Electronic Supplementary Material

Fine-root growth in a forested bog is seasonally dynamic, but shallowly distributed in nutrient-poor peatColleen M. Iversen · Joanne Childs · Richard J. Norby · Todd A. Ontl · Randall K. Kolka · Deanne J. Brice · Karis J. McFarlane · Paul J. Hanson

Online Resource 1 Supplemental minirhizotron methods.

Minirhizotron Installation and Image Collection and AnalysisMinirhizotrons were cellulose acetate butyrate tubes (Bartz Technology, Santa Barbara,

CA, USA) that were ~ 2 m in length, with plastic endcaps. To install the minirhizotrons, an initial shallow hole was made in the peat by first using a piezometer (5.08 cm diameter) with a drive point. The minirhizotron was then inserted into the pre-made hole, capped with the metal cap to prevent fracturing, and pounded to achieve the desired depth using a post-hole driver. The outside of the minirhizotron reaching above the peat surface was covered with pipe insulation to prevent light contamination and to regulate temperature (Fig. S1, below). A rubber stopper was inserted into the top of the tube and electrical tape was used to cover the indexing hole to prevent water intrusion, and the insulation and stopper were covered with a white PVC cap. A hose clamp surrounding the tube and insulation was secured with zip ties to an adjacent piece of thin-wall metal conduit that was driven ~2.5 m into the peat to prevent vertical or horizontal movement of the minirhizotron with frost-heave or changes in water table height. Annual measurements of the angle of minirhizotron tubes and weekly analysis of root images indicated little movement of the tubes.

Images were collected from ~90 frames (~20.5 mm wide by 14.2 mm long) per minirhizotron, depending on the depth of the tube. A smaller portion of each frame (~19.1 mm wide by 13.8 mm long) was digitized to obtain root length and diameter. While internal tube condensation was an issue in the bog (during the filming process warm, moist air was pushed into a minirhizotron kept cool by surrounding peat), this issue was resolved by frequent wiping of the inner surface of the tube with a soft cloth. No windows were completely obscured by condensation, and no images were discarded from the analysis. Minirhizotron images from the current analysis were taken beginning 10 months after tube installation (Table S1, below).

Observed tube area was corrected for window size, peat surface height (which differed across tubes), and for actual angle of installation, which ranged from 40 to 56 degrees from vertical (average = 48.0 ± 3.4 SD, n = 24 tubes). Nearly all roots observed with the minirhizotrons were less than 2 mm in diameter, with the exception of three roots observed in 2011 that were ~2.5 mm in diameter.

Fig. S1 (a) Minirhizotron image collection from a hollow (lower minirhizotron, where the cellulose acetate butyrate minirhizotron tube is surrounded by black pipe insulation, and a minirhizotron camera is inserted with images being captured by the computer system) and a hummock (upper tube, also surrounded by black pipe insulation); PVC caps and stoppers have been removed for imaging, and (b) minirhizotron image appearance in the S1 bog. Individual images are ~ 20 mm × 14 mm, shown from a depth of ~10 to 13 cm captured from the same minirhizotron on July 25, August 1, and August 10, 2011. The elongation of a new, white fine root from one week to the next can be observed in the middle row of images; a framework of older, brown, suberized roots can also be seen.

(a)

(b)

Table S1 Timing of minirhizotron image collection and the number of new roots observed.

YearFirst

sampling date

Last sampling date

No. sessions

Plant functional type

Microtopography

No. new roots

observed

2010 21 July 2010 21 October 2010 2

Tree Hummock NAHollow NA

Shrub Hummock NAHollow NA

Sedge Hummock NAHollow NA

2011 10 May 2011 21 October 2011 14

Tree Hummock 9-123Hollow 0-33

Shrub Hummock 6-395Hollow 3-42

Sedge Hummock 0-1Hollow 0-25

2012 9 May 2012 15 September 2012 15

Tree Hummock 1-33Hollow 1-10

Shrub Hummock 0-59Hollow 0-8

Sedge Hummock 0-13Hollow 0-11

Notes: Due to imminent weather, image collection took 2 days to complete on some sampling dates. The range for number of new fine roots observed is the minimum and maximum across the entire depth profile for each of 12 minirhizotron tubes in each microtopographic position (this metric includes the extension of existing roots). NA indicates that fine-root dynamics were not determined during the year of installation in 2010 (imaging on the two dates was to check on soil contact with the tube surface).

