JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, G01004, doi:10.1029/2005JG000061, 2007
Mercury in vegetation and organic soil at an upland boreal forest site in Prince Albert National Park, Saskatchewan, Canada H. R. Friedli,1 L. F. Radke,1 N. J. Payne,2 D. J. McRae,2 T. J. Lynham,2 and T. W. Blake2 Received 6 June 2005; revised 25 April 2006; accepted 21 August 2006; published 18 January 2007.
[1] We studied an upland boreal forest plot located in the Prince Albert National Park, Saskatchewan, Canada, to measure the total mercury content in vegetation and organic soil with a view to assessing the potential for mercury release during forest fires. The study area consists of two stands of vegetation regrown after fires 39 and 130 years ago, with different carbon and mercury stocks in vegetation and organic soil. The mercury concentrations in ng g 1 (dry weight) were measured for moss (90–110), leaves (8), needles (10), bark (16–38), lichen (30–227), bole wood (2) and for organic soil layers (120–300). The combined mercury stock increased from 1.01 ± 0.28 to 3.45 ± 0.87 mg m 2 for the two stand ages; 93–97% of the mercury resided in the organic soil to the mineral layer. The mercury input to the ecosystem is from wet and dry deposition and is trapped in the organic soil layers as indicated by the high organic soil mercury concentrations and low mercury concentration in the underlying mineral layer. Extrapolation from the data measured for the two subplots to all boreal forests suggests a massive mercury stock in boreal forests (15,000 to 44,000 t). This is a low estimate because boreal lowlands have still higher mercury densities. Not all of the organic soil mercury was acquired since the last burns; some predates the more recent fires. The mercury being predominantly located in the organic soil makes fire severity the most important parameter for mercury release. The anticipated accelerated warming in northern latitudes would increase severity, frequency and burn area of future fires and result in large pulses of mercury to the atmosphere and further stress to the environment. Citation: Friedli, H. R., L. F. Radke, N. J. Payne, D. J. McRae, T. J. Lynham, and T. W. Blake (2007), Mercury in vegetation and organic soil at an upland boreal forest site in Prince Albert National Park, Saskatchewan, Canada, J. Geophys. Res., 112, G01004, doi:10.1029/2005JG000061.
1. Introduction and hydrology [Lindberg, 1996; St. Louis et al., 2001]. Mercury enters boreal ecosystems mostly by wet and dry [2] The cycling of mercury between atmosphere and deposition of particulate and ionic mercury onto live veg- biosphere is particularly important for boreal ecosystems etation and soil surfaces, and by stomatic assimilation of because of the huge amount of biomass contained in boreal gaseous elemental mercury (GEM) [Erickson et al., 2003; forests and peat lands [Kasischke, 2000]. The carbon stocks Frescholtz et al., 2003]. Depending on atmospheric con- are high because of the slow decomposition rate in the cold centrations, GEM can be exchanged in or out of stomata northern climate and low fire frequency of 60–200 years [Hanson et al., 1995]. Deposited mercury can be incorpo- [Stocks and Kauffman, 1997]. Positive correlations have rated into plant tissue, photo chemically reduced to and been reported between stored carbon and mercury in boreal released as GEM [Poissant et al., 2004] or washed off in ecosystems [Grigal, 2003; Grigal et al., 2000; Harden et throughfall. Xylem sap contribution to mercury in plants is al., 2004]: they are of great scientific and public interest minor except in soils with high mercury content [Bishop et because of global warming, which is accelerated in northern al., 1998]. Upon deposition to the ground in throughfall or latitudes [Ra¨isa¨nen, 1997] and likely results in heightened contained in senesced leaves, needles, bark and dead wood, wildfire activity [Stocks et al., 2000] and increased mercury mercury is sequestered by reduced sulfur groups in the and carbon release to the atmosphere. Mercury poses health humic matter of the organic soil [Stumm and Morgan, 1995; hazards to humans, particularly pregnant women and chil- Skyllberg et al., 2003]. dren [U.S. Environmental Protection Agency, 2004]. [4] During wildfires part or most of the mercury in the [3] Mercury is present in boreal ecosystems as a result of fuels is released. The release during biomass burning has multiple processes involving atmosphere, vegetation, soils, been demonstrated in experimental burns [Friedli et al.,
1 2001, 2003a; Mailman and Bodaly, 2005], observed in National Center for Atmospheric Research, Boulder, Colorado, USA. prescribed burns [Veiga et al., 1994; Artaxo et al., 2000; 2Canadian Forest Service, Sault St. Marie, Ontario, Canada. Woodruff et al., 2001; Harden et al., 2004] and in wildfires Copyright 2007 by the American Geophysical Union. [Brunke et al., 2001; Friedli et al., 2003a, 2003b]. In most 0148-0227/07/2005JG000061 years sections of boreal forests in Siberia/Mongolia, Canada
