Appendix 1. Detailed Methods

Aboveground stand characteristics

To quantify carbon (C) and nitrogen (N) pools for individual tree components, we used

component-specific equations developed by Alexander and others (2012) and multiplied the

component biomass by the C and N concentration (foliar C and N concentrations reported in this

study and wood, bark, and twig values obtained from H. Alexander, unpublished). No

component-specific allometry data was available for shrub foliage, so we estimated foliage to be:

total biomass – (branch + stem). For a portion of the smallest shrubs, this calculation yielded a

negative number, possibly due to our shrubs being smaller than those used to develop the

equations. For these shrubs, we calculated component-specific biomass by multiplying the total

biomass by an estimated proportional contribution of foliage and stems (Mack and others 2008).

Shrub C and N concentration data was obtained from H. Alexander (unpublished). Standing dead

tree and shrub biomass was calculated the same way as live biomass, except that the wood

biomass values were multiplied by 0.5 to account for the observation that many dead trees had

snapped, losing the crown and upper stemwood, which we did not quantify in this study. We also

assumed that wood C and N concentration did not differ between live and dead wood and applied

the same values to both.

Tree cross-sections used for aging the stands and estimating aboveground tree net primary

productivity (ANPPtree) were collected at diameter at breast height (DBH, 1.4 m) from a total of

15 trees per species. In the lab, cross-sections were dried and sanded to obtain a smooth, clear

surface, then scanned. Ring number and width were determined using WinDendro software

(Regent Instruments Inc., Quebec, Canada). To calculate ANPPtree, the mean width of the

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preceding 5 years’ growth was used with the wood allometric equations to estimate secondary

growth.

To estimate specific leaf area, a subsample of approximately 100 fresh spruce needles of the

current year’s growth and 10 fresh birch leaves were scanned using WinRhizo software (Regent

Instruments Inc., Quebec, Canada). For the spruce needles, projected leaf area estimates were

multiplied by 1.55 to account for their rhombus shape (Bond-Lamberty and others 2003). For

nutrient analysis, foliage was dried at 60oC, then ground using a wiley mill (Thomas Scientific

model 3383-L10, Swedesboro, NJ). A portion of this material was analyzed for percent C and N

using a Costech Analytical ECS 4010 Elemental Analyzer (Valencia, CA) at the University of

Florida and an additional subsample was shipped to the Louisiana State University AgCenter for

analysis of total phosphorus (P), calcium, magnesium and potassium. Briefly, 5 mL of

concentrated nitric acid was added to 0.5 g of ground sample. After 50 minutes, 3 mL of

hydrogen peroxide was added and the sample was left to digest for 2.75 hours on a heat block.

Samples were then cooled and diluted, then run on a Spectro ARCOS iCAP inductively coupled

plasma spectrometer (Germany).

Downed woody debris was quantified using the line intercept method (Brown 1974) along three,

10 m transects placed at random locations across each plot. Fine woody debris was categorized

into 5 size classes and the number of intercepts of each of class were recorded and converted to

wood mass using multiplier values reported in Nalder and others (1997). The diameter of coarse

woody debris (≥ 7 cm) was also recorded and converted to an area basis as outlined in Ter-

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Mikaelian and others (2008). Biomass estimates were converted to C and N pools using the same

concentration information used to estimate standing tree wood biomass.

Soil Characterization

To estimate moisture content, a weighed subsample of each composite SOL sample was dried at

60oC and each mineral sample at 110oC, then re-weighed. Subsamples of both horizon types

were dried at 60oC and ground prior to analysis of C and N concentration using the same

approach detailed previously for the foliage and litter analysis. Exchangeable base cations were

measured by mixing 50 mL of 1 M ammonium chloride (Robertson and others 1999) with 5 g of

field moist organic horizon sample and 10 g of mineral soil. Samples were shaken for 1 h on a

shaker table, then filtered through a GF/A filter via vacuum filtration and frozen until analysis at

the Louisiana State University AgCenter (Baton Rouge, LA) on a Spectro CIROS inductively

coupled plasma spectrometer (Germany). Mehlich P concentration was determined using a

double acid (0.05 M hydrochloric acid (HCl) + 0.0125 M sulfuric acid) extraction procedure,

with 20 mL of acid solution added to 5 g of air dried soil and shaken for 5 minutes (Kuo 1996).

