the influence of partial harvesting and forest floor disturbance on nutrient availability and...

The influence of partial harvesting and forest floordisturbance on nutrient availability and understoryvegetation in boreal mixedwoods

Brent R. Frey, Victor J. Lieffers, Alison D. Munson, and Peter V. Blenis

Abstract: The impacts of partial cut systems on nutrient availability and understory vegetation are poorly understood.To examine these responses, white spruce dominated stands in the boreal mixedwood of Alberta were clear-cut orpartial-cut and the forest floor treated by slash burning, mixing, mounding, or scalping in a split-plot design. Soil nutri-ent availability (ion exchange resin), net N mineralization (in situ incubations), and vegetation (density and cover)responses were assessed. With the exception of higher Mg availability in the clearcuts, differences in nutrient availabil-ity were driven by forest floor disturbance and not harvest method. Relative to controls, burning increased availabilityof NH4

+, NO3–, and P, and scalping increased Ca and Mg but diminished K. Controls had low levels of NO3

–. Themixing treatment substantially reduced net N mineralization. In terms of vegetation, partial cuts reduced root suckeringby Populus spp. (Populus tremuloides Michx., Populus balsamifera L.) relative to clearcuts. Burning and moundingstimulated fireweed (Epilobium angustifolium L.) cover, while scalping increased Populus spp. sucker density. In con-trast, mixing largely reduced vegetation establishment, likely because of the destruction of roots and rhizomes and re-duced N supply. Nutrient availability and vegetation establishment were more strongly controlled by forest floordisturbance than by partial canopy retention.

Résumé : Les impacts des systèmes de coupe partielle sur la disponibilité des nutriments et la végétation en sous-étagesont peu connus. Dans le but d’étudier ces réactions, des coupes à blanc et des coupes partielles ont été pratiquéesdans des peuplements dominés par l’épinette blanche dans la forêt boréale mélangée de l’Alberta et quatre traitementsde préparation de terrain ont été appliqués selon un plan expérimental en tiroir. Ces traitements comprenaient le brû-lage des déchets de coupe, le mélange du sol, la mise en buttes et le scalpage du sol. La disponibilité des nutrimentsdans le sol (échange d’ions sur résine), la minéralisation nette de N (incubations in situ) et les réactions de la végéta-tion (densité et couverture) ont été mesurées. À l’exception de la disponibilité accrue de Mg dans la coupe à blanc, lesdifférences dans la disponibilité des nutriments étaient dues aux perturbations de la couverture morte et non à la mé-thode de récolte. Relativement au traitement témoin, le brûlage des déchets de coupe a augmenté la disponibilité deNH4

+, NO3– et P et le scalpage a augmenté celle de Ca et Mg mais diminué celle de K. Les témoins avaient un niveau

de NO3– faible. Le mélange du sol a substantiellement réduit la minéralisation nette de N. Concernant la végétation, la

coupe partielle a réduit le drageonnement de Populus spp. (Populus tremuloides Michx. et Populus balsamifera L.)comparativement à la coupe à blanc. Le brûlage et la mise en buttes ont favorisé le couvert d’épilobe à feuilles étroites(Epilobium angustifolium L.) tandis que le scalpage a augmenté la densité des drageons de Populus spp. Par contre, lemélange du sol a fortement réduit l’établissement de la végétation, probablement à cause de la destruction des racineset des rhizomes et de la faible quantité de N. La disponibilité des nutriments et l’établissement de la végétation ont étéplus fortement contrôlés par la perturbation de la couverture morte que par la rétention du couvert.

[Traduit par la Rédaction] Frey et al. 1188

Introduction

In mixedwood boreal forests of Canada, establishment ofwhite spruce following logging is difficult (Lieffers and Beck1994). The primary reason for this is the invasion of shade-intolerant vegetation following logging (Hogg and Lieffers

1991a) or fire (Dyrness and Norum 1983), which greatly re-duces the access of conifer seedlings to light, nutrients, andwater (Munson et al. 1993; Thevathasan et al. 2000). Soilmicrosite conditions are also critical to the establishmentand growth of planted white spruce. Cold soil temperaturesand nutrient-deficient microsites can impair the growth of

Can. J. For. Res. 33: 1180–1188 (2003) doi: 10.1139/X03-042 © 2003 NRC Canada

1180

Received 3 April 2002. Accepted 29 January 2003. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on26 May 2003.

