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Foliar Responses of Understorey Abies lasiocarpa to Different Degrees ofRelease Cutting in a Betula papyrifera and Conifer Mixed Species StandJ. R. Wang; T. Letchford; P. G. Comeau; D. Coopersmith

Online Publication Date: 01 December 2000

To cite this Article Wang, J. R., Letchford, T., Comeau, P. G. and Coopersmith, D.(2000)'Foliar Responses of Understorey Abieslasiocarpa to Different Degrees of Release Cutting in a Betula papyrifera and Conifer Mixed Species Stand',Scandinavian Journal ofForest Research,15:6,611 — 620

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Scand. J. For. Res. 15: 611–620, 2000

Foliar Responses of Understorey Abies lasiocarpa to Dif ferentDegrees of Release Cutt ing in a Betula papyrifera and ConiferMixed Species Stand

J. R. WANG1, T. LETCHFORD1, P. G. COMEAU2 and D. COOPERSMITH3

1Red Rock Research Station, Ministry of Forests, 18435 Forest Nursery Road, Prince George, BC, Canada V2N 5Y7; 2BC

Ministry of Forests, Research Branch, P.O. Box 9519, Station Provincial Government, Victoria, BC, Canada V8W 9C2

and 3BC Ministry of Forests, Prince George Forest Region, 1011 4th Ave., Prince George, BC, Canada V2L 3H9

Wang, J. R., Letchford, T., Comeau, P. G. and Coopersmith, D. (1Red Rock Research Station,Ministry of Forests, 18435 Forest Nursery Road, Prince George, BC, Canada V2N 5Y7, 2BC

Ministry of Forests, Research Branch, P.O. Box 9519 Station Provincial Government,

Victoria, BC, Canada V8W 9C2, 3BC Ministry of Forests, Prince George Forest Region, 10114th Ave. Prince George, BC, Canada V2L 3H9). Folia responses of understorey Abies

lasiocarpa to different degrees of release cutting in a Betula papyrifera and conifer mixed species

stand. Received November 25, 1999. Accepted February 27, 2000. Scand. J. For. Res. 15:611–620, 2000.

Foliar responses of subalpine �r [Abies lasiocarpa (Hook.) Nutt.] to thinning were studied in

a 35-yr-old mixed stand of paper birch (Betula papyrifera Marsh.) and conifers. The stand

regenerated naturally after a wild�re with a canopy dominated by paper birch (average height9.8 m) and an understorey dominated by subalpine �r (average height 1.6 m). The stand was

thinned to four densities of birch: 0, 600 and 1200 stems ha¼1 and control (unthinned at

2300–6400 stems ha¼1) in the autumn of 1995. The understorey conifers, mainly subalpine �r,were thinned to 1200 stems ha¼1. The study used a completely randomized split-plot design.

Three sample trees were systematically selected from each treatment replicate and each tree

stratum (upper, intermediate and lower understorey). One-year-old and older age class needleswere collected from one south-facing branch within the �fth whorl from the tree top. Thinning

of paper birch signi�cantly (pB0.001) increased leaf area and dry weight per 100 needles forintermediate and short trees except in the 0 birch treatment. Understorey subalpine �r trees in

600 stems ha¼1 birch (T3) had the largest leaf area and leaf dry weight per 100 1-yr-old

needles. Speci�c leaf area (SLA) decreased from unthinned (T1) to 0 birch (T4). Lowerunderstorey trees had the largest SLA. One-year-old needles had signi�cantly higher N, P and

K concentrations in all the thinning treatments. These responses are consistent with the shade

tolerance of subalpine �r. The results suggest that when managing a paper birch–conifersmixed-wood forest it may be of bene�t to understorey conifers to leave a birch canopy as a

nursing crop. Key words: Abies lasiocarpa, Betula papyrifera, foliar nutrients, mixed woods,

thinning.