Samples were then filtered through Whatman No. 5 filter paper and P concentration was

measured colorimetrically using a BioTek Instruments PowerWave XS microplate reder

(Winooski, VT). Soil pH was measured using a Thermo Scientific Orion 2 Star pH meter. For

organic soils, 5 g of air-dried material were mixed with 50 mL of deonized water in a cup and for

mineral soils 20 mL of water were added. Each sample was well mixed, allowed to equilibrate

for 30 minutes, then pH was recorded once the value stabilized.

To calculate potential N mineralization and nitrification, ammonium (NH4+) and nitrate (NO3

-)

were extracted from a subsample of the initial composite soils, as well as for the 30-day and 90-

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day incubations detailed in the Soil C and N fluxes section of the main text and below. Briefly, 5

g of moist organic soil and 10 g of mineral soil were mixed with 50 mL of 2 M potassium

chloride for 1 hr (Robertson and others 1999). Samples were then filtered through a GF/A filter

via vacuum filtration and immediately frozen until analysis on an Astoria-Pacific International

Autoanalyzer (Clackamas, OR) at the University of Florida.

Near total element digestion was performed on all mineral soils (0-10 cm plot composite samples

and all deeper, ≤ 1 m mineral soil increments) by ALS Minerals (Reno, NV) using a four acid

near total digestion method. First, 0.25 g of ground sample was digested with HCl, perchloric,

nitric, and hydrofluoric acids. The residue was brought to volume with dilute HCl, then analyzed

on an inductively coupled plasma atomic emission spectrometer.

To radiocarbon date the soils, green moss was first removed from the SOL surface of black

spruce SOLs using scissors. The core was then cut in half vertically. One half of the core was

carefully cut into 5 cm depth increments and each segment was analyzed for moisture content,

bulk density, and C and N using the same methods as described for SOL processing previously.

The second half was carefully divided into 1 cm depth increments. A small portion of these

segments were then smeared on a piece of paper to qualitatively determine the presence/absence

of charcoal within the increment (Jones and others 2013). When the charcoal layer was found,

the increment containing the charcoal and at least 2 increments on both sides of this increment

were chosen for further processing and radiocarbon analysis. Moss macrofossils were carefully

removed with a dissecting scope from the chosen increments in the black spruce SOLs and a

bulk soil sample (excluding roots) was also obtained. The birch SOL did not contain any

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charcoal or mosses, so only a root-free bulk soil sample was analyzed. Cellulose was extracted

from the macrofossil samples (Gaudinski and others 2005) and all macrofossil and bulk soil

samples were converted to graphite at 650oC with an iron catalyst in a hydrogen atmosphere

(Vogel and others 1987). Graphite samples were then sent to the UC Irvine W.M. Keck Carbon

Cycle Accelerator Mass Spectrometry Laboratory for analysis. Values obtained for the bulk and

macrofossil samples from a given depth increment in spruce stands differed little and here we

only report the bulk values. Also, we were unable to keep the black spruce SOL core from block

C intact and therefore it was excluded from the analysis.

Ion exchange resins were rinsed with deionized water after returning to the lab and refrigerated

until analysis. In preparation for extraction, resin bags were rinsed for 20 seconds with nano-pure

water to remove all remaining soil particles. Each bag was placed in a tube with 30 mL of a

mixture of 0.1 M HCl and 2 M sodium chloride and shaken for 1 hr on a shaker table. Extractant

was then drip-filtered through a Whatman GF/A filter and frozen until further analysis.

Phosphate was measured colorimetrically within 1 week of extraction using a spectrophotometer

microplate reader (PowerWave XS Microplate Reader, Bio-Tek Instruments, Inc., Winooski,

VT) using the ascorbic acid molybdenum-blue method (Murphy and Riley 1962). The remaining

sample was frozen until analysis of NH4+ and NO3

- using a segmented flow autoanalyzer

(Astoria-Pacific, Inc., Clackamas, OR) at the University of Florida.