B.R. Frey,1,2 V.J. Lieffers, and P.V. Blenis. Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1,Canada.A.D. Munson. Centre de recherche en biologie forestière, Faculté de foresterie et de géomatique, Université Laval, Sainte-Foy,QC G1K 7P4, Canada.

1Corresponding author (e-mail: [email protected]).2Present address: Department of Renewable Resources, 4-42 Earth Sciences Building, University of Alberta, Edmonton, AB T6G 2E3,Canada.

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Frey et al. 1181

white spruce seedlings (Grossnickle 2000). Warmer micro-sites or those with greater nutrient availability can directlyincrease conifer seedling growth (Brand and Janas 1987;Delong et al. 1997); however, they might ultimately diminishseedling growth by promoting invasion of early successionalspecies that compete aggressively with conifer seedlings(Munson et al. 1993; Thevathasan et al. 2000).

Site preparation treatments are commonly employed tolimit competing vegetation and improve the seedling micro-site (Lieffers et al. 1993; Landhäusser and Lieffers 1999).Because the forest floor is the principal reservoir of nutrientsin boreal forests (Van Cleve et al. 1983), site preparation cansubstantially affect nutrient cycling and plant growth. Signif-icant increases in nitrogen availability in the forest floor aretypically noted after clear-cutting (Binkley 1984). Mechani-cal site preparation (MSP) may increase (Vitousek et al.1992) or decrease (Munson and Timmer 1995) rates of Nmineralization. Burning the forest floor may increase or de-crease N and P availability, depending on burn intensity(Dyrness and Norum 1983). Soil pH generally increases fol-lowing burning, but cation exchange capacity may increaseor decrease depending on the amount of forest floor loss(Feller 1982). Mechanical treatments (Lieffers and Beck1994) such as blading, disc trenching, mounding or mixing,or fire (Feller 1982) have been used to create more favour-able microsites for white spruce growth; treatment objectivesmay be to reduce competition (Örlander et al.1990) and (or)reduce slash and forest floor thickness, thereby increasingtemperature, moisture, microbial activity, mineralizationrates, and nutrient availability (Prescott et al. 2000). Asnoted above, however, considerable confusion remains con-cerning the exact effects of MSP and fire on microsite condi-tions, particularly in relation to the amount of forest floorremoved or disturbed.

Partial canopy retention has been suggested as an alterna-tive strategy to slow the invasion of light-demanding species(Lieffers et al. 1993, 1999). Competitors such as Epilobiumangustifolium L. and Calamagrostis canadensis (Michx.) P.Beauv. tend to be less vigorous and abundant under densercanopies that limit light availability (Lieffers and Stadt1994). The reduction in insolation by the residual trees of apartial canopy relative to complete canopy removal (clear-cut) should also result in lower soil temperatures (Barg andEdmonds 1999; Londo et al. 1999), and thus, likely lowermicrobial activity and mineralization rates — all possibly af-fecting plant response. In addition, the residual trees willlikely continue to take up nutrients after harvest. As a resultof these complex interactions, the impacts of partial cut sys-tems on organic matter and nutrient dynamics (Prescott1997) or competing vegetation (Lieffers et al. 1993) arepoorly understood, particularly in combination with MSP inthe understory.

The objectives of this study were to assess microsites cre-ated by different forest floor disturbances within clear-cutand partial cut stands (i) to assess nutrient availability andmineralization and (ii) to assess the response of understoryvegetation.

Materials and methods

Study area and designThe experiment was initiated in May 1999 at the

EMEND (Ecosystem Management Emulating Natural Dis-turbance) project site northwest of Peace River, Alberta(56°45′ N, 118 o20′ W). Stands dominated by mature whitespruce (Picea glauca (Moench) Voss) developing on mesicsites were identified for the study. The white spruce compo-nent comprised at least 75% of the preharvest basal area ineach of the stands, with smaller components of trembling as-pen (Populus tremuloides Michx.) and (or) balsam poplar(Populus balsamifera L.). Common understory species in-cluded low-bush cranberry (Viburnum edule (Michx.) Raf.),wild rose (Rosa acicularis Lindl.), fireweed (Epilobiumangustifolium), hairy wild rye (Elymus innovatus Beal), andbunch berry (Cornus canadensis L.). The soils were well toimperfectly drained, Dark Gray Luvisols or Orthic LuvicGleysols, developed on fine-textured glaciolacustrine parentmaterial with an average forest floor depth of 13 cm.

