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METALS IN ROOTS, STEM, AND FOLIAGE OF FOREST TREES Walter C. Shortle USDA Forest Service Northeastern Forest Experiment Station Durham, NH 03824 ABSTRACT Eight metals (Ca, K, Mg, Mn, Fe, - Zn, Cu. Mo in decreasing molar concentrations) are considered essential for tree growth and function. They account for less than one percent of tree mass. They must be obtained from soils derived from the earth's crust in which the eight most abundant metals are A1, Na, Ca, Fe, Mg, K, Ti, Mn. To obtain the essential metals from among the common metals requires mechanisms for selectively accumulating some metals, while discriminating against others, by biologically regulated energy-consuming processes. Of major concern are forests in which soil pH is in the 3 to 4.5. range. In this range A1 ions can be released from soil solids by inputs of strong inorganic ions, such as sulfate and nitrate. Increased vulnerability of large trees may be triggered as A1 ions attain equimolar concentration with Ca ions in the absorbing fine roots. Awareness that a fatal tree disease was developing in the spruce-fir forests of Germany began in the mid- 1 970s with descriptions of a dieback and decline of silver fir (Abies alba Mill.) generally referred to as ~fir-dying." Trees with dying crowns had a marked decrease in sapwood basal area (portion of the transverse area of secondary xylem that is functional sapwoocI) in the lower stem, and a marked increase in "wetwood" (wound initiated infection) in the lower stem and upper secondary roots (Bauch et al. 1979~. Firs with dying crowns and decreasing sapwood basal areas due to the spread of wetwood had been declining in cambial activity since the 1 950s, long before crown symptoms were observed. Dendroclimatological studies indicated that this suppression of cambial growth was not related to weather patterns (Eckstein et al. 1983~. By the early 1 980s, a dieback and decline syndrome was observed in Norway spruce (Picea abies (L.) Karst), although the symptoms were not as pronounced as in silver fir (Schutt and Cowling 1985~. Spruce accounts for 40 percent of the commercial forest in West Germany, fir only about 2 percent. Concern now changed from "fir-dyinn" to "forest-dying" in central Europe and in other countries including Canada. The relationship of this sudden loss of tree health to pollution associated with the 20tli century economy has been scientific investigations and a matter of great public interest. conclusions have yet been reached about the actual impact of deposition on forests, the possibility clearly exists that forests eastern North the United States and stresses created by air the subject of many Although no definite air pollution and acidic of central Europe and America are at risk from air pollution, either by direct chronic effects on foliage or indirect cumulative effects on fine roots in soil. 275
276 A mechanism has been proposed that links some soil related effects of acidic deposition to suppression of cambial growth, reduction of sapwood basal area, crown degradation, and eventual mortality (Shortle and Smith 1988~. The mechanism involves the gradual replacement of essential metal cations, calcium and magnesium, in the rooting zone of spruce-fir stands with aluminum, which is released into soil solution by adding strong anions to acidic soils (pH 3 to 4.5) where spruce and fir commonly grow (Ulrich 1983~. The molar ratio of aluminum to calcium in absorbing fine root tissue increases and at an equImolar ratio the uptake of calcium becomes limited (Bauch and Schroeder 1982, Stienen et al. 1984, Bauch et al. 1985a, Shortle and Stienen 1988~. Adding calcium to decrease the ratio in soil and in fine roots restored short-term calcium uptake in isotope studies (Schroeder, et al. 1988), partially reversed damage in seedling studies (Stienen and Bauch 1988), and reversed the declining cambial growth of mature trees in liming studies (Bauch et al. 1985b). The timing and dose of calcium additions are obviously important factors in obtaining a favorable response. Calcium is the most abundant metal in spruce, fir, and northern hardwood trees (Young and Carpenter 1967~. The high calcium requirement for mature trees was recognized as a major difference between trees and crop plants (Rennie 1955~. Unlike other major essential elements (nitrogen, phosphorus, and potassium), calcium is not recovered from older living sapwood as it is converted into a heartwood core in which calcium will be sequestered until the wood decays (Bamber and Fukazawa 1985~. Although the uptake of calcium and other major essential cations may vary considerably from site to site, the amount incorporated into functional sawood appears to remain relatively constant (Shortle and Bauch 1986~. If calcium uptake is restricted by interference from aluminum, and the rate of incorporation is conserved, then a prolonged suppression of cambial growth is the expected outcome. Prolonged cambial suppression would be seen as a series of narrow growth rings. The conversion of wider growth rings into the heartwood core at the inner sapwood boundary is no longer balanced by the addition of new sapwood rings at the