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Online Resource 2 Specific root length (SRL, m/g) in relation to the diameter of tree and ericaceous shrub fine roots collected from the S1 bog. Data from voucher specimens were collected separately for P. mariana and L. laricina (tree species) and C. calyculata and R. groenlandicum (ericaceous shrub species), but combined by plant functional type (tree and shrub) to develop these relationships, as species cannot be distinguished in minirhizotron images. In most cases, first and second order roots were not separated due to time constraints (for R. groenlandicum, which had extremely fine roots, the first three absorptive root orders were not separated). With few exceptions, the first three orders of shrub roots were less than 200 µm, and the first three orders of tree roots were greater than 200 µm. For the first three root orders of both tree species and shrub species (in red circles) sampled from voucher specimens, root diameter was negatively related with specific root length, following a negative power relationship (SRL = 3.72 × D-1.95, R2=0.94, P < 0.0001 for trees and SRL = 0.07 × D-3.86, R2=0.90, P < 0.0001 for shrubs, where D is root diameter and SRL is specific root length). Data for fourth and higher order roots (black circles) sampled from voucher specimens are also shown, although these were not included in the power functions used in scaling minirhizotron observations. Note the differences in y-axes between panels a and b, where ericaceous shrub roots had a much greater SRL than tree roots.

Root diameter (mm)

0.0 0.1 0.2 0.3 0.4 0.5

Specific root length(m

/g)

0

200

400

600

800

1000

1200

1400

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root

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th(m

/g)

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60

80

100Orders 1-3Orders 4+

(b) Shrub roots(a) Tree roots

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Online Resource 3 Relationship between fine-root length production and nearby tree basal area for the dominant plant functional types in 2011 and 2012. Each data point is the total length production for trees (a) or shrubs (b), where each point was an individual minirhizotron, where hummocks are filled circles connected by dashed lines, and hollows are open circles. Data collected in 2011 are in grey; data collected in 2012 are in blue. (a) A significant portion of the variation in the length production of tree roots in the hummocks in 2011 was explained by nearby tree basal area density, where (log) Length production = 0.42 + 0.41 × (log) BA (Adj. R2 = 0.54, P = 0.004) and BA is nearby tree basal area (i.e., trees within 1.5 m of paired minirhizotrons) per unit ground area. The regression is indicated by a solid grey line (back-transformed from log function). There were no significant linear relationships between root length production and nearby tree basal area in any other case, for trees or shrubs (P > 0.08). (b) The length production of shrub fine roots had a unimodal relationship with nearby tree basal area density across hummocks and hollows.

Tree basal area within 1.5 m of minirhizotron pair(cm2/m2 ground area)

0 5 10 15 20 25

Fine

-roo

t len

gth

prod

uctio

n(k

m m

-2 g

roun

d ar

ea y

r-1)

0

5

10

15

20

25(a) Tree fine roots

0 5 10 15 20 25

Fine-root length production(km

m-2 ground area yr -1)

0

5

10

15

20

25(b) Shrub fine roots

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Online Resource 4 The timing of the production of new shrub fine roots and shrub leaf out (a) and the timing of the production of tree fine roots and wood, as well as tree leaf out (b) in 2011. Data for fine roots and wood are presented as means ± 1 standard error. Root growth phenology (solid lines in panels a and b) was determined from minirhizotron images. The timing of leaf out was determined from manual observations and daily images (Hanson et al. 2015b) where the dashed lines for shrubs R. groenlandicum and C. calyculata, as well as trees L. laricina and P. mariana are annual leaf out. The phenology of wood growth was determined from dendrobands, and cumulative tree basal area index (BAI) is indicated by a dotted line in panel b. Environmental data collected from nearby monitoring stations includes photosynthetically active radiation (PAR) measured at 2 m above the peat surface (c), peat temperature measured in the top 5 cm of hummocks (d), and water table level (WT) measured in a hollow (e), where 0 cm was the hollow surface (the surface of the hummocks where minirhizotrons were installed were on average 13 cm above the hollow surface).