The experimental design was a splitplot, with harvest in-tensity as the main plot factor and forest floor disturbance asthe subplot factor. Harvest treatments were applied in threeseparate blocks, dispersed over an area of approximately10 000 ha. Blocks contained stands of similar age, ecosite,and stand cover type (as described above). Within each block,harvest treatments were randomly assigned to main plotsthat were approximately 10 ha in size. The two harvest treat-ments were (i) complete removal (clearcut or CC) and(ii) 50% basal area removal (partial cut or PC) by harvestingin the N–S direction, 5-m wide strips spaced 15 m apart, ac-companied by removal of about 33% of the basal area fromthe retention strips. In the PC, the harvester remained in thecut strip (the machine corridor) and reached into the reten-tion strip for removal of trees; consequently, there was nomachine traffic in the retention strips. Stands were harvestedduring the winter of 1998–1999.

The forest floor disturbance treatments (subplots) consistedof light burn, scalp, mix, and mound plots, as well as an un-disturbed control plot. Forest floor disturbance treatmentswere approximately 2 m × 2 m plots buffered by at least 1 mfrom any adjacent plot and were applied four times in eachCC and PC. The results for the four plots of each distur-bance treatment were treated as subsamples and averagedbefore analysis. Forest floor disturbance plots were locatedon sites determined to be uniform in microtopography, slope,soil type, and forest floor depth. Within the CC, forest floordisturbance plots were located in the central area of theclearcut, away from forest edges. Within the PC, forest floordisturbance plots were located in the central area of the15 m wide retention strip. MSP plots were installed in lateMay 1999 with an excavator using two attachments (amounding bucket for scalp and mound treatments and aMeri-Crusher high-speed horizontal drum mulcher for themixing treatments). The mounding treatment created in-verted, mineral-capped mounds that were approximately 1 min diameter with 15 cm thick mineral-caps. For this treat-ment only, four adjacent mounds created one mound plot.The scalping treatment removed the majority of the forestfloor, leaving an average of 2 cm of H layer above the min-eral soil. The mixing treatment fragmented and mixed theforest floor with the upper 2–3 cm of mineral soil. For sitepreparation in the PC, the excavator remained in the ma-chine corridor, and treatments were installed by reachinginto the retention strip. The burning treatment was carried



out during the last week of May and first week of June usinga propane torch. Burn depth was used to characterize theburning treatment, since the depth of burn is considered tobe the most important factor affecting understory plant re-sponse in the boreal (Schimmel and Granström 1996). Burnpins placed in the plot before burning indicated that theaverage depth of burn was approximately 2 cm, with con-sumption limited primarily to the surface litter–moss layer,thereby creating a scorched and blackened surface.

Environmental measurementsCanopy light transmission in the PC stands averaged 48%,

based on measurements of photosynthetically active radia-tion above the shrub layer. Measurements were made using ahand-held integrating radiometer (Sunfleck ceptometer,model SF-80, Decagon Devices Inc., Pullman, Wash.) underuniformly overcast conditions and calibrated against simulta-neous open-sky, photosynthetically active radiation measure-ments made in nearby clearings, using a photosyntheticallyactive radiation sensor (LI80S, LI-COR Inc., Lincoln, Nebr.)linked to a data logger (CR21X, Campbell Scientific Co.,Ltd., Logan, Utah).

For assessing soil moisture, mineral and organic soil cores(each 2 cm in diameter and approximately 7 cm long, takendirectly above and below the organic–mineral interface)were removed from each forest floor disturbance plot, oncein July and once in August 1999. All treatments were sam-pled on the same day, at least 4 days after a rain event. Soilmoisture content was determined gravimetrically; cores weredried for 48 h at either 105°C (for mineral cores) or 68°C(for organic cores). Soil moisture content (%) was calculatedas the mass of moisture loss divided by the dry mass of thesample, multiplied by 100. The July and August measure-ments of soil moisture were averaged for each plot to obtainan average measure of soil moisture for each treatment.

Soil temperatures in each forest floor treatment plot wereassessed on clear days by fixing a portable digital thermom-eter (Universal Enterprises, Beaverton, Ore.) to copper–constantan thermocouples set at depths of 20 cm below thetreatment surface in each plot. This depth was chosen be-cause it represented the primary rooting zone of plants inthese stands. All treatment plots were sampled on the sameday between 1430 and 1700, once in June, July, and August1999. The June–August temperature measurements were av-eraged for each plot to provide an average summer tempera-ture for each treatment.

