Black spruce seedlings in a Kalmia–Vacciniumassociation: microsite manipulation to exploreinteractions in the field

Nelson Thiffault, Brian D. Titus, and Alison D. Munson

Abstract: We established a field trial on an ericaceous-dominated clearcut in Quebec to determine the effect of Kalmiaangustifolia L., Vaccinium angustifolium (Ait.), and V. myrtilloides (Michx.) on the growth and physiology of blackspruce (Picea mariana (Mill.) BSP) seedlings and on soil characteristics over the first two growing seasons. Plots un-dergoing one of three treatments (shrub removal, humus removal, or undisturbed control) were planted with blackspruce seedlings that were either unfertilized or spot fertilized at time of planting. In some of the undisturbed controlplots, we also used 15NH4

15NO3 to compare uptake of broadcast N fertilizer by vegetation. The ericaceous shrubs had asignificant negative impact on seedling growth. Growth reductions were not related to water stress, soil temperature, orsoil moisture. Extractable NH4-N and P concentrations in mineral soil tended to decrease in the presence of ericaceousshrubs, but effects were not significant. Seedling foliar N concentration was also reduced in the presence of ericaceousshrubs. Of the total amount of 15N fertilizer found in vegetation, 64% was immobilized in Vaccinium spp., 31% in Kal-mia, and 5% in black spruce, but spruce took up more 15N per unit of root biomass than the ericaceous shrubs. Kalmiahad consistently higher predawn xylem water potentials than black spruce.

Résumé : Nous avons entrepris une expérience sur une station récemment coupée au Québec et dominée par les érica-cées, afin d’étudier les effets de ces arbustes (Kalmia angustifolia L., Vaccinium angustifolium (Ait.), et V. myrtilloides(Michx.)) sur la croissance et la physiologie de plants d’épinette noire (Picea mariana (Mill.) BSP) et sur les caracté-ristiques du sol pendant les deux premières saisons de croissance. Nous avons reboisé des parcelles exemptes de végé-tation, d’humus et témoins avec des plants d’épinette noire qui ont reçu ou non un engrais lors de la mise en terre.Dans certaines parcelles témoins, nous avons également utilisé du 15NH4

15NO3 pour comparer l’absorption de l’engraispar la végétation. Les éricacées ont significativement diminué la croissance des plants. Cette réduction n’était pas liéeau stress hydrique, à la température ou à l’humidité du sol. Les éricacées ont eu tendance à réduire les concentrationsde NH4-N et de P du sol minéral, mais les effets n’étaient pas significatifs. Les éricacées ont réduit la concentrationfoliaire en N des plants. Sur la quantité totale de 15N récupéré dans la végétation, 64 % a été immobilisé dans le Vac-cinium spp., 31 % dans le Kalmia et 5 % dans l’épinette noire, mais l’épinette a absorbé davantage de 15N par unité debiomasse racinaire que les éricacées. Le Kalmia avait constamment des potentiels hydriques préaube supérieurs à ceuxde l’épinette.

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Introduction

Reforestation success frequently depends on the natureand abundance of competing vegetation, and on the silvi-cultural treatment used to enhance seedling survival andgrowth. Competition for light is often responsible for planta-tion failure on rich sites (Jobidon 2000), whereas competi-tion for nutrients and water tends to be more important on

poorer sites (Örlander et al. 1996). The ericaceous shrubKalmia angustifolia L. (sheep laurel or lambkill; hereafterreferred to as Kalmia) is found in eastern Canada and thenortheastern United States (Ebinger 1997) and tolerates awide range of ecological conditions (Titus et al. 1995).When Kalmia is present as an understory shrub in commer-cial stands of black spruce (Picea mariana (Mill.) BSP) orjack pine (Pinus banksiana Lamb.), it proliferates rapidly by

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Received 31 October 2003. Accepted 4 March 2004. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on27 August 2004.

N. Thiffault.1,2 Centre de recherche en biologie forestière, Faculté de foresterie et de géomatique, Université Laval, Sainte-Foy,QC G1K 7P4, Canada.B.D. Titus. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, 506 West Burnside Road, Victoria,BC V8Z 1M5, 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: nelson.thiffault@mrnfp.gouv.qc.ca).2Present address: Ministère des Ressources naturelles, de la Faune et des Parcs du Québec, Direction de la recherche forestière,2700 rue Einstein, Sainte-Foy QC G1P 3W8, Canada.

asexual reproduction following removal or destruction of theoverstory by cutting or wildfire (van Nostrand 1971). Rhizo-matous growth and resprouting at the base of stems are theusual mechanisms of reproduction in this species (Mallik1993) and, where it is already present, ensure its rapidspread on recently disturbed sites. In Canada, Kalmia is rec-ognized as a competitive species that can have major effectson the growth of natural or artificially established coniferseedlings in some ecosystems in Newfoundland, Nova Sco-tia, Ontario, and Quebec (van Nostrand 1971; Anonymous1972; Wall 1977; English and Hackett 1994; Jobidon 1995).

Many hypotheses have been proposed to explain Kalmiainterference with newly established conifers. Under labora-tory conditions, phenolic acids produced by Kalmia nega-tively affect early root development of black sprucegerminants (Zhu and Mallik 1994). Tannins extracted fromKalmia foliage and twigs reduce soil nitrogen availability(Bradley et al. 2000). In the field, black spruce seedlings lo-cated within a 1-m radius of Kalmia have lower biomass andfoliar nutrient concentrations than seedlings located fartheraway and are colonized to a lesser extent by beneficial my-corrhizal fungi and to a greater extent by pathogens(Yamasaki et al. 1998). Although there is no convincing di-rect evidence for allelopathic interference with conifers(Inderjit and Mallik 2002), a recent field study suggests thatnon-nutritional (i.e., potentially allelopathic) mechanismsplay an important role in Kalmia interference with spruceseedling growth (Yamasaki et al. 2002).

Direct or indirect allelopathic influences of Kalmia onseedling root growth should affect seedling physiologicalprocesses and hence seedling growth. There is a relationshipbetween water stress and the photosynthetic rates of conifers(Brix 1979), with the feedback regulation between growthand photosynthesis resulting in reduced rates of both whenone or the other is affected (Grossnickle 2000). If allelo-chemicals reduce spruce root growth (Zhu and Mallik 1994)and hence hinder water absorption, initial seedling growthand survival may be reduced (Burdett 1990). Although Kal-mia impacts on black spruce nutrition and growth are nowwell documented, no study has yet investigated the influenceof Kalmia on seedling water relations, a path by which seed-ling growth may also be limited.

Competing vegetation can reduce soil moisture levels,which in turn can negatively affect seedling physiology andgrowth (Spittlehouse and Stathers 1990). Competing vegeta-tion can also reduce soil temperature (Spittlehouse andStathers 1990), as can the presence of even thin humus lay-ers because of the low thermal diffusivity of organic matter(Balisky and Burton 1997). As with reduced soil moisture,lower soil temperatures can have a negative impact on seed-ling performance (Spittlehouse and Stathers 1990). However,the potential impacts of Kalmia on soil temperature and wa-ter content have not yet been evaluated.

In Quebec, Kalmia interferes with seedling establish-ment mainly on sites with a mor humus over coarse- tomedium-textured glacial deposits (Jobidon 1995), where itis often found in association with species of the genusVaccinium. Although Vaccinium spp. are not normally con-sidered to be competitors on regenerating sites in easternCanada, their significant cover with Kalmia on these prob-

lematic sites means that field studies on Kalmia inherentlyinclude Vaccinium spp. The association of these ericaceousshrubs therefore needs to be taken into consideration onthese sites, even if the main competitive influences on coni-fer seedlings have traditionally been attributed to Kalmiaalone.

Our first objective was therefore to determine the effect ofericaceous shrubs on soil characteristics and on growth andphysiology of newly planted black spruce seedlings on a sitewith Kalmia and Vaccinium spp. present. Our second objec-tive was to compare the relative ability of these species toacquire resources, especially N and water. We compared un-disturbed control conditions with those following two micrositemanipulation treatments on an ericaceous-dominated regen-erating site in northwestern Quebec: (i) eradication ofericaceous shrubs with herbicides (to remove direct compet-itive influences); and (ii) shrub plus humus removal throughscalping (to remove indirect influences, largely arising fromhumus quality). These two treatments were designed to elu-cidate some of the scientific principles inherent in applyingtwo main silvicultural options for enhancing seedling growthon Kalmia-dominated sites, herbicide application and scari-fication. A third option, fertilization, can also be used to in-crease nutrient availability to conifer seedlings, regardless ofsite preparation treatments applied. The effect of spot fertil-ization at planting on conifer seedling growth was thereforealso compared in all three treatments. In a separate sub-component of the trial, the competitive uptake of broadcastN fertilizer among Kalmia, Vaccinium spp., and spruce seed-lings under undisturbed site conditions was also measured.

Overall, we hypothesized that (i) the presence of erica-ceous shrubs increases black spruce seedling water stress;(ii) Kalmia maintains higher water potential than do blackspruce seedlings; (iii) ericaceous shrubs take up N fertilizermore competitively than does black spruce; and (iv) thepresence of ericaceous shrubs reduces soil nutrient concen-tration and availability, soil temperature, and soil water con-tent.

Materials and methods

Study siteWe established an experimental plantation on a cutover in

northwest Quebec (48°29′37 ′′N, 76°55′40 ′′W) in the balsamfir (Abies balsamea (L.) Mill.) – white birch (Betula papy-rifera Marsh.) bioclimatic region (Saucier et al. 1998). Theregion has a subpolar subhumid continental climate with amean annual temperature of 2.5 °C and a growing season of150–160 days. Mean annual precipitation is 950 mm, with30% falling as snow. The previous stand originated after afire 70 years earlier, and was composed of black spruce andjack pine. Dominant and co-dominant trees were between 12and 17 m high and conifer canopy cover ranged from 61 to80%. The stand was clear-cut in the summer of 1999 andproduced 100 m3·ha–1 of merchantable wood. The soil was aHumo–Ferric Podzol (Soil Classification Working Group1998) characterized by an 8 cm deep mor humus and devel-oped from a moderately well-drained fluvio-glacial deposit.The texture of the surface mineral horizons (0–15 cm) wasloamy sand (74% sand, 22% silt, 4% clay). An evenly dis-

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tributed, low-density cover of Kalmia (9%; range 0%–40%)and Vaccinium spp. (V. angustifolium (Ait.) and V. myrtilloides(Michx.)) dominated the cutover. Ground lichens (mainlyCladina spp.) were also present.

Experimental design and treatmentsWe designed the field trial as a split-plot factorial, with 10

replicate blocks established over approximately 0.2 ha(Fig. 1). Each block (5 m × 22 m) was divided into threemain plots (5 m × 6 m) separated by 2-m buffers. Withineach block, we randomly applied the following soil modifi-cation treatments to each of the three main plots: (i) un-treated control (C) with vegetation and humus undisturbed;(ii) ericaceous shrub removal (SR) through the application inSeptember 1999 of glyphosate (Vision, Monsanto CanadaInc.; 2% v/v in water, applied to dripping point) combinedwith a surfactant (Sylgard 309®, Dow Corning Canada Inc.;0.25% v/v in water) (English and Titus 2000); and (iii) scalp-ing (HR), in which all vegetation plus the organic layerswere removed mechanically and manually in August 1999 toexpose the mineral Ae horizon. We designed these three

treatments to separate the influences of humus and erica-ceous vegetation on seedling growth, physiology, and soilproperties. During the summers of 2000 and 2001, we manu-ally weeded SR and HR plots to control resprouting vegetation.

In early June 2000, we planted 20 containerized blackspruce seedlings in each main plot of the 10 blocks (seed or-igin 48°51′N, 78°01′W). Seedlings were grown in 45–110containers (45 cells of 110 cm3 each per container) at theprovincial nursery at Trécessons using standard nurserypractices for production of 2+0 stock in Quebec. At the timeof planting, seedlings (n = 300) were 28.7 cm (±4.2 cm) tall,with ground-level diameter of 3.3 mm (±0.5 mm). Theywere planted 1 m apart in four rows of five seedlings each,with 1 m between rows. We split main plots longitudinallyand randomly chose one subplot to be fertilized at the timeof planting (FTP) with a slow-release fertilizer package(Silva Paks, RTI, Salinas, California). We buried Silva Paks5 cm deep and about 2 cm away from each seedling in thetwo adjacent seedling rows of each subplot (i.e., 10 seed-lings total). Each Silva Pak contained 9 g of fertilizer(26.3% total N, 12.0% available phosphoric acid, and 6.0%

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Fig. 1. Experimental design. Each block (1–10) measures 5 m × 22 m and is divided into three main plots (C, SR, and HR). Eachmain plot measures 5 m × 6 m and is divided into two subplots (without FTP, with FTP). Each subplot contains 10 containerizedblack spruce seedlings. In control plots (C, without FTP) of four randomly selected blocks, we established four 1 m × 2 m quadratsand applied double-labelled ammonium nitrate (15NH4

15NO3 at 2.5% atom%) at a rate of 100 kg·ha–1 in each quadrat.

soluble potash, equivalent to 2.4, 0.5, and 0.5 g elementalN, P, and K, respectively). The N, P, and K sources werepolymer-coated to provide 24.9% slow-release N, 5.3%slow release available P, 5.1% slowly available potash, and6.0% S. Within each subplot in each of the 10 blocks, halfof the seedlings (i.e., five) were identified by tags forlong-term growth measurements. Untagged seedlings wereused for destructive sampling to determine xylem water po-tential, foliar nutrient concentration, and seedling biomass.

Soil temperature and water contentSoil temperature at 10 cm and volumetric soil water con-

tent (SWC) in the top 15 cm of soil were monitored at themain plot level (C, SR, HR) in five replicate blocks over thefirst two growing seasons (June to November in 2000 and2001). Soil temperature was measured with thermistors(temperature probe model 107BAM, Campbell Sci., Logan,Utah). Soil water content was assessed with time domainreflectometry (TDR) probes (CS615 water content reflecto-meter, Campbell Sci.). We placed TDR probes at a 30° an-gle, with the rods completely inserted in the mineral soil.Data were averaged hourly and recorded with a CR10 datalogger (Campbell Sci.).

Soil nutrientsMixedbed ion exchange resins (IONAC NM-60 H+/OH–,

J.T. Baker, Phillipsburg, New Jersey) were used to assesssoil nutrient availability at the main plot level in 2000 and2001. Ion exchange resins (IER) were washed and charged(Thiffault et al. 2000) before being placed (15 mL of resinper bag, 50% weight-based humidity, 0.27 g·mL–1 of resins,dry mass) in square bags of nylon polyester fabric (8 cm ×8 cm). Once sealed, these resin bags were rinsed in de-ionized water and stored cold (~5 °C) until used. Three resinbags were buried (horizontally, 10 cm below the soil sur-face) in each of the three main plots (C, SR, HR) of five rep-licate blocks at the beginning of each growing season (earlyJune 2000 and 2001) and retrieved in late October after onegrowing season. Adsorbed ions were extracted using a 2 mol/LNaCl solution and analyzed colorimetrically by spectro-photometry (FIA Quickchem, Lachat, Milwaukee, Wiscon-sin) and plasma atomic emission spectrometry (ICAP-9000,Thermo Instruments, Franklin, Massachusetts). Results wereexpressed on a per-bag basis (after Giblin et al. 1994;Kjønaas 1999; Burgess and Wetzel 2000).

Nine random samples taken from the surface mineral hori-zon (10 cm) were collected (with the use of a garden shovel)in the main plot of each replicate block at the end of the sec-ond (2001) growing season, and were grouped into threesamples per plot for chemical analysis. Prior to analysis, soilsamples were dried and ground (to pass through a 2-mmmesh screen). Mineral N (NH4-N and NO3-N) was extractedwith a 2 mol/L KCl solution and measured colorimetricallyby spectrophotometry (FIA Quickchem). Extractable P, K,Ca, and Mg were extracted with a Mehlich-III solution (SenTran and Simard 1993) and measured by inductively coupledplasma analysis (ICAP-9000).

Seedling growth measurementsAll tagged seedlings were measured for total height (cm)

and ground-level diameter (mm) immediately after planting

and again at the end of the second growing season. Wecalculated 2-year height and diameter increments by sub-tracting initial dimensions from final ones. We visually esti-mated Kalmia cover (by 5% cover class) in a 0.5-m radiusaround each tagged seedling in the control plots at the timeof planting and again after two growing seasons. We alsomeasured the distance between each tagged seedling and thenearest Kalmia stem (taller than 0.1 m).

Xylem water potentialDuring the second growing season (summer 2001), we

measured predawn (0300) and midday (1300) xylem waterpotentials (XWP) in unfertilized black spruce seedlings inall main plots of five randomly selected blocks (logisticalconstraints precluded XWP measurement of fertilized seed-lings); and in Kalmia plants located in the unfertilized con-trol subplot of the same blocks (predawn measurements weremade on 26, 28 and 30 July, and 10 and 12 August; middaymeasurements on 25, 27, 28 July, and 9, 11, 12 August). Xy-lem water potential in 1-year-old shoots was measured usinga portable pressure chamber (PMS Instruments, Corvallis,Oregon), following the method described by Ritchie and Hinkley(1975). Cut shoots were kept in paper bags until measuredwithin 20 min of excision. Different spruce seedlings andKalmia plants were used on each measurement date.

Foliar nutrient concentrationsAt the end of the second growing season (October 2001),

we sampled current-year shoots from black spruce seedlingsto determine foliar nutrient concentrations. One compositesample from three seedlings in each subplot was collected ineach of the 10 blocks. Samples were stored frozen,oven-dried at 65 °C for 48 h and ground to pass through a40-mesh screen. Following H2SO4 digestion (H2O2/Se)(Walinga et al. 1995), Kjeldahl N was measuredcolorimetrically by spectrophotometry (FIA Quickchem) andP, K, Ca, and Mg by inductively coupled plasma analysis(ICAP-9000).

15N uptake and recoveryEarly in the second growing season (mid-June 2001), we

established one rectangular quadrat (1 m × 2 m) in the unfer-tilized control subplot of four randomly chosen blocks anddivided it into eight subquadrats (0.5 m × 0.5 m) (Fig. 1).Each quadrat was centred on two adjacent seedlings.Double-labelled ammonium nitrate (15NH4

15NO3 at 2.5%atom%, Icon Services Inc., Summit, New Jersey) was appliedto each quadrat at a rate of 100 kg N·ha–1 (57.14 g of15NH4

15NO3 in 2 L of distilled water, uniformly sprayed un-der pressure with a manual garden sprayer). Fertilizer wasapplied at dusk (1900), followed by 2 L of distilled water torinse foliage, for a total water input equivalent to 2 mm ofrain. In October 2001, the two black spruce seedlings ineach 15N-fertilized quadrat were excavated in each of thefour blocks (eight seedlings total). Aboveground Kalmia andVaccinium spp. components were cut at ground level andcollected in four randomly selected subquadrats in all quad-rats. Vegetation (above- and belowground, Kalmia andVaccinium spp.) and humus were sampled in one of the re-maining subquadrats in each quadrat. We separated Kalmiaand Vaccinium spp. roots in the lab, based on their attach-

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ment to aboveground stems and on their colour. All col-lected material was kept frozen until processed.

Black spruce seedlings were oven-dried at 65 °C for 48 h,partitioned into needles, twigs, stems, and root systems; andweighed and ground to pass through a 40-mesh screen.Aboveground parts of Kalmia and Vaccinium spp. wereoven-dried, partitioned into stems and leaves, weighed, andground. Belowground parts of Kalmia and Vacciniumspp. were separated from humus by gentle washing withwater, and were then oven-dried, weighed, and ground. Ni-trogen isotope mass and atomic percent were determined on5-mg samples with an Isotope Ratio Mass Spectrometer(Europa Scientific Integra, PDZ Europa, Cheshire, UK) atthe Davis Stable Isotope Facility (University of California,Davis, California). Total 15N uptake and percent of 15N re-covery were calculated separately for black spruce, Kalmia,and Vaccinium spp. on a 1-m2 basis. Data from the two blackspruce seedlings from each quadrat were averaged. The eightseedlings that received 15N fertilization were omitted fromgrowth, XWP, and foliar nutrient concentration measure-ments related to soil modification (C, SR, HR) or fertiliza-tion (with FTP, without FTP) effects.

Statistical analysesAnalyses of variance (ANOVA) for a split-plot design us-

ing the three soil modification treatments (C, SR, HR) asmain plots and the two fertilization treatments (with FTP,without FTP) as subplots was used to test for treatment ef-fects on 2-year growth increments of black spruce heightand diameter (10 blocks). The effects of soil modification(C, SR, HR) and species on black spruce XWP, as well as

the effects of species (black spruce, Kalmia) on XWP, wereanalyzed separately using ANOVA for split-plot in time, withsoil modification and species as the main plots, respectively,and time as the subplot for both analyses (five blocks).Analysis of variance for repeated measurements (ANOVAR)was used to test for treatment and time of season effects onfirst and second season soil temperature and SWC profiles(five blocks). For this analysis, treatment effects were deter-mined separately for three periods within each year (9 June– 18 July; 19 July – 7 September; 8 September – 11 Novem-ber). The effect of treatments on nutrient sorption by resins(five blocks) and extractable soil nutrient concentration (10blocks) were determined using ANOVA for a completelyrandomized block design. Total plant 15N uptake data werenatural log-transformed to meet requirements of normalityand homoscedasticity before analysis by ANOVA for com-pletely randomized block designs with species as the treat-ment (four blocks). For these results (15N uptake data), wepresent back-transformed means and confidence intervals (af-ter Bernier-Cardou and Bigras 2001). Percent 15N recoverydata were rank-transformed and analyzed using theKruskal–Wallis nonparametric rank test.

Results

Soil temperature and water contentRemoving ericaceous shrubs by herbicide application (SR)

did not significantly (p = 0.082) influence soil temperatureduring the first and second growing seasons compared withundisturbed plots (Figs. 2A and 2B). However, shrub andhumus removal by scalping (HR) had a significant effect(p < 0.05) on soil temperature during most of the first grow-ing season (Fig. 2A) and two-thirds of the second growingseason (Fig. 2B). Generally, scalping increased soil tempera-ture during the summer (9 June – 7 September) and de-creased soil temperature during the remaining months of thegrowing season. The SWC profiles in the first year differedbetween the three soil modification treatments (treatment ×time, p = 0.004), but the daily analysis did not show any sig-nificant difference (p = 0.234) (Fig. 3A). During the firstpart of the second season (9 June – 18 July), SR plots hadlower SWC than the control plots. Humus scalping (HR) ex-acerbated this effect (Fig. 3B). From early August until theend of the season, SWC was higher in the SR plots than inthe control and least in the HR treatments (Fig. 3B).

Soil nutrientsDuring both the 2000 and 2001 seasons, NO3-N, NH4-N,

and phosphorous sorption by IER buried 10 cm beneath thesoil surface were below the detection limit for these nutri-ents (0.2, 1.0, and 0.2 mg·bag–1, respectively). Humus scalp-ing (HR) reduced seasonal K sorption by resins during thefirst and second years (Table 1). Calcium and Mg sorptionby resins were similar in all treatments during the first sea-son, but were significantly lower in the SR and HR plotscompared with the control plots during the second season.

Extractable NO3-N concentration in the top 10 cm of themineral soil was negligible and below detection limits(<1 mg·kg–1). Shrub removal (SR) or humus removal (HR)did not affect extractable NH4-N and Ca concentrations (Ta-ble 2). Extractable K concentrations increased when shrubs

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Fig. 2. Effect of ericaceous shrubs and humus on soil tempera-ture in (A) 2000 and (B) 2001 (C, control; SR, ericaceous shrubremoval with herbicide; HR, scalping).

were removed; P and K concentrations decreased when hu-mus was scalped. Extractable Mg concentration was the samein control and SR plots, and 66% lower in HR plots.

Spruce seedling height and diameter growthAt the time of planting, mean distance from planted seed-

lings to the nearest Kalmia stem was 25 cm (range 4–112 cm).After two growing seasons, mean Kalmia cover density hadincreased from 9% at planting to 14% (range 0–60%) anddistance to the nearest Kalmia stem had decreased to 14 cm(range 1–73 cm). Overall seedling mortality at the end of the

second season was 2%, and occurred only in the control andthe SR treatments. Removal of ericaceous shrubs had a sig-nificant and positive impact on spruce seedling height anddiameter growth over 2 years (Table 3). The presence ofericaceous shrubs in the control plots reduced 2-year spruceseedling height and diameter increments by 25% and 32%,respectively, compared with SR plots. When shrub removalwas associated with humus removal (HR), spruce height anddiameter growth were similar to that in the control plots.Fertilization at time of planting doubled 2-year spruce heightand diameter growth (Table 3). No soil modification × fertil-ization interaction was found.

Xylem water potential (XWP)Throughout the second growing season, predawn XWP

was lower in black spruce seedlings than in Kalmia plants(Fig. 4A). However, midday XWP of the two species wassimilar. Scalping negatively affected black spruce predawnXWP during the second season, but had no effect on middayXWP (Fig. 4B). Ericaceous shrub removal alone did not in-fluence spruce seedling XWP.

Foliar nutrient concentrationSeedlings in SR plots had greater foliar N concentrations

than seedlings in control plots, both with and without fertil-ization at planting (Table 4). Fertilization increased foliar Nconcentration from 12.5 g·kg–1 to 17.1 g·kg–1 in control plots,but had no significant effect in either the SR or HR plots.Phosphorus foliar concentration was not influenced by fertil-ization, but increased with SR to 2.1 g·kg–1, compared with1.8 g·kg–1 in control plots. With fertilization, SR or HR didnot affect spruce foliar K concentration. Compared withseedlings in the control plots, spruce foliar Ca and Mg con-centrations increased with SR but decreased with HR. Fertil-ization also resulted in a decrease in foliar Ca and Mgconcentrations.

15N uptake and recoveryVegetation took up 19% of the 15N applied. Kalmia and

Vaccinium spp. uptake was 7 and 11 times greater, respec-tively, than that of black spruce seedlings (Table 5). How-ever, 15N uptake per unit of root biomass was six timesgreater for black spruce compared with the two ericaceousshrubs, which did not differ from each other (Table 5).

Discussion

Spruce seedling growth can be reduced in the presence ofKalmia (Yamasaki et al. 1998, 2002), and a number of hy-potheses have been suggested to explain this growth “check”(Damman 1971; Zhu and Mallik 1994; Inderjit and Mallik1996b; Yamasaki et al. 1998, 2002; Bradley et al. 2000).Other ericaceous shrubs often grow in association with Kal-mia (Titus et al. 1995), and some of these have also been im-plicated in seedling growth check (e.g., Ledumgroenlandicum Oed. in Inderjit and Mallik 1996a, 1997).Our trial was established on a site where Vaccinium angusti-folium and V. myrtilloides grew in association with Kalmia.Although some Vaccinium species can interfere with spruceregeneration in Europe (Pellissier and Trosset 1989;

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Fig. 3. Effect of ericaceous shrubs and humus on soil water con-tent (SWC) in (A) 2000 and (B) 2001 (C, control; SR, ericaceousshrub removal with herbicide; HR, scalping).

Nutrient (mg·bag–1)

Treatment K Ca Mg

Year 1C 6.9 (1.1)a 3.5 (0.8)a 1.0 (0.2)aSR 6.5 (1.1)a 1.8 (0.8)a 0.7 (0.2)aHR 1.0 (1.1)b 0.9 (0.8)a 0.2 (0.2)a

p=0.006 p=0.086 p=0.050Year 2C 11.6 (1.3)b 5.5 (0.7)b 2.4 (0.3)cSR 9.6 (1.3)b 2.4 (0.7)a 1.1 (0.3)bHR 0.9 (1.3)a 0.9 (0.7)a 0.2 (0.3)a

p<0.001 p=0.003 p=0.001

Note: NH4-N, NO3-N,and P were below detection limits. Data presentedas mean (SE). For a given year, means followed by the same letter arenot significantly different at α = 0.05 (Fisher’s protected LSD test). C,control; SR, ericaceous shrub removal with herbicide; HR, scalping.

Table 1. Soil nutrient availability during the first and secondgrowing seasons, estimated using ion exchange resin bags.

Jäderlund et al. 1997), members of this genus have not beenconsidered problematic within the range of Kalmia in NorthAmerica. However, their close association with Kalmia ondry boreal sites such as that in our study means that we can-not focus on Kalmia alone. We therefore refer to the effectsof our treatments on ericaceous shrubs, not just on Kalmia.

Removal of ericaceous shrubsHerbicides can be used to obtain successful long-term

control of Kalmia (English and Titus 2000). However, ques-tions remain about the ability of humus produced by Kalmiato supply nutrients to conifer seedlings, even if Kalmia iseradicated (Bradley et al. 1997a). Our field trial clearly dem-onstrates that, regardless of the interference mechanisms in-volved, spruce growth increased significantly after control ofKalmia and Vaccinium (Table 3). The living ericaceousplants appear to be necessary for interference mechanisms tobe strongest, and humus quality seems less important. Whenwe removed the vegetation and humus through scalping andplanted seedlings in bare mineral soil, seedling growth wasreduced compared with when we eradicated the vegetationand was not different from that in the control treatmentwhen Kalmia and Vaccinium were present. This suggeststhat humus is important for sustaining seedling growth, evenif its quality was influenced by ericaceous litter inputs. Anynegative influences of humus on germinants observed in lab-

oratory studies (Mallik and Newton 1988) did not result inoperationally unacceptable spruce growth in the field.

Reduced nutrient availability is often associated with thepresence of Kalmia and has been attributed to the buildup ofrecalcitrant humus (e.g., Damman 1971; Bradley et al.1997a, 1997b, 2000). However, we found that vegetationcontrol had no detectable effect on soil nutrient availabilityin the rooting zone in the first year following treatment. Inthe second year after treatment, Ca and Mg availability de-creased. Availability of NO3-N, NH4-N, and P could not bedetermined with IER, as concentrations were below detect-able limits (both years). Although nutrient adsorption capa-bility by IER can be reduced by excessive soil drying(Kjønaas 1999), DeLuca et al. (2002) also used IER andfound decreasing levels of inorganic N with increasing erica-ceous cover. The extremely low levels of NH4-N and NO3-Non our site may therefore have resulted from the high coverof ericaceous plants rather than from methodologicalartefacts. Elimination of Kalmia and Vaccinium plants alsohad no effect on extractable NH4-N, P, Ca, or Mg concentra-tions in the top 10 cm of the mineral soil after 2 years, al-though K concentrations increased (Table 2). As with resultsfor IER, extractable NO3-N was below detection limits.However, there may have been insufficient time after vegeta-tion control for humus quality and resultant nutrient avail-ability to change substantially.

Although we did not detect changes in soil nutrient avail-ability following vegetation control, seedling growth andcurrent-year foliar concentrations of N, P, Ca, and Mg in-creased (Table 4), suggesting increased conifer seedling up-take of nutrients. Conversely, we observed decreased Ca andMg absorption by IER with vegetation control but increasedseedling foliar concentrations. Control of Kalmia and Vacci-nium spp. led to no change in available K concentration be-neath the humus (as measured with IER bags), an increase inextractable K concentration in the mineral soil beneath, anda decrease in spruce foliar K concentration. It is possiblethat conifer uptake of K was not sufficient for the increasedseedling growth rates, and foliar K concentrations thereforedecreased through dilution (Timmer 1991). On the basis ofsoil measurements alone, we thus rejected our hypothesisthat the presence of ericaceous shrubs reduces soil mineralnutrient concentrations and availability, at least in the shortterm. However, given the ability of other spruce and erica-ceous species to bypass mineral N uptake (Näsholm et al.1998), enhanced foliar N concentrations in black spruce maybe the result of increased organic N uptake. Dissolved or-

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Thiffault et al. 1663

Nutrient (mg·kg–1)

Treatment NH4-N P K Ca Mg

C 6.6 (0.9)a 11.2 (1.2)ab 15.6 (0.9)b 21.7 (4.8)a 5.9 (0.9)bSR 8.6 (0.9)a 13.8 (1.2)b 19.1 (0.9)c 15.0 (4.8)a 5.4 (0.9)bHR 6.5 (0.9)a 8.5 (1.2)a 7.1 (0.9)a 6.3 (4.8)a 1.9 (0.9)a

p=0.145 p=0.021 p<0.001 p=0.104 p=0.010

Note: NO3-N was below detection limits. Data presented as mean (SE). Means followed by the same letterare not significantly different at α = 0.05 (Fisher’s protected LSD test). C, control; SR, ericaceous shrub re-moval with herbicide; HR, scalping.

Table 2. Mineral soil (upper 10 cm) extractable nutrient concentrations at the end of the secondgrowing season.

Treatment GIh (cm) GId (mm)

C 3.8 (0.3)a 1.3 (0.1)aSR 5.1 (0.3)b 1.9 (0.1)bHR 4.3 (0.3)a 1.5 (0.1)a

p=0.002 p<0.001Without FTP 2.8 (0.3) 1.1 (0.1)With FTP 6.0 (0.3) 2.1 (0.1)

p<0.001 p<0.001

Note: Data presented as mean (SE). C, control; SR,ericaceous shrub removal with herbicide; HR, scalping;FTP, fertilization at time of planting. For the C, SR, andHR treatments, means followed by the same letter are notstatistically different at α = 0.05 (Fisher’s protected LSDtest).

Table 3. Two-year height (GIh) and diameter (GId)growth increments of black spruce seedlings plantedwith or without ericaceous shrub and humus, andwith or without controlled-release fertilizer applica-tion at time of planting.

ganic N can be a very important soil pool on boreal blackspruce sites (Smith et al. 1998) and, although we did notmeasure it, controlling ericaceous vegetation may have in-creased dissolved organic N availability to conifer seedlings.

Controlling vegetation had no effect on soil temperature(Fig. 2). Soil temperature can affect both soil nutrient miner-alization processes (Kladivko and Keeney 1987; Leiros et al.1999) and seedling growth (Örlander et al. 1990; Jobidon2000). Soil water content can also affect seedling growthand soil nutrient processes. Although SWC was not affectedby vegetation removal in the first year after treatment (Fig. 3),it was reduced early in the growing season (before July 19)and increased from early August onwards in the second year,

compared with the control treatment. However, these effectsdid not have an impact on XWP of spruce seedlings (Fig. 4).We therefore also rejected our hypothesis that the presenceof ericaceous shrubs reduces soil temperature and water con-tent.

Fine roots are responsible for 90% of water and nutrientabsorption (Chung and Kramer 1975), and impairment oftheir growth or function could be expected to reduce waterand nutrient uptake. We initially hypothesized that the pres-ence of Kalmia would increase black spruce seedling waterstress via potential allelopathic inhibition of spruce fine rootgrowth or function. However, because we found that removalof ericaceous shrubs had no effect on spruce XWP and foliar

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1664 Can. J. For. Res. Vol. 34, 2004

Fig. 4. Predawn and midday xylem water potentials during the 2001 growing season of (A) planted black spruce seedlings and natu-rally established Kalmia angustifolia plants, and (B) black spruce seedlings planted in plots with or without ericaceous shrubs andhumus (C, control; SR, ericaceous shrub removal with herbicide; HR, scalping).

nutrient concentrations generally increased, we suggest thatspruce fine root growth and function were not significantlyimpaired by the presence of these shrubs.

Humus removalIf the quality of humus that develops under Kalmia–Vaccinium

associations contributes to seedling growth check (Bradleyet al. 1997a, 1997b), then mechanical site preparation to re-move vegetation and associated humus is a silvicultural op-tion (Prévost 1997; Thiffault et al. 2004). However, ourresults show that if seedlings are planted in bare mineral soilwith no humus retained, seedling growth is no better than ifsites were not disturbed (Table 3). This may be because hu-mus removal results in the loss of an important source of nu-trient capital from the site (Munson et al. 1993; Prescott etal. 2000). While availability (by IER) of K, Ca, and Mg wasgreatly reduced in the seedling rooting zone when the humuswas removed (NH4-N, NO3-N, and P were not detectable),seedling foliar N concentration increased and P and K wereunchanged, even though K availability in the rooting zonedecreased. Reductions in Ca and Mg availability in the root-ing zone were associated with reduced foliar concentrations.It remains to be seen whether acceptable seedling growth inthe scalping treatment can be sustained in the long term fol-lowing the removal of nutrient capital in the humus.

Increases in soil temperature (Fig. 2) and decreases in soilwater content (Fig. 3) are typical responses to removal of the

insulating mat of organic humus by scalping (Örlander et al.1990). Cowles (1982) also observed that exposing mineralsoil by complete removal of the lichen mat creates unfavour-able growth conditions for conifer seedlings by increasingevaporative loss and thus reducing soil water availability.The differences in soil water content that we found resultedin lower predawn XWP in the seedlings (Fig. 4). This, inturn, can impair seedling growth by exacerbating transplant-ing shock (Margolis and Brand 1990).

Fertilization responseKalmia-induced check of spruce seedlings can arise from

nutrient deficiencies, among other interference mechanisms(Yamasaki et al. 2002). On similar ericaceous-dominatedsites in the United Kingdom (Taylor and Tabbush 1990) andBritish Columbia (Prescott et al. 1996), fertilizer was used toovercome establishment and nutritional problems. We there-fore used spot fertilization (Silva Paks) for efficient N deliv-ery to planted seedlings.

Spot fertilization approximately doubled 2-year seedlinggrowth (Table 3). It also increased foliar N concentrations inundisturbed control plots, but had no effect when shrubs orhumus were removed (Table 4). Potassium, Ca, and Mg fo-liar concentrations decreased with fertilization, while P re-mained constant. Paquin et al. (1998) also found thatseedling growth and N concentration in current-year needlesof newly planted black spruce seedlings increase after fertil-ization. The lack of P and the negative (K, Ca, Mg) effectsof fertilizer on foliar concentrations that we observed maybe the result of a dilution effect (e.g., Timmer 1991) becauseof increased needle biomass.

Comparison of spruce and Kalmia performanceIn keeping with our hypothesis, predawn XWP was found

to be lower (more negative) in black spruce seedlings than inKalmia plants, suggesting that there was greater water stressin spruce than in Kalmia. Black spruce root growth is rela-tively slow (Lamhamedi and Bernier 1994). Seedling rootcontact with soil is not optimal during the first growing sea-son, since root systems are largely confined to thepeat-based root plug (Bernier 1993), where resistance to wa-ter flow is more important (Örlander and Due 1986). In con-trast, Kalmia roots are extensive (Mallik 1993; Titus et al.1995) and therefore access a large soil volume from whichto absorb water. It is also possible that Kalmia can maintaina constant water potential even when soil water availabilitybecomes limiting, as has been observed in K. latifolia L.(Lipscomb and Nilsen 1990). In a separate trial from spotfertilization at time of planting, we tracked broadcast fertil-izer N uptake by ericaceous shrubs and black spruce by ap-plying 15N at operational rates (100 kg N·ha–1). Kalmia,Vaccinium spp., and black spruce seedlings represented 40%,58%, and 2%, respectively, of the total plant biomass in theundisturbed control treatment. Of the 15N taken up by vege-tation (i.e., 19% of the total 15N applied), 64% was immobi-lized by Vaccinium spp., 31% by Kalmia, and 5% by blackspruce, even though the uptake of 15N per unit of root bio-mass by ericaceous shrubs was six times lower than forblack spruce. We therefore rejected our hypothesis that Kal-mia takes up fertilizer N more efficiently than black spruce,

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Thiffault et al. 1665

Treatment N (g·kg–1) K (g·kg–1)

Without FTPC 12.5 (0.7)a 5.8 (0.2)bSR 20.1 (0.7)c 5.1 (0.2)aHR 16.8 (0.7)b 6.2 (0.2)b

With FTPC 17.1 (0.7)a 5.1 (0.2)aSR 19.9 (0.7)b 5.0 (0.2)aHR 18.5 (0.7)ab 5.2 (0.2)a

p=0.003* p=0.026*

Treatment P (g·kg–1) Ca (g·kg–1) Mg (g·kg–1)

C 1.8 (0.05)a 3.6 (0.3)b 1.1 (0.04)bSR 2.1 (0.05)b 4.4 (0.3)c 1.2 (0.04)cHR 1.8 (0.05)a 2.9 (0.3)a 0.8 (0.04)a

p=0.003** p<0.001** p<0.001**Without FTP 1.9 (0.04) 4.1 (0.2) 1.1 (0.03)With FTP 1.9 (0.04) 3.1 (0.2) 0.9 (0.03)

p=0.934*** p<0.001*** p<0.001***

Note: Mean (SE) data are presented for main treatment effects when in-teractions are not significant (p = 0.05) and for interactions only (Zar1999) when interactions are significant (p < 0.05) using ANOVA by nutri-ent. Within each nutrient, means of each combination of treatments thatare followed by the same letter are not significantly different (α = 0.05)using Fisher’s protected LSD test. C, control; SR, ericaceous shrub re-moval with herbicide; HR, scalping; FTP, fertilization at time of planting.

*p (soil modification × FTP).**p (soil modification).***p (FTP).

Table 4. Nutrient concentrations in black spruce current-yearfoliage 2 years after planting.

but concluded instead that competition for soil nutrientsprobably takes place because of the amount of overall nutri-ent uptake by the large root biomass of ericaceous shrubs ascompared with black spruce.

We found that black spruce absorbed 0.5% of the applied15N, which is similar to the 0.1% and 0.4% absorbed in thefirst and second growing seasons, respectively, by white spruce(Picea glauca) seedlings in Saskatchewan (Staples et al. 1999).In our study, 18% of the 15N was taken up by ericaceousshrubs, as compared with 5% and 6% in the first and secondgrowing seasons, respectively, by competitors (mainlyPopulus tremuloides Michx. and grass species) in Saskatche-wan (Staples et al. 1999). The majority of the 15N we ap-plied (81%) was therefore not taken up by plants, a result inkeeping with observations elsewhere (e.g., Preston and Mead1994; Staples et al. 1999), and was probably rapidly immo-bilized in an organic form in the soil (Chang et al. 1997).

Conclusion

Our results have implications for plantation establishmenton Kalmia-dominated sites. Broadcast fertilization should beavoided because the extensive root systems of Kalmia andassociated species ensure that ericaceous shrubs dominatenutrient uptake processes. Also, most of the fertilizer appliedremained inaccessible to plants and was probably immobi-lized in an organic form in the soil. However, as has beenobserved in other ecosystems, spot fertilization at the time ofplanting with slow-release fertilizer placed near seedlingroot plugs can promote initial seedling growth, thus reducingthe time to canopy closure and shading out of Kalmia. Hu-mus removal in this study positively affected the soil thermalregimes. Scalping, however, reduced soil nutrient and wateravailability adjacent to seedlings. Mechanical scarificationtechniques that expose mineral soil in trenches or patcheswithout exporting organic matter from the site would likelyhave similar beneficial effects on soil temperature as thosewe observed.

Acknowledgements

We acknowledge the help of Francis Cadoret for his skilledwork in the field and for preparing samples for analyses. Wethank staff members of the ministère des Ressources

naturelles, de la Faune et des Parcs (MRNFP) who per-formed the chemical analyses, Robert Jobidon (MRNFP) forinsightful discussions, Jean Noël (MRNFP) for help inediting the figures, and Neil Anderson (Reforestation Tech-nologies International, Salinas, California) for supplying theslow-release fertilizer. We are grateful to staff members ofAbitibi-Consolidated Inc. and the MRNFP for their help inlocating the experimental area. We thank Darwin Burgess(Natural Resources Canada, Canadian Forest Service) andMarcel Prévost (MRNFP) for proofreading a draft of thispaper, and two anonymous reviewers who provided helpfulcriticism of the manuscript. This project was funded by theMRNFP, in consultation with the Fonds québecois da la re-cherche sur la nature et les technologies through a researchgrant to A.D. Munson. It formed part of the doctoral studiesof the senior author. Use of trade, firm, or corporation namesis for information only and does not constitute endorsementby the MRNFP or the Canadian Forest Service.

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