Prop

ortio

n of

max

imum

root

pro

duct

ion

(Hum

moc

k su

rfac

e)

0.0

0.2

0.4

0.6

0.8

2011

Cum

ulative tree BAI

(cm2)

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0.6

0.9

1.2

Aver

age

daily

tem

pera

ture

in

hum

moc

k su

rfac

e (°

C)

05

10152025

Day of year

120 140 160 180 200 220 240 260 280

Daily W

T level(cm

)

-40

-20

0

20

PAR

(mol quanta m

-2 d-1)

0

20

40

60

Day of year

120 140 160 180 200 220 240 260 280

0.0

0.2

0.4

0.6

0.8(a) Shrub root phenology

(c) PAR

(d) Peat temperature

(e) Water table level

(b) Tree root phenology

R. groenlandicumC. calyculata

L. laricinaP. mariana

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Online Resource 5 Radiocarbon dating of fine roots sampled from throughout the peat profile in the SPRUCE experimental plots in 2012.

Species Microtopography

Peat depth (cm) CAMS # ∆14C Conventional 14C

age (yr BP)95.4 % (2s) calibrated age

rangesCalibration

Data

Calibrated years (AD)Larch Hummock 20 to 10 164298 44 ± 3 NA 2006 2011 1,2Larch Hummock 10 to 0 164299 59 ± 4 NA 2002 2007 1

Larch Hollow 0 to -30 164293 52 ± 3 NA 2005 2009 1

Spruce Hummock 10 to 0 164297* 41 ± 4 NA 2007 2012 1,2

Shrub Hummock 20 to 10 164300 59 ± 3 NA 2003 2007 1Shrub Hummock 10 to 0 164301 56 ± 3 NA 2004 2008 1

Shrub Hollow 0 to -30 164294 77 ± 3 NA 2000 2003 1

Calibrated years (BC)Shrub Hummock -50 to -100 164302 -374 ± 3 3705 ± 40 1975 2204 3Shrub Hummock -100 to -200 164303 -465 ± 2 4970 ± 25 3665 3796 3

Shrub Hollow -50 to -100 164295 -365 ± 2 3590 ± 25 1887 2022 3Shrub Hollow -100 to -200 164296 -426 ± 2 4395 ± 30 2918 3095 3Notes: Radiocarbon values are reported in ∆ 14C notation, corrected for 14C decay since 1950 (Stuiver and Polach 1977). Measured δ13C values of fine roots collected from ingrowth cores were used to correct for mass-dependent fractionation (data not shown). Measured ∆ 14C values had an average accelerator mass spectrometry precision of less than 3‰. Two sigma calibrated age ranges were determined by matching root 14C values to atmospheric 14C calibration curves using Calib v7.1 (http://calib.qub.ac.uk/calib/) for roots that were older than 1950 AD and using Calibomb (http://calib.qub.ac.uk/CALIBomb/) for roots younger than 1950 AD. Both programs were developed and are made available through the 14CHRONO Centre at Queens University Belfast and give a distribution of possible solutions based on the variation in the atmospheric record over time and errors associated with the calibration. The INTCAL13 atmospheric 14C calibration record was used for the pre-1950 AD time period (Reimer et al. 2013) and the Northern Hemisphere Zone 1 record was used for 1950–2009 AD (Hua et al. 2013). For roots with two-sigma age ranges that overlapped the end of the Northern Hemisphere Zone 1 record in 2009, the maximum age was determined by matching the values to annual growing

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season averages calculated for Niwot Ridge for 2009–2011 AD (Lehman et al. 2013). All young roots (from the last decade) have a possible age of 1956-1958 AD.