Resin bag preparation, installation, and extractionResin bags consisted of 35 mL of mixed-bed cation plus

anion type 1 IONAC NM-60 H+–OH– (16–50 mesh) ex-change resin (J.T. Baker, Phillipsburg, N.J., U.S.A.) placedin nylon bags and tied shut. Bags were then prepared follow-ing the procedure of Thiffault et al. (2000). Three resin bagswere placed separately within the central area of each forestfloor disturbance plot in mid-June 1999, following the place-ment procedure of Munson and Timmer (1995). A spadewas inserted at 45° to create a slit in the soil, and the bagswere placed flat against the soil surface in the upper 5 cm ofthe mineral soil (directly below the organic–mineral inter-face for burn, control, mix, and scalp treatments and directlyabove the inverted organic layer in the mineral cap of themound treatment). The spade was then removed, the soil

pushed down to ensure good contact, and resin bags wereflagged for later retrieval. Bags were removed in October1999.

In the lab, bags were washed thoroughly with deionizedwater. Each bag of resin was opened and the resin contentswere extracted by shaking for 1 h in 100 mL of 2 M NaClsolution. Extracts were filtered (Fisherbrand P4, Fisher Sci-entific, Pittsburg, Pa.), and the three extracts for each forestfloor disturbance plot were pooled and then analyzed forammonium (NH4

+), nitrate (NO3–), and phosphorus (P) us-

ing a Technicon Autoanalyzer (Tarrytown, N.Y.), and for po-tassium (K+), magnesium (Mg2+), and calcium (Ca2+) byatomic absorption spectrophotometry (Perkin-Elmer 503,Wellesley, Mass.).

Nitrogen mineralizationRates of N mineralization were assessed using an in situ

incubation method (Eno 1960). Two cores of both mineralsoil and forest floor (FH layer) were removed from each for-est floor disturbance plot and placed in 25 µm thick polyeth-ylene bags (Leeson Polyfilm Manufacturing Ltd., Edmonton,Alta.). Cores were 7 cm long × 5 cm diameter, taken directlyabove or below the mineral–organic interface. Because ofthe removal of the forest floor in the scalp treatment, it wasnot possible to incubate an organic (FH) core. After bagging,cores were returned to the holes from which they were takenand litter was replaced on top. A second series of cores ofthe same size — two mineral and two FH — were removedfrom a nearby location for measurement of initial NH4

+ andNO3

– and frozen until analysis. Removal of initial cores andinstallation of incubated cores was done in early July (6–10)1999. Buried cores were removed in the first week of Octo-ber 1999 and frozen until analysis.

For analysis, cores were thawed and pairs from the sameforest floor disturbance plot were bulked and mixed by hand,removing any stones, chunks of wood, or large roots. Dupli-cate subsamples of approximately 10 or 20 g of ovendrymass equivalent (for organic and mineral, respectively) wereextracted in 100 mL of 2 M KCl and placed on a shaker ta-ble for 1 h. Another set of duplicate subsamples was taken atthe same time and oven-dried for moisture determination(105°C for mineral samples and 68°C for organic samples).Extractions were filtered (Whatman 40, Whatman Inc.,Clifton, N.J.) and analyzed for NH4

+ and NO3– using a

Technicon Autoanalyzer. Nitrogen mineralized during theincubation was calculated by subtracting initial NH4

+ andNO3

– from measured levels in the incubated cores.

Vegetation samplingIn late July 1999, percent cover of herbaceous plants was

visually estimated using four corner plots and one centerplot (each 50 cm in diameter) placed in each disturbanceplot. In August 2000, percent cover of E. angustifolium,V. edule, and Populus spp., as well as root sucker density ofPopulus spp., was evaluated within a 2.5-m2 area delineatedby either a single square quadrat centered in the plot (forburn, control, mix, and scalp treatments) or four circularplots centered on mounds.

Statistical analysesAll data, with the exception of V. edule cover, NH4

+ avail-ability, and soil moisture, were log transformed to meet as-

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1182 Can. J. For. Res. Vol. 33, 2003



sumptions of homogeneity of variance. The statistical modelused in the analysis was

Yijk = µ + Bi + Hj +Eij + Fk + HFjk +eijk

where Bi represents block (3 blocks, i = 1…3), Hj representsharvest intensity (CC and PC, j = 1, 2), Fk represents forestfloor disturbance (burn, control, mix, mound, and scalp, k =1…5), and E and e were the error terms for main plot andsub plot factors, respectively (Table 1). Block was a randomfactor, while harvest intensity and forest floor disturbancewere fixed factors in the model. For comparisons betweenthe harvest treatments, the CC treatment was considered tobe the control and the PC treatment the alternative. Tukey’sHSD test was used for all multiple comparisons. Signifi-cance was evaluated at p < 0.10 out of a concern that thelimited replication would result in a high number of type IIerrors at p < 0.05. No significant interactions were detected(except for net N mineralization in the mineral soil). Conse-quently, the effects of harvesting reflect averages over allforest floor disturbance treatments, and the effects of forestfloor disturbance reflect averages over all harvesting treat-ments. Finally, all significant increases or decreases in aproperty are relative to the control, unless stated otherwise.

