post-fire forest floor development along toposequences of white spruce - trembling aspen mixedwood...

Post-fire forest floor development alongtoposequences of white spruce – trembling aspenmixedwood communities in west-central Alberta

T.I. Little, D.J. Pluth, I.G.W. Corns, and D.W. Gilmore

Abstract: After wildfire in the boreal forest, storage of organic carbon (C) begins with the accumulation of forest floormaterial. Soil properties of Gray Luvisols were studied to determine the differences in development along threetoposequences. Our central hypothesis is that slope position does not influence the amount of accumulated organic Cand total nitrogen (N) in the forest floor. Organic C and the C/N ratio in the forest floor and in A and B horizons in-creased from the crest to the toe of the slope. The forest floor contributed 2.0 ± 0.4 kg C·m–2 (mean ± SE) at the crestto 3.5 ± 0.5 kg C·m–2 at the toe. Throughout the solum, the C/N ratio was lower at the top of the slope compared withthe toe (p < 0.05), and there were no differences among slope positions for in situ net N mineralization rates. Leafarea index, used as a proxy for net primary productivity, was greater (p < 0.05) at the toe compared with the crest po-sition, and it was negatively correlated with forest floor total N concentration (r = –0.35,p = 0.027). These results,from mixedwood stands approximately 90 years after the last major fire disturbance, indicate that slope position doesinfluence forest floor organic C by horizon volume (p = 0.02), but not total N concentration (p = 0.07). Despite the ap-parently lower N availability at the toe position, it exhibited the greatest potential productivity.

Résumé: Après un feu dans la forêt boréale, le stockage du carbone organique (C) débute avec l’accumulation desmatériaux qui constituent le parterre forestier. Les propriétés des luvisols gris ont été étudiées pour déterminer les dif-férences dans le développement le long de trois séquences topographiques. Notre hypothèse principale est que la posi-tion sur la pente n’influence pas la quantité de C organique et d’azote (N) total accumulés dans le parterre forestier. LeC organique et le rapport C/N dans le parterre forestier et dans les horizons A et B, ont augmenté du sommet au piedde la pente. Le parterre forestier contribuait 2,0 ± 0,4 kg C·m–2 (moyenne ± erreur type) au sommet jusqu’à 3,5 ±0,5 kg C·m–2 au pied. Partout dans le solum, le ratio C/N était plus faible au sommet qu’au pied de la pente (p < 0,05)et il n’y avait pas de différence dans le taux de minéralisation nette in situ entre différentes positions sur la pente.L’indice de surface foliaire, utilisé comme indice de productivité primaire nette, était plus élevé (p < 0,05) au piedqu’au sommet et il était négativement corrélé avec la concentration de N total dans le parterre forestier (r = –0,35,p =0,027). Ces résultats, qui proviennent de peuplements mélangés approximativement 90 ans après la dernière perturba-tion majeure due au feu, indiquent que la position sur la pente influence le C organique dans le parterre forestier sur labase du volume par horizon (p = 0,02), mais non la concentration de N total (p = 0,07). Bien que la disponibilité de Nsoit apparemment plus faible au pied de la pente, c’est là qu’on retrouve la plus forte productivité potentielle.

[Traduit par la Rédaction] Little et al. 902

IntroductionSoil organic matter (SOM) in post-disturbance, boreal for-

est ecosystems is concentrated in the forest floor as physi-cally and biochemically altered plant and animal litter.Boreal forest floors are somewhat transient because of their

rapid transformation by fire as a recurring natural distur-bance. In the western Canadian boreal forest, fire cycles of40–60 years (Mann and Plug 1999) and 50–100 years(Nalder and Wein 1999) have been reported. During the pe-riod between fire events, forest floors develop through or-ganic matter production and decomposition. These processesare directly and indirectly mediated by ecosystem conditionsthat vary with topographic position, primarily soil drainageclass, in forested landscapes (Donald et al. 1993). Topogra-phy also influences fire severity (Hungerford et al. 1991).Reduction in forest floor thickness ranged from 27%, for themost lightly burned areas, to 77% on upper slope positions,for moderately to heavily burned areas, in a predominantlyblack spruce (Picea mariana (Mill.) BSP) – feathermossecosystem in Alaska. The rate of litter decomposition on alandscape scale is regulated by interrelated factors: (i) mi-croclimate, i.e., soil temperature and moisture; (ii ) litterquality, which includes both physical (e.g., particle size) andchemical characteristics (e.g., phenolics, nutrient concentra-tions); and (iii ) soil faunal and microbial communities

Can. J. For. Res.32: 892–902 (2002) DOI: 10.1139/X02-007 © 2002 NRC Canada

892

Received 1 May 2001. Accepted 19 December 2001.Published on the NRC Research Press Web site athttp://cjfr.nrc.ca on 8 May 2002.

T.I. Little 1 and I.G.W. Corns.2 Canadian Forest Service,Natural Resources Canada, Northern Forestry Centre, 5320 -122 Street Edmonton, AB T6H 3S5, Canada.D.J. Pluth. Department of Renewable Resources, Universityof Alberta, Edmonton, AB T6G 2E3, Canada.D.W. Gilmore. Department of Forest Resources, Universityof Minnesota, North Central Research and Outreach Centre,Grand Rapids, MN 55744, U.S.A.

1Corresponding author (e-mail: [email protected]).2Deceased.

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(Hobbie et al. 2000; Prescott et al. 2000). The effects ofthese factors may vary in nature and intensity with topo-graphic position.

The primary nutrient limitation to boreal ecosystem pro-ductivity is nitrogen (N), unless atmospheric N inputs aresignificant. Since the source for inorganic forms of soil N ismineralization of organic forms, the forest floor and the un-derlying SOM-enriched mineral horizon represent a largepotential source of plant-available N. However, the carbon(C) and N of forest floors are prone to transformations byheat energy from fire. SOM is destructively distilled be-tween 200 and 300°C, is charred between 300 and 400°C,and is oxidized (soil C loss) above 450°C (Hungerford et al.1991). Up to 50% of soil N may be volatized at 300 to500°C, although an immediate post-fire increase in NH4

+

and condensation of organic compounds in the underlyingmineral horizon have been observed (Hungerford et al.1991).