Soil C and N fluxes

To estimate soil carbon dioxide (CO2) respiration (90-day) and potential net N mineralization

and nitrification (30- and 90-day), an approximately 3 cm x 3 cm intact piece of each SOL

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horizon was removed from each intact core so as to include the entire vertical length of the

horizon. Subsamples were then placed in a 32 oz. mason jar with glass beads underlying

perforated aluminum foil on the bottom of the jar. For each horizon, the subsamples originating

from the 3 field sampling locations within a given plot were put into a single jar, creating 1

composite sample per plot, per horizon. For each 10 cm mineral soil core, a small portion of soil

was removed using a spatula vertically along the length of the core. Once in jars, deionized water

was added to each sample to approximate field capacity and samples were placed in the dark at

15oC. Soil CO2 respiration was measured using an automated soil incubation system equipped

with an infrared gas analyzer (Li-820, Licor, Inc, Lincoln, NE) (Bracho and others unpublished).

To estimate 14CO2, CO2 was first removed from each jar by scrubbing with magnesium

perchlorate and soda lime for 5 min. Jars were then left at 15oC for 3-5 days to allow CO2 to

accumulate. The headspace air was then pulled through a zeolite molecular sieve trap (Alltech

13X, Alltech Associates, Deerfield, IL) to absorb the CO2, baked, reduced to graphite (Hicks

Pries and others 2013), then sent to the UC Irvine W.M. Keck Carbon Cycle Accelerator Mass

Spectrometry Laboratory for final analysis.

REFERENCES

Alexander HD, Mack MC, Goetz S, Beck PSA, Belshe EF. 2012. Implications of increased

deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests.

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Bond-Lamberty B, Wang C, Gower ST. 2003. The use of multiple measurement techniques to

refine estimates of conifer needle geometry. Canadian Journal of Forest Research-Revue

Canadienne De Recherche Forestiere 33: 101-105.

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Brown JK. 1974. Handbook for inventorying downed woody material. Ogden, Utah, USA:

USDA Forest Service, Intermountain Forest and Range Experiment Station.

Gaudinski JB, Dawson TE, Quideau S, Schuur EAG, Roden JS, Trumbore SE, Sandquist DR,

Oh SW, Wasylishen RE. 2005. Comparative analysis of cellulose preparation techniques for

use with 13C, 14C, and 18O isotopic measurements. Analytical Chemistry 77: 7212-7224.

Hicks Pries CE, Schuur EAG, Crummer KG. 2013. Thawing permafrost increases old soil and

autotrophic respiration in tundra: Partitioning ecosystem respiration using δ13C and Δ14C.

Global Change Biology 19: 649-661.

Jones BM, Breen AL, Gaglioti BV, Mann DH, Rocha AV, Grosse G, Arp CD, Kunz ML, Walker

DA. 2013. Identification of unrecognized tundra fire events on the north slope of Alaska.

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Kuo S. 1996. Methods of soil analysis. Bartels JM editor. Methods of soil analysis part 3-

chemical methods. Madison: Soil Science Society of America, p893-894.

Mack MC, Treseder KK, Manies KL, Harden JW, Schuur EAG, Vogel JG, Randerson JT,

Chapin FS, III. 2008. Recovery of aboveground plant biomass and productivity after fire in

mesic and dry black spruce forests of interior alaska. Ecosystems 11: 209-225.

Murphy J, Riley JP. 1962. A modified single solution method for determination of phosphate in

natural waters. Analytica Chimica Acta 26: 31-&.

Nalder IA, Wein RW, Alexander ME, deGroot WJ. 1997. Physical properties of dead and

downed round-wood fuels in the boreal forests of Alberta and Northwest Territories.

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Robertson GP, Coleman DC, Bledsoe CS, Sollins P editors. 1999. Standard soil methods for

long-term ecological research. New York: Oxford University Press.

Ter-Mikaelian MT, Colombo SJ, Chen JX. 2008. Amount of downed woody debris and its

prediction using stand characteristics in boreal and mixedwood forests of Ontario, Canada.

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graphite for AMS. Nuclear Instruments & Methods in Physics Research Section B-Beam

Interactions with Materials and Atoms 29: 50-56.