Calibration data sets: 1Northern Hemisphere Zone 1 annual growing season averages from Hua et al. (2013); 2Niwot ridge annual growing season averages for 2009-2011 calculated from Lehman et al. (2013); 3Intcal13 from Reimer et al. (2013).

*-2 sigma calibrated age for 164297 was less than the 2011 value (36 per mil) in Lehman et al. (2013), so it was set to 2012.

References

Hua Q, Barbetti M, Rakowski A (2013) Atmospheric radiocarbon for the period 1950-2010. Radiocarbon 55:2059‒2072

Lehman SJ, Miller JB, Wolak C, Southon J, Tans PP, Montzka SA, Sweeney C, Andrews A, LaFranchi B, Guilderson TP, Turnbull JC (2013) Allocation of terrestrial carbon sources using CO2-C14: Methods, measurement, and modeling. Radiocarbon 55:1484‒1495

Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Ramsey CB, Buck CE, Cheng H, Edwards RL, Friedrich M, Grootes PM, Guilderson TP, Haflidason H, Hajdas I, Hatte C, Heaton TJ, Hoffmann DL, Hogg AG, Hughen KA, Kaiser KF, Kromer B, Manning SW, Niu M, Reimer RW, Richards DA, Scott EM, Southon JR, Staff RA, Turney CSM, van der Plicht J (2013) IntCal13 and Marine13 Radiocarbon age calibration curves 0-50,000 years cal BP. Radiocarbon 55:1869‒1887

Stuiver M, Polach HA (1977) Reporting of 14C data - Discussion. Radiocarbon 19:355‒363

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Online Resource 6 Fine-root biomass production density of the dominant plant functional types on the S1 bog across gradients of microtopography and tree basal area density and throughout the peat profile extrapolated from minirhizotron observations of root length production and root diameter and relativized to peat volume.

Production (Minirhizotrons)

MR no.Tree BA within

1.5 m radius (cm2/m2)

Shrub cover (%) Micro-topography Upper depth

(cm)Lower depth

(cm)Tree root

density (g m-3 yr-1)Shrub root density

(g m-3 yr-1)Sedge root density

(g m-3 yr-1)

2011 2012 2011 2012 2011 20122 0.0 52 Hummock -- -- -- -- -- -- -- --

-- -- -- -- -- -- -- --13 10 301 417 502 192 0 010 0 475 1630 842 305 0 00 -10 18 230 75 23 0 0

-10 -66 4 31 0 0 0 0

1 Hollow 0 -10 0 0 0 0 0 0-10 -20 157 54 0 0 0 0-20 -30 0 0 0 0 0 0-30 -85 63 0 0 0 0 0

Hummock 13 -10 254 863 464 168 0 0Hollow 0 -30 52 18 0 0 0 0

20 0.0 53 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 0 0 0 0 010 0 11 0 35 0 0 00 -10 41 0 54 6 0 0

-10 -67 147 7 24 0 0 0

19 Hollow 0 -10 556 140 129 0 0 0-10 -20 193 267 33 21 0 0-20 -30 470 0 44 0 0 0-30 -89 375 0 0 0 0 0

Hummock 13 -10 23 0 39 3 0 0Hollow 0 -30 406 136 69 7 0 0

22 0.0 59 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 498 199 148 0 0 010 0 396 429 118 101 0 00 -10 89 38 97 79 0 0

-10 -67 89 1 8 0 0 0

21 Hollow 0 -10 210 309 56 164 0 0-10 -20 0 22 0 0 0 0-20 -30 27 0 27 0 0 0

1

-30 -95 25 0 4 0 0 0

Hummock 13 -10 276 229 113 78 0 0Hollow 0 -30 79 110 28 55 0 0

24 0.0 61 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 839 170 323 168 0 010 0 234 142 16 12 0 00 -10 331 0 0 0 0 0

-10 -69 41 0 0 0 0 0

23 Hollow 0 -10 584 0 1 6 0 0-10 -20 0 376 20 12 0 0-20 -30 0 23 0 0 0 0-30 -89 9 5 7 0 0 0

Hummock 13 -10 355 84 49 27 0 0Hollow 0 -30 195 133 7 6 0 0

6 1.5 47 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 0 0 0 0 010 0 174 97 173 110 80 5470 -10 3074 202 151 64 0 176

-10 -68 28 0 4 0 0 0

5 Hollow 0 -10 0 0 0 0 0 0

-10 -20 0 0 0 0 0 0-20 -30 0 0 0 0 0 0-30 -78 0 0 0 0 0 0

Hummock 13 -10 1412 130 141 76 35 314Hollow 0 -30 0 0 0 0 0 0

10 2.3 34 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 3 0 0 0 010 0 18 58 63 30 0 00 -10 908 145 24 3 0 0

-10 -69 18 0 0 0 0 0

9 Hollow 0 -10 0 30 0 20 0 0-10 -20 0 0 1 0 0 0-20 -30 129 0 15 0 43 0-30 -84 0 0 2 0 0 0

Hummock 13 -10 402 89 38 14 0 0Hollow 0 -30 43 10 5 7 14 0

2

12 3.2 34 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 1401 0 0 0 010 0 508 39 13 0 0 00 -10 224 0 119 0 0 0

-10 -65 73 0 20 0 0 0

11 Hollow 0 -10 0 134 26 0 0 0-10 -20 0 39 3 0 0 0-20 -30 0 845 0 38 0 0-30 -86 0 1 0 0 0 0

Hummock 13 -10 318 200 57 0 0 0Hollow 0 -30 0 340 10 13 0 0

8 4.2 56 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 135 60 206 14 0 010 0 6925 6 198 1 0 00 -10 718 5 92 29 0 0

-10 -56 14 34 0 0 0 0

7 Hollow 0 -10 0 2543 0 0 0 0-10 -20 17 0 5 0 50 0-20 -30 0 0 0 0 0 0-30 -75 0 0 6 0 0 0

Hummock 13 -10 3341 13 153 15 0 0Hollow 0 -30 6 848 2 0 17 0

4 7.0 41

Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 0 278 5 0 010 0 40 10 337 11 0 00 -10 159 0 608 34 0 0

-10 -56 52 23 85 6 0 0

3 Hollow 0 -10 547 216 40 17 658 81-10 -20 563 20 10 6 366 348-20 -30 94 77 1 0 0 38-30 -83 33 0 0 0 0 0

Hummock 13 -10 87 4 447 20 0 0Hollow 0 -30 401 105 17 7 341 156

18 13.2 48 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 234 501 672 37 0 0

3

10 0 3661 71 483 79 0 00 -10 637 0 609 8 0 0

-10 -72 314 0 19 0 0 0

17 Hollow 0 -10 637 886 14 19 0 0-10 -20 519 0 34 0 0 0-20 -30 0 0 77 0 0 0-30 -81 61 0 20 0 0 0

Hummock 13 -10 1899 96 562 42 0 0Hollow 0 -30 385 295 42 6 0 0

14 16.8 46 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 0 2007 2 13 0 010 0 2956 219 659 42 0 00 -10 754 140 48 15 0 0

-10 -74 33 0 13 0 0 0

13 Hollow 0 -10 1104 127 27 3 0 0-10 -20 174 0 2 0 0 0-20 -30 0 0 0 0 0 0-30 -75 0 0 1 0 0 0

Hummock 13 -10 1613 418 307 26 0 0Hollow 0 -30 426 42 9 1 0 0

16 25.9 36 Hummock -- -- -- -- -- -- -- ---- -- -- -- -- -- -- --13 10 596 929 0 0 0 010 0 1598 375 269 28 0 00 -10 405 340 311 54 0 0

-10 -76 236 8 33 2 0 0

15 Hollow 0 -10 1049 250 96 12 0 0-10 -20 174 67 9 0 0 0-20 -30 0 0 0 0 0 0-30 -83 14 0 0 0 0 0

Hummock 13 -10 949 432 252 36 0 0Hollow 0 -30 408 106 35 4 0 0

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Notes: R. groenlandicum was dominant shrub, ranging from 23 to 43% cover across all locations (C. calyculata ranged from 0 to 23% and Kalmia polifolia was less than 5% across all locations, and Vaccinium species were less than 10% across all locations). Sedge species averaged less than 7% cover across all locations. Minirhizotron hummock depth increments assumes the average difference between hummock and hollow of 13 cm (calculated across 30 hummock-hollow pairs for minirhizotron installation in the SPRUCE experiment) to calculate distance of minirhizotron measurement above the hollow surface.

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Online Resource 7 The β function for newly-produced roots quantified using minirhizotrons, as well as fine-root standing biomass determined from peat cores across gradients of microtopography and tree basal area density.

Year Micro-topography β (Minirhizotrons) β (Standing biomass)

Tree Shrub Spruce Larch Shrub

2011Hummock 0.923 ± 0.006

(R2 = 0.72)0.903 ± 0.008 (R2 = 0.66) -- -- --

Hollow 0.892 ± 0.016 (R2 = 0.24)

0.884 ± 0.021 (R2 = 0.25) -- -- --

2012Hummock 0.860 ± 0.027

(R2 = 0.20)0.886 ± 0.019 (R2 = 0.47)

0.947 ± 0.005(R2 = 0.98)

0.972 ± 0.006(R2 = 0.71)

0.946 ± 0.007(R2 = 0.96)

Hollow 0.750 ± 0.050 (R2 = 0.04)

0.726 ± 0.078 (R2 = 0.10)

0.872 ± 0.028 (R2 = 0.94)

0.920 ± 0.025 (R2 = 0.57)

0.893 ± 0.013 (R2 = 0.93)

2013 Hummock -- -- -- -- --Hollow -- -- -- -- --

Notes: β is the depth coefficient (Jackson et al. 1996) ± 1 standard error, with the adjusted R2 in parenthesis. Minirhizotrons reached a peat depth averaging -67 cm ± 6 cm SD and -84 cm ± 6 cm SD, depending on whether they were positioned in hummocks or hollows, respectively. In 2011, n = 11 and 7 minirhizotron locations were included in the analysis for tree roots and n = 11 and 6 locations for shrub roots in hummocks and hollows, respectively and in 2012, n = 10 and 9 locations included in the analysis for tree roots and n = 10 and 7 locations for shrub roots in hummocks and hollows, respectively). Peat cores reached a depth of ~ -250 cm, but depth distributions were only calculated to -30 cm depth, given the 14C age of fine roots in deeper peat that indicated a high proportion of dead roots in peat deeper than -30 cm. Thus, the given β for fine-root standing biomass in peat cores is potentially skewed more shallowly than if deeper depths were included.

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Online Resource 8 Resin-available nutrients sampled from a limited array of resin-access tubes installed in hollows in the south end of the bog in 2011 and 2012, and a larger array of resin-access tubes installed in hummocks and hollows in the SPRUCE experimental plots in 2013.