Topographic position is a fixed landscape parameter on amillennial time scale and has several associated soil andplant-community properties influencing soil developmentand boreal ecosystem functioning. Fire also transforms theforest floor, perhaps differentially by topographic position.Transformations affected by fire and by processes of forestfloor formation have ramifications for plant nutrition andglobal biogeochemical cycling of C. A topographically re-lated spatial pattern in a forest floor and in the underlyingmineral horizon may be imprinted by fire heating, but it isassumed in this study that the forest-floor properties follow-ing a severe fire are determined after closure of the upper-most tree canopy.

The aim of this study was to answer the following ques-tions: (i) what are the sizes of soil organic C and N pools inselected boreal mixedwood ecosystems initiated as a resultof wildfire at least 60 years earlier in west-central Alberta;(ii ) how are the pools spatially distributed along topo-sequences of the mixedwood ecosystems; and (iii ) how iscurrent forest productivity related to concentrations of vari-ous forms of soil organic C and N?

Materials and methods

Study areaThe study area (54°40′N, 118°13′W, 761–813 m a.s.l.) is

approximately 70 km southeast of Grande Prairie, Alberta. Itfalls within the Lower Foothills Natural Subregion(Beckingham et al. 1996) and lies on the border of the Wa-piti Plains and the Alberta Benchland Plateau physiographicregions of the Interior Plains (Twardy and Corns 1980). Thisarea has rolling relief (Agriculture Canada Expert Commit-tee on Soil Survey 1987) and is composed of glacio-lacustrine, till, and glaciofluvial materials deposited duringthe late Pleistocene (Twardy and Corns 1980).

Soils conform to the specifications of the Donnelly soilgroup and are predominantly Gray Luvisols (Twardy andCorns 1980; Agriculture Canada Expert Committee on SoilSurvey 1987) or Haplocryalfs (Soil Survey Staff 1998). Soilsat specific study sites ranged from Brunisolic and OrthicGray Luvisols at the top of the slope to Orthic Luvic Gleysolin the lower slope positions.

Regional climate of the study area is continental withlong, cold winters and mild summers (Twardy and Corns1980). The climate of the Lower Foothills Natural Subregionis characterized by a mean annual temperature of 3.0°C, andtotal annual precipitation of 464 mm (Beckingham et al.1996). Grande Prairie, Alberta, is the nearest full-timeweather station to the study area (Grande Prairie A, stationNo. 3072920, 55°11′N, 118°53′W, 666 m a.s.l.). The meanannual temperature for Grande Prairie is 1.6°C, and the totalannual precipitation is 450.2 mm (308.6 mm rainfall) basedon 30 years of data (Environment Canada 1998).

The study area is classified as boreal mixedwood, whichis characteristic of the Lower Foothills Natural Subregion(Beckingham et al. 1996). Predominant vegetation on the topslope positions is typically white spruce (Picea glauca(Moench) Voss) and trembling aspen (Populus tremuloidesMichx.) with an understory dominated by the shrubs greenalder (Alnus crispa (Ait.) Pursh), bracted honeysuckle(Lonicera involucrata(Richards.) Banks), and prickly rose(Rosa acicularisLindl.). Plant communities in top slope po-sitions are primarily categorized as Lower FoothillsEcophases e2, e3, or e4 (Beckingham et al. 1996). Whitespruce and black spruce dominate the bottom slope posi-tions, which have a Labrador tea (Ledum groenlandicumOeder), feathermoss (generallyPtilium, Pleurozium, andHylocomiumspp.), orSphagnumground cover. The bottomslope positions correspond to Lower Foothill Ecophases i3and h1 (Beckingham et al. 1996).

Sampling designA toposequence was defined as a consecutive series of

topographic positions (Beckingham et al. 1996). Topo-sequences were selected to meet several criteria. First, theywere examined to ensure there were no recent disturbances,such as human activity, disease, insect damage and (or) de-foliation, or fire since stand origin. Second, the forest was tobe dominantly spruce and aspen mixedwood. Third, thestands were to be uniform in age along a given topose-quence. Finally, trees along toposequences should not be de-grading as a result of old age, but near to the optimumrotation age with a full canopy (Table 1).

Toposequence sites were subdivided into three lineartransects then subdivided into transect positions and plots.Three nearly parallel transects separated laterally by 15 to30 m were established for replicated sampling of slope posi-tion within a toposequence site. Plots were the sampling lo-cations along each transect within a toposequence site, andtheir locations were determined by the spatial rate of changein slope angle. Specifically, one plot was established perslope change of 0.5 m height along a 10-m distance. Follow-ing plot establishment, each plot was assigned to a topo-graphic (slope) position defined by the form criteria inBeckingham et al. (1996). In the terrain of this study, thelower slope position was not designated on all topose-quences, as a result of the close proximity of adjacent plots.Therefore, only the crest, upper, middle, and toe positionswere used to compare toposequence sites and slope posi-tions. In all comparisons, the middle slope positions had agreater sample size because one site (site 1) had two transectpositions that met the form criteria of middle slope.

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Also, following completion of soil morphological descrip-tion and sampling, specific mineral soil horizons were ag-gregated into one of three horizon categories based ongenetic features (Agriculture Canada Expert Committee onSoil Survey 1987). The aggregate categories were (1) A ho-rizon, including all A horizon variations (Ah, Ahe, Ae, etc.)occurring in a soil profile, the AB transition horizon if pres-ent, and the Bm horizon developed in the A horizon zone ofpolygenetic soil profiles, e.g., the Bm in a Brunisolic GrayLuvisol; (2) B horizon, including variations of the horizon(Bt1, Bt2, Btnj, Bg, etc.), and the BC transitional horizon ifpresent; (3) C horizon, containing all variations of the hori-zon, e.g., Ck, Cca, Cg, etc.

Ecological response variablesStand data were collected from plots. Stand-level vari-

ables such as effective leaf area index (LAI), basal area(BA), tree density, and age were used to assess similarity oftoposequences. BA, mean annual increment of BA (BAI),and LAI were also used as estimates of net primary produc-tivity >1.3 m above the forest floor. Aboveground net pri-mary productivity is a measure of forest productivity.Effective LAI, BA, and BAI will be referred to as ecologicalresponse variables because they serve as surrogates for netprimary production (Gilmore and Seymour 1996).