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Figure 1. Fire chronology for the study area: dates and areas of most recent burns. and Alaska suffer from massive wildfires, consuming on from invading fire from north of the park, or, conversely, average about 4, 2 and 0.3 million ha, respectively, each protect commercial forest property north of the park from a year [Lavoue´etal., 2000]. The emissions from boreal fire originating inside the park. The study area is composed wildfires, both particulate and gaseous combustion prod- of pine (46%), which is transitional between the immature ucts, have regional and global environmental impact be- (C4) and mature (C3) pine types. Boreal spruce (C2) stands cause they frequently are injected into the stratosphere (6%) are located, primarily, along the southern perimeter of [Fromm and Servranckx, 2003; Massie et al., 2003], where study area. Mixed wood (M1) stands (16%) dominated by they become subject to long-range transport and chemical mature conifers and aspen are located in the eastern region transformations, and in case of mercury, to conversion of of the unit. The rest of the study area (32%) consists of GEM into particulate and ionic mercury, which have much immature deciduous-dominated mixed wood (M1), imma- shorter life times than GEM [Schroeder and Munthe, 1998]. ture deciduous (D1) and shrub land communities. The Long-range transport of plumes originating from wildfires ground is covered with live Plueurozium or leaf/needle and containing carbon monoxide [Wotowa and Trainer, litter and a small number of vascular live plants. 2000; Lamarque et al., 2003] and mercury [Sigler et al., [7] The study area consists of two subplots: 114 ha of 2003] has been reported. forest last burned in 1870 (11%, stand age 133 years, [5] The objective of this paper is to quantify the mercury designated as ‘‘Old Stand’’) in the south east corner of the stocks in a previously unexplored upland boreal forest plot plot, and 949 ha of vegetation last burned in 1964 (89%, in Prince Albert National Park in Saskatchewan, Canada, stand age 39 years, designated as ‘‘Young Stand’’) and to assess the potential for mercury release during future (Figure 1). The ‘‘Old Stand’’ is represented by a 100-m wildfires. The research describes the mercury distribution in transect at N 54° 15.839; W 106° 09.1390 (TS-2) located in the standing forest, downed wood and in the organic soil for the mature mixed wood (M-1) fuel subplot. The larger subplots with different stand ages. The release potential ‘‘Young Stand’’ portion is represented by two 100-m trans- during a forest fire is explored on the basis of laboratory ects at N 54° 16.3480; W 106° 11.2530, (TS-1) and N 54° release experiments and the mercury profiles observed in 17.2640; W 106° 10.2900 (TS-3), both located in immature the organic soil and vegetation. jack pine (C-4) regrown since the 1964 fire. To broaden the range of stand ages we partially characterized the oldest 2. Plot Description forest in the park, called Treebeard, last burned 180 years ago and located at N 53° 58.2400; W 106° 17.5960, approx- [6] The study area consists of 1063 ha of forest in the imately 40 km outside and south of the study area. northeast upland region of the Prince Albert National Park (PANP) in Saskatchewan, located between Wassegam and Tibiska Lake. The topography consists of hummocky mo- 3. Methodology rainal uplands, the elevation increasing from the Tibiska 3.1. Sampling for Mercury in Soil and Vegetation Lake shore (502 m) to an interlake plateau at 570 m. The [8] Four sites along each transect were selected to repre- study area was selected by park management for a pre- sent the prevalence of surface coverage type: two sites scribed burn to form a fire break to protect the park lands covered with live moss and two with needle/leaf litter.
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Although not quantitatively determined, moss and needle/ [12] Woody debris was quantified using the line intersect leaf litter coverage is about equal, although the young method [van Wagner, 1968; McRae et al., 1979] using the stands are slightly more moss-covered. The mercury sam- 100-m PCQ transects. Intersections were recorded for all 1 1 pling was done independently from the fuel collection six woody debris diameter classes (0 =2, =2 –1, 1–3, 3–5, described on the next section. Organic material and mineral 5–7 and >7 cm) along the whole transects. soil samples were collected from 30 cm  30 cm squares [13] Ground fuel samples consisting of litter, the fermen- and transported in polyethylene bags with airtight seals. tation and humus layer, together with overlying moss were Practices to avoid mercury contamination included wearing sampled at seven locations along the 100-m transect (0, 20, clean room vinyl gloves, using precleaned plastic tools and 40, 50, 60, 80, 100 m). At each location the material in a double-bagging all samples. 30  30 cm area was removed, bagged and labeled. Litter [9] The dominant surface samples were litter (leaves and and moss were bagged separately, and organic soil was needles, small twigs), live vascular plants (rarely present) removed in layers 2 cm in depth. These samples were later and moss layers. The organic soil layers (O horizons) were oven-dried (72°C for 48 h) and weighed to provide fuel collected as horizontal slices and assigned Of (fibrous), Om loading (kg m 2) and soil densities (g cm 3). (mesic) and Oh (humic) designations, on the basis of the 3.2.2. Fuel Data Analysis approximate degree of decomposition. The underlying min- [14] Point-centered quarter (PCQ) measurement on trees eral soil consisted of clay (fine-grained sand at the Tree- with DBH>3 cm were analyzed to calculate the average beard site) and in all cases there was a very distinct distance from the sampling point to the individual tree, and demarcation between organic and mineral soil layers. The the proportion (%) of stems of each species. Stem density mineral layer itself was sampled at about 2, 4 and 6 cm (stems m 2) was calculated by taking the inverse square of depth, starting from the top of the mineral layer but only a the average distance (m) between the sampling point and few samples were analyzed for mercury. Samples collected individual trees. Average tree height (m) and DBH (cm) from standing vegetation were leaves and needles (aspen, were calculated for each overstory species. Average above- white pine), bark (aspen, jack pine) and lichen. Tree core ground tree weights (kg) were calculated from average tree samples were taken from trembling aspen and white pine. statistics (DBH and height) and species specific regression To avoid mercury losses upon drying, mercury analyses equations [Doucet et al., 1976; Ker, 1980; Singh, 1982, were carried out in the state the samples were collected. The 1986]. Overstory standing fuel load (kg m 2) was calculated mercury content is expressed on dry weight determined on from average tree weights by species, the proportion of sample aliquots dried at 72°C for 48 hours. For the purpose stems of this species, and stem densities. Understory of calculating the mercury stocks, concentrations were fuel load was assessed from measurements of trees with assumed to be the same for understory and overstory plant DBH < 3 cm, using a similar procedure to that employed components. for overstory trees [Baskerville, 1965; Doucet et al., 1976; Harding and Grigal, 1985; Telfer, 1969; Young and 3.2. Fuel Assessment Methodology Carpenter, 1967]. Foliage and bark dry weight proportions 3.2.1. Fuel Measurements were estimated from average tree statistics and species [10] Mercury may be present in any of the combustible specific regression equations [Baskerville, 1965; Doucet et materials present in the boreal forest biome. For this reason al., 1976; Harding and Grigal, 1985; Johnstone and 2 each fuel type was quantified (kg m ). These fuel measure- Peterson, 1980]. Woody debris fuel load (kg m 2) was ments, together with mercury concentration measurements calculated using the procedures described by van Wagner 1 (ng g ), allowed the total mercury content in combustible [1968], who gives the following equation: material to be estimated. Three types of fuel were quanti- fied: (1) standing fuel included predominantly live trees and Fuel load ¼ p2 S Sd2=8L; shrubs, which comprised needles, leaves, twigs, branches, bark and xylem; (2) woody debris included woody material where S is the specific gravity, d is the diameter of the lying on the ground, comprised of twigs, branches, tree woody debris crossing the line and L is the total line length. limbs and boles, and often referred to as dead and downed Species and debris size class gravities were employed, and material; and (3) moss, litter and organic soil. all woody debris was assumed to be older than one year in [11] Standing fuel was quantified using the point-centered age [McRae et al., 1979]. Ground fuel load was calculated quarter (PCQ) method [Cottam and Curtis, 1956] for trees for component parts, i.e., moss, litter and organic soil by with diameter at breast height (DBH) >3 cm. This method 2-cm layers. involves measuring the distance of the closest single tree [15] To compare our results with reference data expressed from the chosen sampling point in each of four quadrants. in carbon mass rather than soil or vegetation mass, a factor The tree species was recorded and the DBH measured by of 0.50 was applied for standing and dead vegetation [Atjay caliper. Tree height measurements were also made, using a et al., 1977], and 0.37 for organic soil [Smith and Heath, 2 m measuring stick for tree heights up to 4 m; for taller 2002], all based on dry weight. We recognize that the trees the height was estimated by eye. The eleven sampling carbon content in soil depends on the degree of decompo- points were spaced at 10 m intervals along a 100-m transect. sition (depth) and differentiated measurements would be All trees and shrubs with DBH< 3 cm within a concentric required in a more detailed analysis. 2 m radius circle were recorded, along with their height, stem diameter at ground level and species. These size 3.3. Analytical Methods for Total Mercury classes (< or > 3 cm DBH) are referred to as overstory [16] The samples of soil and vegetation, as collected in and under story, respectively. the field with no drying to avoid mercury losses, were
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Table 1. Mercury Content for Plant Components, Organic Soil, (http://www.asp.ucar.edu/ friedli/table.htm). Mercury levels and Mineral Soila in the organic layer increase from the surface to a maximum Young Stand, Old Stand, Treebeard, at about mid depth and show a discontinuity in mercury Substrate 39 Years 133 Years 180 Years concentration at the organic layer/mineral layer interface, Moss 94.5 (2) 108.0 90.6 which is very distinct in this ecosystem (Figure 2). This Aspen leaves - 8.0 - profile shape is qualitatively independent of stand age and Spruce needles 9.9 - - predominant surface coverage, moss or needle/leaf litter. Aspen bark - 15.9 - The profiles shown in Figure 2 are divided into moss, Jack pine bark 38.6 - - Lichen 30.6 74.0 227.1 organic soil and the underlying mineral soil. A dead moss Litter (leaves, needles) 68.3 (3) - 127.1 layer was discernible in only two cases. The decomposing Aspen wood - 2.08 - layers are shown combined as ‘‘Deep Organic.’’ White spruce wood 1.86 - - Organic layers 100–160(13) 120–300(4) 160–250(5) 4.2. Fuel Assessment Mineral soil 9.2 (4) 8.8 (4) 25.2 [19] In the old stand (130 years, 11% of plot area) the a 1 Mercury content is given in units of ng g , dry weight. Numbers in overstory is comprised of white spruce [Picea glauca parentheses designate the number of samples analyzed. (Moench) Voss] (64% of stems, mean DBH 21 cm), trembling aspen [Populus tremuloides Mich.] (25%, mean analyzed for total mercury by Frontier Geosciences, Inc. DBH 31 cm) and balsam fir [Abies balsamea (L.) Mill.] (Seattle, Washington), using the FGS-069.1 ‘‘THg Analysis (11%, mean DBH 17 cm). Standing fuel mass totaled and Calibration’’ method, which is the basis for EPA 15.4 kg m 2 (7.7 kg m 2 C), of which the understory Method 1631c. Subsamples were homogenized before anal- comprised only 2.3%. This overstory biomass is similar to ysis. Data quality was evaluated via replicate digests and the value measured in a northern Ontario mature boreal matrix spikes and audit solutions traceable to the National mixed-wood forest (14 kg m 2 [Lee et al., 2002]). Estimated Institute of Standards and Technology. Percent relative DBH specific foliage and bark dry weight proportions were differences (PRD) were ±0.7% to ±10.4 %. The detection 10 and 7% for white spruce, 2 and 22% for the trembling limits (estimated as 3 times the standard deviation for aspen and 17 and 9% for balsam fir. independent blanks analyzed in triplicates) were 0.14 to 1 [20] The young stand subplot (39 years, 89% of plot area) 0.33 ng g . The mercury concentration obtained for the is represented by two transects (TS-1 and TS-3): both are ‘‘as collected’’ samples were converted to dry weight values primarily comprised of jack pine [Pinus banksiana Lamb.] by correcting for the individual moisture contents. Flett (68 & 73% of stems, mean DBH’s 6.8 & 6.4 cm) and white Research Ltd. (Winnipeg, Manitoba, Canada) analyzed spruce (30 and 27%, mean DBH’s 4.5 & 4.2 cm), and the some soil and the tree core samples. Soil samples were standing fuel mass totaled 9.4 and 10.2 kg m 2 (4.7 to treated with a nitric/sulfuric acid digestion at 130°Cfor 5.1 kg m 2 C) of which the understory comprised 4.4 and 16 h, while the tree cores were similarly digested at 150°C. 4.6%. This overstory biomass is consistent with published Analysis of the resulting solution aliquots was by a purge values for jack pine stands [Doucet et al., 1976; Maclean and trap method similar to that outlined in EPA method and Wein, 1976]. Estimated DBH-specific foliage and 1631. Duplicate and matrix spikes were included with every bark dry weight proportions averaged 3 and 11% for jack ten samples, and duplicate reference material (MESS II pine and 14 and 12% for the white spruce. sediment for mineral soil, DORM-2 for wood) for every [21] The total live aboveground fuel mass increased 1.5 digestion lot of 20 samples. Detection limits were 1–2 ng 2 2 1 fold from about 10 to 15 kg m (5 to 7.5 kg m C) Hg g dry weight. because more fuel is contained in the mature trees (Table 2). 3.4. Mercury Release Experiments Foliage and bark weights in the older stand show a twofold increase. Concurrently the overstory species composition [17] Field samples of organic soil (55% moisture content) were exposed for various time intervals (5–45 min) and changed from mostly jack pine and white spruce to jack temperatures (100–300°C) in an electrically heated oven pine, balsam fir and trembling aspen, as is expected for with recirculating air passing over the samples. No ignition boreal succession [Weir and Johnson, 1998]. Including occurred. down/dead fuel (as in Table 2) does not materially change the relationships. The aboveground carbon in PANP (5 to 7.5 kg m 2) compares with values from Alaska (4.02 to 4. Results 5.32 kg m 2 [Kasischke et al., 2000]) and for Saskatch- 4.1. Mercury in Plant Components and Organic Soil ewan/Manitoba (0.9 to 5.6 kg m 2 [Gower et al., 1997]). [18] Mercury concentrations in plant components as pre- [22] The organic soil mass increased from an average of 2 2 sented in Table 1 are in the same ranges given in the review 6 to 8.4 kg m (2.2 to 3.1 kg m C) or 1.5-fold for by Grigal [2003] and data from Mailman and Bodaly stand age increasing from 39 to 130 years. This is the [2005] and Moore et al. [1995], but there is large variation result of increased organic soil depth to the mineral layer and only limited data for the study area are available. The and increasing density with depth. Carbon stocks of 11 and 2 highest mercury concentrations measured in this upland 18 kg m (Alaska [Kasischke et al., 2000]), 9.3 ± 2 boreal ecosystem are found in the organic soil. The results 7.0 kg m (Pacific NW [Jobbagy and Jackson, 2000]), 2 of the characterization of the soil layers collected from the 2.9–3.3 kg m (Alaska and Pacific Northwest [Smith and 2 2 old and young stands in the study area (and a comparison Heath, 2002]), 40 kg m (black spruce) and 6.6 kg m with the oldest site in the park, Treebeard) provide details of (jack pine) in Saskatchewan and Manitoba [Gower et al., the mercury distribution in the organic soil, litter and moss 1997] have been reported. The value for the PANP
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Figure 2. Mercury content (ng g 1, dry weight) in surface litter, moss, and the organic soil layers as a function of soil depth, and total mercury pool for each sample area (mg m 2). The plots are arranged by stand age (year since last burn) for moss- and needle-covered sites and are color coded for content identity. The number after the location designation (e.g., TS-1 30-m) refers to the position of the sample measured from the head of the transect. The fermenting and decomposing layers are listed as ‘‘Deep Organic.’’ The mercury concentration in the mineral soil layers is also indicated. research plot is at the lower end of the data range. Soil understory and overstory and for the organic soil above the carbon stocks vary greatly in response to location (upland, mineral layer (Table 3). Aboveground mercury in the lowland, and latitude), stand age, vegetation and climate understory and overstory is the sum of mercury contained (carbon accumulation and decay) and hydrological out- in foliage, bark, lichen and wood (standing and dead/ flow. In our plot, 31–41% of the total carbon resided in downed). Understory mercury is small: in the two young the organic soil layer. transects it is 2.8 and 3.2 mgm 2 (TS-1 and TS-3), the corresponding value for the old stand is 1.9 mgm 2. The 4.3. Mercury Stocks overstory stock is much larger: for the young stand they are [23] Mercury stocks were calculated for the two stand 54.5 and 60.8 mgm 2 for the two transects and 81.3 mgm 2 ages, separately for the aboveground mercury pools in the for the old stand. The mercury stock in the aboveground
Table 2. Aboveground and Organic Soil Mass for Two PANP Subplots With 39- and 130-Year Stand Age, and the Percent Fraction of Carbon Contained in Soila Aboveground Vegetation Total Soil Stand Age Location Standingb Dead/Down Mass Carbon Mass Carbon Percent of Carbon in Soil 39 TS-1 9.4 (0.3,0.9) 4.6 14.0 7.0 6.1 2.26 32.3 39 TS-3 10.2 (0.4,1.1) 0.3 10.5 5.3 5.8 2.15 40.6 130 TS-2 15.4 (0.9,1.1) 4.5 19.9 9.95 8.4 3.11 31.1 aAboveground and organic soil mass are given in kg m 2. Carbon mass is estimated as 37% of the dry weight of organic soil mass. bGiven in parentheses are (foliage, bark).
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Table 3. Distribution of Mercury in Foliage, Bark, Lichen, and Bole Wood in the Overstory and Understory of Subplots of 39- and 130-Year Stand Agea
Young Stand (TS-1, TS-3) Old Stand TS-1 TS-3 TS-2 % Pool % Pool % Pool Overstory Foliage 6.0 3.3 6.5 3.9 11.2 9.1 Bark 66.6 36.3 69.5 42.3 58.2 47.3 Lichen 1.0 0.5 1.0 0.6 2.0 1.6 Bolewood 26.4 14.4 23.0 14.0 28.6 23.3 Total 100 54.5 100 60.8 100 81.3
Understory Foliage 15.5 0.5 13.2 0.4 21.1 0.4 Bark 62.7 1.7 63.5 2.1 52.6 1.0 Lichen 0.1 0.0 0.0 0.03 0.0 0.04 Bolewood 21.7 0.6 22.3 0.7 26.3 0.5 Figure 3. Fraction of mercury in the organic layer as a Total 100 2.8 100 3.2 100 1.9 a 2 function of soil depth: a graph to illustrate the mercury Stocks are given in mgm . release potential from wildfires burning increasingly deeper into the organic soil, reflecting different fire severities. vegetation is 1.5 times larger for the higher stand age, The release curves represent the mercury profiles for tran- paralleling vegetation mass increase (1.5 times), and resides sects TS-1 and TS-3 (combined, 39-year stand age), TS-2 mostly in the overstory (>93%). About 65% of the above- (133-year stand age) and Treebeard (180-year stand age). ground mercury resides in bark, about 25% in bole wood, and the bulk of the remainder is in foliage (Table 2). Lichen (1.01 mg m 2). The soil stock thus estimated is 13 kg and and moss, although high in mercury content, contribute little the aboveground contribution is 0.3 kg, for a total of 13.3 kg to the stock because of their low mass fraction. The sequestered mercury. Applying the values from our study distribution among plant parts has important consequences area to all boreal ecosystems (1,509 Â 106 ha [Kasischke et for mercury emissions: twigs, needles and bark, which al., 2000]) yields a stock of 15,200–44,100 t. This value combined contain most of the mercury in aboveground fuel, range is likely to be an underestimate because boreal are extensively consumed by fire and release the contained ecosystems also contain bogs, fens and permafrost with mercury. much deeper organic layers and high mercury contents. [24] The mercury stock in the organic soil (moss, litter, Grigal [2003] estimated the boreal stock as 30,300 t. To put fermentation and humus layers to the mineral layer) is the these figures in perspective: Mason and Sheu [2002] esti- sum of the mercury in individual layers in the mercury mated the atmospheric reservoir as 5000 t; in other words, sample locations and is based on the mercury content of boreal forests may have sequestered at least three to nine each layer, volume and density (Table 4). The pool density times as much mercury as is contained in the atmosphere. (stock) for the old stand is 2.92 ± 0.87 mg m 2 and 1.01 ± 0.28 mg m 2 for the younger stand. In the young stand 4.4. Mercury Release From Heated Soil 93–96% of the mercury resides in the organic soil, the [26] To estimate the potential mercury release from soil fraction is about 97% for the old stand. For a boreal plot in during a wildfire we carried out laboratory experiments to Ontario, Canada, 5–9% of the mercury pool was found in simulate the process. Heating subsets of samples of organic vegetation, 91–95% in soil [Hintelmann et al., 2002], soil (55% moisture) in air at 100 and 300°C for various time mirroring the data from our study area. Nater and Grigal intervals showed nearly complete loss of mercury after only [1992], Grigal [2003] and Grigal et al. [2000] reported 5 minutes of heating at 300°C. At 300°C and 30 minutes values for Great Lakes ecosystems similar to the PANP full mercury and substantial mass loss occurred and the results. However, much larger stocks have been reported for sample was partly charred although not ignited. By contrast, areas influenced by high local deposition rates associated at 100°C after 45 minutes of heating only about 10% of the with anthropogenic pollution, for example, in the Acadia mercury but all moisture were lost. Biester and Zimmer National Park (18 mg m 2 [Amirbahman et al., 2004]) or [1998] reported that in their experiments humus-bound NE Bavaria, Germany (17 mg m 2 [Schwesig and Matzner, mercury was released at 150°C. Assuming that all mercury 2001]). to the burn depth is released and that the mercury in the [25] The soil mercury pool contained in the 1063 ha plot lower layers is not affected, plots were generated for was estimated as the sum of the two subplots: 119 ha are old fractional mercury release as a function of burn depth stand forest, (2.92 mg m 2) and 949 ha are young stand (Figure 3) for all mercury sampling sites. The data quanti-
Table 4. Summary of the Properties (±1 Std. Dev.) for the Organic Soil in the Old and Young Stands in the PANP Research Plot Location Depth, cm Hg Concentration, ppm Bulk Density, g cm 3 Hg Stock, mg m 2 Percent Hg in Organic Soil Young stand 8.0 ± 1.47 0.096 ± 0.09 0.062 ± 0.016 1.01 ± 0.28 93.3–95.7 Old stand 10.0 ± 1.86 0.201 ± 0.117 0.083 ± 0.019 2.92 ± 0.87 96.7–97.4
6of9 G01004 FRIEDLI ET AL.: MERCURY IN BOREAL VEGETATION AND SOIL G01004 tatively indicate that burn depth, which reflects the severity but mercury accumulation data for bogs [e.g., Coggins et of a fire, controls the degree of mercury release. al., 2006; Givelet et al., 2003; Benoit et al., 1998; Engstrom and Swain, 1997] and for lakes [e.g., Heyvaert et al., 2000] 5. Discussion can serve as surrogates. The published data indicate con- sistently drastically increased accumulation rates relative to [27] This study concerns the distribution of mercury in the historical background during recent industrial times different components of a boreal forest at different stand ( 1930–1990) which overlap with the regrowth periods ages, and estimates of the mercury release due to different in our study area. The 1964 burn coincides with the historic fire regimes. peak deposition rates. Engstrom and Swain [1997] reported [28] The main result is the recognition of large mercury accumulation rates of 25–70 mgm 2 yr 1 during the peak 2 stocks and their location in the ecosystem: 2.92 ± 0.87 mg m deposition rate in the 1960–1970s in eastern Minnesota. We 2 and 1.01 ± 0.28 mg m for the 130 and 39 year stand averaged the accumulation rates for 20 year increments age subplots, respectively, and 93+% of the mercury located between 1876 and 1996 measured for five bogs by Coggins in the organic soil. As stand age increases, both carbon and et al. [2006] and Givelet et al. [2003] and integrated the mercury increase, although at different rates because the values over the growth periods for the old and young stands. mercury accumulation is affected by deposition rate, a The accumulation estimates are 2266 and 857 mgm 2, deposition rate which varied greatly during the growth compared to the measured stocks of 2920 ± 870 and 1010 ± period of the research plot. The understory mass is very 280 mgm 2. The average accumulation rates for the old and small in these dark very dense forests and its mercury stock young stand ages were 17.4 and 22.0 mgm 2 yr 1, is minor but important because it is susceptible to extensive compared to current ( 10 mgm 2 yr 1 [Seigneur et al., combustion and mercury release during a fire. The mercury 2004]) and preindustrial ( 4 mgm 2 yr 1 [U.S. Environ- content in live vegetation is typical for plants growing in mental Protection Agency, 2004] deposition rates. The areas far from anthropogenic sources. The large contribu- difference between the measured mercury stock and what tion to the mercury stock by bark was unexpected and would be expected from historic deposition (with the requires verification with additional data. Research on caveats above) does suggest that some past fires did not gingko biloba bark [Sanjo et al., 2004] indicated different burn down to the mineral layer and thus part of current pool mercury content for inner and outer bark, and xylem, is mercury retained after the last burn. The fact that boreal suggesting a more complex picture. The same is likely true wildfire occasionally do burn down to the mineral layer is for boreal species and as bark is partially burned during illustrated by the presence of a very thin carbon veneer at wildfires, more attention needs to be given to both mercury the mineral layer interface observed at some of the sampling distribution and release characteristics. Lichen and mosses sites. are known to accumulate mercury efficiently [Rasmussen, [32] It is important to note that the components with the 1 1995] so that the high content (227 ng g ) observed in the highest mercury content, i.e., leaves/needles, bark and soil, oldest forest (Treebeard, 180 years stand age) can be are also the components predominantly combusted and rationalized but their contribution to the stock is minor stripped of mercury during boreal fires. Most fires in the because their mass fraction is low. PANP area are lightning-started stand-replacing crown fires [29] Our research area follows a predicable behavior with that involve soils, leaving a patchwork of different burn increasing stand age: plant succession, growth in carbon and depths [Weir et al., 2000]. This is a common behavior of mercury stocks, shifts in mercury and carbon from under- fires in boreal forests [Kasischke et al., 1995a; Richter et al., story to overstory, and growth of mercury and carbon in the 2000]. Harden et al. [2004] found during the Frostfire organic soil. The observed size of the carbon stock is typical experiment in Alaska an average the 64.3% reduction of for upland conditions and stand age although the soil carbon depth of the organic layer, corresponding to a 78.6% loss of fraction is low compared to overall boreal values. mercury, the rest remaining in the unburned soil. Boreal [30] How the mercury is distributed in the organic soil is crown fires consume foliage, bark, twigs and organic soil, very important because it determines the degrees of release but little bole wood [Stocks and Kauffman, 1997]. during fire. The mercury profiles in the organic soil are [33] The size of the mercury stock, its distribution in the consistent for all samples: increasing concentration with ecosystem, and the dynamics of the fire ultimately deter- depth with a maximum located between surface and the mine the extent of mercury release during a wildfire. During mineral layer. Similar profiles were also observed for a some surface wildfires, detritus can present a barrier to heat boreal ecosystem in Ontario, Canada [Hintelmann et al., transfer to the organic and mineral soil layers [Pyne et al., 2002] and this observation supports the notion that organic 1996] and the resulting surface temperature may not soil acts as an efficient chemical trap for mercury [Miretzky cause substantial mercury release. However, Massman and et al., 2005] and that the bonding of mercury to reduced Frank [2004] observed 400 and 100°C at 2 and 30 cm sulfur contained in the humic layers [Skyllberg et al., 2003] depth, respectively, in a surface fire. When organic soil is limits mercury migration through the organic layer. ignited, smoldering combustion temperatures >300°Cdo [31] It is not possible to read the soil profiles directly in occur. Our laboratory data on heating of organic soil terms of deposition trends. Deposition and accumulation are indicated complete release of the sequestered mercury in related through a number of elaborate physical processes. organic soil at 300°C after 5 min heat exposure. Since >95% As an indicator of such a relationship we compared the of mercury resides in the organic soil, the majority of observed mercury stocks with what might be expected from the mercury release is controlled by the average burn depth, historic deposition rates, integrated over the growing life of as is obvious from the release versus burn depth curves the stands. Local deposition data for PANP are not known (Figure 3).
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[34] Published data on the relationship of fire, global Baskerville, G. (1965), Estimation of dry weight of tree components and warming and carbon balance for boreal forests [Kasischke total standing crop in conifer stands, Ecology, 46, 867–869. 2 Benoit, J. M., W. F. Fitzgerald, and A. W. H. Damman (1998), Biogeo- et al., 1995b] project 3.5 and 5.6 kg m organic soil carbon chemistry of an ombrotrophic bog: Evaluation of use as an archive of losses because of increased fire activity. The huge mercury atmospheric mercury deposition, Environ. Res., 78(2), 118–133. (and carbon) stocks in boreal ecosystems present a large Biester, H., and H. Zimmer (1998), Solubility and changes of mercury binding forms in contaminated soil after immobilization treatment, potential for mercury release even under today’s climatic Environ. Sci. Technol., 32, 2755–2762. conditions, but the projections for accelerated warming in Bishop, K. H., Y. H. Lee, J. Munthe, and E. Dambrine (1998), Xylem sap as the northern latitudes will increase fire frequency and a pathway for total mercury and methyl mercury from soil to tree canopy in a boreal forest, Biogeochemistry, 40, 101–113. severity, as well as area burned, and the potential for Brunke, E.-G., C. Labuschagne, and F. Slemr (2001), Gaseous mercury massive mercury releases to the environment becomes emissions from a fire in the Cape Peninsula, South Africa, during January vastly greater with serious consequences for the health of 2000, Geophys. Res. Lett., 28(8), 1483–1486. people locally and globally. Coggins, A. M., S. G. Jennings, and R. Ebinghaus (2006), Accumulation rates of the heavy metals lead, mercury and cadmium in ombrotrophic peatlands in the west of Ireland, Atmos. Environ., 40, 260–278. Cottam, G., and J. T. Curtis (1956), The use of distance measures in phy- 6. Conclusions tosociological sampling, Ecology, 37, 451–460. Doucet, R., J. V. Berglund, and C. E. Farnsworth (1976), Dry matter pro- [35] The upland plot in this Saskatchewan boreal forest 2 duction in 40-year-old Pinus banksiana stands in Quebec, Can. J. For. contains large stocks of mercury (1–3.5 mg m ) which Res., 6, 357–367. constitute a serious threat for large mercury pulses to the Engstrom, D. R., and E. B. Swain (1997), Recent decline in atmospheric atmosphere during present and future wildfires. The size of mercury deposition in the upper Midwest, Environ. Sci. Technol., 31, 960–967. the combined stock (in live and downed vegetation and soil) Erickson, J. A., M. S. Gustin, D. E. Schorran, D. W. Johnson, S. E. Lindberg, depends on stand age, i.e., the time since the last fire, and and J. S. Coleman (2003), Accumulation of atmospheric mercury in the residual mercury remaining from previous burns. More forest foliage, Atmos. Environ., 37, 1613–1622. Frescholtz, T. F., M. S. Gustin, D. E. Schorran, and G. C. J. Fernandez than 95% of the mercury resides in the organic soil, which (2003), Assessing the source of mercury in foliar tissue of quaking aspen, acts as an effective chemical trap. The amount of mercury in Environ. Toxicol. Chem., 22(9), 2114–2119. live and downed vegetation is small and is released in Friedli, H. R., L. F. Radke, and J. Y. Lu (2001), Mercury in smoke from crown fires, together with, depending on fire severity, a biomass fires, Geophys. Res. Lett., 28(17), 3223–3226. Friedli, H. R., L. F. Radke, J. Y. Lu, C. M. Banic, W. R. Leaitch, and J. I. fraction of the much larger soil stocks. The organic soil MacPherson (2003a), Mercury emissions from burning of biomass from contains mercury from direct wet and dry deposition temperate North American forests: Laboratory and airborne measure- (throughfall), from mercury contained in senesced needles, ments, Atmos. Environ., 37, 253–267. Friedli, H. R., L. F. Radke, R. Prescott, P. V. Hobbs, and P. Sinha (2003b), leaves and bark (litterfall) and mercury not released during Mercury emissions from the August 2001 wildfires in Washington State previous fires. For our research plot the mercury concentra- and an agricultural waste fire in Oregon, and atmospheric mercury budget tion in live vegetation reflects an atmospheric mercury estimates, Global Biogeochem. Cycles, 17(2), 1039, doi:10.1029/ 2002GB001972. background devoid of large point sources. The predicted Fromm, M. D., and R. Servranckx (2003), Transport of forest fire smoke accelerated climate warming at northern latitudes is expected above the tropopause by supercell convection, Geophys. Res. Lett., to increase frequency, severity and burn areas of wildfires in 30(10), 1542, doi:10.1029/2002GL016820. the future, which will release increasingly more mercury to Givelet, N., F. Roos-Barraclough, and W. Shotyk (2003), Predominant anthropogenic sources and rates of atmospheric mercury accumulation the atmosphere. The consequences of such events are more in southern Ontario recorded by peat cores from three bogs: Comparison local deposition of particulate and ionic mercury causing with natural ‘‘background’’ values (past 8000 years), J. Environ. Monit., additional ecological stress to humans and animals, and large 5, 935–949. Gower, S. T., J. G. Vogel, J. M. Norman, C. J. Kucharik, S. J. Steele, and fluxes of GEM to the global atmospheric pool. T. K. Stow (1997), Carbon distribution and aboveground net primary production in aspen, jack pine, and black spruce stands in Saskatchewan [36] Acknowledgments. We acknowledge with thanks the support of and Manitoba, Canada, J. Geophys. Res., 102(D24), 29,029–29,041. Parks Canada in permitting us to work in the Prince Albert National Park, Grigal, D. F. (2003), Mercury sequestration in forests and peatlands: A and those Parks Canada staff involved with our research there, notably Jim review, J. Environ. Qual., 32, 393–405. Weir. Support to Friedli and Radke from Electric Power Research Institute Grigal, D. F., R. K. Kolka, J. A. Fleck, and E. A. Nater (2000), under contract P-2044 and from the National Center for Atmospheric Mercury budget of an upland-peatland watershed, Biogeochemistry, Research (NCAR) is gratefully acknowledged. NCAR is sponsored by 50, 95–109. the National Science Foundation. Thanks go to Eric Presto (Frontier Hanson, P. J., S. E. Lindberg, T. A. Tabberer, J. G. Owens, and K.-H. Kim Geoscience, Inc.) for his advice on sample collection and handling. NCAR (1995), Foliar exchange of mercury vapor: Evidence for a compensation colleagues Wilfred Thompson, Sylvia Murphy, Steve Massie and Jim point, Water Air Soil Pollut., 80, 373–382. Greenberg provided helpful reviews and improvements of the manuscript. Harden, J. W., J. C. Neff, D. V. Sandberg, M. R. Turetsky, R. Ottmar, Comments by anonymous reviewers were especially helpful and are G. Gleixner, T. L. Fries, and K. L. 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