Soil nitrogen dynamics and nutrition of pine following silvicultural treatments in boreal and Great Lakes-St. Lawrence plantations

Pores;~;ology Management Forest EcologyandManagement76 (1995) 169-179

Soil nitrogen dynamics and nutrition of pine following silvicultural treatments in boreal and Great Lakes-St. Lawrence plantations Alison D. Munson a,*, Victor R. Timmer b a Centre de Recherche

en Biologic Forest&e, Facultk b Faculty of Forestry, University

de Foresterie ef Giomatique, Universit6 Laval, Sainte-Foy, Que. GlK of Toronto, 33 Willcocks St., Toronto, Ont. MSS 383, Canada

7P4, Canada

Accepted 15February 1995

Abstract Six years after establishment of a boreal forest plantation(Ontario, Canada),the impact of intensivesilvicultural treatmentson soil nitrogen(N) reserves,N availability andnutrition of jack pine(Pinus bunk&ma Lamb.)wasexamined. Treatmentsof scarification(blade removalof entire forest floor), fertilizer application(annually) and vegetationcontrol (annuallyfor 4 years)were appliedin a factorialdesign,andthe sitewassubsequently plantedto native conifer species.In the sixth seasonafter transplanting,no significantimpactsof treatmentson total N reservesin the humusor surfacemineral soil were noted.Scarificationmarkedlyreducedboth soil ammonium(NH,-N) and nitrate(NO,-N) availability, especially early in the growing season.The positive effect of vegetationcontrol on NH,-N and NO,-N availability was also significantlyreducedby scarification.VegetationcontrolsignificantlyincreasedNH,-N and NO,-N availability in the mid to late growingseason,andhighestlevelsof NO,-N (ten timesgreaterthancontrol) werenotedwith combinedfertilization andvegetationcontrol. We comparedthe nutritionaland growth response of jack pine on the borealsite with white pine (P~~u.sstrobes L.) response to the sametreatmentson a moresouthernGreat Lakes-St. Lawrenceforestsite. Response to treatmentswasinterpretedby vector analysis,usingcrownbiomassestimates, ratherthanneedlebiomass,to integratelonger term response to treatment.For both species,treatmentsincludingvegetationcontrol by herbicideresultedin greatercrown biomassand N accumulation;the greatestresponsewasobservedfollowing the three treatmentscombined:scarification, fertilization and vegetationcontrol. White pine hada strongerrelative response to treatmentsthan the borealspecies,jack pine, supportingthe hypothesisthat the borealspeciesdoesnot showasgreatan acclimationresponse to changingresource availability. Keywords:

Nitrogen;Fertilizer;Herbicide; Scarification; Pinus

banksiana

1. Introduction In 1986 and 1987, a series of three experimental plantations was establishedin the forests of Canada’s ’ Corresponding author.

major climatic zones to characterize the ecophysiological responseof native pines and sprucesto factorial combinations of scarification, fertilizer application and vegetation control (Brand and Janas, 1988; Brand, 1991). A growth analysis approachwas subsequently applied to evaluate tree responseand accli-

0378-1127/95/$09.50 0 1995ElsevierScience B.V. All rightsreserved SSDI0378-1127(95)03547-8

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Ecology

mation to changes in resource availability (light, water and nutrients) following the treatments (Margolis and Brand, 1990; Brand, 1991). Studies carried out in 1989 and 1990 on the most southern of the three sites, in the Great Lakes-St. Lawrence forest region (Rowe, 19721, demonstrated the impact of these treatments on soil microbial biomass and microbial community structure (Ohtonen et al., 1992) and on soil microclimate, soil fertility and tree nutrition (Munson et al., 1993). The present study examined the impact of silvicultural treatments on soil nutrient reserves and seasonal nitrogen availability on a second site, in the boreal forest region (Rowe, 19721, near Foleyet, northern Ontario, 6 years after plantation establishment. Foliar nutrition of jack pine (Pinus banksiuna Lamb.) was also characterized in relation to soil nutrients. General trends in soil and tree response were compared with results obtained on the more southern Great Lakes forest site with white pine (Pinus strobus L.), 4 years after planting (Munson et al., 1993). In order to improve our capability to predict ecosystem response to harvest disturbance and subsequent silvicultural treatments, it is necessary to repeat experiments under different climate and soil conditions; the observed differences in response may help us to discern critical processes controlling ecosystem function in the different situations (Franklin, 1989). It is also important to continue to monitor response to silvicultural treatments over long time periods. In the present study, we hypothesized that the cooler climate and soil on the boreal site would lessen the degree of soil N response to harvest disturbance and treatments compared to response observed on the Great Lakes site (Munson et al., 1993). The greater buildup of soil organic matter on the boreal site (Brand, 1991) indicated reduced decomposition and N mineralization rates. Vitousek et al. (1982) found that sites with relatively low N availability and low litterfall N prior to disturbance showed relatively small ammonium (NH,-N) and nitrate (NO,-N) responses to disturbance. Although the experimental plots described here were subject to prolonged fertilizer and herbicide treatments, we would still expect that soil N response to the same treatments should be less on the boreal compared with the Great Lakes-St. Lawrence forest site.

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Secondly, we hypothesized that long term tree nutritional response and growth response to treatments would also be less on the boreal site, since the intensity and duration of the N response to treatment may be reduced under boreal conditions. The interpretation of tree nutritional response to treatment by foliar analysis becomes less straightforward over time, since total biomass response becomes more important than individual needle response. Hence, we present a new method for incorporating total crown biomass into nutritional interpretation by vector analysis (Timmer and Stone, 19781, comparing the boreal species, jack pine, with the Great Lakes native pine, white pine. The growth response of jack pine may also be less than white pine since boreal species are adapted to more nutrient-poor soil conditions. Consequently, growth response to changing soil nutrient conditions associated with harvest and silvicultural treatments may be less (fhapin et al.. 1986).

2.1. Study sites and plantation

estabiishment

The boreal forest site is near Poleyet (48”22’N, 82”27’W) in the Clay Belt region of Ontario, Canada. The forest cover before harvest was a mixedwood forest of white spruce (Picea glauca [Moench] Vossl, black spruce (Picea mariuna (Mill) B.S.P.), white birch (Betula papyrifera Marsh.) and trembling aspen (Populus tremubides Michx.). The tree stand was harvested between 1968 and 1970 and replanted to black spruce, which failed owing to heavy vegetation competition. This competing vegetation was manually removed before plantation establishment in 1987. The soil is a humo-ferric podzol (Canadian Soil Survey Committee, 1978) of silty loam texture. moderately well-drained, with a humus layer of 5-10 cm depth. For comparison, the Great Lakes forest site is at the Petawawa National Forestry Institute near Chalk River, Ontario (45”57’N, 77“34’W), and before harvest was dominated by aspen, white birch, white spruce (Picea gleucu [Moenchl Voss) with smaller components of yellow birch (Be&u alZeghaniensis Britton), basswood (Tilia-americana L.1, white pine and balsam fir ( Abies balsamaa IL.1

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Mill). A 10 ha area of forest was clearcut in 1985 for plantation establishment in 1986. The soil is classified as an orthic humo-ferric podzol (Canadian Soil Survey Committee, 1978) of sandy loam texture, well-drained, with a 2-5 cm thick fibrimor humus form. The experimental design in both cases was a split plot factorial design, originally with three levels of soil surface modification, two levels of soil fertility and two levels of vegetation competition in plots of 20 m X 40 m. For the present study, one soil surface modification treatment using plastic mulch to raise soil temperature was eliminated owing to minimal effects noted in earlier analyses of response. The other two treatments of soil surface modification consisted of either complete removal of humus layer by blade scarification to expose mineral soil (S) or humus layer left undisturbed. Soil fertility levels were either no treatment or increased (F) by annual application of Osmocote slow release fertilizer (176-10, plus micronutrients; nitrogen as 9.1% ammonium, 7.9% nitrate) at rates starting at 30 g per tree in the first year, increasing to 90 g in the fourth year. Vegetation competition was at two levels: no control of competition or annual application of Roundup@ (N-phosphonomethyl glycine) at 2.0 kg ha-’ for a period of 4 years (H). The boreal site was also treated with herbicide in 1986, the year before planting. 2.2. Soil and foliar sampling and analyses

For total nutrients (C, N), five cores (7 cm diameter, to 10 cm depth) per plot were sampled along a randomly located transect and then composited for analyses. Composite samples were air-dried and finely sieved (0.25 mm) in preparation for C and N analyses. Kjeldahl N and oxidizable C by the Walkley-Black method were determined. For soil N availability measurement, ion exchange resins were used, following the methods of preparation and extraction of Krause and Ramlal (1987). A mixedbed cation/anion resin was measured by volume (15 ml) into nylon polyester bags (8 cm X 8 cm), which were then sealed by heat. Resin bags were installed in early June (5 June 1992) on the boreal site, and recovered and new bags reinstalled the following dates (6 weeks per period): 15 July and

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25 August. The final set of bags was recovered 4 October 1992. Bags were installed at 10 cm in the mineral soil by creating a slit with a straight blade shovel at a 45” angle, and laying the bag flat against the soil surface before closing the slit. In scarified plots (S), the mineral soil was at the surface, while on undisturbed plots the mineral soil was covered by a humus layer of variable thickness. The same resins could not be obtained in 1992 as used for the Great Lakes site; we had used AG3-X4A (anion) and BioRex (cation, both from BioRad), and for the boreal site we used a mixedbed cation/anion resin (JTBaker). Foliar sampling of current and l-year-old needles of five trees per plot was carried out in the first week of October on both sites. Three subsamples of 50 needles from each sample were dried at 65°C for 48 h, and then weighed for needle mass determination. Dried material (40 mesh) was ground, and digested in a H,O,/ Se solution before analyses for macronutrients (Parkinson and Allen, 1975). Total N in the digest solution was measured using the FIAstar Tecator spectrophotometer (Tecator, Hoganas, Sweden) and P and cations were measured using ICP analyses (inductively coupled plasma, Plasma 40, Perkin-Elmer, Norwalk, CT). Vector analysis followed the method developed by Timmer and Stone (1978). Crown biomass estimates (dry mass) were made by substituting height and diameter measures into equations for biomass prediction, developed by Ouellet (1983). The equation used for both jack pine and white pine was y = p,DpzHP3 + E

where D is diameter (cm) and H is height (m). For jack pine the estimated coefficients for crown ovendry mass are & = 0.10569, pz = 2.69203 and & = - 0.99737. For white pine the estimated coefficients for crown oven-dry mass are & = 0.4717, & = 3.12228 and & = - 1.10248. 2.3. Statistical analyses

All data were tested for variance homogeneity and logarithmic transformations were used where necessary to meet assumptions for homogeneity. A general linear model for split-plot experimental design (Statistical Analysis System Institute Inc., Cary, NC) was

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used to test for significant treatment and interaction effects. The general model for the analysis was

significant and were not evident in the mineral soil. Total N in the mineral soil ranged from 1.2 g kg- ’ for the control treatment to 0.9 g kg-’ for treatments including scarification, while total C varied from 19.3 g kg-’ in the control to 14.8 g kg-’ for the treatment including scarification and vegetation control. Soil pH was 4.8 for the humus layer and 5.0 for mineral soil; silvicultural treatments did not significantly affect pH.

~jkr=p+Ri+Si+RS;j+Fk+H,+SF,k+SHj, + FHkr + SFHjkl + RSFHij,,

where R is replicate (i = 1, 2, 3 or 41, S is scarification ( j = 0 or 11, F is fertilizer application (k = 0 or l), H is vegetation control (I = 0 or 1).

3. Results

3.2. Soil nitrogen availability

3.1. Total soil C and N reserves

Scarification had a negative impact on total season NH,-N availability (Fig. l), while fertilization increased NH,-N levels in the soil. SciKification also reduced the positive effect of vegetation control on NH,-N availability. The negative effect of scarification was also evident for soil nitrate (NO,-N). Scarification generally reduced total season nitrate availability and also diminished the positive effect of vegetation control on soil nitrate. Fertilization as a single treatment markedly increased NO,-N levels in the soil, generally to a greater extent than the increase in NH,-N. The highest levels of soil NO,-N were associated with the combined treatment of fer-

On the boreal site, we did not observe significant differences among treatments in total soil N, C, or the C/N ratio in the humus or mineral soil, 6 years after plantation establishment and initial treatment. In the forest humus layer, there was a tendency for total N to be reduced after vegetation control (8.7 g kg-’ for control vs. 8.0 g kg-’ with vegetation control) and for lower total C following fertilization (180.2 g kg-’ for control vs. 154.5 g kg-* after fertilization), which is reflected in a lower humus C:N. However, these trends were not statistically

2oom

, 17500 -

q NH,-N n NOa-N

1500012500

-

NHQ-N s (P
100007500

N03-N s (p
C

F

H

FH

S

SF

Treatment Fig. 1. Total season ammonium (NH,-N) and nitrate (NO,-N) availability, measured the surface mineral soil (at 10 cm, from 5 June to 4 October 1992). S, scarification; MeanfSE, n=S.

SH

SFH

using three series of ion exchange resin bpgs buried in F, fertilization; H, vegetation control ti61 B&e.

A.D. Mmson,

V.R. Timmer/

Forest Ecology

tilizer application and vegetation control; this level was more than 10 times that measured in control plots. When scarification was applied with these two treatments, NO,-N levels were considerably reduced. The seasonal pattern of N availability indicated different dynamics for NH,-N and NO,-N availability (Fig. 2). Ammonium levels were generally lower

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early in the season and highest in September (Figs. 2(a)-2(c)). Fertilization had a consistently positive effect on availability throughout the growing season. The negative impact of scarification alone was most evident in the earliest sampling period (Fig. 2(a)). Vegetation control alone increased NH,-N levels in the surface soil only in the mid-growing season.

1400 a

1200

3 en 3

1000

z,

June

SID
a

800 600

C

F

H

FH

S

SF

SH SFH

C

F

H

FH

S

SF SH SFH

C

F

H

FH

S

SF

SH SFH

C

F

H

FH

S

SF SH SFH

C

F

FH

S

SF

SHSFH

1400 a

1200

;

1000

2

800

f

600

g

400 200 0

1400 1200 iii p” a t

1000 800

2,

600

f

400 200 0 H

Treatment

C

F

H

FH

S

SF

SH SFH

Treatment

Fig. 2. Seasonal availability of ammonium (NH,-N) and nitrate (NO,-N), measured using three series of ion exchange resin bags buried in the surface mineral soil (at 10 cm, from 5 June to 15 July; 16 July-25 August; 26 August-4 October 1992). S, scarification; F, fertilization; H, vegetation control with herbicide. Mean + SE, n = 5.

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As for NH,-N, the negative effect of scarification on NO,-N availability was most marked early in the growing season (Fig. 2(d)). Scarification also consistently reduced the positive effect of vegetation control, and fertilization and vegetation control, on soil NO,-N levels. The positive impact of vegetation control on NO,-N availability was significant during the last sampling period (mid-August through September). 3.3. Foliar nutrition and growth

response

Only N concentration in current foliage of jack pine was affected by silvicultural treatments; N tended to decrease with vegetation control, and scarification increased this negative effect. Concentrations of P and K in current needles were not affected by silvicultural treatments (Table 1). Foliar concen-

Table 1 Impact of single and factorial treatments on nutrient concentrations (mg g- ’ ) of current foliage (sampled October) of jack pine and white pine Treatment Jackpine C F H FH S SF SH SFH

a

N 16.2(0.1) 16.7cO.2) 14.8(1.1) 16.5(0.3) 14.3(0.6) 13LxO.3) 14.8(0.5) 15.2cO.4) < 0.034)

14.90.0) 13.5(1.0) 19.2U.6) 20.4(0.4) 17.0(1.0) 14.2c1.7) US(2.6) 17.6c1.5)

’

1.6(0.0) 1.6(0.0) 1.6(0.0) 1.tiO.O) l.S(O.1) lS(O.0) l&0.2) 1.5~0.0)

S.o(O.2) 5.4(0.1) 5.2cO.l) 5.tiO.l) 5.4(0.4) 5.2cO.2) 4.8cO.2) 5.3(0.1)

NS

NS

2JNO.O)

6.7cO.4) 6.1cO.4) 7.5CO.4) 6.2cO.3) 6.0(0.5) 6.2cO.2) 6.4cO.6) 6.0(0.1)

2.0(0.2) 1.9(0.1) 1.8(0.1) l.S(O.0) 1.8tO.O) lB(O.2) 1.7(0.1)

H, vegetation

control;

F, fertilizer

effects and associated

probabilities

79

Table 2 Impact of single and factorial treatments on growth parameters of jack pine and white pine. Values in parentheses are the standard error Treatment

Jack pine C F H FH S SF SH SFH

a Current needle mass (50 needles)

(mg)

l-year-old needle mass (50 needles) (mg)

453 (50) 418 (18) 501(9) 400 (8) 374 (36) 411 (25) 385 (27) 372 (31)

5Y2 592 552 603 494 576 631 666

H(P White pine c F H FH s SF SH SFH

297 228 532 541 337 284 454 571

i 0.018)

b

(16) (21) (20) (48) (43) (4) (41) (21)

Crown biomass increment (kg year-’ f

(75) (277) (54) (55) (71) (40) (70) (33)

NS

85 (5) 101 (6)

98 (8) 107(11) 128 (191 119(6) 90 (2) 149 (18)

0.04 0.05 0.93 I. 14 0.10 0.10 0.97 1.31

K

H(P < 0.0001) S x H(P < 0.002) F x H(P < 0.007) a C, control; cation. b Treatment

76 (199.5) 169-I

H (P < 0.0001) FxH(P
S X H(P White pine C F H FH S SF SH SFH

P

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addition;

S, scarifi-

from ANOVA.

a C, control; cation. b Treatment

F, fertilizer

addition;

effects and associated

S (I’ < 0.045) F(P<0.030) H, vegetation probabilities

control;

S, scarifi-

from ANOVA.

trations of N, P and K in l-year-old jack pine needles were consistently lower than current year levels (data not shown), and were not affected by treatments. Current needle mass of jack pine was reduced by the vegetation control treatment, while l-year-old needle mass did not show variation with treatment (Table 2). The calculated estimates of crown biomass (based on height and diameter) showed very different responses than needle mass. A comparison of mean annual increments of crown biomass of the control trees since planting shows that productivity was generally comparable for jack pine on the boreid site ad white pine on the southern Great Lakes site; however, jack pine tended to show greater response to scarification, and white pine to ve@&n c&trol

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Forest Ecoloa

(Table 2). The relative responsiveness to treatments in terms of dry matter accumulation and N content in the crown was higher for white pine than jack pine,

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crown

biomass

1000

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although response patterns were generally similar (Fig. 3). The largest growth responses were associated with vegetation control, especially when com-

Relative 100

76 (1995)

(Control

= 100)

2000

a

I

.sFH 110-

105'

OF

I oo- Wontrol w?= l S 95-

90 0

I

I

500

1000

Relative

OH

I

I

1

1500

2000

2500

N content

(Control

Relative

3000

= 100)

crown

biomass

2000

(Control

= 100)

3000

120-

110-

0

500

1000

1500 Relative

2000

N content

2500

(Control

3000

3500

4000

4500

= 100)

Fig. 3. Effects of silvicultural treatment on nitrogen concentration, content and crown biomass of: (a) jack pine in the boreal forest region; (b) white pine in the Great Lakes-St. Lawrence forest region. Nutrient status of trees on control plots was adjusted to 100 for comparison with trees ore treated plots. S, scarification; F, fertilization; H, vegetation control with herbicide.

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bined with fertilization and scarification. Scarification with or without fertilization resulted in small increases in growth, more so with jack pine than white pine. Nitrogen fertilization alone was ineffective in increasing growth of either species. The fertilizer treatments failed to significantly enhance N uptake, although tissue concentrations in the crown were slightly elevated. Greater nutrient response was evident when fertilization was combined with scarification.

2

250

e

200

y i

3.5273

+ 2.9097x

R2

*

2 150 z

NH~ -N NVbag)

Fig. 4. Correlations between resin exchangeable ammonium (NH,-N) and nitrate (NO,-N) duriug: (a) 5 June-16 July; (b) 16 July-25 August; (c) the whole season.

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4. Discussion Ammonium levels in control plots remained consistently low throughout the growing season.However, in general, there were greater changes in soil NH,-N levels in responseto treatmentson the boreal site, than observed for the Great Lakes-St. Lawrence site (Munson et al., 1993). Mid-seasonNH,-N levels were highest on plots that combined scarification and fertilizer addition, or the three treatments combined. This positive effect of scarification on availability may result from the absenceof an organic layer that would normally be effective in immobilizing added NH,-N, thereby reducing availability in the surface mineral soil. A late seasonincrease in NH,-N following treatments that included vegetation control may signal reduced plant demand (especially tree uptake on theseplots), once the seasonalshootgrowth is terminated. The increasein NH,-N levels does not seem to stimulate nitrification later in the growing season,perhaps due to decreasing soil temperature. In control plots, NO,-N levels were generally low. but showed a tendency for higher levels early in the season.This pattern is typical of northern hardwood forests (Likens et al., 1970; Zak et al.. 1990). and although less marked on this boreal site, it is nevertheless observable. Six years after manual brush removal and planting, nitrate is still measurableon plots that received no further treatment after plantation establishment. Nitrification was also observed on control plots 4 and 5 years after harvest by clearcutting and planting on the more southern Great Lakes site (Munson et al., 1993), however on less acidic soils. On the boreal control plots. reduced levels of soil NO,-N as the seasonadvancesmay be associatedwith increasing plant uptake of NH,-N, reducing available substratefor nitrifiers. Addition of nitrogen as fertilizer (in the form of both NH,-N and NO,-N) had a consistent positive effect throughout the growing season,which may be both a direct effect (NO,-N) or an indirect effect of increased NH,-N supply. Vegetation control combined with fertilizer addition increasedNO,-N levels more than ten-fold. This response was similar in magnitude to that observed following the sametreatment on the Great Lakes site (Ohtonen et al., 1992). This effect was strongest from mid-July onward and contradicts our hypothesis that nitrification response

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should be more conservative on the boreal compared with the Great Lakes site. In both ecosystems, response to vegetation control alone was much less, suggesting a synergy of increased substrate availability (NH,-N) and improved soil temperature and moisture conditions (Ohtonen et al., 1992; Munson et al., 1993). This hypothesis is supported by a strong relationship between soil NH,-N availability and NO,-N availability throughout the season (Fig. 4(c)). The nitrate response was much reduced when vegetation control and fertilizer addition were combined with scarification, underlining the importance of the humus N reserves in this ecosystem. Scarification consistently reduced the positive effect of vegetation control on NO,-N levels in the mineral soil, again signalling a main source of NO,-N production in the organic matter. This is not surprising, considering the total N reserves indicated in organic vs. surface mineral soils and referring to previous studies that confirm the importance of humus N reserves in boreal forest ecosystems (Cole and Rapp, 1981; Van Cleve and Alexander, 1981). The correlation between NH,-N and NO,-N availability (Fig. 4) supports previous findings that nitrification is controlled by NH,-N availability (Jones and Richards, 1977; Robertson, 1982). The strong nit&cation response to fertilization also supports this hypothesis. The seasonal trend of this correlation contributes to our interpretation of N dynamics. We hypothesize that the lower correlation early in the growing season (Fig. 4(a)) compared with the strongest correlation mid-season (Fig. 4(b)), was due to changes in plant competition for NH,-N during the season. Early season plant demand for N will be low, hence NH,-N is less likely to limit nitrification, whereas a high mid-season demand by plants will provoke a substrate limitation to the process of nitrification. The NO,-N response on the boreal site was of the same order of magnitude as on the Great Lakes-St. Lawrence site, while the NH,-N response was relatively greater on the boreal compared with the more southern site. Annual fertilizer N inputs provoke a much stronger response than one would predict using the criteria described by Vitousek et al. (1982). Climatic differences and greater humus accumulation would suggest a slower N cycle in the boreal, hence more conservative N response. However, reducing

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plant uptake while increasing N inputs has resulted in a similar large nitrification response. On more acid soils and conifer-dominated boreal sites in the same region of the Clay Belt, there was no nitrification response to urea fertilization in humus during short term laboratory incubation (Munson and Timmer, 1991). In the present situation, the nitrifier population has had several years to respond to increased NH,-N availability, the humus pH is higher (4.7 vs. 4.01, and sites were previously dominated by a mixedwood (including poplar and white birch) rather than pure conifer forest. In the Alaskan boreal landscape, higher growth potential poplar and birch showed more rapid nutrient uptake and production of higher quality litter with a faster decomposition rate (Chapin, 1987). In the southern boreal of western Quebec, Par& and Bergeron (1993) recently demonstrated the positive effect of birch litter on N mineralization. The deciduous component of the forest cover, then, may be an important variable to predict higher potential N response to disturbance. Recent reviews (Johnson and Ball, 1990; Johnson, 1992) also suggest that multiple inputs of fertilizer may cause a more significant buildup of nitrifier populations than a single fertilizer treatment. Although annual fertilizer application is not currently an operational practice, it is important to consider the potential for NO,-N losses by leaching associated with more intensive silviculture that combines vegetation control with fertilizer application. Reductions in microbial biomass associated with herbicide application may also contribute to losses (Ohtonen et al., 1992), since microbial immobilization of N has been shown to be an important conservation mechanism on a range of sites (Vitousek and Matson, 1985; Groffman et al., 1993), with a seasonal change in importance related to vegetation dynamics (Zak et al., 1990). Analysis of tree nutritional response shows that 6 years after plantation establishment, jack pine shows limited foliar response to treatments. Positive height growth response of jack pine to vegetation control and to scarification (D. Burgess, unpublished data, 1994) is not evident as needle mass response. Vector analysis indicated a positive response of 4-year-old white pine to vegetation control, in terms of N concentration, content and current needle dry mass on the Great Lakes site (Table 1; Munson et al.,

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1993). All treatments that included vegetation control showed a similar reaction, with strongest response to combined vegetation control and fertilization. We hypothesize that the lack of foliar response of jack pine at 6 years was due to growth response in terms of needle numbers (canopy expansion), instead of needle size. Canopy expansion, which increases photosynthetic capacity, becomes more evident in longer term studies of tree response to fertilization (Binkley, 1986). The strong canopy response observed, based on crown biomass estimates (Fig. 3) indicates the importance of measuring whole-tree, in addition to needle level response, in order to characterize long term response to treatment; individual needle response is generally most reliable in the first year after treatment (e.g. Timmer and Ray, 1988). Crown biomass responses were closely related to N accumulation in the crown. Nitrogen concentration after herbicide treatment was increased as much as 29% in white pine compared with 12% in jack pine. Interpretation of the vector nomograms suggest that repeated herbicide applications increased N availability, thus reducing the competition for soil N, and promoting tree growth and N uptake. The greater relative response of white pine compared to jack pine (based on relative crown biomass, Fig. 31, supports our hypothesis that the more southern pine species can acclimate more effectively to changing resource availability, therefore increasing productivity to a greater extent than the boreal species. However, differences in climate and relative N availability over the entire growing season may also be confounded with tree response. In general, the degree of foliar response to treatments (foliar mass and changes in concentration, content), and crown biomass response was less on the boreal site than noted at four years on the Great Lakes site. These results support our hypothesis that the boreal pine species has a weaker acclimation response to changes in resource availability, due to an adaptation to a more resource-poor environment. Since foliar responses of white pine and jack pine were comparable 2 years after planting (Brand, 19911, differences in species acclimation on the two sites may become more evident with plantation development.

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5. conclusions

By interpreting changes in NH,-N and N&-N availability in response to the different types of silvicultural perturbation we can elucidate the factors controlling N mineralization and nitrification in these boreal soil conditions. The nitrification response seemed to be closely related to NH,-N availability, showing a seasonal pattern that indicates vegetation competition for NH,-N. The negative impact of scarification on soil NH,-N and NO,-N availability appears to be more important on the boreal site compared with the Great Lakes site, perhaps related to the more important humus development in the former conditions. However, growth response to scarification has been neutral or positive to date, indicating that soil availability is not reduced below the level of tree demand. As the plantation develops and nutrient demand increases, this balance should be monitored carefully, especially on the boreal site, wh-ereclimate may place a greater limitation on N mineralization. In both experimental plantations, the most important changes in soil N fertility and tree nutrition generally result from vegetation control combined with fertilizer addition, while the greatest crown biomass is associated with combined scarification, fertilization and vegetation control. In general, the degree of tree nutrient response (both foiiar and crown biomass response) to treatments was less on the boreal site compared with the Great Lakes site. This difference in response may be due to a more buffered response of the boreal pine species to treatments, i.e. a weaker acclimation response associated with adaptation to a relatively resource-poor environment. The treatment impacts will be monitored over a longer term to improve interpretations of environmental changes and subsequent acclimation to silvicultural treatments.

Acknowledgments We would like to thank Brad Miller and Lucie Thibodeau for field work, and Bee Sarafyn, Yuaxin Teng and Real Mercier for assistance with ~laboratory analyses. The work was carried out in collaboration with the Optimum Productivity Group at the

A.D. Munson,

V.R. Timmer/Forest

Ecology

Petawawa National Forestry Institute, Ontario. Research funding was provided by the Vegetation Management Alternatives Program, Ontario Ministry of Natural Resources, and by a research grant to the senior author from the Natural Sciences and Engineering Research Council of Canada.

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