stocks, chemistry, and sensitivity to climate change of dead

STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE

OF DEAD ORGANIC MATTER ALONG THE CANADIAN BOREAL

FOREST TRANSECT CASE STUDY

C. M. PRESTON1, J. S. BHATTI2, L. B. FLANAGAN3 and C. NORRIS1

1Pacific Forestry Centre, Natural Resources Canada, Victoria, BC Canada V8Z 1M5E-mail: [email protected]

2Northern Forestry Centre, Natural Resources Canada, Edmonton, AB, Canada T6H 3S53Department of Biological Sciences, University of Lethbridge, Lethbridge, AB, Canada T1K 3M4

Abstract. Improving our ability to predict the impact of climate change on the carbon (C) balance ofboreal forests requires increased understanding of site-specific factors controlling detrital and soil Caccumulation. Jack pine (Pinus banksiana) and black spruce (Picea mariana) stands along the BorealForest Transect Case Study (BFTCS) in northern Canada have similar C stocks in abovegroundvegetation and large woody detritus, but thick forest floors of poorly-drained black spruce stands havemuch higher C stocks, comparable to living biomass. Their properties indicate hindered decompositionand N cycling, with high C/N ratios, strongly stratified and depleted δ13C and δ15N values, highconcentrations of tannins and phenolics, and 13C nuclear magnetic resonance (NMR) spectra typicalof poorly decomposed plant material, especially roots and mosses. The thinner jack pine forest floorappears to be dominated by lichen, with char in some samples. Differences in quantity and quality ofaboveground foliar and woody litter inputs are small and unlikely to account for the contrasts in forestfloor accumulation and properties. These are more likely associated with site conditions, especiallysoil texture and drainage, exacerbated by increases in sphagnum coverage, forest floor depth, andtannins. Small changes in environmental conditions, especially reduced moisture, could trigger largeC losses through rapid decomposition of forest floor in poorly drained black spruce stands in thisregion.

1. Introduction

Worldwide, nearly half of forest ecosystem carbon (C) is found in boreal forests,with less than half of this C as living biomass and the greater proportion in soiland detrital pools (Bhatti et al., 2002, 2003; Goodale et al., 2002; Kurz and Apps,1999). However, there are large variations in C density within boreal forests, thatare related not only to climatic gradients, but also to factors such as vegetation,stand age, disturbance history, topography, aspect and soil type (Bhatti et al., 2002;Hobbie et al., 2000; McGuire et al., 2002; Yu et al., 2002). The Boreal ForestTransect Case Study (BFTCS) in central Canada (Price and Apps, 1995) providesan opportunity to examine the interacting factors controlling stand developmentand C storage in an area expected to be highly sensitive to climate change (Bhattiet al., 2003; Camill, 1999; Hogg et al., 2002; Price et al., 1999a,b).

Within the transect, site conditions, mainly soil texture, drainage and presenceof permafrost, and their subsequent interactions with vegetation, result in a mosaic

Climatic Change (2006)DOI: 10.1007/s10584-006-0466-8 c© Springer 2006

C. M. PRESTON ET AL.

of forest and wetlands (Price and Apps, 1995; Harden et al., 1997; Trumbore andHarden, 1997; Gower et al., 1997). Fire is also a major disturbance and driver of Cdynamics in this large, remote, mainly unmanaged area (Harden et al., 2000; Amiroet al., 2001); globally, it is estimated that up to one-third of boreal forest primaryproductivity is consumed by fire (Harden et al., 2000; Wirth et al., 2002).

Recent assessments of the C balance and sink potential of boreal forests haveemphasized the need for better information on litter, root and soil C (Schulze et al.,1999; Goodale et al., 2002; Liski et al., 2003), as well as a broader understandingof the physical, chemical and biological mechanisms that control C sequestrationin soil organic matter (Hobbie et al., 2000). This should reduce the uncertainty inpredicting release of CO2 through increased decomposition of soil C, especially atnorthern latitudes (Hobbie et al., 2000; Kirschbaum, 2000; Sanderman et al., 2003).

Despite their importance in the global C cycle, there is little information on theorganic chemistry of litter, forest floor and mineral soil of boreal regions, with lim-ited application of the geochemical techniques widely used for soil and sedimentcharacterization (e.g., Preston, 1996, 2001; Hedges et al., 2000; Kogel-Knabner,2000, 2002; Schmidt and Noack, 2000). Previous studies of soil organic matterchemistry within the BFTCS (Preston et al., 2002b) and similar forest ecosystems(Lorenz et al., 2000; Czimczik et al., 2003) indicated some distinct characteristicsassociated with high influence of mosses and lichens, accumulation of forest floortannins in poorly drained sites, presence of black (pyrogenic) C, and strong strati-fication of δ15N values in thick black spruce (Picea mariana) forest floor. Withinthe BFTCS, it may be expected that the potential response to changing climate willbe sensitive to site-specific differences in C chemistry (Hobbie et al., 2000).

This project started in 2000 to assess C stocks and fluxes along the BFTCS, insupport of enhanced modelling and carbon accounting tools for Canadian forests.We report here C stocks in tree biomass, woody detritus, and forest floor, andannual litter inputs for jack pine (Pinus banksiana) and black spruce sites along theBFTCS. We have used several chemical techniques to link the striking differencesin these forests to molecular-level chemistry of their litterfall and forest floor.

2. Methods

2.1. FIELD SITES

The BFTCS (Figure 1) (Price and Apps, 1995; Halliwell and Apps, 1997)from Prince Albert, Saskatchewan (53◦11.7′N, 106◦13.9′W) to Gillam, Manitoba(56◦25.2′N, 94◦16.1′W) is along the original Boreal Ecosystem-Atmosphere Study(BOREAS) transect, and is also one of the high latitude transects of the InternationalGeosphere Biosphere Programme (IGBP) (McGuire et al., 2002). Its orientationis along an ecoclimatic gradient, ranging from agricultural grasslands in south-ern Saskatchewan through the boreal forest in the central portion to the tundra


Figure 1. Location of study sites within the BFTCS.

in northern Manitoba (Yu et al., 2002). Mean annual temperature and precipita-tion at four sites along the transect are shown in Figure 2. In the northeast endof BFTCS, low temperatures limit growth; the southwest is warmer, but growth ismainly limited by low moisture availability (Bhatti et al., 2002).

The dominant tree species are black spruce and jack pine. In the understory, inaddition to black spruce seedlings and saplings, some moss species tend to occurmainly in black spruce stands, including Pleurozium schreberi, Ptilidium crista-castrensis and Hylocomium splendens as well as undifferentiated Sphagnum andother moss species. Jack pine germinates and establishes only in full light on mineralsoils, and thus tends to be found early in succession on fire-disturbed sites. Jack pinestands tend to have shrubs Vaccinium vitis-idaea, V. myrtilloides and Arctostaphylosuva-ursi, mosses Dicranum polysetum and Polytrichum sp., and the reindeer lichensCladonia cornuta, Cladina mitis, and C. stellaris. Alnus rugosa (alder) occurs inthe shrub layer of both stand types.