Results

Soil temperature and moistureSoil temperature was significantly affected by canopy re-

moval (Table 2), with temperatures about 2°C higher in theCC than in the PC harvested stands. Forest floor disturbancealso affected soil temperature (Table 3), with mounding pro-ducing the highest temperatures followed by scalping, mix-ing, burning, and control plots. Soil moisture contents inboth the organic and mineral layers were lower in moundsthan in the other treatments (Table 3).

Nutrient availability (resin bags)Burn treatments had more NH4

+ than any other forestfloor treatment (Fig. 1a), and the burn and scalp treatmentshad more NO3

– than controls (Fig. 1b). The availability of Pwas greatest in the burn treatment; control and mix treat-ments were intermediate; and mound and scalp treatmentshad the lowest P availability (Fig. 2a). Available Ca levelswere higher for burn, mix, and scalp treatments comparedwith control and mound treatments (Fig. 2b). Magnesiumavailability was greater in scalps than in burn, control, ormound treatments, and greater in mix and burn treatmentsthan in control and mound treatments (Fig. 2c). In addition,harvest intensity affected Mg availability (Table 2), Mg be-ing notably higher in the CC than in the PC harvest treat-ment. K availability was higher in the burns relative tocontrol, mound, and scalp treatments, and control and mixtreatments had higher K availability than mounds and scalps(Fig. 2d).

Net N mineralizationIn the organic horizon, mound treatments showed higher

net N mineralization than burn and mix treatments, while netN mineralization was lower in the mix treatment than in thecontrol (Fig. 3). In the mineral soil layer, there was a signifi-cant interaction between forest floor disturbance and harvesttype (p = 0.0354). In the CC, net N mineralization was

higher in the burn and control treatments than in the mix andmound treatments, while in the PC, there were no differ-ences among the treatments.

VegetationPercent cover of herbaceous species (Fig. 4a) in the year

of harvest was significantly affected by forest floor distur-bance. Mix and mound treatments had lower herbaceouscover than burn, control, and scalp treatments. Epilobiumangustifolium cover was elevated in the burn and mound

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Frey et al. 1183

Source* df

Main plotB 3 – 1 = 2H 2 – 1 = 1E (2 – 1)(3 – 1) = 2Main plot total 5SubplotF 5 – 1 = 4H × F (2 – 1)(5 – 1) = 4e 2(3 – 1)(5 – 1) = 16Subplot total 24

Total (3)(2)(5) – 1 = 29

*B, block; H, harvest; E, main plot error; F,forest floor disturbance; e, subplot error.

Table 1. ANOVA table showing thedegrees of freedom associated with mainplot factors, subplot factors, and their asso-ciated error terms.

Fig. 1. NH4+ (p = 0.0023) and NO3

– (p = 0.0077) availability asaffected by forest floor disturbance, measured by ion exchangeresin bag uptake for the period June–October 1999 (n = 6; 3blocks × 2 harvest treatments). Means with standard error barsare presented. Columns with the same letter are not significantlydifferent.



treatments relative to the controls (Fig. 4b). Viburnum edule,in contrast, was highest in the burn and control plots andmost reduced in the mix treatment (Fig. 4c). Populusspp. cover was higher in the scalp treatment than in mix andmound treatments (Fig. 4d) and was also higher in CC thanin PC stands (Table 2). The density of root suckers ofPopulus spp. was likewise higher in the CC than in the PC(Table 2) and higher in the scalping treatment than any otherforest floor disturbance (Table 3).

Discussion

While there was greater availability of Mg in the CC, har-vest intensity had limited effects on nutrient responses. Kimet al. (1996), Prescott (1997), and Burgess and Wetzel (2000)also noted few clear differences in N mineralization and nu-trient availability among different levels of harvest. Compen-sation among the environmental factors affecting microbialactivity may reduce the differences between harvest intensi-ties (Yin et al. 1989; Prescott 1997), e.g., more optimalmoisture conditions in the PC could compensate for moreoptimal temperatures in the CC. Furthermore, forest floordisturbance treatments in the PC likely damaged roots of re-tention trees, and thereby lowered uptake, reducing differ-ences between the CC and PC. Limited replication of theharvest treatments may also have weakened the power of thetest.

Suckering by Populus spp. was clearly higher in the CCthan in the PC, a response that is well documented (Schierand Smith 1979). Canopy retention, however, did not reduce

cover or density of other plants relative to the CC, despitecanopy light transmission of only 48% in the PC. The weakeffect of canopy likely reflects the overriding effect of theforest floor disturbance treatments (discussed below) in driv-ing plant regeneration. Still, undisturbed control plots in theCC did not develop greater vegetation cover relative to theundisturbed control plots in the PC (where light was lim-ited). We suspect this was related to relatively higher sur-vival of plants in the retention strips of the PC where therewas no machine traffic (Thomas et al. 1999). Though growthof understory vegetation was likely higher in the CC, highersurvival of plants in the PC would have contributed to den-sity and cover measurements that were similar to the CC(where damaged plants resprouted from reserves) (Moola etal. 1998; Thomas et al. 1999). Since understory vegetationcover is well related to forest canopy cover and light avail-ability (Lieffers and Stadt 1994; Messier et al. 1998), weexpect that greater differences between PC and CC will de-velop with time.

BurningDespite the limited consumption of forest floor by the

burning treatment, nutrient availability, as measured by ionexchange resins, increased substantially. Higher nutrientavailability is generally observed following fire (Wells et al.1979); ash inputs from slash consumption contribute tohigher P, Ca, Mg, and K availability (Feller 1982; Macadam1987) and short term increases in NH4

+ (Raison 1979) andNO3

– (Bauhus et al. 1993). Nonetheless, in white sprucestands in Alaska, similar low intensity burns caused by wild-

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1184 Can. J. For. Res. Vol. 33, 2003

Harvest type*

Variable CC PC p†

Soil temperature (°C) 12.9 (1.3) 11.1 (1.2) <0.0001Magnesium (µg/resin bag) 152 (45) 86 (21) 0.0278Populus spp. density (stems/ha)

Year 1 93 200 (59 600) 60 800 (61 300) 0.0253Year 2 74 800 (21 000) 53 900 (12 800) 0.0164

Populus spp. cover, year 2 (%) 9 (5) 3 (2) 0.0368

*Means for each harvest type are shown with standard errors in parentheses.†Values from ANOVA.

Table 2. Significant effects of harvest type (CC, clearcut; PC, partial cut) on soiltemperature at 20 cm, magnesium availability, and Populus spp. density and cover.

Forest floor disturbance*

Variable Burn Control Mix Mound Scalp p†

SoilTemperature (°C) 10.5c (0.4) 9.9d (0.4) 10.9c (0.3) 15.4a (0.6) 13.2b (0.5) <0.0001% moisture content

Organic 102a (12.9) 108a (12.9) 105a (17.8) 60b (4.0) NA 0.0108Mineral 29.9a (3.3) 28.3a (2.8) 29.3a (3.4) 12.7b (0.9) 27.9a (4.0) <0.0001

Populus spp. density(stems/ha)

Year 1 34 600b (14 600) 38 100b (25 200) 17 200b (6 600) 44 000b (27 000) 250 600a (64 200) 0.0003Year 2 25 400b (8 400) 36 500b (20 800) 17 500b (6 200) 25 400b (13 300) 122 400a (57 100) 0.0023

Note: Means followed by the same letter are not significantly different. NA, not applicable.*Means for each forest floor disturbance are shown with standard errors in parentheses†Values from ANOVA.

Table 3. Effects of forest floor disturbance on soil temperature at 20 cm depth, soil moisture content, and Populus spp. sucker density.



fire did not increase nutrient availability (with the exceptionof P) (Dyrness et al. 1989). However, our sites were har-vested before burning, and the presence of nutrient-rich

branch and foliage slash likely contributed to elevated nutri-ent availability (Feller 1982).

Burning did not increase net N mineralization, despite in-creases in temperature and nutrient availability, both ofwhich are known to stimulate microbial activity and furtherenhance nutrient release following disturbance (Bunnell etal. 1976; Covington and Sackett 1986; Krause and Ramlal1987). Temperature increases were perhaps too small (<1°C)to significantly stimulate mineralization rates. While burningdid not apparently enhance net N mineralization, NO3

– pro-duction, as measured by resins, was stimulated. The highNO3

– availability in the burns was likely driven by theincreased nitrifier activity that follows slash burning(Jurgenson et al. 1981), a process apparently stimulated byincreased base cation availability (Bauhus et al. 1993; Ste-Marie and Paré 1999).