INTRODUCTION

Successful timber production depends on our ability

to prescribe effective silvicultural treatments. This

requires an understanding of tree foliar responses to

treatments such as thinning. The effects of thinning

on growth and leaf nutrient concentration of residual

trees have been studied in pure even-aged stands

(Brix 1981, O’Hara 1988, Mugasha et al. 1991,

Hokka et al. 1996). Less work has been reported on

thinning broadleaf and conifer mixed-wood stands

(Pothier & Margolis 1990, 1991), especially when the

target species is in the understorey. Thinning trials

have frequently been undertaken to allow the predic-

tion of speci�c stand type responses to a particular

silvicultural treatment, leading to a large accumula-

tion of information. However, such trials did not

advance understanding of the underlying mechanisms

for the silvicultural treatment responses. Without

such understanding, reliable and accurate predictions

about stand responses to management actions cannot

be made.

Productivity of individual trees usually increases

rapidly after thinning. Gower et al. (1992) found that

foliar biomass, foliar ef�ciency (FE¾ increase in

biomass:foliar biomass) and levels of foliar N in-

creased after thinning. However, Lavigne (1988)

found that the foliar ef�ciencies, as measured by

annual stem growth per unit of foliar weight, of

thinned stands were not signi�cantly greater than

those of unthinned stands even with reduced competi-

tion. It was estimated that seasonal photosynthesis

© 2000 Taylor & Francis. ISSN 0282-7581

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J. R. Wang et al.612 Scand. J. For. Res. 15 (2000)

might be as much as 21%higher in residual trees of a

thinned stand of lodgepole pine as a result of in-

creased leaf hydration and additional light (Donner

& Running 1986).

Changes in foliar characteristics (leaf area, speci�c

leaf area, nutrient concentration, etc.) help to explain

the observed growth responses and may lead to a

diagnostic index that is useful in predicting the re-

sponse potential of unthinned stands. In a stand of

black spruce [Picea mariana (Mill.) B.S.P.] on a newly

drained peatland site, Mugasha et al. (1991) found

that foliar N and P concentrations and contents

increased two growing seasons after thinning. Hokka

et al. (1996) studied the effects of thinning on foliar

nutrient status of Scots pine (Pinus sylvestris L.)

stands on drained boreal peatlands. They found that

thinning increased foliar P concentration, needle size

and contents of N, P and K per 100 needles four

growing seasons after thinning. Pang et al. (1987)

reported that heavy thinning signi�cantly increased

foliar N and Mg concentrations in a stand of Dou-

glas �r [Pseudotsuga menziesii (Mirb.) Franco] in

British Columbia, Canada. It was concluded that

thinning improved the foliar nutrient status by reduc-

ing the competition for nutrients. Thinning also in-

creased leaf photosynthesis and N concentration of

paper birch (Wang et al. 1995), but reduced the foliar

nutrient concentrations of loblolly pine, possibly ow-

ing to dilution from greater growth (Ginn et al.

1991). However, most thinning studies were carried

out in pure stands. Therefore, it is unclear whether

similar responses exist for conifers in a mixed-wood

stand, especially for understorey conifers such as

subalpine �r in this study.

Subalpine �r [(Abies lasiocarpa (Hook.) Nutt.] is

one of the dominant climax tree species in the sub-

boreal spruce (SBS) biogeoclimatic zone of British

Columbia (Meidinger & Pojar 1991). Although it

may grow in pure stands, it is usually mixed with

other species such as white spruce [Picea glauca

(Moench) Voss], lodgepole pine (Pinus contorta

Dougl.), aspen (Populus tremuloides Michix.) and pa-

per birch (Betula papyrifera Marsh.). Subalpine �r is

shade tolerant. Advanced regeneration is one of its

constant features in the SBS, making it suitable for

mixed-wood management. Although not considered

as desirable as spruce, the species is used for lumber

and pulp. In British Columbia, mixed paper birch

and subalpine �r stands are not considered of com-

mercial value and the sites that they occupy are

classi�ed as non-satisfactorily regenerated (NSR). As

a result of this policy, such forest stands would be

destroyed and replanted to pure conifers. Because of

biodiversity and sustainable management concerns

this practice has been questioned and criticized. To

aid future management of these stands, the effects

were studied of releasing the understorey subalpine �r

through thinning overstorey paper birch and some

understorey subalpine �r to different densities. To the

authors’ knowledge, there is no information about

foliar nutrients and morphological characteristics in

response to thinning these types of mixed-wood

stands.