outer sapwood boundary (the vascular cambIum), thus the sapwood basal area decreases as observed in declining spruce and fir trees. Mature spruce and fir trees with smaller sapwood basal areas have smaller crowns (Marchand 1984~. Large trees with small crowns and low sapwood basal areas are more vulnerable to attack by pathogens and insects (Shigo 1 985a,b). Such trees would also have less energy available to compensate for the adverse aluminum to calcium ratio through additonal fine root production. The range of soil pH values observed in the humus and underlying mineral soil in spruce-fir stands across northern New England (Shortle and Stienen 1988) was similar to those observed in many stands in Germany (Stienen et al. 1984~. Extracts of humus (organic soil horizons) collected from Camels Hump, Vermont, and Mount Abraham (25 km to the south), where red spruce are dying, had a molar Al:Ca ratio of 13.0 (Taylor et al. 1986) add 6.0 (Shortle anti Stienen 1988), respectively; those from Acadia National Park, Maine, and Beddington (60 km to the north), where red spruce appeared healthy, had a molar Al:Ca ratio of 1.2 and 0.S, respectively. When the humus layers from Camels Hump and Acadia were used in an experiment with rain chemistry, mist chemistry, and ozone, soil was the only treatment affecting both the above-ground and below-ground biomass of red spruce seedlings (Taylor et al. 1986~. No treatment specific symptoms of visible needle injury were observed. Experiments with spruce seedlings under various cultural conditions in which the molar Al:Ca ratio was 1 indicated that this condition can affect the development of fine roots and shoots (Schier 1985, Stienen and Bauch 1988~. Molar ratios of Al:Ca in spruce and fir fine root tissue were determined from collections made from humus and mineral soil sampled across New England in 1985 and
277 1986. In brief summary, at Mount Abraham, Vermont, Al:Ca in fine root tissue taken from the lower humus, the major rooting zone of spruce and fir, was 3.4 (spruce, 1985), 1.7 (fir, 1985), and 4.1 (spruce, 1986~. Fine root tissue in the underlying mineral soil at Mount Abraham was 3.1 (spruce, 1986~. At Beddington, Maine, Al:Ca in fine root tissue taken from the lower humus was 0.1 (spruce, 1985), 0.3 (fir, 1985), and 0.1 (spruce, 1986~. Fine root tissue in the underlying mineral soil at Beddington was 1.3 (spruce, 1986~. Fine root production was sparse in the mineral soil from both sites. Values for seven additional locations between Mount Abraham and Beddington were intermediate, but more like Beddington than Mount Abraham at this time. However, at the Crawford Notch, New Hampshire, location where the largest spruce are dead and dying, spruce fine root samples taken in 1987 had an Al:Ca ratio of 1.1 for humus and 3.7 for mineral soil. Limited growth of yellow birch (Betula alleghaniensis Britton), is attributed to aluminum in the po~zols of New England (Hoyle 1971~. The humus layer supported good growth of younger, smaller trees, but did not compensate for the inadequacy of the mineral subsoil to support growth of larger trees, especially in the presence of aluminum. At high elevation sites, it appears that aluminum may now be affecting the uptake in the lower humus, which is usually a safe place for fine root absorption. The molar Al:Ca ratio of fine root tissue may be a useful indicator of the influence of acidic deposition on spruce-fir forests on acid soils (pH 3 to 4.5~. Under existing acidic conditions, forests are at risk from the continued input of strong anions derived from emissions of sulfur and nitrogen. Above-ground tissues are highly conserved with respect to major metal ion concentrations with stemwood being more stable than foliage. These parts of the tree appeared to respond to changing soil conditions by changing rates of tissue development from cambium and buds. Changes in cambial activity, sapwood basal area, and the spread! of infection in forest trees can be monitored by electrical measurements (Shigo and Shortle 1985~. Sampling absorbing fine roots (0.2 mm diem X<5 mm length) in amounts sufficient to determine molar ion ratios is not feasible on a routine basis over large geographic areas. However, some simpler analyses applied in sequence on samples of soil from the zone of major fine root activity may be useful as an indicator of atmospheric effects in the forest. Presumptive tests for low acidity and high ionic strength could be made by simple electrical measurements followed by determination of soluble or exchangeable metal ion ratios, such as Al:Ca and Al:Mg on acidic soils, or Ca:K on less acidic soils, by routine chemical analysis. Coupling such measurements with periodic electrical measurements of cambial activity and internal infections could help locate areas of progressive decline and impending mortality, or of recovery following episodes of decline and mortality. REFERENCES Bamber, R.K., and K. Fukazawa. 1985. Sapwood and heartwood: A review. Forestry Abstracts 46:567-580. Bauch, J., P. Klein, A. Fruhwald, and H. Brill. 1979. Alteration of wood characteristics - in Abies -alba Mill. due to "fir-dying" and considerations concerning its origin. Eur. J. For.Pathol. 9:321-331.
278 Bauch, J., P. Rademacher, W. Berneike, J. Kroth, and W. Michaelis. 198 5a. Breite und Elementgehalt der Jahrringe in Fichten aus Waldschadensgebieten. Pp. 943- 959 in Waldschaden-Einflussfaktoren und ihre Bewertung. Dusseldorf, FRG: VD1 Berichte 560. Bauch, I., and W. Schroeder. 1982. Zellularer Nachweis van Elementen in den Feinwurzeln gesunder und erkrankter Tanne (Abies alba Mills. Forstwissenschaftliches Centralblatt 101 :285-294. Bauch, J., H. Stienen, B. Ulrich, and E. Matzuer. 1985b. Einfluss einer Kalkung bzw. Dungung auf den Elementgehalt in Feinwurzeln und das Dickenwachstum von Fichten aus Waldschadensgebieten. 43:1148-1150. Allgemeine Forst Zeitschrift Eckstein, D., R.W. Aniol, and J. Bauch. 1983. Dendroklimatologische Untersuchungen zum Tannesterben. Eur. J. For. Pathol. 13:279-288. Hoyle, M.C. 1971. Effects of the chemical environment on yellow birch root development and top growth. Plant and Soil. 35:623-633. Marchand, P. J. 1984. Sapwood area as an estimator of foliage biomass and projected leaf area for Abies balsamea and Picea rubens. Can. J. For. Res. 14:85-87. Rennie, P. J. 1955. The uptake of nutrients by mature forests growth. Plant and Soil 7:49-95. Schier, G. A. 1985. Response of red spruce and balsam fir seedlings to aluminum toxicity in nutrient solutions. Can. J. For. Res. 15:29-33. Schroeder, W.H., J. Bauch, J., and R. Endeward. 1988. Microprobe analysis of Ca exchange and uptake in the fine roots of spruce: influence of pH and aluminum. Trees 2~3~: (in press). Schutt, P., and E.B. Cowling. 1985. Waldsterben, a general decline in forests in central Europe: symptoms, development and possible causes. Plant Disease 69:548-558. Shigo, A. L. 1985a. Compartmentalization of decay in trees. Sci. American 252:96- 103. Shigo, A. L. l985b. Wounded forests, starving trees. J. Forestry 83~11~:668-673. Shigo, A. L., and W.C. Shortle. 1985. Shigometry: A reference guide. USDA For. Serv. Agric. Handb.646. 48 p. Shortle, W. C., and I. Bauch. 1986. Wood characteristics of Abies balsamea in New England states compared to Abies aZba from sites in Europe with decline problems. IAWA Bull. n.s. 7:375-387. Shortle, W.C., and K.T. Smith. 1988. Aluminum-induced, calcium deficiency syndrome in declining spruce-fir forests Science 240:1017-1018.
279 Shortle, W.C., and H. Stienen. 1988. Role of ions in the etiology of spruce decline. In Proc. Research Symposium, The effects of atmospheric pollution on spruce and fir forests in the eastern United States and the Federal Republic of Germany, October 18-23, 1987. Burlington, VT. USDA For. Serv. Gen. Tech. Rep. NE- (in press). Stienen, H., R. Barckhausen, H. Schaub, and J. Bauch. 1984. Mikroskopische und rontgenenergiedispersive Untersuchungen an Feinwurzeln gesunder und erkrankter Fichten (Picea abies (L.) Karst.) verssschiedener standarte. Forstwissenschaftliches Centralblatt 103:262-274. Stienen, H. and J. Bauch. 1988. Element determination in tissues of spruce and fir seedlings from hydropanic and soil cultures simulating acidification and deacidification. Plant and Soil 106:231-238. Taylor, G.E., Jr., R.J. Norby, S.B. McLaughlin, A.H. Johnson, and R.S. Turner. 1986. ~~ ~ rubens Sarg.) carOon d~ox~de assimilation and growth of red spruce (Picea seedlings in response to ozone, precipitation chemistry and soil type. Oecologia (Berlin) 70:163-176. Ulrich, B. 1983. A concept of forest ecosystem stability and of acidic deposition as a ctriving force for destabilization. Pp. 1-32 in Ulrich, B., Pankrath, J. (eds.~. Effects of air pollution in forest ecosystems. Dordrecht, Holland: D. Reidel Publishing Co. Young, H.E., anc! P.M. Carpenter. 1967. Weight, nutrient element and productivity studies of seedlings and saplings of eight tree species in natural ecosystems. Tech.Bull. 28. Orono, ME: Maine Agric. Expt. Stn., Univ. of Maine. 39 p.
278 Bauch, J., P. Rademacher, W. Berneike, J. Kroth, and W. Michaelis. 198 5a. Breite und Elementgehalt der Jahrringe in Fichten aus Waldschadensgebieten. Pp. 943- 959 in Waldschaden-Einflussfaktoren und ihre Bewertung. Dusseldorf, FRG: VD1 Berichte 560. Bauch, I., and W. Schroeder. 1982. Zellularer Nachweis van Elementen in den Feinwurzeln gesunder und erkrankter Tanne (Abies alba Mills. Forstwissenschaftliches Centralblatt 101 :285-294. Bauch, J., H. Stienen, B. Ulrich, and E. Matzuer. 1985b. Einfluss einer Kalkung bzw. Dungung auf den Elementgehalt in Feinwurzeln und das Dickenwachstum von Fichten aus Waldschadensgebieten. 43:1148-1150. Allgemeine Forst Zeitschrift Eckstein, D., R.W. Aniol, and J. Bauch. 1983. Dendroklimatologische Untersuchungen zum Tannesterben. Eur. J. For. Pathol. 13:279-288. Hoyle, M.C. 1971. Effects of the chemical environment on yellow birch root development and top growth. Plant and Soil. 35:623-633. Marchand, P. J. 1984. Sapwood area as an estimator of foliage biomass and projected leaf area for Abies balsamea and Picea rubens. Can. J. For. Res. 14:85-87. Rennie, P. J. 1955. The uptake of nutrients by mature forests growth. Plant and Soil 7:49-95. Schier, G. A. 1985. Response of red spruce and balsam fir seedlings to aluminum toxicity in nutrient solutions. Can. J. For. Res. 15:29-33. Schroeder, W.H., J. Bauch, J., and R. Endeward. 1988. Microprobe analysis of Ca exchange and uptake in the fine roots of spruce: influence of pH and aluminum. Trees 2~3~: (in press). Schutt, P., and E.B. Cowling. 1985. Waldsterben, a general decline in forests in central Europe: symptoms, development and possible causes. Plant Disease 69:548-558. Shigo, A. L. 1985a. Compartmentalization of decay in trees. Sci. American 252:96- 103. Shigo, A. L. l985b. Wounded forests, starving trees. J. Forestry 83~11~:668-673. Shigo, A. L., and W.C. Shortle. 1985. Shigometry: A reference guide. USDA For. Serv. Agric. Handb.646. 48 p. Shortle, W. C., and I. Bauch. 1986. Wood characteristics of Abies balsamea in New England states compared to Abies aZba from sites in Europe with decline problems. IAWA Bull. n.s. 7:375-387. Shortle, W.C., and K.T. Smith. 1988. Aluminum-induced, calcium deficiency syndrome in declining spruce-fir forests Science 240:1017-1018.