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Appendix 2. Forest Stand Characteristics for Each Spatially Interspersed Block of Black Spruce and Paper Birch across the Study Site

Black spruce Paper birch

Block A Block B Block C Block A Block B Block C

Tree age (years) 45.4(0.7) 38.5(2.3) 44.0(1.0) 49.8(0.8) 47.9(0.8) 46.8(1.3)Density (stems m-2) All stems 8.22(1.43) 8.29(0.94) 5.43(0.47) 0.56(0.05) 1.21(0.13) 1.04(0.08) Plot target species BD 4.72(13.2) 4.68(1.04) 1.7(0.32) 0.02(0.02) 0.06(0.04) 0.1(0.01) DBH 2.95(0.44) 3.05(0.32) 3.14(0.41) 0.50(0.05) 0.88(0.09) 0.80(0.10) Other species BD 0.03(0.01) 0.04(0.01) 0.002(0.002) 0.02(0.01) 0.23(0.13) 0.11(0.02) DBH 0.22(0.05) 0.25(0.11) 0.06(0.01) 0.01(0.004) 0.02(0.01) 0.02(0.01) Tall shrubs 0.31(0.07) 0.27(0.09) 0.58(0.07) 0.00(0.00) 0.02(0.02) 0.10(0.03)Basal area (m2 ha-1) All stems 20.99(3.24) 19.81(1.18) 24.60(2.85) 29.41(1.58) 25.67(1.62) 30.82(4.27) Plot target species BD 4.10(1.15) 4.07(0.90) 1.41(0.20) 0.005(0.004) 0.03(0.03) 0.02(0.01) DBH 12.84(3.11) 10.98(1.03) 18.04(2.37) 29.31(1.59) 24.43(1.37) 28.76(4.28) Other species BD 0.18(0.09) 0.12(0.05) 0.002(0.002) 0.07(0.06) 0.20(0.09) 0.10(0.03) DBH 2.42(0.69) 2.38(1.16) 1.39(0.49) 0.02(0.01) 0.12(0.07) 0.08(0.05) Tall shrubs 1.44(0.48) 2.26(.068) 3.76(0.59) 0.00(0.00) 0.89(0.55) 1.85(0.45)Live tree biomass (kg m-2) All stems 4.92(1.08) 4.62(0.54) 6.39(0.91) 12.06(0.80) 9.88(0.67) 12.04(1.81) Plot target species BD 0.22(0.06) 0.21(0.05) 0.08(0.01) 0.0001(0.0001) 0.002(0.002) 0.001(0.001) DBH 3.66(0.83) 3.20(0.29) 5.02(0.64) 12.04(0.80) 9.43(0.59) 11.51(1.79)

Other species BD 0.02(0.01) 0.01(0.004) 0.0001(0.0001) 0.01(0.01) 0.01(0.004) 0.01(0.002) DBH 0.78(0.24) 0.77(0.38) 0.50(0.20) 0.01(0.004) 0.03(0.02) 0.02(0.01) Tall shrubs 0.24(0.10) 0.42(0.14) 0.80(0.20) 0.00(0.00) 0.41(0.27) 0.50)(0.10)

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Appendix 2. Continued

Proportion biomass Plot target species 0.80(0.03) 0.76(0.05) 0.80(0.04) 1.00(0.001) 0.96(0.03) 0.95(0.01) Other species 0.15(0.02) 0.14(0.06) 0.07(0.02) 0.001(0.001) 0.004(0.002) 0.002(0.001) Tall shrubs 0.04(0.02) 0.10(0.03) 0.12(0.02) 0.00(0.00) 0.04(0.03) 0.05(0.01)

All values are mean (SE), n = 5 plots per block.

Trees ≥ 1.4 m in height are included in the rows labeled DBH (diameter at breast height) and trees < 1.4 m tall are included in the BD

(basal diameter) category.

Other species includes all measure trees that were not the focus species studied within the given plot.