This paper reports the effects of thinning a dense

understorey of subalpine �r and an overstorey of

paper birch in a birch–conifer mixed wood. Although

the target species of the project are both the domi-

nant overstorey paper birch and understorey sub-

alpine �r, this paper focused only on the understorey

subalpine �r. The objectives of the study were: (1) to

quantify foliar responses to thinning in needle mor-

phology and nutrient status of subalpine �r; and (2)

to determine how the morphological and nutritional

acclimation to thinning was affected by thinning in-

tensity and tree stratum. These changes may poten-

tially in�uence the effectiveness of unit needle area

for light interception and carbon assimilation, and

ultimately the growth of leave trees. It was hypothe-

sized that foliar dry mass per 100 needles, new foliar

mass and foliar nutrient concentrations of subalpine

�r would increase in response to thinning.

MATERIALS AND METHODS

Study site

The study site is located in central British Columbia

(30 km east of Prince George, 760 m a.s.l. 53°54Æ N,

122°24Æ W) in the SBS biogeoclimatic zone. The stand

regenerated naturally following a 20000 ha wild�re in

August 1961. Paper birch was the dominant over-

storey species, with 2300 stems ha¼1 and an average

height of 9.8 m before thinning. There was a small

component of lodgepole pine, interior Douglas �r

[Pseudotsuga menzisii var. glauca (Beissn.) Franco]

and white spruce. The understorey was dominated by

subalpine �r with 33000 stems ha¼1 and an average

height of 1.6 m before thinning. The shrub layer

included black huckleberry (Vaccinium mem-

branaceum Dougl. Ex Torr.) and thimbleberry (Rubus

pavi�orus Nutt.). Common herbs were bunchberry

(Cornus canadensis L.), queen’s cup (Clintonia

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Folia responses of A. lasiocarpa to thinningScand. J. For. Res. 15 (2000) 613

uni�ora Kunth), twin�ower (Linnaea borealis L.) and

false Solomon’s seal [Smilacina racemosa (Nutt.) S.

Wats.].

Annual precipitation averages 410 mm, with June

and August being the wettest months. Annual aver-

age snowfall exceeds 2.4 m. Average maximum and

minimum temperatures are ¼7.5 and ¼16.6°C in

January and 22.0 and 8.1°C in July, respectively

(Anon. 1982).

Experimental design

The study used a completely randomized split-plot

design. Four treatment levels of paper birch densities

were created through thinning: (1) T1, untreated con-

trol (no birch thinning); (2) T2, 1200 birch stems

ha¼1; (3) T3, 600 birch stems ha¼1; and (4) T4, 0

birch stems ha¼1 (complete removal of birch). Treat-

ment plots were 80× 80 m with a 40× 40 m perma-

nent measurement subplot located in the centre,

leaving a 20 m wide buffer zone around the measure-

ment plot. Treatments were replicated three times.

Nine plots were thinned in October 1995 and three

were thinned in April 1996. The authors believe that

fall thinning was equivalent to the spring thinning

because biological activities are expected to be very

low during the intervening winter. Understorey

conifers were thinned to 1200 stems ha¼1 in all plots.

The order of preference for leaving understorey

conifers was: (1) white spruce; (2) Douglas-�r; and (3)

subalpine �r. This order was chosen in order to

preserve the small numbers (about 50 trees ha¼1) of

white spruce and Douglas �r that were on site, to

avoid converting the understorey to pure subalpine

�r. Residual trees were selected with priority given

�rst to location, then to tree form and then to size.

All thinning debris was left on the site. After thin-

ning, all residual trees within the measurement sub-

plot were number-tagged and their diameters at

breast height (DBH) or basal diameters (BD, if less

than 1.5 m tall) and heights were measured.

