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Mercury in Vegetation and Organic Soil at an Upland Boreal Forest Site in Prince Albert National Park, Saskatchewan, Canada H

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Mercury in Vegetation and Organic Soil at an Upland Boreal Forest Site in Prince Albert National Park, Saskatchewan, Canada H 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 gÀ1 (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 mÀ2 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 G01004 1of9 G01004 FRIEDLI ET AL.: MERCURY IN BOREAL VEGETATION AND SOIL G01004 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. 2of9 G01004 FRIEDLI ET AL.: MERCURY IN BOREAL VEGETATION AND SOIL G01004 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 (0À =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.
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