for the Northern Plateau Climatic Division of New York. The model was calibrated using the 1890-1950 period, and model predictions run from 1951 to 1976. The dark line is the actual indice record, the light line is the estimated indice record. The climate model was Index = -0.261 Jp - 0.300 Ap + 0.180 Op + 0.202 Np + 0.291 Dp + 0.155 Mc + 0.209 Sc. R2 = 0.502; R:dj = 0.436, OF = 53. (Jp, previous July temperature; A, August; N. November; D, December; Mc, current flay temperature; S. September). (From Cook 1985). The number of rings at breast height ranger from 142 to 297, with most in the range 200 to 250.
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221 Conkey (1979, 1984a,b, 1985) showed an abrupt and sustained change in red spruce maximum latewood density in high-elevation, old-growth red spruce in Maine after 1962 that she tentatively attributed to the effects of shortened growing seasons caused by a series of cool spring temperatures. The interpretation of latewood- density chronologies is a promising new tool in assessing the response of trees to their environment, but it is difficult to tell at present what factors are most important in determining density patterns. Conkey's findings are another signal of changes in tree ring properties probably related to temperature beginning in the 1960s. The tree ring evidence is consistent with the contention that red spruce have been deteriorating for approximately 20 to 25 years and that the deterioration was probably synchronous across the northern Appalachians. One acceptable interpretation of the available evidence is that, sometime around 1960-1965, the balance of stand conditions and environmental factors stressed red spruce beyond its ability to recover completely and that a decline ensued. Another is that continual or increasing stress to which red spruce is particularly susceptible has caused widespread injury, mortality, and collapse of the canopy in high-elevation stands. We are unaware of any clear evidence that balsam fir are deteriorating except as a secondary consequence of gap formation and the attendant increase in microclimatic stress (i.ee, increased susceptibility to wind and winter damage). Tree ring records for balsam fir generally do not show the abrupt change in the 1960s (Figures 6.8 to 6.10) observed in red spruce except at the front of naturally occurring fir waves at elevations above the spruce-fir zone (Marchand 1984). SUMMARY OF POSS IBLE CAUSES Dieback symptoms are most pronounced above 900 m, which is an environment subject to natural stresses such as wind, cold winter temperatures, and nutrient-poor soils and to high levels of pollutants such as acidic and heavy metal deposition. Sorting out the factors that have actually contributed to the large-scale deterioration of red spruce is complicated by the lack of information on pathogens, the range of known natural stresses and potential pollution-related stress factors, the background
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222 of mortality caused by normal stand dynamics, and the secondary effects of stand destabilization. The breakup of the high-elevation stands in some areas appears to have proceeded as a positive-feedback system--the gaps increase climatic stress and susceptibility to wind, then blowdown enlarges the disturbance patches and exposes more trees. It appears that the breakup of the deteri- orating stands was triggered by rather extensive mortality in spruce, and the observation that red spruce in all size classes now show the development and progression of dieback argues that at least some of the factors causing spruce mortality are still operating. Although the for- mation of disturbance patches (particularly by windthrow) is an important determinant of vegetative patterns, the synchronized spruce mortality across a broad region, and mortality in all size classes, are different from the disturbance phenomena currently recognized as contributing to present-day vegetative patterns is subalpine spurce-fir forests (Foster and Reiners 1982). Of interest in a historical context are reports of widespread mortality in red spruce between 1871 and 1890 in eastern North America (Hopkins 1891, 1901). These suggest that large-scale mortality in red spruce populations can be unrelated to pollutant deposition. The episodes summarized by Hopkins (1891, 1901) occurred at roughly the same time in West Virginia, New York, Vermont, New Hampshire, Maine, and New Brunswick and were tentatively attributed to a spruce beetle (called Dendroctonous piceaperda by Hopkins (1901)) - acting as a secondary agent following some other stress. A rigorous investigation of the nature, extent, and symptoms of previous episodes of spurce mortality might provide useful information for comparison with the recent conditions of spruce decline. In evaluating the possible causes of the current red spruce deterioration we recognize the following pos- sibilities at the present time. Further research and evaluation of each is warranted . 1. Stand dynamics resulting from aging or successional characteristics. 2. Drought. 3. Biotic disease. Repeated or severe winter damage due to climatic change. 5. Pollutant effects. 6. Combinations of pollutant and natural stresses.