The immediate changes after thinning were in-

creased growing space for the residual trees and

increased photosynthetically active radiation (PAR)

for the understorey subalpine �r trees. Four quantum

sensors (model LI-190SA; Li-Cor, Lincoln, NE,

USA) were installed in May 1996 in one plot each of

treatments T1, T2 and T3, 1 m above the ground at

four systematically (10 m from plot centre to N, S, E

and W) chosen points. A single quantum sensor was

installed in the T4 treatment (in the centre of plot 10)

above the subalpine �r canopy as an open sky read-

ing. All quantum sensors were connected to datalog-

gers (Campbell Scienti�c, Logan, UT, USA) which

recorded hourly averages of 1 min readings. During

the 1996 growing season, T1 received about 10% of

full light and T2 and T3 received 30%and 60%of full

light, respectively (Fig. 1).

Foliar sampling and analysis

Foliage was sampled prior to bud �ush on May 7, 8

and 9 in 1997, equivalent to sampling one growing

season after thinning (Ballard & Carter 1985, Brække

1996, Mugasha et al. 1991). Foliar samples were

collected from all 12 plots (4 treatments× 3 repli-

cates). Within each plot, three sample points were

systematically chosen at the plot centre and two

diagonal corners of the 40× 40 m permanent mea-

surement subplot. Subalpine �r trees were divided

into three strata (upper, intermediate and lower) ac-

cording to their heights in the understorey. Three

subalpine �r trees, one from each tree stratum, were

selected near each sample point. DBH and total

height of all sampled trees were measured (Table 1).

Short trees did not reach breast height, and therefore

BD was measured. One south-facing branch within

the �fth whorl from the tree top was cut from the

main stem from each sample tree (Ballard & Carter

1985). Sample branches were labelled, placed in black

plastic bags, transferred to the laboratory at Red

Rock Research Station and stored at 4°C. One-year-

old needles were separated by clipping off all 1-yr-old

twigs from the branch. Needles older than 1 yr were

combined into a second sample for each branch.

Foliage samples were dried at 70°C for 48 h and

weighed to the nearest 0.01 g. Before drying, a sub-

sample of 100 needles was taken from each 1-yr-old

needle sample and scanned for projected leaf area,

Fig. 1. Understorey (1 m above ground) photosynthetically

active radiation (PAR) as a percentage of open sky PAR bythinning treatment.

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J. R. Wang et al.614 Scand. J. For. Res. 15 (2000)

Table 1. Height and diameter at breast height (DBH) or basal diameter (BD) for upper (U), intermediate (I) and

lower (L) understorey subalpine �r trees after different thinning treatments

No. of samples Height (m) DBH (cm)Tree stratum BD (cm)Treatment

UpperT1 9 6.790.35 7.390.42

9 3.290.31 3.593.15Intermediate

Lower 9 0.990.09 1.790.02UpperT2 9 5.790.31 6.690.44

9 3.390.18 3.890.26Intermediate

Lower 9 0.990.07 1.890.019 6.190.34 7.190.54T3 Upper

9 3.490.20 3.990.18Intermediate

Lower 9 1.190.09 2.190.02Upper 9 5.490.16 6.090.29T4

9 3.290.17 3.790.19Intermediate

9 1.190.08 1.890.01Lower

Data are shown as means9SE.

T1, control; T2, 1200 stems ha¼1 birch; T3, 600 stems ha¼1 birch; T4, 0 stems ha¼1 birch.

needle length and width using WinNeedle (Regent

Instruments, Quebec, Canada). Speci�c leaf area

(SLA) of 1-yr-old needles was calculated as projected

leaf area per unit of needle dry weight. All needle

samples (108 1-yr-old needle and 108 older age

classes) were ground for chemical analyses using a

Braun type KSM-2 coffee grinder. Foliar analyses

were completed at the British Columbia Ministry of

Forests, Analytical Chemistry Laboratory in Victo-

ria. N and P were determined colorimetrically using

�ow-injection analysis and K, Ca and Mg were deter-

mined by atomic absorption spectrophotometry. Fo-

liar nutrient content per 100 needles was calculated

by multiplying nutrient concentration (% dry weight)

by dry weight per 100 needles.