Burning promoted higher E. angustifolium cover in thefirst year, but otherwise had little impact on the density orcover of V. edule or Populus spp. Epilobium angustifoliumaccumulates nutrients at a relatively high rate (Dyrness andNorum 1983; van Andel and Vera 1977) and is stimulated byfertilization (Reinikeinan in Myerscough 1980), so increasedcover was likely related to elevated nutrient availability inthe burns. Aspen suckering was not stimulated by burning,as has been observed on other boreal mixedwood sites(Peltzer et al. 2000), likely because of insufficient distur-

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Frey et al. 1185

Fig. 2. P (p < 0.0001), Ca (p = 0.0002), Mg (p < 0.0001), andK (p < 0.0001) availability as affected by forest floor distur-bance, measured by ion exchange resin bag uptake for the periodJune–October 1999 (n = 6; 3 blocks × 2 harvest treatments).Means with standard error bars are presented. Columns with thesame letter are not significantly different.

Fig. 3. Net N mineralization in the organic horizon (p = 0.0004)as affected by forest floor disturbance, measured for the periodJune–October 1999 (n = 6; 3 blocks × 2 harvest treatments).Means with standard error bars are presented. Columns with thesame letter are not significantly different. NA, not applicable.

Fig. 4. Percent herbaceous cover in July 1999 (p < 0.0001) andcover of Epilobium angustifolium (p = 0.0076), Viburnum edule(p = 0.0406), and Populus spp. (p = 0.0368) in 2000, as affectedby forest floor disturbance (n = 6; 3 blocks × 2 harvest treat-ments). Means with standard error bars are presented. Columnswith the same letter are not significantly different.



bance to the root system in the present study (Lavertu et al.1994).

MixingThe mixing treatment substantially increased Ca and Mg

availability relative to the control. This increase has beennoted previously (Krause and Ramlal 1987; Schmidt et al.1996), attributable to the higher concentration of base cat-ions in the mineral soil (Schmidt et al. 1996). Although mix-ing may promote decomposition and enhance tree seedlingnutrition by improving aeration (Prescott et al. 2000; Mallikand Hu 1997), our mixing did not promote higher NH4

+ orNO3

– availability and actually resulted in lower net N miner-alization. Keenan et al. (1994) also found that decompositionand mineralization were either unaffected or diminished bymixing. Possibly, incorporation of labile C substrates (litterand logging residue) increases microbial immobilization ofN (Vitousek et al. 1992; Thibodeau et al. 2000). Conse-quently, while mixing may enhance microbial activity (Mallikand Hu 1997), N mineralization will likely not be enhanceduntil a later stage of decomposition.

Vegetation establishment, particularly by V. edule and her-baceous plants, was lowest in the mixing treatment. Incoastal British Columbia, Keenan et al. (1994) also foundthat mixing was highly effective in reducing vegetation com-petition. High speed mixing of the forest floor materials andupper 2–3 cm of mineral soil likely fragmented and de-stroyed most roots and rhizomes. However, some plants didsprout, probably from below the mixing depth. The deeprooting of E. angustifolium (Moss 1936; Myerscough 1980;Calmes and Zasada 1982) may explain its improved estab-lishment relative to other species such as V. edule. Sproutingcould also occur from root or rhizome segments; howeverthey are less likely to sprout because of low reserves(Broderick 1980; Hogg and Lieffers 1991b). Root and rhi-zome damage, coupled with low N mineralization, likely ex-plains why this treatment resulted in the lowest regrowth ofcompeting vegetation.

MoundingThere was a trend towards higher net N mineralization in

the mounds, at least compared with the burn and mix treat-ments. The warm temperatures, soil mixing, and cycles ofwetting and drying likely contributed to increased microbialactivity and nutrient release in the mounds (Örlander et al.1990; Johannson 1994; Lundmark-Thelin and Johansson1997). Our results contrast somewhat with those ofSmolander et al. (2000) in Sweden, who noted equivalent ordiminished rates of N mineralization in mounds. The smallersize of their mounds and measurement of N mineralizationdeeper within the mound likely limited the temperature ef-fect, and substantial incorporation of residues in their moundsmay also have favoured N immobilization. Availability ofother nutrients in our study, particularly Ca, Mg, K, and P,was consistently low in the mounds, although this was likelyan artifact of placing the resin bags in the mineral cap abovethe organic horizon. In other treatments in our study, resinbags were below the organic horizon and would have ab-sorbed ions through diffusion and downward movement ofwater. Had resin bags been placed in the buried organic

layer, nutrient availability would likely have been higher, asnoted in other studies (Lundmark in Örlander et al. 1990).