LAI was measured with a LAI-2000 Plant Canopy Ana-lyzer (LI-COR Inc., Lincoln, Nebr.). Within-plot measure-ments were taken from nine locations (eight at every 45°azimuth along the plot perimeter plus one at the plot center)to estimate effective LAI per 10 × 10 m plot. Measurementswere taken under conditions of continuous and uniformcloud cover a few days prior to the first evidence of leafabscission. LAI was adjusted by BA (LAI adj. BA) for spe-cies composition (e.g., proportion of coniferous component)of the sample site. The adjustment was necessary since theLI-COR LAI-2000 underestimates conifer LAI by 35 to 40%(Gower and Norman 1991). The forest stand parameters ofBA (m2·ha–1), tree height (m), tree diameter outside bark(cm), sapwood area (cm2), and age at 1.3 m above ground

level were collected from sample trees selected by the pointsample method (Husch et al. 1982) from plot centers. Onlyplot centers nearest to the midpoints of the crest, lower, toe,and depression positions of the slope were sampled to pre-vent repeated measurement of large sample trees from adja-cent within-transect positions. Tree age was estimated fromtree cores extracted at 1.3 m.

Soil propertiesSoils were characterized by a pedon description and from

horizon samples within each plot along the center transect(Agriculture Canada Expert Committee on Soil Survey1987). The intensity of soil sampling varied by horizondepth according to the general knowledge of horizontal andvertical spatial variability of soil properties (Metz et al.1966; Cameron et al. 1971; Blyth and MacLeod 1978;Amponsah 1998).

Forest floor and mineral horizon samples were collectedfor determination of organic C and total N. The forest floorsamples were first ground to approximately 1-mm pieces ina Wiley mill and then ground to <2µm in a ball grinder. Themineral horizon samples were processed by a wooden roll-ing pin to <2 mm before further grinding. Ovendried soilsamples were treated with 6 M HCl before organic C and to-tal N were determined by the Dumas combustion method us-ing a Carlo Erba NA 1500 Analyzer (McGill and Figueiredo1993).

Organic C and total N pools of the forest floor and min-eral horizons were calculated for each horizon (m/v) usingindependent estimates of element concentration, soil horizonthickness, and bulk density (e.g., this reduces to kg C·m–2 ofsoil horizon). Horizon thickness for the plots along the lat-eral transects was assumed to be the same as that reported inthe morphological description for the specific center transectposition and toposequence site. Although sampling intensitywas greatest for the forest floor and A horizons, only meanvalues of each were used to calculate one horizon estimateper plot. C/N ratios are based on mass.

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894 Can. J. For. Res. Vol. 32, 2002

Toposequence attribute Unit Site 1,n = 15 Site 2,n = 12 Site 3,n = 12

Physiographic attributesAspect South Southeast NorthMaximum slope length m 427 145 300Mid elevation m a.s.l. 777 816 768Total change in elevation m 24 8 12

Forest attributesMaximum tree age at 1.3 m Age deciduous, age coniferous 104, 88 (crest, lower)a 93, 78 (crest, middle)a 80, 94 (middle, toe)a

Stand composition by BA % deciduous, % coniferous 66, 34 45, 55 40, 60Stand density trees/ha 1826 1293 2500BA m2·ha–1 41±2 38±2 30±3BAI m2·ha–1·year–1 0.55±0.04 0.59±0.03 0.65±0.06LAI m 2·m–2 2.7±0.2 3.1±0.2 3.1±0.1

Soil attributesForest floor thickness cm 6.2±0.7 5.8±0.6 6.0±1.1Forest floor organic C kg·m–2 2.8±0.4 1.9±0.2 2.1±0.5A horizon organic C kg·m–2 2.3±0.4 2.2±0.6 1.4±0.2

Note: BA, basal area at 1.3 m height; BAI, mean annual basal area increment at 1.3 m height; LAI, leaf area index above 1.3 m height.aSlope positions at which the oldest trees were found.

Table 1. Summary of physiographic, forest, and soil attributes of the three toposequence sites in 1998 (mean ± SE).



Net N mineralization rates were estimated by in situ incu-bation of unmixed core samples of forest floor (225 cm2)and of the upper 10 cm of the A horizon. About one-half ofa core sample was retained for an estimate of initial inor-ganic N and the remainder was returned, in a 11µm thickpolyethylene bag, to its original horizon position (Dow Cus-tomer Service, personal communication). The in situ incuba-tion of the forest floor material was duplicated in space (twoper plot) and time (first set installed 27 to 31 July and re-moved 12 to 15 August; second set installed 12 to 15 Augustand removed 31 August). One A horizon core sample wasincubated per plot between 27 July and 31 August 1998. Nocomprehensive soil temperature data for the study sites wereavailable. However, at 53.5°N, 114.2°W, mean August soiltemperature was the seasonally highest at 14°C at 10 cmdepth in Gray Luvisolic soils under a closed aspen canopy,indicating that near-surface soil temperatures would likelypeak during early to mid-August in our study area (Macyk etal. 1978). All incubated soil samples were stored in the fieldat 3 to 6°C for 4 days or less, then stored at –9°C in the lab-oratory, until extraction of NH4

+ and NO3–. The KCl-

extracted NH4+ and NO3

– concentrations were quantified byautoanalyzer (Maynard and Kalra 1993).

Bulk density was determined for the forest floor and min-eral horizons. The forest floor was sampled by systemati-cally removing 900 cm2 and recording undisturbed horizonthickness. Mineral horizons were sampled by Uhland corer(331 cm3). Dry bulk density was determined according tothe procedure outlined by Culley (1993). Mineral soil sam-ples were dried at 105°C, and organic horizon material (L, F,and H) at 75°C, to constant mass.

Statistical analysesA two-way analysis of variance (ANOVA) was used to

test the effects of slope position on ecological response vari-ables and soil properties. The assumptions for the analysis ofvariance were satisfied, i.e., random experimental errors,normally and independently distributed with a zero meanand common variance (Steel and Torrie 1980; McClave andDietrich 1991). ANOVA analyses were performed usingSPSS 6.1.1 (SPSS, Inc. 1995).

Results

Toposequence sitesThe maximum age for toposequence sites was used to es-

timate the time since the last presumed stand-originating fire(Table 1). Maximum mean time elapsed since the last stand-originating fire was 92 ± 12 years (mean ± SE) based on themaximum age of aspen. Maximum deciduous ages occurredin the crest, upper, and middle slope positions on all topo-sequences.

Boreal stands in which no single species exceeds 80% ofthe BA are considered mixedwood (MacDonald 1995). Per-cent stand composition was calculated from toposequenceBA (Table 1), and all three toposequence sites qualify as bo-real mixedwood stands by this criterion. Mean conifer spe-cies composition, by BA, for all toposequences by slopeposition was 30, 36, 42, and 79% for crest, upper, middle,and toe positions, respectively. Deciduous trees were mostlytrembling aspen, and coniferous trees were typically white

spruce. Subdominant tree species were balsam poplar(Populus balsamiferaL.) and black spruce.

LAI (Fig. 1), organic C in the B horizon, and forest floortotal N (g·100 g–1) had significant interactions of topose-quence site and slope position (Tables 2 and 3). The LAI in-teraction (p = 0.002) was expressed as 2.90 ± 0.39 m2·m–2 atthe toe of the slope of site 3, which was much lower thanvalues obtained for the same slope positions of site 1 (3.87 ±0.39 m2·m–2) and site 2 (3.69 ± 0.35 m2·m–2). The mean BAof site 3 was about 11 m2·ha–1 less than that of sites 2 and 3(p = 0.01). The interaction of site and slope position for or-ganic C in the B horizon (p = 0.006) was due to the toe ofsite 3 having about 9 kg·m–2 more C than any other slopeposition on any site. The mean organic C of site 3 was

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Little et al. 895

1

2

3

4

5

Crest

Upper Slope

Middle Slope

Toe

0

(a)

1

2

3

4

5

0

(b)

1

2

3

4

5

0

(c)

1

2

3

4

5

LA

I

0

(d)

Site 1 Site 2 Site 3

LA

ILA

ILA

I

Fig. 1. Leaf area index (LAI, m2·m–2) according to slope posi-tion and toposequence site. Values are means ± SEs.



1.7 kg·m–2 more than that of the other two sites (p < 0.001).The interaction of site and slope position for forest floor to-tal N (p = 0.019) was due to the crest position of site 1 hav-ing a higher value (1.99 g·100 g–1) than the crest of sites 2and 3 (1.21 and 1.45 g·100 g–1, respectively). Overall, the to-tal N of the forest floor was greater at site 1 (1.72 g·100 g–1)compared with sites 2 and 3 (1.29 and 1.53 g·100 g–1, re-spectively; p < 0.000). The mean C/N ratio of the forestfloor was three times greater at site 2 than at site 3, and inthe A horizon, the C/N ratios at sites 2 and 3 were abouteight times greater than that at site 1. There were no otherdifferences detected among sites by BAI (Table 2) or for allthe remaining soil properties by horizon (Table 3).

As the results from the two-way ANOVA showed that thethree toposequences were generally similar, the data from allthree sites were pooled by slope position for further analysis.

Slope positionLAI was the only ecological response variable that was

different among slope positions (p = 0.002; Table 2). Over-all, the mean LAI of the toe slope position was 3.5 m2·m–2,which was at least 0.6 m2·m–2 greater (p < 0.05) than that ofany of the upslope positions (crest, upper, and middle; Ta-ble 4).

Slope position influenced organic C (kg·m–2), C/N ratio,rate of specific net N mineralized (mg·(g total N)–1·14 d–1),rate of net mineralized N (mg·m–2·14 d–1), and the NO3-N/NH4-N ratio. Organic C differed (p < 0.02) between twoor more slope positions in the solum horizons, with the toeposition having the highest value (Table 5, Fig. 2). The Ahorizon total N was greater at the toe, 0.19 ± 0.05 g·100 g–1,than at the top of the slope, 0.10 ± 0.01 g·100 g–1 (Table 5,p = 0.030). Forest floor and A horizon C/N values at the toe

were 4.6 and 2.5 greater, respectively, than those at the topslope positions (Table 6). There was no change in rate ofspecific net N mineralization by slope position (Table 3), butthe trend for the rate of net N mineralization in the A hori-zon at the toe was at least 49 mg·m–2·14 d–1 less than that atthe top of the slope (Table 6). No differences were detectedamong slope positions for forest floor or A horizon for NO3-N/NH4-N ratio, which approached zero values in the forestfloor (Table 7).

Soil properties and the ecological response variablesPearson’s correlation was used to determine the strength

of the relationships between leaf area ecological responsevariables and soil properties. Both LAI and LAI adjusted forthe proportion of conifer BA in the stand (LAI adj. BA)were used in the correlation matrices, since the LI-CORLAI-2000 has been shown to underestimate conifer LAI byup to 40% (Gower and Norman 1991). Soil properties forwhich the effect of slope position was strongest according tothe two-way ANOVA (Table 3) also showed the strongestcorrelation with leaf area ecological response variables(Fig. 3). The correlation of LAI and C/N ratio of the forestfloor and A horizon produced significant Pearson’s correla-tion coefficients (r) (Figs. 3a and 3b), and their values wereincreased by using LAI adj. BA (Figs. 3c and 3d). Organic Cwas weakly correlated with the leaf area ecological responsevariables, since allp values were >0.05 (not shown), but to-tal N (g·100 g–1) had anr = –0.35 (p = 0.027) for LAI andr = –0.49 (p = 0.001) for LAI adj. BA.

Discussion

Toposequence sites were relatively similar according tomost of the examined ecological response and soil propertyvariables. Among sites, BA and LAI were not different (p >0.01), nor was BAI different (p = 0.444). Of the 15 soil hori-zon properties examined, only four were different amongsites (p = 0.05). Those four properties were the B horizonvolumetric organic C (kg·m–2), forest floor and A horizonC/N ratio, and forest floor total N (g·100 g–1). The generalsimilarity of the toposequences suggests that slope aspectwas not a significant site property for tree growth, especiallyin comparison with slope position. Based on this informa-tion, the slope positions were compared by pooling the dataof the three toposequences (Tables 4 to 7).