Data analysis

The effects of thinning and tree stratum were

analysed using a split-plot design with thinning treat-

ment as the main plot and tree stratum as the split

plot. Needle morphological characteristics and nutri-

ent variables were analysed with SYSTAT 6.0 (SPSS,

Chicago, IL, USA, 1998). Signi�cant differences (hB

0.05) between thinning treatments and between tree

strata were determined with Bonferroni multiple

comparisons (Sokal & Rohlf 1995). Signi�cant differ-

ences between nutrient concentrations of 1-yr-old

needles and older age class needles were determined

by paired sample t -tests.

Further interpretation of changes in foliar dry

mass, nutrient concentration and content was

achieved by adopting the vector analysis technique

re�ned by Timmer & Stone (1978). For each nutrient,

the average values for foliar nutrient concentration

and content from treated plots are shown relative to

the average value for the control plots, which is set at

coordinate (100, 100). The direction and magnitude

of the vector of each element connecting the control

and treated values is used to diagnose the tree’s

nutritional status and response to thinning

treatments.

RESULTS

Average heights of sampled trees ranged from 5.4 to

6.7 m for upper, 3.2 to 3.4 m for intermediate and 0.9

to 1.1 m for lower understorey trees, respectively

(Table 1). DBH values ranged from 6.0 to 7.3 cm for

upper and 3.5 to 3.9 cm for intermediate understorey

trees. Basal diameters for lower understorey trees

ranged from 1.7 to 2.1 cm. In general, ANOVA

revealed signi�cant effects of thinning and tree stra-

tum on most variables. No signi�cant interaction

between thinning and tree stratum was detected for

any of the measured variables (Tables 2, 3). There-

fore, only simple effects of thinning and tree stratum

are presented.

Needle width did not respond to thinning treat-

ments and was statistically the same among different

tree strata (Table 2). However, needle length was

signi�cantly affected by both thinning and tree stra-

tum. Trees in 600 stems ha¼1 birch (T3) had the

longest 1-yr-old needles, ranging from 20.7 to 25.6

mm, and the largest leaf area and dry weight per 100

needles (Fig. 2a,b). The 0 birch treatment (T4) had

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Folia responses of A. lasiocarpa to thinningScand. J. For. Res. 15 (2000) 615

the smallest leaf area but dry weight per 100 needles

was similar to that in T1 and T2.

The upper and intermediate understorey trees had

similar leaf area and dry weight per 100 needles (Fig.

2e,f). Lower understorey trees had the smallest leaf

area and dry weight per 100 needles.

There were signi�cant effects of thinning and tree

stratum on SLA (pB0.001), respectively (Table 2).

SLA decreased from unthinned (T1) to 0 birch treat-

ment (T4) (Fig. 2c). Upper understorey trees (U) had

the smallest SLA. In contrast, lower understorey trees

(L) had the largest SLA (Fig. 2d). This pattern of

change in SLA re�ected the changed understorey

light condition (Fig. 1) after thinning.

Thinning and tree stratum signi�cantly affected

foliar N concentration and content of 1-yr-old

needles (Table 3). Trees in T4 had the highest foliar

N concentration (Fig. 3a) and were signi�cantly dif-

ferent from the other thinning treatments. The upper

understorey trees had the lowest N concentration,

ranging from 1.10 to 1.46%, and short trees had the

highest N concentration, ranging from 1.15 to 1.67%,

while intermediate trees had the intermediate N con-

centration, ranging from 1.10 to 1.56% (Fig. 3d).

Paired samples t -tests showed that 1-yr-old needles

had a signi�cantly higher N concentration than older

needles of trees in T3 (t ¾ ¼3.34, pB0.001, n¾26)

and T4 (t ¾ ¼3.374, p¾0.002, n¾26) (Fig. 4a).

Thinning and tree stratum also had signi�cant

effects on foliar N content per 100 needles (Table 3),

although foliar N content (mg) per 100 needles varied

in a different pattern from N concentration. The trees

in T3 had the highest N content, ranging from 7.74 to

13.77 mg per 100 needles (Fig. 5a). This can be

attributed to their large dry weight per 100 needles

(see Fig. 2b). Lower understorey trees had the lowest

N content, despite having the highest N concentra-

tion. This was attributed to their small dry weight per

100 needles (Fig. 2f).

Paired sample t -tests showed that, in all treat-

ments, 1-yr-old needles had signi�cantly (pB0.001)

higher P and K concentrations than older needles

(Fig. 4b,c). In contrast, older age class needles had

signi�cantly higher Ca concentration in all treatments

Table 2. Results of ANOVA for SLA, leaf area per 100 needles and dry weight per 100 needles of understorey

subalpine �r

dfParameter Source pMS F

348.2313TSpeci�c leaf area (cm2 g¼1) 8.662 0.0078Error 1 –40.180 –

St 0.0002 75.7412296.416

0.517T× St 6 27.350 0.902–Error 2 16 30.320 –

0.098T 3 448.108 2.953Leaf area per 100 needles (cm2)

––151.7228Error 114.025946.4732 0.000St

0.296T× St 6 90.490 1.341

Error 2 16 67.486 – –Dry weight per 100 needles (g) T 3 0.407 3.049 0.092

––0.1348Error 1

St 2 2.123 38.479 0.000T× St 6 0.066 1.200 0.356

–Error 2 16 0.055 –

Needle width (mm) T 3 0.065 0.588 0.640Error 1 8 0.110 – –

St 2 0.030 1.162 0.338

T× St 6 0.048 1.831 0.15616Error 2 ––0.026

Needle length (mm) 0.0192.666T 119.7123Error 1 8 44.901 – –

St 0.0142 8.153155.952

0.6570.69613.3136T× StError 2 16 19.127 – –

T, thinning treatment; St, tree stratum as in Table 1. Error 1 is the main plot error and error 2 is the split-plot error.

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J. R. Wang et al.616 Scand. J. For. Res. 15 (2000)

Table 3. Results of ANOVA for foliar nutrient con-

centrations and content of 1-yr-old needles of sub-

alpine �r

Source df MS F pParameter

Concentration (% of dry weight)1.065 6.077N 0.018T 30.175 – –8Error 10.180 24.074 0.000St 20.019 2.529 0.0656T× St

Error 2 16 0.007 – –0.012 5.532 0.0253TP0.002 – –Error 1 80.000 0.019 0.8282St

6 0.000 0.261 0.947T× St0.001 – –16Error 2

3K 0.021 1.774 0.230T0.012 – –Error 1 80.096 17.305 0.0002St0.013 2.270 0.089T× St 60.006 – –16Error 20.131 3.436 0.072Ca T 30.038 – –8Error 10.022 2.566 0.108St 20.014 1.623 0.2056T× St0.009 – –Error 2 160.001 3.793 0.0583TMg0.000 – –Error 1 80.002 4.705 0.0252St0.001 1.144 0.382T× St 60.00016Error 2

Content (mg per 100 needles)T 3 86.484 2.289 0.015N

37.776 – –Error 1 8274.410 33.523 0.0002St

9.557 1.167 0.371T× St 68.186 – –16Error 21.530 2.171 0.169P T 30.705 – –8Error 19.353 46.938 0.000St 20.333 1.673 0.1926T× St0.199 – –Error 2 16

20.956 4.542 0.0393TK4.613 – –Error 1 8

120.183 47.890 0.0002St5.477 2.182 0.099T× St 62.510 – –16Error 24.42 2.813 0.108Ca T 31.572 – –8Error 1

2 43.377 51.175 0.000St1.636 1.930 0.1376T× St

16 0.848 – –Error 23Mg 0.067 1.974 0.197T

0.034 – –8Error 1St 2 1.446 47.203 0.000

0.060 1.973 0.1306T× St0.031 – –Error 2 16

T, thinning treatments; St, tree stratum. Error 1 and error2 are the same as in Table 2.

(Fig. 4d). No signi�cant difference was found be-

tween Mg concentrations of new needles and old

needles except in T2 (Fig. 4e).

Foliar Ca concentration was not signi�cantly af-

fected by thinning or tree stratum (Table 3). Al-

though foliar P concentration was signi�cantly

affected by thinning treatments, there were no signi�-

cant differences in P concentration among tree status.

In contrast, there were no signi�cant differences in

foliar K and Mg concentrations between thinning

treatments, but tree stratum signi�cantly affected fo-

liar K and Mg concentrations (Table 3, Fig. 3c,e).