Twelve field sites (Table I) were established in four areas spanning the length ofthe BFTCS, near Prince Albert and Flin Flon (Saskatchewan), and Thompson andGillam (Manitoba). Sites were located in stands dominated by jack pine or blackspruce, each site comprising three plots of 20 × 20 m2. The Prince Albert Old


Figure 2. Mean annual temperature and precipitation along the BFTCS.

Jack Pine and Old Black Spruce sites (S-TE-OJP, S-TE-OBS), and the ThompsonOld Jack Pine site (N-TE-OJP) are at the original BOREAS Southern and Northernflux tower sites. Our plots are just outside the footprints of the towers, whichare still operating as part of the FLUXNET-Canada research network evaluatingecosystem CO2 and H2O exchange. The Flin Flon plots (F-JM-1 and F-BM-1) werereestablished (F-JM-2 and F-BM-2) at nearby locations in 2001 due to vandalismof the original plots. Soil temperature at the 5-cm depth was observed continuouslyusing thermistors and a data logger at two points in the Southern Old Jack Pine andOld Black Spruce sites (S-TE-OJP, S-TE-OBS) since 1997.

2.2. ASSESSMENT OF C STOCKS

Aboveground biomass of the trees was determined using allometric equations thatwere developed for jack pine and black spruce (Gower et al., 1997). The tree


TABLE ICharacteristics of study sites along the BFTCS

Ecoclimate Main Site Breast ht. Forest floor Moisture Drainageregion species name age (y) depth (cm) regime class

Prince Albert (Southern)

Mid Boreal Jack Pine S-JIH-4 42 3.9 Xeric Well

Mid Boreal Jack Pine S-TE-OJP 73 3.7 Xeric Moderate

Mid Boreal Black Spruce S-BIH-1 55 6.7 Mesic Poor

Mid Boreal Black Spruce S-TE-OBS 90 21.5 Hygric Poor

Flin Flon

High Boreal Jack Pine/Fir F-JM-1 117 3.5 Mesic Imperfect

High Boreal Black Spruce F-BM-1 121 23.2 Mesic Poor

Thompson (Northern)

High Boreal Jack Pine N-JIL-1 80 1.5 Xeric Very rapid

High Boreal Jack Pine N-TE-OJP 74 6.7 Xeric Rapid

High Boreal Black Spruce N-BIH-1 128 21 Subhygric Poor

High Boreal Black Spruce N-BMH-7 120 15 Subhygric Poor

Gillam

High Boreal Jack Pine G-JM-1 78 2.1 Subxeric Rapid

Subarctic Black Spruce G-BM-1 99 9.6 Subhygric Imperfect

dimensions (height H in meters and diameter at breast height D in centimeters)were measured in June 2001 for the three plots on each site. The abovegroundbiomass of individual trees was calculated using the allometric equations. The sumof tree biomass for all the trees corresponds to the aboveground biomass in the plot,and the average for the three plots provides the average aboveground biomass forthe site. The stand age was determined by the dendrochronological method; i.e. thetree rings of 10–15 sample trees.

At each site litter is collected in 36 plastic buckets of 27.3 cm diameter and 30 cmheight with mesh bottoms, and 27 mesh quadrats laid on the forest floor (1 m2 with 1cm2 openings). The latter were designed to capture the coarser material, especiallytwigs. The traps were installed in June 2000, and are emptied once a year. Litter washand-sorted into needles, twigs, bark, male cones, female cones and cone flakes,and deciduous leaves. Annual litterfall values reported here are based on two orthree years of collection. Litter trap data from the Flin Flon and Gillam sites forthe first collection year (2000–2001) were not used in the C stock determination,although the samples were used for litter characterization. Area-based litter massinputs were converted to C, N and tannin inputs using the mean values for eachlitter type and species.

Coarse woody debris (CWD) mass was measured using three 1 × 10 m2 tran-sects in each plot. The LFH samples were collected from three locations using a


20 × 20 cm2 template in each of the triplicate plots. The samples excluding the livevegetation and roots were oven-dried at 70 ◦C to constant weight. Samples wereanalysed for total C by dry combustion using a LECO Carbon Determinator CR12.

2.3. SAMPLING FOR LITTER AND FOREST FLOOR CHEMICAL CHARACTERIZATION

In addition to the survey to determine C stocks, forest floor for more intensivechemical characterization was sampled in summer 2000 from plots in the PrinceAlbert, Thompson and Gillam areas. Jack pine forest floor was sampled as a singlelayer, except for the southern immature stand (S-JIH-4), which was separated intoL and H horizons. The thicker forest floor of the black spruce sites was sampledby L, F, and H layers, except for S-BIH-1 which was sampled as a single layer. Allplots were sampled in duplicate, but for two of the black spruce sites, N-BMH-7and G-BM-1, the H horizon was only represented by a single sample.

After removal of larger pieces such as branches, cones and coarse roots, thesamples were oven-dried at 70 ◦C for 24 h, and ground in a Wiley mill to 2 mm, andthen to 60 mesh (250 μm). The coarse roots picked from the black spruce sampleswere dried and ground in the same way, although yields were not quantified.

Litter collected from the traps in June 2001 was used for the intensive chemicalcharacterization. Twigs and bark flakes were combined into one category, as weremale and female cones. Leaves from understorey species were available in sufficientquantity for analysis from five sites. Needles and twigs were combined to give threesamples per plot, although in some cases, samples were only available from twosubplots. The smaller amounts of cones were combined into one sample per plot.Subsamples of leaves were kept separate for F-JM-1 and combined for G-JM-1and N-TE-OJP. Samples were dried and ground as described earlier. Because of thegreater difficulty in grinding the tough fibers of litter materials, and the small sizeof many samples, all litter samples were then finely ground in a Retch Mixer Mill 2,facilitated by freezing the loaded capsules in liquid nitrogen just prior to grinding.

2.4. CHEMICAL, ISOTOPE AND NMR ANALYSIS

Samples were analysed for total C by dry combustion using a LECO Carbon Deter-minator CR12. Total (N) was analysed by a semimicro-Kjeldahl method (Prestonet al., 2002a). Condensed tannins were analysed as the sum of extractable andresidual fractions using the butanol/HCl assay, and for total phenolics using theFolin-Ciocalteu assay. As described fully elsewhere (Lorenz et al., 2000), sampleswere extracted twice with acetone:water (70:30 by volume) and extractable tanninsdetermined on an aliquot of the combined extracts. Residual tannin was then deter-mined by direct hydrolysis of the extraction residue. As previously, the assay wasstandardized against tannin purified from branch tips of balsam fir (Abies balsamea(L.) Mill.), as black spruce and jack pine tannins were not available.


Total phenolics were determined by taking an aliquot of the same extract anddrying it down with air in a test tube. To this was added 1.0 ml distilled water, 0.5 mlFolin-Ciocalteu reagent (Sigma), and 2.5 ml sodium carbonate solution (20% w/v).The mixture was then transferred to disposable cuvettes and absorbance measuredat 750 nm on a Beckman DU 520 UV/Vis Spectrophotometer. The assay wasstandardized against catechol.

Samples were analysed for C and N stable isotope ratios (δ13C, δ15N) using anelemental analyser (NC2500, CE Instruments, ThermoQuest Italia, Milan, Italy)coupled to a gas isotope ratio mass spectrometer (Delta Plus, Finnigan Mat, Bremen,Germany) operating in continuous flow mode. Due to the high C/N ratio of thesamples, analyses were run separately, with a run for δ13C followed by one forδ15N. A subsample of ground material was sealed in a tin capsule and loaded intothe elemental analyzer for combustion/reduction. Sample weights were typically 1–3 mg for δ13C and 3–10 mg for δ15N. Water generated by combustion was removedby a magnesium perchlorate trap. The carbon dioxide and nitrogen gases generatedfrom the combustion/reduction process were separated in the gas chromatographcolumn of the elemental analyzer and passed directly via a helium gas carrier streamto the inlet of the mass spectrometer for stable isotope analysis. Standard deviationswere similar for laboratory standards and for representative samples, typically 0.1‰or better for δ13C and 0.1–0.2‰ for δ15N.

Solid-state 13C nuclear magnetic resonance spectra with cross-polarization andmagic-angle spinning (CPMAS NMR) were obtained at 75.47 MHz on a BrukerMSL 300 spectrometer. Dry, powdered samples were packed into a zirconiumoxide rotor of 7 mm OD. Acquisition conditions were: 4.7 kHz spinning rate, 1 mscontact time, 2 s recycle time, and 5000–36 000 scans. Chemical shifts are reportedrelative to tetramethylsilane (TMS) at 0 ppm, with the reference frequency set usingadamantane. Most forest floor horizons were sampled and analysed in duplicate butdue to the time requirements for NMR, spectra were acquired for one of each pairplus a few duplicates. More detailed discussion of NMR methods may be foundelsewhere (Preston, 1996, 2001; Preston et al., 2002a,b).

3. Results and Discussion

3.1. SITE CHARACTERISTICS AND C STOCKS

As shown in Table I, jack pine stands on well-drained sandy soils have a thinner dufflayer than the black spruce stands. Thick forest floor is especially associated withpoorly-drained black spruce stands, and this moss-organic layer plays an importantrole in boreal forests (Weber and Van Cleve, 1984; Bonan and Shugart, 1989; Bonan,1992; Harden et al., 1997; Trumbore and Harden, 1997; Nalder and Wein, 1999;Yu et al., 2002). Its low thermal conductivity and high water-absorbing capacityreduce soil temperatures and maintain high soil moisture contents, thereby reducing


Figure 3. Soil temperature (5 cm depth) for the Southern Old Jack Pine and Old Black Spruce sites(S-TE-OJP, S-TE-OBS).

decomposition, nutrient availability and stand productivity and promoting furtheraccumulation of the organic layer. As shown in Figure 3, soil temperature (5 cmdepth) increases more slowly for the Old Black Spruce (S-TE-OBS) site, with thesummer maximum 5 ◦C below that of the Old Jack Pine (S-TE-OJP) site, althoughthe latter is colder in winter.

Our C stocks in forest floor, detritus (snags plus CWD) and aboveground treebiomass (Figure 4) are in general agreement with other studies from this region. Theaverage aboveground biomass C for jack pine and black spruce was estimated to

Figure 4. Carbon stocks (kg C m−2) of tree biomass, detritus (coarse woody debris and snags) andforest floor along the BFTCS.


be 3.4 and 5.2 kg m−2 respectively, comparable to the estimates reported by Goweret al. (1997) and Nalder and Wein (1999). In general for both species, abovegroundbiomass was higher in the southern study sites as compared to northern study sites.

Litter inputs will be reported later in more detail and for a longer collectionperiod. However, the first 2–3 year results (Figure 5a) indicate similar foliar littermasses for jack pine and black spruce, but higher non-woody litter in black spruce,

Figure 5. Foliar (needles and understorey leaves) and woody (cones, twigs, bark) litter inputs (g m−2)of (a) mass, (b) C, (c) N, and (d) condensed tannins for jack pine and black spruce stands along theBFTCS.


and much greater interannual variation in the amounts of non-woody litter (notshown). Again, our results are in general agreement with other studies in this region.Our mean litter inputs for jack pine (120 g m−2) and black spruce (127 g m−2) arecomparable to those reported by Gower et al. (2000) for their BOREAS southernstudy area sites (JP foliar 86.0 g m−2, JP woody 26.6 g m−2, total 112.6 g m−2,23.6% woody; BS foliar 78.5 g m−2, BS woody 24.3 g m−2, total 102.8 g m−2,23.6% woody), and a little higher than their northern study area sites (JP foliar61.9 g m−2, JP woody 17.0 g m−2, total 78.9 g m−2, 21.5% woody; BS foliar 68.4 gm−2, BS woody 35.4 g m−2, total 103.8 g m−2, 34.1% woody). Our foliar data arealso similar to those calculated from Nakane et al. (1997) for two black spruce sites(64 and 76 g m−2) within the southern study area. They also found woody inputs tobe much more variable, 72 and 30 g m−2, respectively for the two sites. Litterfall(foliar and woody) was 30–60 g C m−2 (approximately 60–120 g m−2) for threeblack spruce stands near the northern study area (Wang et al., 2003; Bond-Lambertyet al., 2004).

3.2. FOREST FLOOR CHEMISTRY

3.2.1. Forest Floor C, N and Stable Isotope AnalysisAs shown in Table II, total C of forest floor ranged from 241–463 mg g−1 in jackpine sites and 235–500 mg g−1 in black spruce, typical of forest floor organichorizons (Lorenz et al., 2000; Vance and Chapin, 2001; Preston et al., 2002a, b;Morrison, 2003). Total C and C/N ratio usually decrease with increasing degreeof decomposition, i.e., from L to F to H horizons (Prescott et al., 1995), but thesetrends were not found consistently for the black spruce sites.

By contrast, the natural abundance of 13C consistently increased from L to F toH for all black spruce sites. The δ13C value for the S-BIH-1 site sampled as onehorizon was −26.4‰, similar to F and H horizons from other sites. The widestspread was 2.5‰ in the S-OBS-F site, with smaller differences from 0.9 to 1.5‰in the other sites. Values of δ13C tended to increase from south to north, but moredata would be required to define trends. In a previous study (Preston et al., 2002b),δ13C for a combined LFH forest floor sample in a black spruce stand near Gillamwas −26.0‰, similar to our value of −25.9‰ for the H horizon of the Gillam site.Values of δ15N showed similar trends, increasing from L to F to H horizons, exceptfor Gillam (G-BM-1) where F was more enriched than H. There was also a trendof increasing values from south to north. The largest range was for N-BIH-1, from−1.0‰ (L) to 3.5‰ (H).

For the jack pine S-JIH-4 site, total C and N decreased from L to H, and δ15N,δ13C and C/N ratio increased. The C/N ratio was lowest in the two Thompsonarea sites. Similar to black spruce sites, both δ15N and δ13C tended to increase, ingeneral, from south to north, except that δ15N values were high for the S-JIH-4site. Values reported here are more depleted than those previously reported for jack


TABLE IIChemical and stable isotope analysis of forest floor samples

Tannins

Site C N E + Ra Phenolics δ13C δ15Nname Horizon (mg g−1) (mg g−1) C/N (mg g−1) E/Sb (mg g−1) ‰ ‰

Jack Pine

S-JIH-4 L 389 8.0 49 10.3 0.60 2.1 −28.7 1.6

H 328 6.5 51 5.4 0.64 1.4 −27.7 2.4

S-TE-OJP cS 241 5.8 41 3.4 0.51 0.7 −27.3 −2.7

N-JIL-1 S 243 8.8 28 1.0 0.33 0.4 −27.5 −1.2

N-TE-OJP S 393 12.1 33 1.0 0.00 1.0 −27.5 −1.4

G-JM-1 S 463 9.4 49 2.8 0.56 1.5 −26.2 2.5

Black Spruce

S-BIH-1 S 409 8.7 47 21.5 0.62 4.7 −26.4 2.4

S-TE-OBS L 480 8.1 59 21.4 0.48 3.2 −28.8 −1.2

F 470 7.2 65 11.9 0.46 1.7 −27.7 −0.4

H 396 11.0 36 1.9 0.36 0.5 −26.3 1.6

N-BIH-1 L 333 7.1 47 12.6 0.54 1.9 −27.2 −1.0

F 465 7.7 60 15.7 0.47 2.5 −26.4 1.4

H 330 6.3 52 9.4 0.49 1.8 −26.2 3.5

N-BMH-7 L 500 6.9 73 28.9 0.45 5.6 −27.1 −0.3

F 484 6.3 77 23.8 0.43 3.7 −26.5 1.5

H 235 5.1 46 2.8 0.48 0.5 −25.6 2.6

G-BM-1 L 482 7.5 64 16.0 0.53 2.5 −27.3 1.1

F 427 8.1 53 10.7 0.50 2.1 −26.4 4.5

H 272 8.2 33 3.0 0.65 0.7 −25.9 3.7

aSum of extractable (E) and residual (R) tannins.bRatio of extractable (E) to sum of extractable and residual (S) tannins.cS: thin forest floor sampled as a single horizon.

pine forest floor from sites near Prince Albert and Thompson (−26.7 and −26.6‰,respectively (Preston et al., 2002b)).

Few other data are available from this region, although our values are similar tothose from black spruce stands in the BOREAS southern study area (Flanagan et al.,1999), and boreal forest sites in northwestern Canada (Bird et al., 2002b), Alaska(Schuur et al., 2003), and central Siberia (Bird et al., 2002a). A general tendencyfor δ15N and δ13C to increase with depth has been widely observed (Nadelhofferand Fry, 1988; Hogberg et al., 1996; Buchmann et al., 1997; Ehleringer et al., 2000;Feng, 2002; Schuur et al., 2003). The pattern can become particularly striking incolder forest soils, where slow decomposition and lack of earthworm activity resultin a thick forest floor with highly depleted values, and increasing enrichment with


depth in the mineral soil. The forest floor then largely reflects the depleted isotopicsignature of the litter inputs, whereas the deeper mineral horizons and especially thefiner size fractions (Preston et al., 2002b; Bird et al., 2002b) reflect the enrichmentattributed mainly to conversion of plant residues to microbial biomass. For N, thistrend can be reinforced by plant uptake of organic forms of N, short-circuiting thecycle of immobilization and mineralization to ammonium and nitrate (Hogberget al., 1996; Northup et al., 1998). For black spruce, the increases of δ15N and δ13Cfrom L to F to H horizons are likely associated with N-limitation, tight N-cycling andlimited microbial transformation of plant inputs (Hogberg et al., 1996; Amundsonet al., 2003). Two other factors may influence the isotopic composition of the forestfloor. In addition to N2 fixation by alder, δ15N values may be influenced by N2

fixation in feather moss and lichens (Dawson, 1983; DeLuca et al., 2002b). Thelow δ13C values, especially in the L layers, are due in part to high inputs of mosses,which have a more depleted 13C signature than boreal tree and shrub components(Brooks et al., 1997; Flanagan et al., 1999; Schuur et al., 2003).

3.2.2. Forest Floor Tannins and PhenolicsCondensed tannins are polymers that bind proteins, and occur widely in foliage,bark and roots of many higher plants, often in higher concentrations than lignin(Preston, 1999; Lorenz et al., 2000; Hernes et al., 2001; Kogel-Knabner, 2002;Kraus et al., 2003a,b). Note that we use the term lignin only for the polymer basedon phenylpropane units, not for the operationally-defined acid-insoluble residueof proximate analysis, often referred to in the ecological literature as lignin orKlason lignin (Preston et al., 2000; Kogel-Knabner, 2002). The black spruce siteshad high tannin levels, up to 28.9 mg g−1 for the sum of extractable and residualfractions, and generally decreasing with depth. Lower concentrations were found inthe jack pine sites, with the highest values for S-JIH-4, with two distinct forest floorhorizons. For both black spruce and jack pine stands, extractable tannins comprisedaround 35–64% of the total. Tannin concentrations in forest floor are typically <10mg ml−1 (Preston, 1999; Kranabetter and Banner, 2000; Lorenz et al., 2000), andthen mostly in the residual form, or so low that they are only detected by directhydrolysis of the total sample.

Condensed tannin analysis is best standardized against tannins from the sameplant species, as structural variations affect the response to the butanol/HCl assay(Kraus et al., 2003a). As jack pine and black spruce standards were not available,we used tannin purified from balsam fir. Some of the tannins in forest floor maybe derived from understorey species, which could cause further variability in theassay response. Therefore, while the absolute values for black spruce versus jackpine forest floor should be compared with caution, it is unlikely that this wouldobliterate the differences between the ecosystems. The black spruce forest floorsamples also tend to be higher in total phenolics, and total phenolics and tanninswere highly correlated for both stand types (not shown). Second, as discussed later,the differences in tannin concentrations are consistent with the NMR spectra and


finally, high tannin concentrations were also found in two black spruce stands innorthern Ontario (Lorenz et al., 2000).

The influence of tannins in hindering decomposition and reducing N availabilityseems to be exacerbated in nutrient-limited ecosystems, where nutrients can becomeincreasingly sequestered in thick organic horizons, in some cases resulting in thedominance of ericaceous shrubs over trees (Kraus et al., 2003b). Reduction ofenzyme activity by tannins has been suggested as a factor in low N availabilityand microbial activity in Alaskan forests (Flanagan and Van Cleve, 1983; Vanceand Chapin, 2001). Tannins may be also responsible for the inhibition of methaneconsumption attributed to unidentified phenolics in aqueous extracts of forest soils,including samples from aspen and jack pine sites in the BOREAS northern studyarea (Amaral and Knowles, 1997). The high tannin levels may result from poordrainage in black spruce stands along the transect, as the usual very low levelswere found at drier black spruce sites in northern Quebec (unpublished results).The particular structure of black spruce tannin (procyanidin), with two OH groupson the B ring (Lorenz and Preston, 2002) may also contribute to its persistence, asthe prodelphinidin variant with three OH groups is more susceptible to reaction ofthe B-ring with soil organic matter Hernes et al. (2001).

3.3. LITTER CHEMISTRY

3.3.1. Litter Carbon and NitrogenPoor quality and slow decomposition of black spruce forest floor has long beenconsidered a factor in low rates of nutrient cycling (Flanagan and Van Cleeve, 1983;Vance and Chapin, 2001). Therefore it is important to see whether differences inforest floor properties reflect differences in quality of aboveground litter inputs(Table III). As expected for plant materials, all litter samples are over 500 mg g−1

C, and most have high C/N ratios >100. For the Flin Flon sites, litter needle Nconcentrations (7.7 mg g−1 for F-JM-1, 6.9 mg g−1 for F-BM-1) were similar toreported values for black spruce in Alaska (6.2 mg g−1, Flanagan and Van Cleve,1983) and jack pine in northern Ontario (6.5 mg g−1, Morrison, 2003). They arealso comparable to old foliage (i.e., attached non-current needles) of jack pine andblack spruce at the BOREAS study areas (6.0–7.6 mg g−1) reported by Gower et al.(2000), except for their higher value for jack pine (1.02 mg g−1) at the southernstudy area. For the other sites, however, our litter needle N concentrations aregenerally lower (3.4–5.3 mg g−1), with the lowest values (<4 mg g−1) for jackpine. These preliminary data do not indicate substantive differences in litter qualityof jack pine versus black spruce with regard to total N and C/N ratios. Leaves fromthe understorey species had the highest N and lowest C/N ratios, but these representa very small proportion of the litter inputs.

There were wide variations in the δ15N and δ13C values, although the range ofδ13C was within that expected from the C3 photosynthesis pathway utilized by most


TABLE IIIChemical and stable isotope analysis of littertrap samples

Tannins

Site C N E + Ra Phenolic δ13C δ15Nname (mg g−1) (mg g−1) C/N (mg g−1) E/Sb (mg g−1) ‰ ‰

(a) Jack Pine

NeedlesS-JIH-4 540 3.4 157 25.9 0.72 14.1 −28.5 −4.0

S-TE-OJP 563 3.9 144 41.8 0.70 15.5 −27.9 −4.0

F-JM-1 545 7.7 71 12.5 0.39 12.7 −28.0 −4.2

N-JIL-1 546 3.9 140 17.5 0.42 12.4 −28.2 −3.2

N-TE-OJP 555 3.7 151 17.5 0.42 13.3 −28.1 −3.5

G-JM-1 573 4.1 138 45.3 0.72 22.2 −28.0 −2.7

Mean JP N 554 4.5 124 26.8 0.63 15.0 −28.1 −3.6

Twigs

S-JIH-4 519 4.6 113 12.2 0.69 9.4 −28.2 −6.1

S-TE-OJP 537 4.3 125 9.9 0.59 8.1 −27.1 −5.9

F-JM-1 544 4.7 115 5.9 0.44 12.8 −27.5 −4.4

N-JIL-1 520 4.4 119 3.9 0.31 7.3 −27.6 −4.6

N-TE-OJP 538 3.9 136 3.0 0.14 6.1 −27.7 −5.8

G-JM-1 559 3.7 153 6.4 0.39 7.9 −28.3 −3.2

Mean JP T 536 4.3 126 6.9 0.51 8.6 −27.7 −5.0

Cones

S-JIH-4 518 2.9 177 67.8 0.86 26.2 −28.4 −4.6

S-TE-OJP 538 6.4 84 40.0 0.61 11.9 −27.4 −4.2

F-JM-1 520 3.1 168 14.0 0.51 11.8 −25.5 −5.0

N-JIL-1 528 5.0 105 18.5 0.29 9.7 −27.5 −3.4

N-TE-OJP 529 4.3 122 49.1 0.65 15.0 −26.5 −3.3

G-JM-1 534 2.4 227 101.9 0.81 23.2 −27.9 −2.7

Mean JP C 528 4.0 131 48.6 0.72 16.3 −27.2 −3.9

Leaves

F-JM-1 520 11.0 47 11.8 0.11 11.1 −29.5 −3.4

N-TE-OJP 556 14.7 38 4.6 0.00 8.1 −28.4 −4.3

G-JM-1 530 19.3 27 7.0 0.26 7.7 −29.4 −1.6

Mean JP L 535 15.0 36 7.8 0.13 9.0 −29.1 −3.1

(b) Black Spruce

NeedlesS-BIH-1 528 4.7 97 26.4 0.64 10.7 −27.3 −2.1

S-TE-OBS 543 4.7 117 28.5 0.51 12.1 −28.0 −5.1

F-BM-1 547 6.9 79 15.4 0.45 16.9 −27.2 −3.3

N-BIH-1 544 4.3 126 17.0 0.45 20.8 −27.3 −4.9

(Continued on next page)


TABLE III(Continued)

Tannins

Site C N E + Ra Phenolic δ13C δ15Nname (mg g−1) (mg g−1) C/N (mg g−1) E/Sb (mg g−1) ‰ ‰

N-BMH-7 541 5.3 103 15.3 0.30 16.0 −27.2 −5.6

G-BM-1 549 4.4 126 17.0 0.49 18.6 −27.9 −6.2

Mean BS N 542 5.0 108 19.9 0.49 15.9 −27.5 −4.5

Twigs

S-BIH-1 519 4.8 108 19.1 0.79 14.3 −26.9 −4.4

S-TE-OBS 549 5.2 105 40.2 0.86 20.4 −26.5 −6.7

F-BM-1 540 4.7 114 10.5 0.58 16.1 −25.8 −4.8

N-BIH-1 545 4.2 131 11.2 0.58 18.4 −26.2 −5.6

N-BMH-7 542 5.3 102 9.8 0.45 16.0 −26.0 −5.7

G-BM-1 557 5.1 110 11.0 0.50 16.4 −26.5 −5.8

Mean BS T 542 4.9 111 17.0 0.71 16.9 −26.3 −5.5

Cones

S-BIH-1 529 6.0 105 98.9 0.79 21.5 −26.2 −3.0

S-TE-OBS 551 3.7 149 153.7 0.83 24.1 −26.2 −5.5

F-BM-1 577 3.1 185 12.3 0.25 9.5 −25.2 −5.4

N-BIH-1 550 2.4 227 122.4 0.72 17.8 −26.2 −6.3

N-BMH-7 554 3.9 141 131.8 0.69 17.7 −26.3 −5.8

G-BM-1 553 3.2 174 141.3 0.81 22.2 −26.2 −7.2

Mean BS C 552 3.7 148 110.1 0.76 18.8 −26.0 −5.5

Leaves

F-BM-1 546 12.1 45 7.6 0.00 6.9 −29.2 −0.9

S-BIH-1 509 8.8 58 7.7 0.04 5.1 −29.3 −0.9

Mean BS L 527 10.4 51 7.7 0.02 6.0 −29.2 −0.9

aSum of extractable (E) and residual (R) tannins.bRatio of extractable (E) to sum of extractable and residual (S) tannins.

trees, shrubs and temperate grasses (Brooks et al., 1997; Buchmann et al., 1997).For both stand types, needles were in general more depleted in δ13C than twigs andcones, as also found by Schuur et al. (2003) for black spruce, and understory alderleaves had the most depleted δ13C values.

For jack pine litters, values of δ15N tended to increase from south to north,whereas the opposite was true for black spruce litter, in contrast to the trend ofincreasing δ15N in black spruce forest floor. The spread of δ15N values was thusgreatest for Gillam black spruce (G-BM-1), with litters between −5.8 and −7.2‰,and forest floor horizons between 1.1 and 4.5‰. For both forest types, δ15N valueswere greater for forest floor than for litters, regardless of litter type or horizon, butthere was more overlap of values for δ13C. Compared to other litter types from the


same site, the few samples of alder leaves were lowest in δ13C and most enrichedin δ15N. As noted previously for mosses, the lower δ13C of the shrub alder leavescan be related to life form and their lower position in the canopy (Brooks et al.,1997). Their higher δ15N values and total N concentrations are likely the resultof fixation of atmospheric N2, with a higher δ15N than soil N (Vogel and Gower,1998).

Some coarse roots were removed during processing of the black spruce forestfloor samples. This was not done on a quantitative basis, nor was there any identifi-cation by species, and material from duplicate samples was combined for analysis.We present the results (Table IV) simply because there is very little informationon the organic chemistry of roots. Similar to the litter samples, the coarse rootswere over 500 mg g−1 C, and most had C/N ratios >100. The δ15N values of rootswere always lower that those of the corresponding forest floor, and for the mostpart, higher than the aboveground litter samples from the same site. The maximumdifference between roots and forest floor (7.7‰) was found for the F layer of theGillam site (G-BM-1). For δ13C, root values were similar to those of litters andforest floor, and no obvious pattern was seen between the δ13C values of the rootsversus forest floor.

TABLE IVProperties of coarse roots from black spruce forest floor, and char from jack pine forest floor

Tannins

Site C N E + Ra Phenolic δ13C δ15Nname Horizon (mg g−1) (mg g−1) C/N (mg g−1) E/Sb (mg g−1) ‰ ‰

Coarse RootsS-TE-OBS L 530 6.2 85 126 0.81 34.6 −28.0 −3.4

F 534 4.1 132 131 0.78 39.3 −27.1 −3.9

H 514 4.8 108 100 0.91 34.8 −26.5 −1.7

N-BIH-1 L 503 6.1 83 107 0.79 39.1 −26.5 −4.9

F 532 4.7 113 144 0.80 41.7 −26.3 −3.6

H 525 4.5 117 104 0.77 33.5 −26.7 −1.9

N-BMH-7 L 514 4.2 123 152 0.77 50.1 −27.1 −5.7

F 540 4.3 127 132 0.77 45.9 −27.1 −4.7

H 531 4.6 116 107 0.80 35.6 −27.1 −3.7

G-BM-1 L 548 4.5 121 92 0.89 30.6 −27.7 −2.2

F 540 4.2 130 108 0.88 37.9 −27.2 −3.2

Char

G-JM-1-1 T 605 8.2 74 0.40 0.22 3.8 −25.3 2.3

G-JM-1-2 T 605 3.5 175 0.00 0.00 4.9 −25.0 0.3

aSum of extractable (E) and residual (R) tannins.bRatio of extractable (E) to sum of extractable and residual (S) tannins.


3.3.2. Litter Tannins and PhenolicsTannin concentrations in littertrap samples were highly variable, with the highestindividual values found in cones, up to 102 mg g−1 for jack pine and 154 mg g−1

for black spruce. The percentage of extractable tannin also varied widely (4–86%).The very wide range for cones, with some samples as low as 12 mg g−1, may reflectdifferences in the age of cones entering the traps, or in the proportion of male conesthat are lower in tannins (unpublished results). Black spruce needle litter tanninconcentrations (17–29 mg g−1), were similar to those reported previously (25 and38 mg g−1, Lorenz et al., 2000).

Black spruce coarse roots (Table IV) were also high in tannins, with 77–91%extractable by acetone/water. It is unlikely that the black spruce values are unusuallyhigh for coarser roots, as discussed later with the NMR results. Analysis of fine rootsfrom jack pine and black spruce sites is in progress, and future studies should includeanalysis of the inputs from lichens and mosses, nonvascular plants that produce awide range of phenolic secondary compounds (Fahselt, 1994; Ingolfsdottir, 2002),but no lignin or tannins (Wilson et al., 1989; Williams et al., 1998).

3.3.3. Litter InputsAnnual inputs of litter C and N (Figure 5b and c) reflect the patterns of mass inputs(Figure 5a), with black spruce generally having both total inputs, and a higherproportion in woody forms. Thus, in terms of litter quality, black spruce standshad higher inputs of N, but higher inputs of woody components and of tannins(Figure 5d).

Ongoing work includes analysis of the subsequent littertrap collections andcalibration of tannin concentrations with black spruce and jack pine tannins. Thesecond and third litter collections have also been separated into more categories,including bark flakes and male cones. However, it is clear that litters of both speciescontain substantial amounts of tannins, including extractable fractions, and that thedifferences in forest floor stocks and properties are unlikely to be accounted for bythe relatively small differences in quantity and quality of the litter components weexamined.

3.4. ORGANIC CARBON CHARACTERIZATION

Carbon-13 CPMAS NMR has been widely used for direct characterization of soilorganic matter and litter (Preston, 1996, 2001; Preston et al., 2000, 2002a; Kogel-Knabner, 2000; Quideau et al., 2001; Dignac et al., 2002; Sjogersten et al., 2003).NMR spectra show intensity versus chemical shift, expressed as ppm of the obser-vation frequency, relative to the chemical shift of a standard compound. Chemicalshift reflects the electronic environment of the carbon bonds, and increases withincreasing electronegativity and unsaturation. For 13C the main chemical-shift re-gions are ascribed to C in alkyl (0–47 ppm), O- and di-O-alkyl (47–112 ppm),


aromatic (112–140 ppm), phenolic (140–160 ppm), and carboxyl, ketone and ester(160–215 ppm) structures. We present spectra of forest floor and litter componentsof the Southern Old Jack Pine and Old Black Spruce sites, which were typical foreach forest type. Assignments and general interpretations are based on many pre-vious studies as listed earlier, and as the main features are typical of organic matterspectra, they will not be described in great detail.

3.4.1. Forest Floor NMR – Black SpruceSpectra of the L, F, and H layers of the Southern Old Black Spruce site (S-TE-OBS)are shown in Figure 6(a–c). The L and F spectra are very similar, with the largestpeak at 73 ppm in the O-alkyl region, mainly due to carbohydrate. The sharp peakat 105 ppm is also mainly due to the C1 of carbohydrate. The peak in the alkylregion (0–47 ppm) has two maxima, at 30 and 33 ppm, characteristic of CH2 in longchains, while the underlying broader intensity is due to a variety of CH, CH2 andCH3 (methyl) structures. The aromatic and phenolic regions are weak, with featurestypical of tannins rather than lignin, especially the phenolic region, with peaks at145 and 155 ppm. Low lignin content is also consistent with the very weak methoxylsignal at 57 ppm, which appears as a shoulder on the O-alkyl peak. Carboxyl, amideand ester C produce the peak at 174 ppm. This region is mainly associated with theamide C of proteins and the carboxyl groups of microbial and plant lipids. The Land F spectra are typical of poorly decomposed material, retaining the sharp peaksof plant litter inputs, as discussed in the next section. Similar NMR spectra, andhigh tannin contents were found for the forest floor of two black spruce sites innorthern Ontario (Lorenz et al., 2000).

The H horizon shows loss of O- and di-O-alkyl intensity, relative to the otherregions, and broadening of signals mainly in the aromatic and phenolic regions. Thisis typical of increasing decomposition, but without the expected increase in alkyl/O-alkyl ratio. The difference in the H horizon may also be due to greater influence ofCWD, which becomes higher in aromatic (lignin) C with increasing decomposition(Preston et al., 2002a). This detrital pool ranged from 0.15 to 1.91 kg C m−2 inour sites (detritus component, Figure 4), and for jack pine was comparable to theforest floor C pool. In addition to visible CWD, old buried, decomposed logs havea long-term influence on forest floor chemistry (Prescott et al., 1995; Preston et al.,2002a).

The spectrum of coarse roots from the F horizon is shown in Figure 6d; the L andH root samples were similar. The coarse roots mainly differed from L and F forestfloor in having lower alkyl C. Peaks characteristic of tannin were also very clear,consistent with the high tannin contents measured chemically. While our resultsrepresent a limited sampling, they are consistent with other NMR (Preston et al.,2002a; Rosenberg et al., 2003) and histochemical (McKenzie and Peterson, 1995)studies of roots.

The L and F spectra of this site thus retain many characteristics of the litterand root inputs, as did H spectra of some of the other black spruce sites. Lignin


Figure 6. 13C CPMAS NMR spectra of L, F, and H horizons from the Southern Old Black Sprucesite (a, b, c) and coarse roots from F horizon (d).

contents appear to be low for all litter, root, and forest floor samples, with aromaticcomponents derived substantially from tannins. Feathermoss and sphagnum inputsare substantial for these black spruce ecosystems (Harden et al., 1997; Trumboreand Harden, 1997; Flanagan et al., 1999; Hobbie et al., 2000; Yu et al., 2002).Similar to the lichen spectrum presented in the next section, sphagnum is high in


carbohydrate, and very low in aromaticity (Bergman et al., 2000). The NMR spectraof the forest floor are more sharply resolved and even lower in aromatic C than thelitterfall inputs shown in the next section, as previously found for two black sprucestands in Ontario (Lorenz et al., 2000). The L and F forest floor spectra shown, andsimilar ones obtained in this study are therefore consistent with a high proportionof poorly decomposed mosses and black spruce roots.

3.4.2. Forest Floor NMR – Jack PineThe forest floor from the Southern Old Jack Pine site (S-OJP, Figure 7a) was veryhigh in O-alkyl (mainly carbohydrate) C, and remarkably low in aromatic andphenolic C. Similar spectra were previously observed for jack pine forest flooralong the BFTCS (Preston et al., 2002b), and for Scots pine in Siberia (Czimcziket al., 2003). This may indicate the dominance of lichen in the accumulation oforganic matter in the forest floor of these ecosystems. Similar to previous reports(Preston et al., 2000; Czimczik et al., 2003), the spectrum of lichen (Figure 7b) isvery high in O- and di-O-alkyl C, indicating mainly carbohydrate structures. Theapparent dominance of lichen structures may be due to low amount of rooting inthe forest floor, and rapid decomposition of surficial jack pine litter.

In addition to plant detrital inputs, black C generated in forest fires may con-tribute to a long-lasting pool of recalcitrant organic matter (Harden et al., 1997,2000; Schulze et al., 1999; Hobbie et al., 2000; Wirth et al., 2002). The aromaticstructures produced by charring have a broad peak at 125–130 ppm (Baldock andSmernik, 2002; Czimczik et al., 2002, 2003; Preston et al., 2002a, b). This wasobserved in spectra of the lower (H) horizon of S-JIH-4, and the Gillam site (G-BM-1), confirmed by spectra of duplicate samples. Figure 7c shows one of the samplesfrom G-BM-1 along with the spectrum of charred woody fragments picked fromthe sample (Figure 7d).

3.4.3. Litter NMRSpectra were obtained for the needle, twig, and cone litters from the southern fluxtower sites, as shown in Figure 8(a–c) for black spruce and Figure 8(d–f) for jackpine. All spectra are dominated by carbohydrate peaks at 73 and 105 ppm, anddiffer mainly in the proportion of alkyl C (0–47 ppm), and degree of splitting ofthe phenolic region (140–165 ppm). The needle spectra are similar, except thatphenolic peaks are better resolved for black spruce. The black spruce needles havea split peak at 145 and 155 ppm more characteristic of tannin, whereas for jackpine, the peak at 147 pm with a shoulder at 152 ppm indicates a higher proportionof lignin. The black spruce twig litter is higher in alkyl C, probably due to a higherproportion of bark. The jack pine twig sample was the lowest in aromatic andphenolic C, and also the lowest in tannin content (5 mg g−1) for this subsample.Spectra of cones for the two species were similar.

Spectra of the needle litter are similar to those shown elsewhere for jack pine andblack spruce (Lorenz et al., 2000; Preston et al., 2000), and Norway spruce (Lorenz


Figure 7. 13C CPMAS NMR spectra of (a) forest floor from Southern Old Jack Pine site, (b) lichen,(c) Gillam G-JM-1 jack pine forest floor, (d) char fragments picked from (c).

et al., 2000; Dignac et al., 2002). The jack pine twig litter is similar to fine woodydebris (<1 cm) in British Columbia coastal forests (Preston et al., 2002a), but thehigh alkyl C content of black spruce twig litter may be due to higher bark content.As noted previously, this and other studies have found a higher proportion of woodylitterfall for black spruce than for jack pine in this region, and this coupled with


Figure 8. 13C CPMAS NMR spectra of needle, twig and cone litters from black spruce (a–c) and jackpine (d–f) littertraps.

a difference in their organic composition may contribute to slower decompositionand nutrient cycling in black spruce stands.

4. Implications of Site Chemistries

4.1. CONTRASTING FOREST FLOOR CHEMISTRIES

As discussed previously, site conditions and their interactions with vegetation andfire strongly influence stand development and C storage within the range of cli-matic conditions along the transect, and these further influence water and energyexchange (McGuire et al., 2002). The C stocks in forest floor of black spruce standsare comparable to their living tree biomass. The organic “fingerprint” of the forestfloor is consistent with a poorly decomposed mixture of mosses and litter inputs,the latter likely dominated by roots rather than by aboveground inputs. Inefficient


decomposition is reflected in the strongly stratified stable isotope composition, andhigh concentrations of condensed tannins. The latter may be due to poor drainageand low temperatures that impede leaching and decomposition of tannins, possiblyenhanced by the structure of black spruce tannin. These high tannin concentrationsare likely to further reduce N availability and hinder decomposition. Tannins arequickly degraded by contact with mineral surfaces, but would become increasinglyisolated from their influence as tannin-rich roots proliferate mainly in the increas-ingly thick forest floor. It has been suggested that activity of phenolic compoundsshould be reduced by adsorption to char (DeLuca et al., 2002a), but effectivenessof this process may also be reduced by the thickness of the forest floor. Partialconsumption of black spruce forest floor by fire results in charcoal layers, althoughchar was not observed in our samples. Its presence was expected, but the sam-pling strategy was not designed for this purpose, and occurrence can be patchy.Thus, several feedbacks operate through which the black spruce sites develop theircharacteristic site conditions.

The thinner jack pine forest floor was very low in aromatic C, and appeared tobe dominated by lichen inputs. This is consistent with rooting being concentratedin the mineral soil, but also implies that aboveground litter is rapidly decomposed.Several samples contained char, determined both from visual observation and itscharacteristic NMR peak. Thus, site factors (drainage and soil texture) and vegeta-tion interact to produce dramatically different C stocks and forest floor dynamicsover distances too short to be affected by climatic differences.

4.2. COMPARABLE LITTER CHEMISTRIES

As noted previously, slow nutrient cycling in black spruce has been linked to poorquality of its forest floor. For our sites, however, it is unlikely that the strikingdifferences in forest floor stocks and chemistry result from large differences inaboveground litter quality per se. Foliar litter inputs were similar in quantity andquality, and black spruce needles decompose readily (Lorenz et al., 2000; Prestonet al., 2000; Trofymow et al., 2002). Black spruce stands tend to have higher inputsof woody litter, with some lower quality factors. These are mainly higher tanninconcentrations in black spruce cones, and higher alkyl C in black spruce twig litter.The C/N ratios were also higher for black spruce cones, although N concentrationswere similar for the woody and non-woody litter components. However, the higherwoody inputs for black spruce were not reflected in the C quality of the forest floor.

4.3. SENSITIVITY TO CHANGING CONDITIONS

As noted in Section 1, there has been much debate over the effect of increasedtemperatures on organic matter decomposition and CO2 release. A key point isthat decomposition slows down with time as substrates become more chemically


complex and less susceptible to enzyme attack, so that fresh material should bemore sensitive to increased temperature than older material (Fyles and McGill,1987; Trofymow et al., 2002). Except for charcoal layers, black spruce forest floorappears to be hardly more than compressed plant material. Its potential for CO2

release should therefore be sensitive to increased temperature, but even more soto changes in drainage, which has long been identified as a key control of soil Cstorage in this region (Harden et al., 1997; Trumbore and Harden, 1997; Price et al.,1999a,b; Bhatti et al., 2002). This effect may be amplified if tannins further inhibitdecomposition in poorly-drained black spruce forest floor.

Litter decomposition studies have focused on foliar inputs, but roots, mossesand lichens may contribute more to forest floor accumulation in these ecosystems.Mosses and lichens generally decompose more slowly than litter of higher plantseven when environmental conditions are the same (Hobbie et al., 2000), althoughFyles and McGill (1987) found exceptions to this pattern. More research is requiredto separate litter physical and chemical quality attributes from environmental factorsfor mosses, lichens and roots, so that their responses can be predicted. By contrast,charcoal would be resistant to decomposition, but more information is needed on thesize of this pool on a landscape scale (Harden et al., 1997), and what proportion of ithas the highly condensed aromatic structure associated with long-term preservation(Schmidt and Noack, 2000; Baldock and Smernik, 2002; Czimczik et al., 2003).

For boreal forest soils, especially for black spruce, it appears that decompositionis constrained mainly by low temperatures and waterlogging, although secondarychemicals such as tannins and sphagnum acid may also contribute some resistance.Thus, as cultivation of mineral soils breaks down aggregates and exposes organicmatter protected from decomposition by mineral association (Krull et al., 2003),small changes in environmental conditions could trigger large C losses throughrapid decomposition of black spruce forest floor. Our observations of high O-alkyl C, especially for jack pine forest floor, and thus high potential susceptibilityto increased decomposition with environmental change are similar to finding bySjogersten et al. (2003) for Fennoscandian birch forest.

4.4. RESEARCH RECOMMENDATIONS

Further research will need to strengthen the links between our understanding ofsite-specific sensitivity of organic matter to decomposition, and our ability to modelthe response of detrital carbon stocks to changing conditions. Scaling-up of patchmodels that account for site hydrology in this region (Price et al., 1999a) willrequire improved mapping of areas covered by poorly-drained black spruce in bo-real Canada. We need to establish the stocks, chemical characteristics, ecologicalfunction and sink strength of black C in boreal forest soils. We also need to clar-ify the influence of environmental versus litter quality controls on decomposition,especially for lichen, mosses and roots. This has to include improvements in our


fundamental understanding of litter chemistry and its changes during decomposi-tion, through spectroscopic and molecular-level chemical analysis (Preston, 1996;Williams et al., 1998; Hedges et al., 2000; Kogel-Knabner, 2000, 2002). While theinformation in this report is limited and preliminary, it is consistent with an earlierstudy (Preston et al., 2002b), and with other analyses of factors affecting C cyclingin boreal forests (Flanagan and Van Cleve, 1983; Bonan and Shugart, 1989; Bonan,1992; Harden et al., 2000; Hobbie et al., 2000). It is reasonable to expect that betterinformation on the chemistry of detrital and soil C pools will enhance efforts topredict their response to climate change.

Acknowledgments

We thank co-op student Heather De Costa for sample analysis, Lana Laird forseparating litter components, Mike Apps for helpful discussions, and especiallyThierry Varem-Sanders for his extraordinary contributions in the field.

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(Received 3 October 2003; in revised form 17 November 2004)

stocks, chemistry, and sensitivity to climate change of dead

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