Year Microtopography

Peat depth relative to

hollow (cm)

Plant-available NH4+

(µg N cm-2 30 d-1)

Plant-available NO3

- (µg N cm-2 30 d-1)

Plant-available PO4

- (µg P cm-2 30 d-1)

2011

Hummock NA NA NA NA

Hollow(n = 3)

-10 6.9 ± 1.1A 0.1 ± 0.1 3.6 ± 0.6-30 79.9 ± 32.0B 0.0 ± 0.0 21.0 ± 8.1-60 26.7± 5.9AB 0.1 ± 0.0 14.2 ± 1.8

2012

Hummock NA NA NA NA

Hollow(n = 3)

-10 2.2 ± 0.4A 0.1 ± 0.1 0.8 ± 0.3-30 7.3 ± 2.2B 0.0 ± 0.0 3.2 ± 0.8-60 8.5 ± 2.1AB 0.0 ± 0.0 1.7 ± 0.5

ANOVA model Depth 2,15: F = 4.7, P = 0.03 Depth 2,15: F = 1.1, P = 0.37 Depth 2,15: F = 2.5, P < 0.11

2013 Hummock(n = 16) +15* 3.5 ± 1.1A 0.2 ± 0.1 1.0 ± 0.3A

-5 15.3 ± 2.8BC 0.2 ± 0.1 4.1 ± 1.2BC

-10 19.1 ± 2.8CD 0.2 ± 0.0 5.6 ± 1.9BC

-30 22.8 ± 2.7CD 0.2 ± 0.0 6.6 ± 1.6C

Hollow(n = 16) -10 9.5 ± 2.1B 0.1 ± 0.0 2.0 ± 0.5AB

-30 19.3 ± 3.1BC 0.2 ± 0.1 4.0 ± 0.9BC

ANOVA modelDepth 3,90: F = 28.4, P < 0.0001Topography 1,90: F= 11.9, P = 0.0009D × T 1,90: F =4.4, P = 0.04

Depth 3,90: F = 0.3, P = 0.80Topography 1,90: F = 0.5, P = 0.48D × T 1,90: F = 3.5, P = 0.06

Depth 3,90: F = 10.0, P < 0.0001Topography 1,90: F = 1.7, P = 0.20D × T 1,90: F = 0.6, P = 0.44

Notes: Data are from ion-exchange resins incubated from 21 June to 25 July in 2011 (two ~biweekly incubations were summed) and 21 June to 24 July in 2012 (~one month-long incubation), and 25 June to 23 July in 2013 (or 19 June to 15 July for second set, ~one month-long incubation for each array of six access tubes, which were averaged within a plot, and then averaged across plots). Shown are the average availability of NH4-N, NO3-N, and PO4, standardized to the resin surface area and a 30-day timeframe, with a standard error given ‘n’ number of resins incubated per depth increment. The ANOVA model for each statistical comparison (2011-2012 or 2013) is shown below the observations, where the subscripted numbers are the numerator and denominator degrees of freedom; also shown are the F-statistics and P-values. Observed increases in nutrient availability with depth were significant (P < 0.05) across 2011 and 2012 (repeated-measures ANOVA) for ammonium, but not nitrate or phosphate. In 2013, ammonium availability increased with peat depth, though the magnitude of the increase was strongest in hummock microtopography (depth × microtopography interaction). Nitrate availability did not change significantly with peat depth or microtopography, while phosphate increased with depth but did not differ across hummock-hollow microtopography. There were no interactions between depth and microtopography for nitrate or phosphate. Differing upper case

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letters indicate significant differences among depth increments, across hummock and hollow topography, within a given year.

*Positive values in the depth column are shallower than the surface of the hollow, and negative values are deeper than the surface of the hollow, which was set to ‘0 cm’ (the surface of the hummocks was on average 25 cm above the hollows across all resin access tubes sampled in 2013).

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Online Resource 9 Average areal percent cover of hummock and hollow microtopography in the S1 bog.

Transect 1 Transect 2 Transect 3 Average

Hummock 63.7 ± 12.0 61.6 ± 15.2 59.9 ± 18.7 61.7 ± 15.1

Hollow 36.3 ± 12.0 38.4 ± 15.2 40.1 ± 18.7 38.3 ± 15.1

Notes: Means ± 1 standard deviation of areal cover of hummocks and hollows estimated in each of ten 4 m × 4 m plots along three 60-m transects.

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