The mean foliar P concentration ranged from 0.19 to

0.24% across the treatments and was the greatest in

T3 (Fig. 3b). The mean foliar K and Mg concentra-

tions varied across tree status from 0.50 to 0.69%,

and 0.06 to 0.09%, respectively.

Thinning signi�cantly affected only foliar K con-

tent per 100 needles, being the greatest in T3 (Fig.

5b). However, tree stratum signi�cantly affected P,

K, Ca and Mg content per 100 needles (Table 3). In

general, the upper and intermediate understorey trees

had the highest foliar P, Ca and Mg contents per 100

1-yr-old needles (Fig. 5c–e).

Vector analysis indicated strong N, P and Ca re-

sponses to the thinning treatments (Fig. 6). Accumu-

lation of foliar N, P and Ca was associated with their

increased concentrations when the stand was thinned

to 0 birch (T4). Mg was unchanged in T4, diluted in

T3 and showed slight luxury consumption in T2.

DISCUSSION

Thinning overstorey paper birch and understorey

subalpine �r in a mixed birch–conifer stand signi�-

cantly affected subalpine �r foliar morphological

characteristics. Understorey subalpine �r trees can

respond in many different ways to the suddenly

changed light environment (Fig. 1). The results from

this study indicate that foliar responses to thinning

were greatest for upper understorey trees, moderate

for intermediate understorey trees and least for lower

understorey trees (Fig. 2). These foliar morphological

responses are in agreement with the thinning objec-

tive to encourage the growth of tall trees rather than

to resuscitate the short ones (Smith et al. 1997).

In general, trees will respond to the changed envi-

ronment caused by thinning in a sequence of events

at different levels. First, foliar morphology and min-

eral nutrition change, then physiological activities

such as photosynthesis increase. Crown size and leaf

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Folia responses of A. lasiocarpa to thinningScand. J. For. Res. 15 (2000) 617

area increase, thus adding to the photosynthetically

active surface. Eventually, these changes are followed

by increased cambial growth. In this study, the foliar

responses of understorey A. lasiocarpa to release cut-

ting were analysed one growing season after the

treatment. Needle primordia number is predeter-

mined in A. lasiocarpa. Thus, the number of needles

was �xed in the bud at the time of application of the

thinning treatments. However, needle morphology

and nutrient concentrations are determined by cur-

rent growing season condition and thus �rst growing

season needle response indicates the most immediate

responses of residual trees to thinning. It is fairly well

documented that �rst-year needles re�ect growth re-

sponses in subsequent years and that speci�c needle

weight of trees is an indicator of their growth capac-

ity (Brække 1996).

Fig. 3. Mean nutrient concentrations of 1-yr-old needles ofthe understorey subalpine �r in different thinning treat-

ments (a, b) and by tree stratum (c–e). T1, T2, T3, T4, and

U, I, L are the same as in Fig. 2. The vertical bars indicate91 SE (n¾9). Bars with the same letter are not signi�-

cantly different (p\0.05) according to Bonferroni multiple

comparison.

Fig. 2. One-year-old needle leaf area (cm2), dry weight (g)per 100 needles and speci�c leaf area (cm2 g¼1) of under-

storey subalpine �r in different thinning treatments (a–c):

T1, control (no birch thinning); T2, 1200 stems ha¼1 birch;T3, 600 stems ha¼1 birch; T4, 0 stems ha¼1 birch; and by

tree stratum (d–f): upper (U), intermediate (I) and lower(L) understorey trees. The vertical bars indicate 91 SE

(n¾9). Bars with the same letter are not signi�cantly

different (p\0.05) according to Bonferroni multiple com-parison.

Light-related alterations in needle dimensions are

relevant adaptive features, contributing to the effec-

tive use of light resource. SLA links mass- and area-

based expressions of leaf photosynthetic rates and it

was found to be closely related to photosynthesis

(Field & Mooney 1986). SLA of trees is indicative of

their growth capacity, as well as being an indicator of

treatment responses of 1-yr-old needles. SLA re�ects

growth response in subsequent years. Signi�cant

growth responses in diameter and height of subalpine

�r trees were found in the autumn of 1999 (Wang et

al. unpubl. data).