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Appendix 3. Total Elemental Concentrations for Mineral Soils (0-100 cm) for Studied Black Spruce and Alaska Paper Birch Stands

Black spruce

Soil Depth Increment (cm)0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100

C % 2.80(0.34) 2.12(1.09) 1.20(0.43) 1.07(0.42) 1.63(0.84) 1.57(1.03) 0.64 ND ND NDN % 0.16(0.02) 0.13(0.06) 0.07(0.02) 0.07(0.02) 0.10(0.05) 0.09(0.05) 0.05 ND ND NDAg (ppm) 12.8(3.6) 11.1(4.5) 19.4(3.2) 27.6(3.5) 19.0(7.5) 17.5(1.6) 17.3 ND ND NDAl % 6.66(0.65) 6.49(0.49) 6.50(0.46) 6.73(0.48) 6.90(0.44) 6.88(0.35) 6.54 ND ND NDAs (ppm) 11(1) 16(4) 13(1) 13(2) 13(0) 15(1) 16 ND ND NDBa (ppm) 757(64) 752(36) 749(35) 761(38) 807(48) 777(31) 834 ND ND NDBe (ppm) 1.3(0.1) 1.3(0.1) 1.3(0.1) 1.4(0.1) 1.5(0.1) 1.4(0.1) 1.3 ND `ND NDBi (ppm) 1(1) 1(1) 1(1) 1(1) 1(1) 2(1) 2 ND ND NDCa % 1.39(0.32) 1.29(0.20) 1.24(0.19) 1.24(0.18) 1.21(0.12) 1.20(0.15) 1 ND ND NDCd (ppm) 0(0) 1(0) 3(0) 4(1) 2(1) 1(0) 3 ND ND NDCr (ppm) 86(10) 82(6) 77(7) 79(3) 82(4) 80(3) 74 ND ND NDCu (ppm) 24(6) 31(4) 32(6) 37(2) 38(2) 36(4) 36 ND ND NDFe % 3.46(0.32) 3.78(0.14) 3.61(0.20) 3.73(0.20) 3.84(0.16) 3.89(0.21) 3.66 ND ND NDGa (ppm) 13(3) 17(3) 17(3) 16(3) 20(0) 20(0) 16 ND ND NDK % 1.47(0.14) 1.46(0.14) 1.52(0.14) 1.61(0.21) 1.62(0.16) 1.65(0.18) 1.46 ND ND NDLa (ppm) 33(3) 33(3) 30(6) 30(6) 33(3) 35(5) 30 ND ND NDMg (%) 1.03(0.11) 1.02(0.07) 1.01(0.09) 1.06(0.09) 1.08(0.08) 1.08(0.06) 1.04 ND ND NDMn (ppm) 487(23) 512(33) 533(61) 533(62) 578(26) 563(47) 509 ND ND NDNa % 1.26(0.16) 1.17(0.06) 1.16(0.13) 1.14(0.13) 1.12(0.04) 1.12(0.14) 1.31 ND ND NDNi (ppm) 29(4) 31(3) 30(3) 32(2) 36(3) 35(3) 32 ND ND NDP (ppm) 665(31) 698(117) 624(53) 610(50) 620(26) 673(3) 694 ND ND NDPb (ppm) 15(1) 13(1) 31(12) 18(2) 15(1) 15(0) 14 ND ND NDS % 0.02(0.003) 0.02(0.01) 0.01(0.01) 0.01(0.01) 0.02(0.01) 0.01(0.00) 0.01 ND ND NDSc (ppm) 14(2) 14(1) 13(1) 14(1) 15(1) 15(1) 14 ND ND NDSr (ppm) 188(30) 177(18) 176(22) 176(19) 175(11) 177(12) 201 ND ND NDTi % 0.43(0.04) 0.42(0.03) 0.40(0.03) 0.43(0.03) 0.43(0.03) 0.44(0.01) 0.41 ND ND ND

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V (ppm) 109(12) 109(8) 104(8) 108(5) 113(6) 110(1) 109 ND ND NDZn (ppm) 76(3) 78(2) 84(2) 88(5) 86(7) 80(2) 89 ND ND ND

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Appendix 3. Continued

Paper birchSoil Depth Increment (cm)

0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100C % 4.13(0.76) 1.54(0.32) 0.91(0.27) 0.60(0.12) 0.54(0.13) 0.65(0.14) 0.50(0.04) 0.51 0.59 0.61N % 0.20(0.03) 0.09(0.01) 0.05(0.01) 0.04(0.01) 0.05(0.00) 0.05(0.00) 0.04(0.01) 0.05 0.05 0.06Ag (ppm) 0.5(0.5) 0.70(0.70) 1.3(0.8) 1.4(1.1) 8.4(8.1) 5.6(5.6) 8.9(8.9) 0 0.5 1.7Al % 6.20(0.18) 6.49(0.20) 6.90(0.33) 6.57(0.13) 6.46(0.12) 6.35(0.17) 6.69(0.24) 6.44 6.25 6.7As (ppm) 10(1) 13(1) 13(1) 15(2) 9(5) 10(2) 12(2) 13 19 15Ba (ppm) 677(18) 707(39) 753(52) 713(73) 717(66) 627(107) 680(170) 870 880 930Be (ppm) 1.1(0.0) 1.2(0.1) 1.2(0.1) 1.2(0.1) 1.2(0.1) 1.1(0.1) 1.2(0.1) 1.3 1.3 1.3Bi (ppm) 1(1) 1(1) 0(0) 1(1) 1(1) 1(1) 4(2) 0 4 0Ca % 1.44(0.36) 1.36(0.33) 1.50(0.35) 1.53(0.32) 1.34(0.30) 1.51(0.15) 1.61(0.21) 1.72 1.65 1.73Cd (ppm) 0(0) 0(0) 0(0) 0(0) 0(0) 0(0) 0(0) 0 0 0Cr (ppm) 105(27) 98(22) 104(18) 98(12) 86(9) 90(5) 97(14) 78 79 83Cu (ppm) 28(8) 29(6) 33(6) 36(4) 36(5) 51(10) 57(22) 34 33 36Fe % 3.42(0.33) 3.71(0.17) 3.95(0.22) 4.03(0.19) 3.98(0.38) 4.58(0.44) 4.70(0.96) 3.58 3.63 3.74Ga (ppm) 13(3) 20(0) 17(3) 20(0) 20(0) 20(0) 20(0) 20 10 20K % 1.22(0.03) 1.26(0.03) 1.32(0.04) 1.21(0.11) 1.33(0.13) 1.12(0.13) 1.24(0.19) 1.43 1.40 1.48La (ppm) 23(3) 23(3) 33(3) 30(6) 30(0) 27(3) 25(5) 30 30 30Mg (%) 1.01(0.16) 1.06(0.14) 1.18(0.19) 1.16(0.13) 1.08(0.14) 1.23(0.06) 1.29(0.13) 1.13 1.09 1.16Mn (ppm) 459(69) 428(38) 504(59) 540(65) 553(68) 702(105) 789(40) 683 785 778Na % 1.20(0.15) 1.21(0.17) 1.33(0.21) 1.29(0.16) 1.17(0.17) 1.17(0.17) 1.24(0.25) 1.45 1.37 1.47Ni (ppm) 34(10) 36(7) 41(9) 40(9) 38(7) 41(3) 45(6) 37 39 41P (ppm) 593(133) 467(68) 520(136) 507(113) 533(146) 520(150) 620(190) 810 800 780Pb (ppm) 12(1) 64(50) 14(1) 14(2) 23(9) 19(8) 13(1) 15 17 17S % 0.01(0.003) 0.01(0.003) 0.01(0.003) 0.003(0.003) 0.003(0.00) 0.003(0.003) 0.01(0.01) 0.01 0.02 0.01Sc (ppm) 14(1) 14(1) 16(2) 16(1) 15(1) 17(1) 18(3) 14 14 15Sr (ppm) 179(36) 177(33) 197(41) 192(35) 173(32) 169(34) 183(48) 226 216 229Ti % 0.45(0.00) 0.41(0.02) 0.46(0.02) 0.47(0.03) 0.45(0.06) 0.49(0.05) 0.50(0.08) 0.42 0.41 0.42V (ppm) 116(9) 113(12) 125(12) 128(9) 123(14) 144(15) 151(31) 115 114 119Zn (ppm) 68(9) 70(6) 77(6) 75(6) 76(5) 77(4) 82(3) 83 85 92

Mean (SE), n = 3 samples per species, per depth increment. Cells containing ND indicate no data available.181182