Among slope positions, differences in ecological responsevariables were primarily observed between the top and bot-tom of the slope (Table 4). No change in LAI was detectedamong crest, upper, and middle slope positions, but LAI wasat least 0.6 m2·m–2 less at the top (crest, upper, and middle)than at the bottom (toe) of the slope.

Soil organic CThe trend among slope positions for stored organic C to

1.20 m depth (Fig. 2) was similar to that of LAI; differencesoccurred primarily between the bottom (21.2 kg·m–2) andtop slope positions (13.1 to 14.6 kg·m–2). In the forest floor,there was 3.5 kg C·m–2 at the toe position, which was a netaccumulation of 1.4, 1.7, and 1.3 kg C·m–2 more than at thecrest, upper, and middle slope positions, respectively (p =0.05). These concentrations are comparable to those at an

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896 Can. J. For. Res. Vol. 32, 2002

Source ofvariation df MS p

LAI (m 2·m–2)T 2 0.762 0.020S 3 1.089 0.002T × S 6 0.827 0.002Error 24 0.164BA (m2·ha–1)T 2 423.528 0.010S 3 74.407 0.419T × S 6 42.046 0.763Error 24 75.972BAI (m 2·ha–1·year–1)T 2 0.024 0.444S 3 0.029 0.400T × S 6 0.007 0.953Error 24 0.028

Note: BA, basal area at 1.3 m height; BAI,mean annual basal area increment at 1.3 mheight; LAI, leaf area index above 1.3 mheight; T, toposequence; S, slope.

Table 2. Two-way ANOVA models forthe comparison of ecological responsevariable values among toposequencesites and slope positions.



80-year-old aspen stand growing on Gray Luvisolic soils de-veloped in till of boreal Saskatchewan, where forest floor or-ganic C was 3.9, 6.0, and 3.8 kg·m–2 at the upper, lower, andtoe slope positions, respectively (Huang and Schoenau1996). Their summary from other studies indicated that soilC storage in southern boreal forest floors of Saskatchewanranged from 1.7 to 5.9 kg·m–2 with topographic position un-specified. The forest floors in our study accounted for 14(crest), 12 (upper), 16 (lower), and 17% (toe) of the organicC stored to a depth of 1.2 m. The boreal forest floors ofHuang and Schoenau (1996) accounted for up to 47% of thetotal organic C to 1 m depth.

Assuming zero residual organic matter from a pre-fire for-est floor, the mean rate of organic C accumulation in thecurrent 90 ± 12 year-old forest floors of our toposequencesranged from 19 (upper) to 39 g·m–2·year–1 (toe) versus from47 (toe) to 75 g·m–2·year–1 (lower) in the 80-year-old,boreal-aspen Saskatchewan toposequence (Huang andSchoenau 1996); whereas a 120-year-old Swedish boreal for-est floor underPinus sylvestrisL. on a level, rapidly drained,nutrient-poor Brunisol averaged only 13 g C·m–2·year–1 (Berget al. 2001). From this comparison among post-fire, borealforest floors, tree species and topographic position appear toinfluence the rate of organic C accumulation.

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Little et al. 897


T × S 6 37.878 0.260Error 24 27.280

A horizon (0–10 cm)T 2 1.660 0.410S 3 3.626 0.138T × S 6 0.740 0.863Error 24 1.793

Net mineralized N (mg·m–2·14 d–1)Forest floor

T 2 28 421.248 0.567S 3 20 831.623 0.736T × S 6 40 868.960 0.555Error 24 48 936.792

A horizon (0–10 cm)T 2 54 971.753 0.062S 3 21 925.796 0.311T × S 6 20 293.088 0.357Error 22 17 321.365

NO3-N/NH4-NForest floor

T 2 0.009 0.727S 3 0.008 0.842T × S 6 0.042 0.241Error 27 0.029

A horizon (0–10 cm)T 2 9.152 0.309S 3 7.812 0.387T × S 6 6.411 0.534Error 23 7.404

Note: T, toposequence; S, slope.

Table 3 (concluded).


Horizon thickness (cm)T 2 1.88 0.832S 3 32.674 0.006T × S 6 6.799 0.380Error 24 6.062Organic C (kg·m–2)Forest floor

T 2 3.408 0.086S 3 5.377 0.015T × S 6 1.272 0.439Error 24 1.253

A horizonT 2 0.434 0.820S 3 9.178 0.015T × S 6 0.819 0.885Error 24 2.164

B horizonT 2 45.282 0.041S 3 51.387 0.016T × S 6 50.769 0.006Error 24 12.333

C/N ratioForest floor

T 2 295.621 0.000S 3 73.376 0.003T × S 6 26.647 0.079Error 24 12.135


B horizonT 2 23.171 0.296S 3 62.502 0.032T × S 6 7.778 0.852Error 24 18.091

Total N (%) or (g·100 g–1)Forest floor

T 2 0.581 0.000S 3 0.076 0.073T × S 6 0.093 0.019Error 24 0.029


Specific net mineralized N

(mg·(g total N)–1·14 d–1)Forest floor

T 2 22.582 0.449S 3 47.854 0.183

Table 3. Two-way ANOVA models forthe comparison of soil property valuesamong toposequence sites and slope po-sitions.



In our toposequences, the mineral horizons of the imper-fectly to poorly drained soils at the toe had greater amountsof stored organic C than those of the well to moderately welldrained soils at the top positions. This contributed to the rel-ative 50% increase of organic C in the soil profiles at the toe(Fig. 2). The A and B horizons at the toe contained greateramounts of organic C than those of the top positions (Ta-ble 5). As an indicator of a difference in the biochemicalcomposition of SOM of these mineral horizons, C/N ratioswere highest at the toe (Table 6). The significant differencesin organic C in soil horizons among the slope positions, par-ticularly the top compared with the toe, suggest processescontrolling SOM transfers and transformations differ in ratewith topographic position. The following discussion dwellson measured and inferred factors that are thought to controlprocess rates in an attempt to explain the observed spatialpatterns in soil organic C (SOC) along the toposequences.

We assume that most organic matter enters these domi-nantly Gray Luvisols of the toposequences as abovegroundand belowground plant litter, and most organic C exits as re-spired C. The observed differences in the amount of SOCamong topographic positions are explainable by factors andprocesses internal to the respective soil–plant systems ofeach slope position: (i) organic C in the forest floor is afunction of a dynamic net balance between rates of commu-nity net primary production and decomposition of litter,(ii ) organic C in the mineral horizons is a function of trans-formations of organic matter and downward transfers withinthe profile.

Evidence for lateral transfer downslope of particulate mat-ter as sediment was absent. For example, A horizon thick-ness did not significantly vary along a toposequence(ANOVA not shown), and displaced plant charcoal was notvisible within the A horizon. However, dissolved and colloi-dal organic C (not measured) may have moved laterallywithin the more permeable A horizon, which overlaid thefiner-textured and less permeable Bt horizon. Within soilprofile transfers are discussed further in this section.

Plant productivity, which generates forest litter, is the pri-mary source of SOC (Van Cleve and Powers 1995). Higherrates of forest litter addition may result in greater accumula-tions. The LAI values (Table 4), if a valid surrogate foraboveground net primary productivity, indicate that the plantcommunity at the toe is most productive. As noted before,the toe, with 6.0 cm of forest floor, has a net accumulationof SOC about 1.7 times greater than the 5 cm thick forestfloor at the crest (Table 5). In general, the decomposition

© 2002 NRC Canada

898 Can. J. For. Res. Vol. 32, 2002

Slope position BA (m2·ha–1)a BAI (m2·ha–1·year–1)b LAI (m 2·m–2)c

Crest 32.2±4.3 0.54±0.05 2.8±0.2aUpper slope 35.6±3.0 0.57±0.11 2.7±0.2aMiddle slope 39.4±1.8 0.58±0.03 2.9±0.1aToe 37.4±2.9 0.69±0.08 3.5±0.2b

Note: Values are means ± SEs. Within-column means with the same letter are notsignificantly different (p > 0.05).n = 12 for the middle slope position, andn = 9 for theother positions.

aBA, basal area at 1.3 m height.bBAI, mean annual basal area increment at 1.3 m height.cLAI, leaf area index above 1.3 m height.

Table 4. Comparison of ecological response variables by slope position withpooled toposequence site data.

Cumulative Organic C (kg·m )-2

20

40

60

80

100

120

0

Site 123 O. GL

BR. GLO. GL Site 1

23 O. GL

GLE. EBO. LG

(d) Toe(c) Middle

(b) Upper(a) Crest

Site

0

123

4

20

40

60

80

100

120

12 16 20 24

0

8 0 4 12 16 20 248

BR. GL

O. GLBR. GL

Site 123 O. GL

BR. GLO. GL

Depth

(cm

)D

epth

(cm

)

Fig. 2. Cumulative organic C (mean with 95% confidence limits)by depth within soil profiles for each slope position. Soil groupsalong the centre transect of toposequence sites are as follows:Orthic Gray Luvisol (O. GL), Brunisolic Gray Luvisol (BR. GL),Orthic Luvic Gleysol (O. LG), and Gleyed Eluviated EutricBrunisol (GLE. EB).



rate of forest litter is controlled by substrate quality, temper-ature, and moisture (e.g., Kimmins 1996). Temperature in-creases of 4 to 10°C have been shown to significantlyincrease rates of litter decomposition and net soil N mineral-ization (Hobbie 1996). In this study, A horizon temperatureat 5 cm depth during August was 1–2°C lower at the toethan at the crest regardless of aspect. Furthermore, the meansummer temperature difference in boreal forest floors be-tween the crest and toe positions were reported to be <4°C(aspen canopy, central Alberta, Macyk et al. 1978). Thus,temperature was not likely a controlling factor of decompo-sition rate along our toposequences, albeit a contributingfactor. The moisture factor has its control upon decomposi-tion over an oxic to anoxic gradient (e.g., Hobbie 2000). Ananoxic state in our toe forest floor was an infrequent occur-

rence, as indicated by short-term measured water contents inthe toe A horizon (Little 2001). On the other hand, thehigher C/N ratios of the toe forest floor and A horizon (Ta-ble 6) and mottling observed in the A horizon indicate peri-ods of anaerobic decomposition. Since lignin decompositionis restricted to aerobic organisms, mainly brown- and white-rot fungi (Berg and Matzner 1997), periodic anaerobism inLuvisolic forest floors and A horizons may contribute to alower limit of decomposition of litter fall and root litter thanestimated as a simple function of litter N concentration. Thislatter relationship is discussed next.

Although the chemical composition of the forest litter orthe SOM was not determined, the C/N ratio of litter may bea crude proxy for lignin concentration. Lignin is one of themost recalcitrant components of boreal forest litter (Berg

© 2002 NRC Canada

Little et al. 899

Crest Upper Middle Toe p

Forest floorOrganic C 2.0±0.4a 1.7±0.4a 2.1±0.3a 3.5±0.5b 0.015Total N 0.08±0.02 0.07±0.02 0.09±0.01 0.12±0.02 0.189A horizonOrganic C 3.9±0.3b 2.7±0.4a 2.8±0.2a 4.9±0.8b 0.015Total N 0.32±0.03b 0.20±0.02a 0.24±0.02a 0.31±0.05ab 0.022B horizonOrganic C 4.9±0.3a 6.7±0.8ab 5.5±0.6a 10.1±2.9b 0.016Total N 0.75±0.04 0.97±0.13 0.88±0.06 0.85±0.15 nab

C horizona

Organic C 3.4 3.5 2.9 2.7 naTotal N 0.46 0.44 0.42 0.35 naTotal 1.2-m soil profileOrganic C 14.1 14.6 13.3 21.2 naTotal N 1.61 1.68 1.63 1.63 na

Note: Values are means ± SEs. Within-row means with the same letter are not significantlydifferent (p > 0.05).n = 12 for the middle slope position, andn = 9 for the other positions.

aC horizon crest and upper,n = 2; middle and toe,n = 3; all to a depth of 1.20 m of mineralprofile.

bna, not applicable.

Table 5. Organic C and total N pools (kg·m–2) for each slope position by soil hori-zon and profile total.


C/N ratioForest floor 28.0±2.3a 26.2±1.4a 25.9±1.5a 32.6±2.0b 0.003A horizon 12.3±1.0a 13.1±0.9a 11.8±0.6a 15.6±0.6b 0.002B horizon 6.7±0.2 7.1±0.6 6.4±0.2 11.7±2.6 0.032C horizona 7.4 7.7 7.2 9.0 nab

Rate of specific net N mineralized (mg·(g total N)–1·14 d–1)Forest floor –0.923±1.621 3.312±2.075 3.343±1.754 0.402±0.789 0.183A horizon, 0–10 cm 1.418±0.505 1.276±0.355 0.907±0.390 0.015±0.307 0.138Rate of net N mineralized (mg·m–2·14 d–1)Forest floor 18.2±67.6 81.9±37.0 100.8±31.8 7.0±115.6 0.736A horizon, 0–10 cm 183.3±61.1 119.9±37.0 122.3±45.3 70.9±51.6 0.311Forest floor + A horizon 181.1±8.9 201.9±55.0 192.5±44.3 77.9±138.0 0.584

Note: Values are means ± SEs. For a given soil N variable, within-row means with the same letter are notsignificantly different (p > 0.05).n = 9 for each slope position by solum horizon cell except for the middleslope wheren = 12. For the C horizon,n = 3.

aValues to a depth of 1.20 m.bna, not applicable.

Table 6. Variables related to soil N availability according to slope position and soil horizon.



and Matzner 1997). In the literature, two litter-chemistry pa-rameters have been correlated with boreal leaf-litter decom-position: (i) lignin/N ratio (e.g., Berg and Matzner 1997) and(ii ) litter total N concentration and end limit to decomposi-tion (Berg et al. 2001). Perhaps the lignin concentration“will determine when litter N will have a rate-enhancing or-suppressingeffect” (Berg and Matzner 1997). Berg et al.(2001) found a negative linear relationship between the endlimit of litter decomposition (the asymptotic value of littermass as a function of time) and initial N concentration ofleaf litter. If this relationship derived for European borealspecies of undisturbed forests with mor humus forms holdstrue for North American boreal species, white spruce needlelitter with an initial N concentration of 7.0 g·kg–1 (Alaskan

upland, Ruess et al. 1996) and black spruce needle litterwith 2.3 g·kg–1 (Alberta peatland, Mugasha et al. 1996)would have an asymptotic limit of 76 and 83% decomposi-tion, respectively. On the other hand, aspen leaf litter at6.0 g N·kg–1 (Alberta Foothills, Taylor et al. 1989) or1.0 g N·kg–1 (Van Cleve et al. 1983) would have 78 and71% limits, respectively. On the basis of these similar litterdecomposition limits inferred from literature values of Nconcentration of fresh leaf fall, our measured differences inthe mass of organic C in forest floors (Table 5) between topand toe positions cannot be explained by the variable pro-portions of conifers (Table 1).

Inferred differences in annual leaf fall between top (70%Populus) and toe (79%Picea) positions also do not explainthe C mass differences in the forest floors. White spruce leaffall and total litter fall were 137 and 187 g·m–2·year–1, re-spectively (Alaskan upland, 36% deciduous, Ruess et al.1996). Aspen leaf fall was 205 g·m–2·year–1 plus59 2 1g m year⋅ ⋅− − of Corylus (E.H. Hogg, personal communi-cation; boreal Saskatchewan, aspen BA = 33.5 m2·ha–1).

A vertical transfer of colloidal C from the forest floor tothe A and B horizons is suggested by the sequence of meanvalues for stored organic C (Table 5) and C/N ratio (Table 6)in the toe soil profiles. Some of the organic C of our B hori-zons at any slope position may be precipitated soluble C.Downward leaching of soluble C and colloidal C occurred asmeasured in gravity–lysimeter samples collected within theupper Bt horizon of a moderately well drained Gray Luvisol(central Alberta, 53.1°N, 114.2°W) developed in till at acrest position and under an aspen – white spruce canopy(Howitt and Pawluk 1985). Their Bt horizon, with4.0 g C·kg–1, had slightly more organic C than the overlyingAe horizon and the underlying BC horizon, as evidence ofilluvial SOM. Polyphenols, polyuronides, and polysaccha-rides determined in canopy drip and horizon leachates ac-counted for transfer of the soluble C. In a laboratoryexperiment, Donald et al. (1993) found sorption of a hydro-phobic fraction of dissolved organic C (DOC) on a Bt sam-ple of a boreal Gray Luvisol only at a toe position. Theyspeculated that DOC was transported by lateral flow. By in-ference, part of the organic C in the subsurface horizons ofespecially the toe position of our toposequences may be at-tributed to vertical and lateral transfers of colloidal and solu-ble C. We assume that another part of the subsurface organicC is biochemically transformed belowground litter.

Soil N and net ecosystem productivityThe greater plant community productivity (LAI) at the

bottom of the slope seems incongruent with the topographictrend in N availability, but the greater humus (SOC) accu-mulation may improve other attributes for vascular plant

© 2002 NRC Canada

900 Can. J. For. Res. Vol. 32, 2002


Forest floora 0.05±0.05 0.01±0.01 0.01±0.08 –0.01±0.02 0.842A horizon, 0–10 cmb 1.99±1.97 –0.35±0.24 –0.13±0.09 –0.10±0.23 0.387

Note: Values are means ± SEs. Negative values occur if net N immobilization occurred in either NO3

or NH4, but not both.aForest floor middle position,n = 12; other positions,n = 9.bA horizon crest position,n = 8; other positions,n = 9.

Table 7. NO3-N/NH4-N ratio according to slope position and soil horizon.

Forest floor A horizon

5 1015 20 25 30 35 40 45

1

2

3

4

5

0R = 0.18

2

C/N

LA

I

(a)

R = 0.092

C/N

12

3

4

5

0

LA

I

(b)

15 20 25

6

12345

0

LA

Iadj.