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223 Considering the available data, aging or competitive pressures seem unlikely to be primary factors causing mortality of red spruce because the observed patterns argue against them: widespread and synchronous mortality; mortality in all age classes; mortality in stands of widely differing basal area, age, composition; and disturbance history. Although most of the deteriorating northern stands are multiaged and contain old spruce (more than 200 years old), few have basal area values characteristic of virgin, old-growth spruce stands, reported to be 45 to 69 m2/ha in New Hampshire and 41 to 59 m'/ha in Maine (Foster and Some stands (see Figure 6.10) in the late nineteenth and early some extent the regrowth of some the late 1800s may have resulted Reiners 1983). were subject to logging twentieth centuries. To stands after cutting in in critical competitive pressure and sharply decreasing ring width by 1960 (Meyer 1929). On the other hand, other stands show no signs of release caused by logging or other major disturbance (e.g., Figure 6.8) but show the same share rina widen decrease over the past 20 years. _ _ ~ _ _ , _ _ The subalpine forest on the west side of Mount Washington is a virgin forest, and it shows dieback, mortality, and decreased ring width in the same way as the rest of the high-elevation sites. (See also Foster and Reiners 1983.) The deterioration of stands harvested intensively less than a century ago argues against the contention that the observed dead spruce result only from the breakup of old-growth stands. Mortality in all size and age classes and in stands with different disturbance histories argues against the idea of "cohort senescence" (mortality of an even-aged canopy) that has been applied to other synchronous episodes of canopy tree death (Mueller-Dombois 1983). However, the previous episode of regionwide spruce mortality and periods of logging could have served as synchronizing events that led to a mu~tiaged cohort, which grew into a highly competitive situation by 1960. Widespread drought in the northeastern United States in the mid-1960s (see Chapter 3) was prolonged and severe (Cook and Jacoby 1977), and as droughts are often associated with declines of other species (Marion 1981, Houston 1981, Burgess et al. 1984) the 1960s drought has been proposed as a possible cause of the current spruce deterioration (Kelso 1965, Tegethoff 1964, Hadfield 1968, Siccama 1974, Johnson and Siccama 1983). However, it is difficult to assess the extent of drought stress in high-elevation forests. Examination of Palmer Drought
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224 Severity Index values (which are, at best, a modest representation of high-elevation drought) shows that the peak drought years were 1964 and 1965 (Johnson and Siccama 1983). The data of Siccama (1974), reproduced in Figure 6.1(d), provide direct evidence that it was not dry in absolute terms at Camels Hump and that growing season precipitation amounts increased substantially at higher elevation. Even though 60 cm of warm season precipitation might be a critically low amount for trees adapted to higher precipitation, the increase in precipitation with increasing elevation argues against drought as a principal cause of deterioration, since the degree of deterioration increases at higher elevation. The persistence of dieback symptoms in smaller size classes (Friedland et al. 1984c) also argues against a drought 20 years past being a principal cause of the continuing deterioration. Finally, the observations of patches of dead and dying spruce in 1962 (Kelso 1965, Tegethoff 1964, Wheeler 1965) indicate that some mortality was occurring before the period of most-intense drought. Based on the reported observations of the past 20 years it appears unlikely that red spruce are deteri- orating because of a biotic agent. Although no primary pathogens nave been reported, no systematic studies have been conducted so this possibility cannot be confidently rejected at present, and its definitive evaluation awaits further research. More likely as a key factor in spruce decline was a second climatic anomaly that occurred during the 1960 s and that could have had a region-wide, adverse influence on red spruce. In Chapter 3 we have described the occurrence of anonamously cold winters across the eastern United States from 1962 to 1971 (e.g., Namias 1970), and several observations suggest that such an occurrence could be of primary importance. Multiple regression analyses of tree ring chronologies (Cook 1985, Conkey 1979) suggest an adverse effect of cold winter tempera- tures and cool spring temperatures on annual ring width, although no physiological connection has been established. Numerous reports of severe winter damage, particularly during the past 20 years, suggest that red spruce is susceptible to damage from winter conditions, particularly after mid-winter thaw (Curry and Church 1952, Pomerleau 1962, Friedland et al. 1984c, Burgess et al. 1984). Early observers of the recent deterioration of spruce in New Hampshire observed severe winter damage in stands with dying spruce and suggested that severe winter
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225 29 27 25 F 2 19 17 _ N EW ENG LAN D 1 889-90 AVG DEC-FEB TEMPS (1888-89 - 1976-77) 15 13 Q.~R R7 . nor 1 904-05 1 933-34 1 976-77 ~ _ _ ~ ~ ~ ~ 1917-18 ~ ~ ~ ~ I I I 1888 1896 11904 1912 1920 1928 1936 1944 1952 1960 1968 1976 1900 YEAR FIGURE 6.12 Trend in winter temperatures in New England (after Diaz and Quayle 1978). The smoothed curve is a third-order polynomial least squares fit. conditions may have contributed to the mortality (Kelso 1965, Wheeler 1965, Tegethoff 1964). Trends in New England winter temperatures (Diaz and Quayle 1978) show that the past 25 years have been characterized by a trend toward colder winters (Figure 6.12). Diaz and Namias (1983) and Conkey (1985) have shown evidence of cool spring temperatures and shortened growing seasons in the 1960s and 1970s. The effects of cool springs and cold winters may be of particular importance to spruce growing near the top of its elevational range where the growing season length may be marginal under average conditions. Acidic substances, heavy metals, and gaseous pollutants alone or in combination could alter life-maintaining processes in forests. Whether they can do so to the extent necessary to cause the mortality reported in the above discussion cannot be rigorously evaluated. There is no indication now that acidic deposition is an
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226 important factor in the red spruce decline. Ring width anomalies occur in the 1960s in areas of relatively low acidic deposition (Roman and Raynal 1980) and in higher elevation studies where deposition is higher, and where trees are in contact with acidic cloud water for a sub- stantial part of the year. The abrupt and synchronous changes in ring width and wood density patterns across such a wide area seem more likely to be related to climate than to air pollution. Acid deposition or other airborne chemicals might play a role in predisposing the trees to the factors that are injurious, and when those factors are clearly defined, the role of airborne chemicals can be better assessed. Testing hypotheses involving interactions of natural and pollution stress will eventually be warranted since it may be difficult to reconstruct the effect of climatic anomalies or biotic disease in a way that will show that those factors alone could have triggered the recent deterioration. Finally, refined procedures for tree ring analysis may provide useful insights on the involvement of climate. REFERENCES Adams, C. C., G. P. Burns, T. L. Hankinson, B. Moore, and N. Taylor. 1920. Plants and animals of Mt. Marcy, New York. Parts I, II, III. Ecology 1:71-94, 204-233, 274-288. 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, N.Y. 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. Conkey, L. E. 1979. Response of tree ring density to climate in Maine, U.S.A. Tree Ring Bull. 19:29-38. Conkey, L. E. 1984a. X-ray densitometry: wood density as a measure of forest productivity and disturbance. Pp. 287-296 of Air Pollution and Productivity of the Forest, D.D. Davis, ed. Washington, D.C.: Izaak Walton League of America.
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Representative terms from entire chapter: