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Journal of Environmental Quality 31:1137-1145 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Heavy Metals in the Environment

Dendrochemical Analysis of Lead and Calcium in Southern Appalachian American Beech

Suzanne Fisher*,a, N.S. Nicholasa and Phillip R. Scheuermanb

a Tennessee Valley Authority, 129 Pine Road, P.O. Box 1649, Norris, TN 37828
b Dep. of Environmental Health, East Tennessee State Univ., Johnson City, TN 37614-0682

* Corresponding author (fishers{at}ornl.gov)

Received for publication July 31, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
The health of the northern hardwood forest in the southern Appalachian Mountains of Tennessee, North Carolina, and Virginia has gained attention from the media and environmental stakeholders due to a purported decline in forest health at higher elevations. This project examined lead (Pb) and calcium (Ca) concentrations in growth rings of an important northern hardwood species, American beech (Fagus grandifolia Ehrh.) at Mount Rogers and Whitetop Mountain, Virginia and attempted to examine concentration relationships with stem growth patterns. Dominant and codominant trees were sampled from 16 research plots at two elevations. Tree cores were crossdated, divided into sections of 10-yr periods, and analyzed using atomic absorption spectroscopy. Lead concentrations correlated negatively with ring width. Elevation and aspect were significantly associated with the Pb concentration, while Ca concentrations were only associated with aspect. Tree core samples taken from higher elevation plots contained higher Pb concentrations than samples collected from lower elevation plots, while the northwest and southwest aspects contained significantly higher amounts of Pb and Ca. Both Pb and Ca concentrations increased during the 1860s and again during the mid-1900s.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
THE HEALTH and subsequent growth pattern of a tree may be influenced by events that occur over its lifespan. Examinations of tree rings through dendrochronology and dendrochemistry supply data integrated over the lifespan of a tree and provide information about growth and associated seasonal effects. In addition to revealing growth trends, the annual rings of a tree may also serve as a chronological guide to past chemical exposures (DeWalle et al., 1995). Certain environmental conditions, such as lowering of soil pH, facilitate the mobilization of toxic metals and possibly replace essential soil nutrients, especially base cations. Excess amounts of these toxic elements may reduce root development and alter the pattern of stem diameter.

If the soil pH decreases below 5.0, lead (Pb), which is normally virtually unavailable for tree uptake, may become soluble and can bind at ion exchange sites, increasing lead's ability to be taken up by roots (Tomlinson, 1983). Several researchers have investigated heavy metal concentrations in tree rings and have noted changes in concentration associated with particular time periods (e.g., Arp and Manasc, 1987; Berish and Ragsdale, 1985; Bondietti et al., 1989, 1990; Eklund, 1995; Guyette et al., 1991; Latimer et al., 1996; Marcantonio et al., 1998; Watmough et al., 1998; Zayed and Loranger, 1992). Using clay soils, Bittel and Miller (1974) showed that Pb was preferentially absorbed over calcium (Ca) at pH levels between 5 and 6. Guyette et al. (1991) observed that Pb and cadmium (Cd) concentrations increased over time in the xylem of eastern red-cedar (Juniperus virginiana L.) trees growing in the mining district of southeast Missouri. The Pb and Cd concentrations reached 11 µmol kg-1 in trees growing in acidic soils (pH < 4.6), but did not increase in trees growing on control sites (pH > 5).

While spruce-fir decline in southern Appalachian forests has received much focus in the past 20 yr (Nicholas et al., 1999) and the health of the northern hardwood forest in the southern Appalachian Mountains of Tennessee, Virginia, and North Carolina has recently received media attention, published data regarding northern hardwood forest health in the southern Appalachian Mountains are scarce. High elevations in the southern Appalachian Mountains are exposed to higher levels of pollutant loading than lower elevations due to increased precipitation and frequent immersion in clouds (Mohnen, 1992). These high elevation forest ecosystems also contain higher frequencies of growth anomalies in trees, including declines in radial growth of red spruce (Bondietti et al., 1990). Several environmental groups have suggested that the hardwood forest condition of Mount Rogers and Whitetop mountain, Virginia has deteriorated in stands above 1370 m elevation, and link this purported decline to high levels of air pollution (Nicholas et al., 2000).

This project was developed to examine if dendrochemical Pb and Ca concentrations in American beech tree rings from Mount Rogers and Whitetop Mountain, in southwest Virginia, exhibit a relationship with growth patterns in northern hardwood species found in the southern Appalachians. In addition, we examined annual growth rings of American beech to determine if there was a correlation between growth and Pb and/or Ca concentrations. Comparing the Pb and Ca concentrations in tree rings with the amount of growth during a particular time period may provide an explanation of growth patterns attributed to abiotic stressors, including those of anthropogenic origin, and biotic stressors, such as changes in soil chemistry. American beech, a common northern hardwood species, was chosen based upon anatomical features examined for dendrochemical analysis by Cutter and Guyette (1993). Positive factors of American beech included low heartwood permeability and moisture content, species pervasiveness, longevity, wide geographic distribution, and foliage type. All criteria considered were based on habitat, xylem, and chemical factors (Cutter and Guyette, 1993).

The objectives of this study were to (i) age and crossdate tree-ring increment cores of American beech sampled from Mount Rogers and Whitetop Mountain, Virginia to determine the annual growth rate; (ii) identify the Pb and Ca concentration, by decade, in annual growth rings of American beech; (iii) analyze associations between growth and Pb and Ca concentration changes through time; and (iv) compare the Pb and Ca concentrations in tree rings at two different elevations (1371 and 1524 m) on Mount Rogers and Whitetop Mountain, Virginia. Calcium was chosen for elemental analysis because it is often displaced by ions that have greater affinity in soil solution, while Pb was analyzed due to the proximity of Interstate 81 to Mount Rogers National Recreation Area.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Mount Rogers National Recreation Area: Site History and Characteristics
Mount Rogers National Recreation Area, which is located in southwestern Virginia, covers 48564 ha (USDA Forest Service, 2001) and includes Whitetop Mountain and Mount Rogers. The U.S. Spruce Lumber Company began the first logging operations on the northeast side of Mount Rogers in 1905 and continued on Mount Rogers and Whitetop Mountain until the 1960s (Pyle and Schafale, 1988). Both Mount Rogers and Whitetop Mountain were dedicated in 1966 as a National Recreation Area managed as the Jefferson National Forest (Saunders, 1979).

Research plots were established in areas considered representative of the northern hardwood forest. Dominant tree species found within the plot areas included sugar maple (Acer saccharum Marshall), red maple (Acer rubrum L.), buckeye (Aesculus flava Sol.), yellow birch (Betula alleghaniensis Britton), and American beech. Dominant understory found on a majority of areas included blackberry (Rubus canadensis L.), catawba rhododendron (Rhododendron catawbiense Michx.), and hobblebush (Viburnum lantanoides Michx.). The soils of Mount Rogers and Whitetop Mountain are Typic or Pachic Haplumbrepts and the greater part of parent material consists of rhyolite (Joslin and Wolfe, 1992). The area receives approximately 1300 mm of annual precipitation, with a mean rainwater pH of 4.2; in addition there is 100 to 500 mm of cloudwater deposition, with a mean pH of 3.5. Joslin and Wolfe (1992) report that the forest on Whitetop is enshrouded in clouds about 30% of the time. Mean annual temperature is 7°C with an average relative humidity of 86%.

Research Plots
Twenty-four research plots were established June through August 1999 as part of a Tennessee Valley Authority–USDA Forest Service research project designed to collect baseline data for future evaluation of northern hardwood forest health. The randomly located 20- x 20-m permanent plots were stratified by elevation (1341 and 1524 m) and aspect (northeast, northwest, southeast, or southwest). A subset of 16 of the TVA–Forest Service plots with overstory American beech was used for this dendrochemical and dendrochronology analysis (Fig. 1) . Plots were established along the contour of the mountain on sites with less than 50% slope. Ten random dominant or codominant trees with intact crowns were cored from each plot for age determination. American beech trees among the 10 selected trees were potential sample trees in this study. A total of 30 tree cores were selected from eight of the lower and eight of the upper elevation plots for the analysis. Increment borers were tested for contamination by rinsing the equipment in a 10% nitric acid solution for examination on an atomic absorption spectrometer. Each sample was individually placed within plastic drinking straws for transportation and labeled with plot and sample number. Samples were immediately stored in a freezer until arrival at the laboratory where they were stored in a 4°C refrigerator until processing.



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  		Fig. 1. Research plots established in 1999 on Mount Rogers and Whitetop Mountain, Virginia and used for dendrochemical and dendrochronological analysis.

 
Ring-width measurements were crossdated by constructing a skeleton plot (Stokes and Smiley, 1968) and using the software program COFECHA (Dendrochronology Program Library, Richard Holmes, ANSI Standard Fortran-77, 1986, Laboratory of Tree-Ring Research, University of Arizona, Tucson). Core samples were read from the bark end of the sample to the pith using a Banister incremental measuring device (Model #380720; Acu-Rite Company, Jamestown, NY). Ring widths were measured using the Turbo Ring Read Program (Van Deusen and Le, 1988) to within 0.001 mm under a stereoscope mounted to a video monitor and converted into Tuscon format.

Long-term growth suppressions of the tree cores were analyzed by examining the skeleton plots for all samples to note periods of suppression and release; however, skeleton plots were used only as an identification tool. A more robust analysis was done to remove age-related growth using the computer program ARSTAN (Holmes et al., 1986). We used an interactive detrending approach by looking at several smoothing splines and varying different splines with variance cutoffs to note which technique was flexible enough to account for the age-related trend, while not removing the signal associated with climate and/or exogenous influences. A conservative standardization approach using a negative exponential curve was chosen to retain this information. Ring width indices were calculated for each year and then averaged by year to produce an ARSTAN master chronology for the total of all trees. Autoregressive time-series modeling was applied by ARSTAN to observe the tree-ring indices after autocorrelation was removed. Standardized ring-width indices were used in subsequent analyses (Vimmerstedt and McClenahen, 1995; Fritts, 1976; Cook et al., 2001).

Sample Processing for Dendrochemical Analysis
Thirty duplicate samples used in the dendrochronological analysis were taken from the same sample trees and used for the dendrochemical analysis. All tree cores were sectioned into 10-yr growth periods by shaving the top layer of wood from the core with a stainless steel razor blade and comparing samples used for the chemical analysis with crossdated samples under a 10x stereoscope. The 10-yr ring sections were then separated with a stainless steel razor blade that was rinsed in 10% nitric acid. Core sections were weighed and dried in an oven for 48 h at 50°C until constant mass was achieved. Samples were then pulverized in a stainless steel Wiley mill with 20-mesh filter and prepared for analysis using a nitric acid–30% hydrogen peroxide digestion technique. Digestion techniques followed a protocol for digestion of soils, sediments, and sludges described in USEPA (1995).

Cation Quantification
Tree core samples were analyzed for Pb and Ca using an atomic absorption spectrometer (Spectra AA-10/20; Varian Techtron, Victoria, Australia) equipped with a flame aspirator and a Model GTA 96 graphite furnace. Certified Pb and Ca standards from Fisher Scientific (Atlanta, GA; lead CAS #1009-74-8, calcium CAS #471-34-1) were used to make all standard solutions. Initial instrument calibration was carried out using five working standard solutions and analyses of Pb and Ca were conducted separately.

Triplicate absorbance readings of each sample were used to determine mean absorbance and to note any machine error. Concentration was determined directly by software in the spectrometer based on the standard calibration curve. Concentrations determined as milligram or microgram of element per gram of tree core material were converted to mol kg-1.

Quality control measures consisted of a working calibration curve, including a calibration blank for all cation measurements. The atomic absorption spectrometer was set to check a reference standard after every 10 samples and the reference standard curve after every 20 samples. Quality control samples included: a sample spike, blank spike, blank, duplicates, and an unknown certified quality assurance and quality control (QA–QC) sample (Environmental Resource Associates, Arvada, CO) for all cations. The QA–QC samples were required to be within a 15% range. Reliability and reproducibility of the atomic absorption spectrometer was evaluated by using Standard Reference Material 1575 (pine needles) available from the National Institute of Standards and Technology (Standard Reference Materials Program, Building 202, Room 204, Gaithersburg, MD 20899).

Data Analysis
The relationship between Pb and Ca concentrations and total decade ring width were tested for correlation using the CORR procedure in SAS. Relationships between elemental concentrations and growth were further evaluated to determine which parameters were significant factors on ring width. Main effects of decade Pb concentration, Ca concentration, age, aspect, elevation, and interacting terms on total decade ring width were statistically analyzed separately using analysis of variance (ANOVA) tests in SAS Version 6.12 (SAS Institute, 1996). The general linear model procedure (GLM) was used for unbalanced data. All ANOVA were run on independent, standardized data unless specified as ranked data. In order to determine if the approximate amount of Pb or Ca present within a decade sample of tree core influenced ring width, concentrations were grouped into class values and tested for main effects on total decade ring width. All class variables were identified in statistical tests as Pb concentration class (Table 1) , Ca concentration class (Table 2) , and decade or age class. The Pb and Ca concentration data were divided into classes by sorting data in ascending order and grouped into class sizes. The approximate age of the tree was analyzed for significance with ring width. Age classes were defined as less than 100 yr old, 101 to 151 yr, and >151 yr. Decade classes were defined by assigning a number to represent 10 yr of data within a particular decade (e.g., 1990 = 1). The GLM procedure was used for unbalanced class data in SAS. In addition, comparison of means and the determination of differences between Pb and Ca concentration classes, width, aspect, and elevation effects were determined by Tukey's studentized range test (HSD). While testing for the effects of elevation and aspect on Pb and Ca concentrations, concentration values were ranked.


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  		Table 1. Mean and range of tree-ring width for eight lead concentration classes.{dagger}

 

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  		Table 2. Mean and range of tree-ring width for calcium concentration class.{dagger}

 
Lead and Ca concentration data examined through time were indexed because trees sampled did not span the same time period and changes in Pb and Ca concentration were highly variable among individual trees (Tables 3 and 4) . This procedure was done to account for the statistical variability that existed between recent decade period data (Guyette and McGinnes, 1986; Eklund, 1995) that contained all 30-tree samples and early decade period data from 1740 to 1800 that contained less than two samples. The concentration index is shown in Eq. [1]:

[1]
was created by multiplying the average cation concentration (a) for a particular decade by the total number of trees sampled (N) divided by the number of trees sampled for the particular decade (T). This weights each decade concentration based on the number of samples gathered.


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  		Table 3. Lead concentration over time.

 

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  		Table 4. Calcium concentration over time.

 
A regression model relating standardized tree-ring widths to monthly climatic variables was developed to model and predict growth according to historical climate data from the National Oceanic and Atmospheric Administration's (NOAA) National Climatic Data Center (National Climatic Data Center, 2000). Average monthly minimum and maximum temperature values (Fahrenheit) were recorded and monthly precipitation values were measured in hundredths of an inch. The index chronology values for ring widths were determined to be non-normal using the Shapiro–Wilks test in the UNIVARIATE procedure of SAS (99% confidence level). Therefore, Spearman correlation analysis using the CORR procedure in SAS (SAS Institute, 1996) was used to compare index chronology widths to monthly and seasonal precipitation and temperature values. All tests were considered significant at the 95% confidence level. Monthly climate data were considered January through December for a particular year and seasonal data were grouped according to growing season, May through October. Early growing season (May–July) and late growing season (August–October) were also tested. Data from the previous year's growing season, beginning in May, were used as a lag variable to account for the dependence of current year tree growth on carbon uptake during the previous year's growing season (Grissino-Mayer and Butler, 1993). The lag variables were also tested for a correlation with index ring widths.

Regional and local data from four weather stations were tested to determine which were the most appropriate for the regression model. Spearman correlations were calculated for monthly climate variables for each weather station and index widths. Spearman correlations were also calculated for the average precipitation and temperature variables together and for combinations of stations and index widths. The correlation analysis determined the most significant climate variables to be May precipitation from the Chilhowie station (Cooperative ID 441675), which is located 15.3 km (9.5 mi) due north of Whitetop Mountain. Data from 1945 through 1999 were used in the regression analysis.

The data were divided into two sections; a calibration period of 1945–1970, which was used in the regression model, and a period of 1971–1999, which was withheld from the calibration model to verify climate and growth models produced (Grissino-Mayer and Butler, 1993). The period of 1945–1999 was chosen because of the data availability. Regression diagnostics using the SAS regression (REG) procedure (SAS Institute, 1996) were conducted. This included the observation of studentized residuals for possible outliers and inspection of Cook's d values influencing regression (Grissino-Mayer and Butler, 1993). The model obtained is shown in Eq. [2]:

[2]
where Ri was the predicted index for year i and Pi was the precipitation average. The residual growth data from the climate model were then compared with the predicted growth values to determine if there were growth anomalies due to factors other than climate.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Element Cation Concentrations and Tree Growth
Using Spearman correlation coefficients, we determined a significant negative relationship (p < 0.01) between total decade ring width and Pb concentrations. An analysis of variance (ANOVA) indicated that variability in Pb explained some (R2 = 0.21) of the variability in tree-ring width (F value = 18.99, Pr > F = 0.0001). In addition, a high statistical significance for the amount of Pb concentration present (F value = 5.45, Pr > F = 0.0001) and ring width was determined. When Pb concentration class values increased, mean ring width per decade decreased. The Pb Concentration Class 1, the smallest amount of lead defined by class separation, had a significantly larger ring width when compared with all other concentration classes (Table 1). Using ANOVA on ranked Pb concentrations, we determined the age class of a tree sample to be significant (df = 2, F value = 16.66, Pr > F = 0.0001).

Ring width was significantly associated with the variability in amount of calcium concentration (Table 2). Concentration Class 1, the lowest Ca concentration class, had significantly larger ring widths than all other concentration classes (Table 2). Tree age was only slightly associated with ring width (F value = 3.75, Pr > F = 0.0538).

Several authors have calculated elemental burdens in an attempt to account for concentration increases that may result from increased incremental concentrations in the narrower annual rings found in the outer portion of tree rings (Baes and McLaughlin, 1984; Berish and Ragsdale, 1985). However, Frelich et al. (1989) report that elemental burdens are not an adequate representation of concentration in tree rings because, while calculating elemental burdens in tree rings, the volumes of the increment cores were not taken into consideration and suggest these calculations may mask the changes in elemental concentration over time. Therefore, to compensate for this occurrence, the weight (kg) of each tree core per decade was accounted for.

An important concern that often challenges dendrochemists is the natural increase in radial trend; therefore, further research upon heavy metal concentrations within tree-ring tissues should be examined to prevent false associations (Watmough et al., 1998).

Influences of Elevation and Aspect on Tree Growth and Cation Concentration
Variation in elevation and aspect explained some of the variability in total decade ring width and/or Ca and Pb concentration (Table 5) . Average ring widths per decade were significantly (p < 0.0001) greater at 1371 than 1524 m. Aspect was a statistically significant factor associated with both Pb and Ca concentrations, whereas elevation was statistically associated only with Pb. Tukey's studentized range tests for comparisons among mean concentrations (mol kg-1) were significantly higher at 1524 than 1371 m. Mean Ca concentration was highest on northwest aspects, while mean Pb concentrations were significantly higher on southwest and northwest aspects (southwest aspect, n = 8; southeast aspect, n = 7; northwest aspect, n = 7; northeast aspect, n = 6).


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  		Table 5. Analysis of variance for the main effects of aspect and elevation on ring width, lead concentration, and calcium concentration.

 
Climate Influence and Tree Growth
Ring-width indices were positively correlated (p < 0.05) with May precipitation data from Chilhowie station. An analysis of variance (ANOVA) was run using the average May precipitation data for the years 1945–1999 and yearly tree-ring indices (F value = 12.67, Pr > F = 0.008). Data were then split into two subsets and a linear regression was run on the calibration period 1945–1970. Using regression diagnostics, the years 1963, 1969, and 1955 were identified as statistical outliers, based on high values obtained in the studentized residuals and Cook's d values. Data from these years were deleted to remove anomalous climate patterns and provide a better model for climate data. Only growth expected from normal climate variations was used and extreme climate events were removed.

The final regression model was used to calculate the tree growth that would have resulted only from climate influences for the years 1971–1999. The regression model for 1945–1970 explained 44% of the total variability (F value = 15.66, Pr > F = 0.0008) in the Arstan chronology tree-ring indices. The resulting equation for 1945–1970 (Eq. [2]) was used to predict growth ring indices for the second half of the data set for years 1971–1999. Comparison of residuals to predicted growth indices indicated that growth occurring throughout the 1960s was below the predicted values (Fig. 2) . A sharp decline in the residuals compared with the expected growth also began in 1982, lasting until 1987 (Fig. 2).



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  		Fig. 2. Measured and predicted values of growth ring indices for American beech generated from final regression model (Ri = 0.873709 + 0.03Pi), where Ri is the predicted index for year i and Pi is the precipitation average.

 
Cation Concentrations Changes through Time
The Pb concentration in American beech samples from Mount Rogers and Whitetop Mountain increased from 1740 to 1999 (Fig. 3) . Concentrations increased beginning in 1900 with an increase again during 1920. Lead concentrations declined in the following decade; however, they rose steadily until 1980, when Pb decreased.



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  		Fig. 3. Lead concentration indices through time sampled from American beech on Mount Rogers and Whitetop Mountain, Virginia.

 
The Ca concentration remained stable in tree-ring samples from 1740 to 1850 (Fig. 4) . Calcium concentrations increased beginning in 1860 and values remained elevated until 1900, when values declined by greater than one-half of the amount present a decade earlier. Calcium concentrations rose again in the following decade, 1910, and remained stable until an increase in 1960. Concentrations decreased between 1960 and 1980, with a second concentration decline during the 1990s (Fig. 4).



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  		Fig. 4. Calcium concentration indices through time sampled from American beech on Mount Rogers and Whitetop Mountain, Virginia.

 

   DISCUSSION AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
REFERENCES
 
Dendrochronological analysis determined that the majority of American beech on Whitetop and Mount Rogers were between the ages of 40 and 110 yr. Decade measurements of ring widths varied over time. However, there were no significant differences among these decadal widths. Ring widths for cores at higher elevation sites were smaller than ring widths of American beech at lower elevation sites, while both Pb and Ca concentrations were highest on northwest and southwest aspects.

Chronology ring widths were inversely correlated with Pb and Ca concentrations. Low Pb concentration classes also corresponded to the widest ring widths. Samples taken from higher elevation plots contained higher Pb concentrations than samples collected from lower elevation plots. One possible suggestion for this relationship with elevation is the enhanced mobility of Pb in soil because of the frequent exposure of higher elevation research plots to cloudwater and rain. However, to validate enhanced mobilization theories, long-term studies that include soil analyses are needed to determine changes in soil chemistry through time. A possible theory concerning the relationship between ring width and aspect could be the pollutant or deposition load brought in from prevailing winds to the northwest and southwest aspects. However, given the unpredictable nature of wind flow patterns within mountainous regions, detailed atmospheric research would need to be conducted in the area to confirm this possibility.

Calcium concentrations appeared to change through time, with an increase that began in 1860 through 1890 and again in the 1960s and 1980s. The increased Ca concentrations during these periods were often followed by periods of decreased Ca concentrations and several researchers have presented explanations for fluctuations in available cation concentrations found in soil and tree rings (e.g., Smith, 1981; Bondietti et al., 1990; Shortle et al., 1997; Watmough, 1997). Increases in Pb and Ca concentrations occurred during the 1860s, which corresponds to the beginning of the Industrial Revolution. The use of energy sources including coal and steam and the invention of the internal-combustion engine introduced large amounts of atmospheric pollutants. While there is no evidence that these pollutants caused changes in soil conditions that increased Ca bioavailability, it is interesting that these events correspond. Other researchers have reported sharp increases in Ca concentrations during the 1960s and 1980s that are followed by periods of decline in concentration (e.g., Bondietti et al., 1989, 1990; Shortle et al., 1997; Watmough, 1997). These researchers have speculated that this temporary increase in Ca uptake may have been caused by increased availability of base cations in the soil (e.g., Bondietti et al., 1990; Shortle et al., 1997). Logging may also enhance the mobility of Ca in forest soils. Decreases in leaf-litter inputs, losses of dissolved organic matter, and leaching of Ca from forest soils may occur following logging (Yanai et al., 1999). Logging of Mount Rogers and Whitetop Mountain occurred beginning in the early 1900s, which corresponds to a decrease in Ca concentrations found in tree rings.

Increased Pb concentrations in the annual rings of trees beginning in the latter half of the 20th century have been documented (e.g., Rolfe, 1974; Kardell and Larsson, 1978; Robitaille, 1981; Guyette et al., 1991; Latimer et al., 1996). The majority of these studies attribute this increase to atmospheric Pb deposition to automobile exhaust. Because both Mount Rogers and Whitetop Mountain are remote from large point sources of pollution, it is difficult to determine if automotive sources could influence the Pb concentrations found in tree rings. However, several researchers report that mountain summits are frequently exposed to air masses from industrial areas and this can result in high rates of hydrogen (H+), sulfur, and heavy metal deposition (e.g., Smith, 1981; Altshuller and Linthurst, 1984). Prevailing winds, precipitation, and cloudwater deposition in combination with frequent exposure to air masses from industrial areas could possibly contribute to these increased rates of deposition at higher elevations (Smith, 1981). Higher average Pb concentrations were found at higher elevations on Whitetop and Mount Rogers. Weathers et al. (2000) and Zechmeister (1993) also noted significant trends for high Pb concentrations at higher altitudes. In addition, it has been noted that for mountains located in the northeastern USA, cloud deposition is significant at elevations greater than 1000 m (Weathers et al., 2000).

Lead concentrations in American beech tree-ring samples from Mount Rogers and Whitetop Mountain, Virginia rose in the 1920s. It is interesting that this time also corresponds to the introduction of leaded gasoline in 1923 (Forget and Zayed, 1995). Ward et al. (1974) also noted a positive correlation between Pb concentrations in trees and Pb emissions from automotive sources in the 1920s. Lead concentrations decreased in the following decades and gradually rose again until 1970, after which the concentrations began to decrease. The U.S. Congress passed legislation in the 1970s that limited use of leaded gasoline and several studies have shown decreased Pb concentrations in tree rings following the banning of leaded gasoline (e.g., Adriano, 1986; Johnson et al., 1995; Latimer et al., 1996; Watmough, 1997; Anderson et al., 2000; Johnson et al., 1995).

Many factors influence Pb toxicity, including differences in tree species, charge of the cation, health of the tree, changes in soil pH, and the climate (Adriano, 1986; McClenahen and Vimmerstedt, 1993). The maximum Pb concentrations found in tree rings on Mount Rogers and Whitetop Mountain, Virginia were between 1.25 x 10-10 to 1.7 x 10-10 mol kg-1, considerably lower than levels found in tree cores from research areas considered to be polluted (Guyette et al., 1991; Eklund, 1995; Latimer et al., 1996).

During the 1960s and again in 1982–1988, residual ring width indices were lower than predicted values (Fig. 2). Because climate variables did not adequately model growth during these time periods, growth may have been influenced by nonclimatic factors. Shortle et al. (1997) suggested that increased Ca concentrations in the northeastern USA occurring in the 1960s coincided with increased atmospheric levels of NOx and SO2. However, long-term deposition data were not gathered for the Mount Rogers and Whitetop Mountain area, and hence we can only speculate about growth trend cause and effect.

Dendrochemical analysis revealed a negative relationship between Pb and Ca concentrations and tree growth. Annual rings provided concentration data for possible environmental exposures throughout the lifespan of American beech trees, showing an increased trend of both Pb and Ca concentration over time. While dendrochemical analysis appears to be an effective tool for determining temporal trends within environmental samples, more research on elemental concentration within tree-ring tissue should be investigated.


   ACKNOWLEDGMENTS
 
This research was supported by the Tennessee Valley Authority's Public Power Institute. The authors would like to thank Michael Gallagher, Creg Bishop, and Dev Joslin for their support in reviewing and preparing this manuscript. We would also like to extend our gratitude to Anita Rose, Alan Mays, Jennie Martin, Mark Wolfe, Roger Tankersley, Jennifer Krstolic, Brent Collier, Patricia Brewer, Cassi Wylie, Larry Shelton, Brian Evenshen, Rob Wilson, USDA Forest Service's Mount Rogers National Recreation Area, and Grayson Highlands State Park, Virginia.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 





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Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal