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 1
Pacific Forestry Centre, Natural Resources Canada, Victoria, BC Canada V8Z 1M5 E-mail:
[email protected] 2 Northern Forestry Centre, Natural Resources Canada, Edmonton, AB, Canada T6H 3S5 3 Department 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 of boreal forests requires increased understanding of site-specific factors controlling detrital and soil C accumulation. Jack pine (Pinus banksiana) and black spruce (Picea mariana) stands along the Boreal Forest Transect Case Study (BFTCS) in northern Canada have similar C stocks in aboveground vegetation and large woody detritus, but thick forest floors of poorly-drained black spruce stands have much higher C stocks, comparable to living biomass. Their properties indicate hindered decomposition and N cycling, with high C/N ratios, strongly stratified and depleted δ 13 C and δ 15 N values, high concentrations of tannins and phenolics, and 13 C nuclear magnetic resonance (NMR) spectra typical of poorly decomposed plant material, especially roots and mosses. The thinner jack pine forest floor appears to be dominated by lichen, with char in some samples. Differences in quantity and quality of aboveground foliar and woody litter inputs are small and unlikely to account for the contrasts in forest floor accumulation and properties. These are more likely associated with site conditions, especially soil texture and drainage, exacerbated by increases in sphagnum coverage, forest floor depth, and tannins. Small changes in environmental conditions, especially reduced moisture, could trigger large C losses through rapid decomposition of forest floor in poorly drained black spruce stands in this region.
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 soil and 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, that are 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 Forest Transect Case Study (BFTCS) in central Canada (Price and Apps, 1995) provides an opportunity to examine the interacting factors controlling stand development and C storage in an area expected to be highly sensitive to climate change (Bhatti et al., 2003; Camill, 1999; Hogg et al., 2002; Price et al., 1999a,b). Within the transect, site conditions, mainly soil texture, drainage and presence of 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 and Harden, 1997; Gower et al., 1997). Fire is also a major disturbance and driver of C dynamics in this large, remote, mainly unmanaged area (Harden et al., 2000; Amiro et al., 2001); globally, it is estimated that up to one-third of boreal forest primary productivity is consumed by fire (Harden et al., 2000; Wirth et al., 2002). Recent assessments of the C balance and sink potential of boreal forests have emphasized 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 understanding of the physical, chemical and biological mechanisms that control C sequestration in soil organic matter (Hobbie et al., 2000). This should reduce the uncertainty in predicting release of CO2 through increased decomposition of soil C, especially at northern latitudes (Hobbie et al., 2000; Kirschbaum, 2000; Sanderman et al., 2003). Despite their importance in the global C cycle, there is little information on the organic chemistry of litter, forest floor and mineral soil of boreal regions, with limited application of the geochemical techniques widely used for soil and sediment characterization (e.g., Preston, 1996, 2001; Hedges et al., 2000; K¨ogel-Knabner, 2000, 2002; Schmidt and Noack, 2000). Previous studies of soil organic matter chemistry within the BFTCS (Preston et al., 2002b) and similar forest ecosystems (Lorenz et al., 2000; Czimczik et al., 2003) indicated some distinct characteristics associated with high influence of mosses and lichens, accumulation of forest floor tannins in poorly drained sites, presence of black (pyrogenic) C, and strong stratification of δ 15 N values in thick black spruce (Picea mariana) forest floor. Within the BFTCS, it may be expected that the potential response to changing climate will be 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, in support of enhanced modelling and carbon accounting tools for Canadian forests. We report here C stocks in tree biomass, woody detritus, and forest floor, and annual litter inputs for jack pine (Pinus banksiana) and black spruce sites along the BFTCS. We have used several chemical techniques to link the striking differences in 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 International Geosphere Biosphere Programme (IGBP) (McGuire et al., 2002). Its orientation is along an ecoclimatic gradient, ranging from agricultural grasslands in southern Saskatchewan through the boreal forest in the central portion to the tundra
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
Figure 1. Location of study sites within the BFTCS.
in northern Manitoba (Yu et al., 2002). Mean annual temperature and precipitation at four sites along the transect are shown in Figure 2. In the northeast end of BFTCS, low temperatures limit growth; the southwest is warmer, but growth is mainly limited by low moisture availability (Bhatti et al., 2002). The dominant tree species are black spruce and jack pine. In the understory, in addition to black spruce seedlings and saplings, some moss species tend to occur mainly in black spruce stands, including Pleurozium schreberi, Ptilidium cristacastrensis and Hylocomium splendens as well as undifferentiated Sphagnum and other moss species. Jack pine germinates and establishes only in full light on mineral soils, and thus tends to be found early in succession on fire-disturbed sites. Jack pine stands tend to have shrubs Vaccinium vitis-idaea, V. myrtilloides and Arctostaphylos uva-ursi, mosses Dicranum polysetum and Polytrichum sp., and the reindeer lichens Cladonia cornuta, Cladina mitis, and C. stellaris. Alnus rugosa (alder) occurs in the shrub layer of both stand types. Twelve field sites (Table I) were established in four areas spanning the length of the BFTCS, near Prince Albert and Flin Flon (Saskatchewan), and Thompson and Gillam (Manitoba). Sites were located in stands dominated by jack pine or black spruce, each site comprising three plots of 20 × 20 m2 . The Prince Albert Old
C. M. PRESTON ET AL.
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 Thompson Old Jack Pine site (N-TE-OJP) are at the original BOREAS Southern and Northern flux tower sites. Our plots are just outside the footprints of the towers, which are still operating as part of the FLUXNET-Canada research network evaluating ecosystem CO2 and H2 O exchange. The Flin Flon plots (F-JM-1 and F-BM-1) were reestablished (F-JM-2 and F-BM-2) at nearby locations in 2001 due to vandalism of the original plots. Soil temperature at the 5-cm depth was observed continuously using thermistors and a data logger at two points in the Southern Old Jack Pine and Old 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 that were developed for jack pine and black spruce (Gower et al., 1997). The tree
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
TABLE I Characteristics of study sites along the BFTCS Ecoclimate region
Main species
Site name
Breast ht. age (y)
Forest floor depth (cm)
Moisture regime
Drainage class
Prince Albert (Southern) Mid Boreal
Jack Pine
S-JIH-4
42
3.9
Xeric
Well
Mid Boreal Jack Pine Mid Boreal Black Spruce Mid Boreal Black Spruce Flin Flon High Boreal Jack Pine/Fir High Boreal Black Spruce Thompson (Northern) High Boreal Jack Pine
S-TE-OJP S-BIH-1 S-TE-OBS
73 55 90
3.7 6.7 21.5
Xeric Mesic Hygric
Moderate Poor Poor
F-JM-1 F-BM-1
117 121
3.5 23.2
Mesic Mesic
Imperfect Poor
N-JIL-1
80
1.5
Xeric
Very rapid
High Boreal High Boreal High Boreal Gillam High Boreal Subarctic
Jack Pine Black Spruce Black Spruce
N-TE-OJP N-BIH-1 N-BMH-7
74 128 120
6.7 21 15
Xeric Subhygric Subhygric
Rapid Poor Poor
Jack Pine Black Spruce
G-JM-1 G-BM-1
78 99
2.1 9.6
Subxeric Subhygric
Rapid 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 aboveground biomass of individual trees was calculated using the allometric equations. The sum of 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 for the site. The stand age was determined by the dendrochronological method; i.e. the tree rings of 10–15 sample trees. At each site litter is collected in 36 plastic buckets of 27.3 cm diameter and 30 cm height with mesh bottoms, and 27 mesh quadrats laid on the forest floor (1 m2 with 1 cm2 openings). The latter were designed to capture the coarser material, especially twigs. The traps were installed in June 2000, and are emptied once a year. Litter was hand-sorted into needles, twigs, bark, male cones, female cones and cone flakes, and deciduous leaves. Annual litterfall values reported here are based on two or three years of collection. Litter trap data from the Flin Flon and Gillam sites for the first collection year (2000–2001) were not used in the C stock determination, although the samples were used for litter characterization. Area-based litter mass inputs were converted to C, N and tannin inputs using the mean values for each litter type and species. Coarse woody debris (CWD) mass was measured using three 1 × 10 m2 transects in each plot. The LFH samples were collected from three locations using a