Mounding significantly increased the establishment ofE. angustifolium. Other studies have suggested that moundinggenerally reduces competitors such as E. angustifolium, de-pending upon the site (Örlander et al. 1990; Macadam andBedford 1998). Inverting the soil may place perennatingstructures of some deeper rooting plants, such as E. angusti-folium, closer to the surface and expose them to higher tem-peratures. The cutting and breaking of rhizomes that occursduring mounding might also have stimulated E. angustifolium(Coates and Hauessler 1986). Once established, it is likelythat growth of E. angustifolium was enhanced by the high nu-trient availability within the mound. Thus, while mounds maygenerally limit competition by V. edule and Calamagrostiscanadensis (Lavertu and Lieffers 1999), they may actually en-hance competition from E. angustifolium.

ScalpingUnexpected increases in NO3

–, Ca, and Mg availabilitywere induced by the scalping treatment. Because the forestfloor is the principal reservoir of nutrients in boreal forests(Van Cleve et al. 1983), decreases in Ca, Mg, K, P, NH4

+,and NO3

– availability are expected following the mechanicalremoval of the forest floor (Krause and Ramlal 1987;Munson et al. 1993, Munson and Timmer 1995). The in-creased nutrient availability in our study could have resultedfrom enhanced mineralization in the upper mineral soil, as-sociated with higher temperatures in the scalps. However,net N mineralization in the upper mineral soil was not en-hanced. Instead, increased nutrient availability was likelydriven by mineralization of the 2-cm H layer retained on thescalps, as nutrient release in this layer is strongly enhancedby soil warming (Verburg et al. 1999). Indeed, without thepartial retention of the forest floor, decreases in Ca, Mg, andNO3

– availability likely would have occurred (e.g., Krauseand Ramlal 1987; Munson et al. 1993).

While scalping did not increase E. angustifolium orV. edule establishment, suckering by aspen and poplar rootswas clearly enhanced. A moderate level of scarification ofthe forest floor and root system generally stimulates suckerproduction by Populus spp. (Maini and Horton 1966;Weingartner 1980; Lavertu et al. 1994), likely attributable toincreased temperatures or root wounding (Lavertu et al.1994). Indeed, scalping wounded many roots and left themwith less soil cover, resulting in higher temperatures. In-creased nutrient availability in the scalps may also have en-hanced suckering, because high NO3

– levels are thought toincrease aspen establishment (Kronzucker et al. 1997) andCa is critical to aspen growth (Alban 1982; Lu and Sucoff2001). High intensity MSP treatments, such as mixing andmounding, appear to diminish sucker establishment (Peltzeret al. 2000), likely because they destroy or severely damageroots, suggesting that deeper scalps that remove roots andrhizomes may offer more effective vegetation control (Lees1970).

Study implicationsNutrient availability and vegetation establishment were

most strongly controlled by forest floor disturbance ratherthan by partial canopy retention. Suppression of competitor

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vegetation may be obtained by mixing, likely because thistreatment destroys the majority of resprouting organs. Lightscalps, in contrast, promote vigorous establishment byPopulus spp., suggesting that deeper scalps may be neces-sary for vegetation control. Also, E. angustifolium may colo-nize mounds by sprouting through the 15-cm mineral cap,indicating that thicker caps may be required for its control.In terms of nutrient availability, mixing can reduce short-term N supply to competitors, whereas other forest floor dis-turbances generally increase nutrient availability. With time,increased foliage development and root growth by retentiontrees may decrease light and soil nutrient and moisture avail-ability, thus causing the CC and PC systems to diverge.However, these responses require further study.

Acknowledgements

We thank Lance Lazaruk, Erin Flynn, Charlene Hahn, andKarin Newman for field assistance; Bill DeGroot and MikeWeber for consultation on burning; Derrick Sidders and RobTaylor for coordination of site preparation; Barb Kischuk forsoil description and assistance with N mineralization cores;and Monica Molina and Chung Nguyen for nutrient analy-ses. Diashowa-Marubeni Int., Canadian Forest Products Ltd.,Canadian Network of Centres of Excellence, SustainableForest Management (NCE-SFM), Canadian Circumpolar In-stitute (Circumpolar/Boreal Alberta Research), and NaturalSciences and Engineering Research Council of Canada(NSERC) provided funding for this project.

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