BA

I

(c)6

12345

0

LA

Iadj.

BA

I

(d)

15 20 25 30 35 40 45

R = 0.342

C/N

5 10

R = 0.152

C/N

15 20 25

6

12

345

0

LA

Iadj.

BA

I

(e)6

12

345

0

(f)

LA

I

Total N (g·100 g )-1

Net N Mineralization Rate(mg·m ·14 d )

-2 -1

0.5 1.0 1.5 2.0 2.5

R = 0.242

R = 0.082

-500 500 1500

Fig. 3. Ecological response variables vs. soil N parameters: leafarea index (LAI, m2·m–2) vs. (a) forest floor C/N (r = 0.43, p =0.007), (b) A horizon C/N (r = 0.29, p = 0.074), and (f) A hori-zon rate of net N mineralization by volume (r = −0.26, p =0.137). Leaf area index adjusted for percent coniferous basalarea (LAI adj. BA, m2·m–2) vs. (c) forest floor C/N (r = 0.58,p = 0.000), (d) A horizon C/N (r = 0.38, p = 0.016), and(e) forest floor total N (r = –0.49,p = 0.001).



growth. One expects less soil N availability when the soilC/N ratio is highest and the trend for net N mineralized isthe lowest (Table 6), which occurs at the bottom of the slopein this study. In this situation, vascular plants would be ex-pected to compete with microorganisms for nutrients, suchas inorganic N, causing a progressive immobilization ofplant nutrients (Prescott et al. 2000). However, as morereadily decomposable fractions of SOM decrease, lignin/N(recalcitrance) increases and humification begins. While theorganic C pools of the forest floor and A and B horizons arelargest at the toe, only the total N pool of the A horizon islargest for the toe position (Table 5). Among the three hori-zons, the C/N ratio is highest at the toe (Table 6). However,within the range of C/N values of 27 (top) to 33 (toe), rates ofnet N mineralization did not vary, indicating a statistical dif-ference is not necessarily meaningful in N mineralization–immobilization. It is, however, important to note that abilityto detect differences in rates of net N mineralization in thisstudy was compromised by the magnitude of the spatial vari-ability (Tables 4 and 6). Nitrogen nutrition of these anthro-pogenically undisturbed mixedwood communities seems tobe predominantly ammonium, regardless of slope positionand near-surface horizon (Table 7).

The scope of this paper limits the soil factors possibly in-fluencing ecosystem productivity to the SOM–nitrogen com-plex. Since the rate of net N mineralization is not differentbetween the top and bottom of the slope, one may speculatethat the higher productivity at the toe site is, at least in part,a result of the beneficial effects of SOM. Although there isan apparent association between SOM and productivity, ithas not been well defined (Fisher 1995). Correlations ofSOC (not shown), C/N ratio, and the rate of net N mineral-ization with ecological response variables were weak. How-ever, among those three soil variables there is some evidencethat forest floor and A horizon C/N ratio can be useful forestimation of LAI (Figs. 3a and 3b). Pearson’s correlationcoefficients for the relationship between LAI adjusted forthe coniferous component of the forest stand and C/N ratioof the forest floor and A horizon were larger (Figs. 3c and3d). There was a stronger negative relationship between LAIand total N concentration on a mass basis (Fig. 3e), in com-parison with the rate of net N mineralized in the A horizon(Fig. 3f, soil volume basis). Although total N concentrationon a mass basis in the forest floor and A horizon differs withslope position (Table 3), the rate of specific net N mineral-ization does not (Table 6), which, along with the highestC/N ratio at the toe, suggests that N is more recalcitrant atthe toe than at the top positions. In boreal Gray Luvisols incentral Alberta supporting fire-origin aspen – white spruce,the specific net N mineralization rate at 22°C did not varyfrom upper to middle slope positions (Offord 1999). Asmentioned above, periodic anaerobic soil in the toe positionmay be decreasing the fungal component of the microbialcommunity to suppress lignin decomposition (Berg andMatzner 1997), thus increasing N recalcitrance, as expressedby lignin/N ratio.

Although differences in LAI of mixedwood communitiesamong slope positions correlate with soil parameters inter-preted for N availability, a causal relationship between LAIand soil N parameters can be questioned. Moreover, non-Nparameters not included in this study may covary with slope

position and thus confound a supposedly simple relationshipbetween soil N and LAI.

Conclusions

Toposequence sites were generally similar according tothe ecological response variables of the aspen – white sprucecommunities and most soil properties examined. However,the ecological response variables and the soil properties didvary among slope positions. LAI of the toe slope positionwas at least 0.6 m2·m–2 greater than that of other slope posi-tions. The mean soil profile organic pools under the 92-year-old stands varied from 13.3 to 21.2 kg C·m–2 to a depth of1.2 m for the top and bottom slope positions, respectively.The organic C pools of the forest floor and A and B hori-zons all contributed to the increase at the toe slope, a trendthat was similar to that of LAI. Trends from inferred valuesof end limit of litter decomposition and of leaf fall mass ap-peared not to explain the largest organic C pool at the toeposition. Inferred vertical transport of colloidal and solubleC from the forest floor to the mineral horizons of the GrayLuvisols could explain the measured increases of organic Cin A and B horizons at the toe. The highest C/N ratio in theforest floor was at the bottom of the slope as was the lowestrate of net N mineralization. LAI adjusted for spruce compo-sition correlated positively with C/N ratio and negativelywith total N concentration of the forest floor and A horizon.These results suggest that soil N may play an influential rolein limiting plant community productivity at the bottom ofthe slope. However, some factors other than soil N probablyhave a greater influence on limiting productivity at the topof the slope on the toposequence sites, e.g., available soilwater (Little 2001).

Acknowledgements

The authors gratefully acknowledge the financial and lo-gistic support of Canadian Forest Products Ltd., and supportfrom the Department of Renewable Resources, University ofAlberta, and the Department of Forest Resources, Universityof Minnesota, as well as the constructive comments of thereviewers and the Associate Editor.

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post-fire forest floor development along toposequences of white spruce - trembling aspen mixedwood...

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