C. M. PRESTON ET AL.
20 × 20 cm2 template in each of the triplicate plots. The samples excluding the live vegetation and roots were oven-dried at 70 ◦ C to constant weight. Samples were analysed 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 intensive chemical characterization was sampled in summer 2000 from plots in the Prince Albert, Thompson and Gillam areas. Jack pine forest floor was sampled as a single layer, except for the southern immature stand (S-JIH-4), which was separated into L and H horizons. The thicker forest floor of the black spruce sites was sampled by L, F, and H layers, except for S-BIH-1 which was sampled as a single layer. All plots were sampled in duplicate, but for two of the black spruce sites, N-BMH-7 and 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, the samples were oven-dried at 70 ◦ C for 24 h, and ground in a Wiley mill to 2 mm, and then to 60 mesh (250 μm). The coarse roots picked from the black spruce samples were 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 chemical characterization. Twigs and bark flakes were combined into one category, as were male and female cones. Leaves from understorey species were available in sufficient quantity for analysis from five sites. Needles and twigs were combined to give three samples per plot, although in some cases, samples were only available from two subplots. 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-1 and N-TE-OJP. Samples were dried and ground as described earlier. Because of the greater difficulty in grinding the tough fibers of litter materials, and the small size of 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 Determinator CR12. Total (N) was analysed by a semimicro-Kjeldahl method (Preston et al., 2002a). Condensed tannins were analysed as the sum of extractable and residual fractions using the butanol/HCl assay, and for total phenolics using the Folin-Ciocalteu assay. As described fully elsewhere (Lorenz et al., 2000), samples were extracted twice with acetone:water (70:30 by volume) and extractable tannins determined on an aliquot of the combined extracts. Residual tannin was then determined by direct hydrolysis of the extraction residue. As previously, the assay was standardized against tannin purified from branch tips of balsam fir (Abies balsamea (L.) Mill.), as black spruce and jack pine tannins were not available.
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
Total phenolics were determined by taking an aliquot of the same extract and drying it down with air in a test tube. To this was added 1.0 ml distilled water, 0.5 ml Folin-Ciocalteu reagent (Sigma), and 2.5 ml sodium carbonate solution (20% w/v). The mixture was then transferred to disposable cuvettes and absorbance measured at 750 nm on a Beckman DU 520 UV/Vis Spectrophotometer. The assay was standardized against catechol. Samples were analysed for C and N stable isotope ratios (δ 13 C, δ 15 N) using an elemental 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 the samples, analyses were run separately, with a run for δ 13 C followed by one for δ 15 N. A subsample of ground material was sealed in a tin capsule and loaded into the elemental analyzer for combustion/reduction. Sample weights were typically 1– 3 mg for δ 13 C and 3–10 mg for δ 15 N. Water generated by combustion was removed by a magnesium perchlorate trap. The carbon dioxide and nitrogen gases generated from the combustion/reduction process were separated in the gas chromatograph column of the elemental analyzer and passed directly via a helium gas carrier stream to the inlet of the mass spectrometer for stable isotope analysis. Standard deviations were similar for laboratory standards and for representative samples, typically 0.1‰ or better for δ 13 C and 0.1–0.2‰ for δ 15 N. Solid-state 13 C nuclear magnetic resonance spectra with cross-polarization and magic-angle spinning (CPMAS NMR) were obtained at 75.47 MHz on a Bruker MSL 300 spectrometer. Dry, powdered samples were packed into a zirconium oxide rotor of 7 mm OD. Acquisition conditions were: 4.7 kHz spinning rate, 1 ms contact time, 2 s recycle time, and 5000–36 000 scans. Chemical shifts are reported relative to tetramethylsilane (TMS) at 0 ppm, with the reference frequency set using adamantane. Most forest floor horizons were sampled and analysed in duplicate but due to the time requirements for NMR, spectra were acquired for one of each pair plus a few duplicates. More detailed discussion of NMR methods may be found elsewhere (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 duff layer than the black spruce stands. Thick forest floor is especially associated with poorly-drained black spruce stands, and this moss-organic layer plays an important role 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 capacity reduce soil temperatures and maintain high soil moisture contents, thereby reducing
C. M. PRESTON ET AL.
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 further accumulation of the organic layer. As shown in Figure 3, soil temperature (5 cm depth) increases more slowly for the Old Black Spruce (S-TE-OBS) site, with the summer maximum 5 ◦ C below that of the Old Jack Pine (S-TE-OJP) site, although the latter is colder in winter. Our C stocks in forest floor, detritus (snags plus CWD) and aboveground tree biomass (Figure 4) are in general agreement with other studies from this region. The average 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) and forest floor along the BFTCS.
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
be 3.4 and 5.2 kg m−2 respectively, comparable to the estimates reported by Gower et al. (1997) and Nalder and Wein (1999). In general for both species, aboveground biomass 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 collection period. However, the first 2–3 year results (Figure 5a) indicate similar foliar litter masses 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 the BFTCS.
C. M. PRESTON ET AL.
and much greater interannual variation in the amounts of non-woody litter (not shown). 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 ) are comparable to those reported by Gower et al. (2000) for their BOREAS southern study 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 foliar 61.9 g m−2 , JP woody 17.0 g m−2 , total 78.9 g m−2 , 21.5% woody; BS foliar 68.4 g m−2 , BS woody 35.4 g m−2 , total 103.8 g m−2 , 34.1% woody). Our foliar data are also 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 to be 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 three black spruce stands near the northern study area (Wang et al., 2003; Bond-Lamberty et al., 2004).
3.2.
FOREST FLOOR CHEMISTRY
3.2.1. Forest Floor C, N and Stable Isotope Analysis As shown in Table II, total C of forest floor ranged from 241–463 mg g−1 in jack pine sites and 235–500 mg g−1 in black spruce, typical of forest floor organic horizons (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 degree of decomposition, i.e., from L to F to H horizons (Prescott et al., 1995), but these trends were not found consistently for the black spruce sites. By contrast, the natural abundance of 13 C consistently increased from L to F to H for all black spruce sites. The δ 13 C value for the S-BIH-1 site sampled as one horizon was −26.4‰, similar to F and H horizons from other sites. The widest spread was 2.5‰ in the S-OBS-F site, with smaller differences from 0.9 to 1.5‰ in the other sites. Values of δ 13 C tended to increase from south to north, but more data would be required to define trends. In a previous study (Preston et al., 2002b), δ 13 C for a combined LFH forest floor sample in a black spruce stand near Gillam was −26.0‰, similar to our value of −25.9‰ for the H horizon of the Gillam site. Values of δ 15 N showed similar trends, increasing from L to F to H horizons, except for Gillam (G-BM-1) where F was more enriched than H. There was also a trend of 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 δ 15 N, 13 δ C and C/N ratio increased. The C/N ratio was lowest in the two Thompson area sites. Similar to black spruce sites, both δ 15 N and δ 13 C tended to increase, in general, from south to north, except that δ 15 N values were high for the S-JIH-4 site. Values reported here are more depleted than those previously reported for jack
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
TABLE II Chemical and stable isotope analysis of forest floor samples Tannins Site name
C N E + Ra Phenolics δ 13 C −1 −1 −1 b Horizon (mg g ) (mg g ) C/N (mg g ) E/S (mg g−1 ) ‰
δ 15 N
‰
Jack Pine S-JIH-4
L H c S S S S
S-TE-OJP N-JIL-1 N-TE-OJP G-JM-1 Black Spruce S-BIH-1 S S-TE-OBS L F H N-BIH-1 L
N-BMH-7
G-BM-1
F H L F H L F H
389 328 241 243 393 463
8.0 6.5 5.8 8.8 12.1 9.4
49 51 41 28 33 49
10.3 5.4 3.4 1.0 1.0 2.8
0.60 0.64 0.51 0.33 0.00 0.56
2.1 1.4 0.7 0.4 1.0 1.5
−28.7 1.6 −27.7 2.4 −27.3 −2.7 −27.5 −1.2 −27.5 −1.4 −26.2 2.5
409 480 470 396 333
8.7 8.1 7.2 11.0 7.1
47 59 65 36 47
21.5 21.4 11.9 1.9 12.6
0.62 0.48 0.46 0.36 0.54
4.7 3.2 1.7 0.5 1.9
−26.4 2.4 −28.8 −1.2 −27.7 −0.4 −26.3 1.6 −27.2 −1.0
465 330 500 484 235 482 427 272
7.7 6.3 6.9 6.3 5.1 7.5 8.1 8.2
60 52 73 77 46 64 53 33
15.7 9.4 28.9 23.8 2.8 16.0 10.7 3.0
0.47 0.49 0.45 0.43 0.48 0.53 0.50 0.65
2.5 1.8 5.6 3.7 0.5 2.5 2.1 0.7
−26.4 1.4 −26.2 3.5 −27.1 −0.3 −26.5 1.5 −25.6 2.6 −27.3 1.1 −26.4 4.5 −25.9 3.7
a
Sum of extractable (E) and residual (R) tannins. Ratio of extractable (E) to sum of extractable and residual (S) tannins. c S: thin forest floor sampled as a single horizon. b
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 to those 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 tendency for δ 15 N and δ 13 C to increase with depth has been widely observed (Nadelhoffer and Fry, 1988; H¨ogberg et al., 1996; Buchmann et al., 1997; Ehleringer et al., 2000; Feng, 2002; Schuur et al., 2003). The pattern can become particularly striking in colder forest soils, where slow decomposition and lack of earthworm activity result in a thick forest floor with highly depleted values, and increasing enrichment with
C. M. PRESTON ET AL.
depth in the mineral soil. The forest floor then largely reflects the depleted isotopic signature of the litter inputs, whereas the deeper mineral horizons and especially the finer size fractions (Preston et al., 2002b; Bird et al., 2002b) reflect the enrichment attributed mainly to conversion of plant residues to microbial biomass. For N, this trend can be reinforced by plant uptake of organic forms of N, short-circuiting the cycle of immobilization and mineralization to ammonium and nitrate (H¨ogberg et al., 1996; Northup et al., 1998). For black spruce, the increases of δ 15 N and δ 13 C from L to F to H horizons are likely associated with N-limitation, tight N-cycling and limited microbial transformation of plant inputs (H¨ogberg et al., 1996; Amundson et al., 2003). Two other factors may influence the isotopic composition of the forest floor. In addition to N2 fixation by alder, δ 15 N values may be influenced by N2 fixation in feather moss and lichens (Dawson, 1983; DeLuca et al., 2002b). The low δ 13 C values, especially in the L layers, are due in part to high inputs of mosses, which have a more depleted 13 C 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 Phenolics Condensed 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; K¨ogel-Knabner, 2002; Kraus et al., 2003a,b). Note that we use the term lignin only for the polymer based on phenylpropane units, not for the operationally-defined acid-insoluble residue of proximate analysis, often referred to in the ecological literature as lignin or Klason lignin (Preston et al., 2000; K¨ogel-Knabner, 2002). The black spruce sites had high tannin levels, up to 28.9 mg g−1 for the sum of extractable and residual fractions, and generally decreasing with depth. Lower concentrations were found in the jack pine sites, with the highest values for S-JIH-4, with two distinct forest floor horizons. For both black spruce and jack pine stands, extractable tannins comprised around 35–64% of the total. Tannin concentrations in forest floor are typically 100. For the Flin Flon sites, litter needle N concentrations (7.7 mg g−1 for F-JM-1, 6.9 mg g−1 for F-BM-1) were similar to reported 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 are also comparable to old foliage (i.e., attached non-current needles) of jack pine and black 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 southern study area. For the other sites, however, our litter needle N concentrations are generally lower (3.4–5.3 mg g−1 ), with the lowest values (100. The δ 15 N values of roots were always lower that those of the corresponding forest floor, and for the most part, higher than the aboveground litter samples from the same site. The maximum difference between roots and forest floor (7.7‰) was found for the F layer of the Gillam site (G-BM-1). For δ 13 C, root values were similar to those of litters and forest floor, and no obvious pattern was seen between the δ 13 C values of the roots versus forest floor. TABLE IV Properties of coarse roots from black spruce forest floor, and char from jack pine forest floor Tannins Site name
C N E + Ra Phenolic δ 13 C Horizon (mg g−1 ) (mg g−1 ) C/N (mg g−1 ) E/Sb (mg g−1 ) ‰
Coarse Roots S-TE-OBS L F H N-BIH-1 L F H N-BMH-7 L F H G-BM-1 L F Char G-JM-1-1 T G-JM-1-2 T a b
δ 15 N
‰
530 534 514 503 532 525 514 540 531 548 540
6.2 4.1 4.8 6.1 4.7 4.5 4.2 4.3 4.6 4.5 4.2
85 132 108 83 113 117 123 127 116 121 130
126 131 100 107 144 104 152 132 107 92 108
0.81 0.78 0.91 0.79 0.80 0.77 0.77 0.77 0.80 0.89 0.88
34.6 39.3 34.8 39.1 41.7 33.5 50.1 45.9 35.6 30.6 37.9
−28.0 −27.1 −26.5 −26.5 −26.3 −26.7 −27.1 −27.1 −27.1 −27.7 −27.2
−3.4 −3.9 −1.7 −4.9 −3.6 −1.9 −5.7 −4.7 −3.7 −2.2 −3.2
605 605
8.2 3.5
74 175
0.40 0.00
0.22 0.00
3.8 4.9
−25.3 −25.0
2.3 0.3
Sum of extractable (E) and residual (R) tannins. Ratio of extractable (E) to sum of extractable and residual (S) tannins.
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
3.3.2. Litter Tannins and Phenolics Tannin concentrations in littertrap samples were highly variable, with the highest individual 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 reflect differences in the age of cones entering the traps, or in the proportion of male cones that are lower in tannins (unpublished results). Black spruce needle litter tannin concentrations (17–29 mg g−1 ), were similar to those reported previously (25 and 38 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 unusually high for coarser roots, as discussed later with the NMR results. Analysis of fine roots from jack pine and black spruce sites is in progress, and future studies should include analysis of the inputs from lichens and mosses, nonvascular plants that produce a wide range of phenolic secondary compounds (Fahselt, 1994; Ing´olfsd´ottir, 2002), but no lignin or tannins (Wilson et al., 1989; Williams et al., 1998). 3.3.3. Litter Inputs Annual 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 higher proportion in woody forms. Thus, in terms of litter quality, black spruce stands had higher inputs of N, but higher inputs of woody components and of tannins (Figure 5d). Ongoing work includes analysis of the subsequent littertrap collections and calibration of tannin concentrations with black spruce and jack pine tannins. The second 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 species contain substantial amounts of tannins, including extractable fractions, and that the differences in forest floor stocks and properties are unlikely to be accounted for by the relatively small differences in quantity and quality of the litter components we examined.
3.4.
ORGANIC CARBON CHARACTERIZATION
Carbon-13 CPMAS NMR has been widely used for direct characterization of soil organic matter and litter (Preston, 1996, 2001; Preston et al., 2000, 2002a; K¨ogelKnabner, 2000; Quideau et al., 2001; Dignac et al., 2002; Sj¨ogersten et al., 2003). NMR spectra show intensity versus chemical shift, expressed as ppm of the observation frequency, relative to the chemical shift of a standard compound. Chemical shift reflects the electronic environment of the carbon bonds, and increases with increasing electronegativity and unsaturation. For 13 C the main chemical-shift regions are ascribed to C in alkyl (0–47 ppm), O- and di-O-alkyl (47–112 ppm),
C. M. PRESTON ET AL.
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 components of the Southern Old Jack Pine and Old Black Spruce sites, which were typical for each forest type. Assignments and general interpretations are based on many previous studies as listed earlier, and as the main features are typical of organic matter spectra, they will not be described in great detail. 3.4.1. Forest Floor NMR – Black Spruce Spectra 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 largest peak at 73 ppm in the O-alkyl region, mainly due to carbohydrate. The sharp peak at 105 ppm is also mainly due to the C1 of carbohydrate. The peak in the alkyl region (0–47 ppm) has two maxima, at 30 and 33 ppm, characteristic of CH2 in long chains, while the underlying broader intensity is due to a variety of CH, CH2 and CH3 (methyl) structures. The aromatic and phenolic regions are weak, with features typical of tannins rather than lignin, especially the phenolic region, with peaks at 145 and 155 ppm. Low lignin content is also consistent with the very weak methoxyl signal at 57 ppm, which appears as a shoulder on the O-alkyl peak. Carboxyl, amide and ester C produce the peak at 174 ppm. This region is mainly associated with the amide C of proteins and the carboxyl groups of microbial and plant lipids. The L and F spectra are typical of poorly decomposed material, retaining the sharp peaks of plant litter inputs, as discussed in the next section. Similar NMR spectra, and high tannin contents were found for the forest floor of two black spruce sites in northern Ontario (Lorenz et al., 2000). The H horizon shows loss of O- and di-O-alkyl intensity, relative to the other regions, and broadening of signals mainly in the aromatic and phenolic regions. This is typical of increasing decomposition, but without the expected increase in alkyl/Oalkyl ratio. The difference in the H horizon may also be due to greater influence of CWD, 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 in our sites (detritus component, Figure 4), and for jack pine was comparable to the forest floor C pool. In addition to visible CWD, old buried, decomposed logs have a 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 and H root samples were similar. The coarse roots mainly differed from L and F forest floor in having lower alkyl C. Peaks characteristic of tannin were also very clear, consistent with the high tannin contents measured chemically. While our results represent 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 litter and root inputs, as did H spectra of some of the other black spruce sites. Lignin
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Figure 6. 13 C CPMAS NMR spectra of L, F, and H horizons from the Southern Old Black Spruce site (a, b, c) and coarse roots from F horizon (d).
contents appear to be low for all litter, root, and forest floor samples, with aromatic components derived substantially from tannins. Feathermoss and sphagnum inputs are substantial for these black spruce ecosystems (Harden et al., 1997; Trumbore and 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
C. M. PRESTON ET AL.
carbohydrate, and very low in aromaticity (Bergman et al., 2000). The NMR spectra of the forest floor are more sharply resolved and even lower in aromatic C than the litterfall inputs shown in the next section, as previously found for two black spruce stands in Ontario (Lorenz et al., 2000). The L and F forest floor spectra shown, and similar ones obtained in this study are therefore consistent with a high proportion of poorly decomposed mosses and black spruce roots. 3.4.2. Forest Floor NMR – Jack Pine The forest floor from the Southern Old Jack Pine site (S-OJP, Figure 7a) was very high in O-alkyl (mainly carbohydrate) C, and remarkably low in aromatic and phenolic C. Similar spectra were previously observed for jack pine forest floor along the BFTCS (Preston et al., 2002b), and for Scots pine in Siberia (Czimczik et al., 2003). This may indicate the dominance of lichen in the accumulation of organic 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) is very high in O- and di-O-alkyl C, indicating mainly carbohydrate structures. The apparent dominance of lichen structures may be due to low amount of rooting in the forest floor, and rapid decomposition of surficial jack pine litter. In addition to plant detrital inputs, black C generated in forest fires may contribute 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 aromatic structures produced by charring have a broad peak at 125–130 ppm (Baldock and Smernik, 2002; Czimczik et al., 2002, 2003; Preston et al., 2002a, b). This was observed in spectra of the lower (H) horizon of S-JIH-4, and the Gillam site (G-BM1), confirmed by spectra of duplicate samples. Figure 7c shows one of the samples from G-BM-1 along with the spectrum of charred woody fragments picked from the sample (Figure 7d). 3.4.3. Litter NMR Spectra were obtained for the needle, twig, and cone litters from the southern flux tower sites, as shown in Figure 8(a–c) for black spruce and Figure 8(d–f) for jack pine. All spectra are dominated by carbohydrate peaks at 73 and 105 ppm, and differ mainly in the proportion of alkyl C (0–47 ppm), and degree of splitting of the phenolic region (140–165 ppm). The needle spectra are similar, except that phenolic peaks are better resolved for black spruce. The black spruce needles have a split peak at 145 and 155 ppm more characteristic of tannin, whereas for jack pine, the peak at 147 pm with a shoulder at 152 ppm indicates a higher proportion of lignin. The black spruce twig litter is higher in alkyl C, probably due to a higher proportion of bark. The jack pine twig sample was the lowest in aromatic and phenolic 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 and black spruce (Lorenz et al., 2000; Preston et al., 2000), and Norway spruce (Lorenz
STOCKS, CHEMISTRY, AND SENSITIVITY TO CLIMATE CHANGE
Figure 7. 13 C 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 woody debris (