The very similar level of all needle morphological

parameters obtained in this study between T1 and T2

is unusual. Greater positive needle responses were

expected from the trees in T2 compared with those in

T1. Although there was a 20% increase in under-

storey light conditions between T1 and T2 (Fig. 1),

subalpine �r did not respond differently. This con�-

rms the �ndings of Wright et al. (1998) that subalpine

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J. R. Wang et al.618 Scand. J. For. Res. 15 (2000)

Fig. 5. Mean nutrient content (mg) per 100 1-yr-old needles

of the understorey subalpine �r in different thinning treat-

ments (a, b) and by different tree strata (c–f). T1, T2, T3,T4, and U, I, L are the same as in Fig. 2. The vertical bars

indicate 91 SE (n¾9). Bars with the same letter are not

signi�cantly different (p\0.05) according to Bonferronimultiple comparison.

�r leader growth increases rapidly between 5 and 20%

of open light and increases little above 20% of open

light. In contrast, all needle morphological character-

istics showed negative responses to complete removal

of overstorey paper birch in T4. The subalpine �r

trees had been growing at low light intensities (10%

of open light in T1) prior to thinning. These trees

may have been injured by a sudden exposure to full

sunlight through thinning in T4, a situation often

referred to as ‘‘thinning shock’’ (Staebler 1956, Har-

rington & Reukema 1983, Smith et al. 1997).

Changes in foliar nutrition following thinning may

relate to several simultaneous processes in forest

ecosystems. Comparison of foliar N, P and K con-

centrations in 1-yr-old needles with those in needles

older than 1 yr showed clearly that the residual trees

had allocated more nutrients to the youngest parts of

their foliage (Fig. 4). Translocation of nutrients from

Fig. 4. Mean foliar nutrient concentrations (% of dry

weight) of 1-yr-old needles (1 yr needles) and needles older

than 1 yr (\1 yr needles) in different thinning treatments.T1, T2, T3 and T4 are the same as in Fig. 1. Signi�cant

differences (at the 5% level) between the needle ages are

marked with asterisks according to paired sample t -tests.The vertical bars indicate 91 SE (n¾27). Solid bars are

1-yr-old needles and hatched bars are \1 yr old needles.

the older needles to 1-yr-old needles does not seem to

be a suf�cient explanation for the increase in nutrient

concentration of 1-yr-old needles, especially for N,

but it cannot be ruled out.

The trees in T4 had signi�cantly higher foliar N

and P concentrations when compared with other

thinning treatments (Fig. 3a,b), which is characteris-

tic of luxury consumption of these two nutrients in

T4 (Timmer & Stone 1978). There are two possible

reasons for this. First, the highest N concentration in

T4 can be caused by reduced competition for nutri-

ents and by increased available N released from the

thinning residues, and from accelerated decompo-

sition of organic matter, called the assart effect

(Kimmins 1997). Secondly, because of the

above-mentioned ‘‘thinning shock’’ effect, trees in T4

did not grow much new foliage. This led to the

accumulation of N and P in needles. Vector analysis

showed that trees in T3 had relatively unchanged

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Folia responses of A. lasiocarpa to thinningScand. J. For. Res. 15 (2000) 619

nutrient concentrations, especially for K, Ca and Mg,

compared with T2 and T4 (Fig. 6). Trees in T3 had

the largest increase in relative nutrient contents be-

cause of the balanced increase in relative dry mass

per 100 needles while maintaining relatively un-

changed nutrient concentrations.

Foliar N concentration increased as tree height

decreased from upper to lower understorey trees (Fig.

3d). These results agree with those reported by Nine-

mets (1997) and Hollinger (1996), who found that N

concentrations were higher in shaded needles than in

sun-exposed needles. In the unthinned stand, where

light and N are assumed to be the limiting factors for

photosynthesis of subalpine �r, the needles of lower

understorey trees should have a higher light use

ef�ciency to survive in low light conditions. Theoreti-

cally, higher light use ef�ciency can be achieved in

two ways: (1) with high N concentration in the leaves

enabling a more powerful light-capturing apparatus

to be established; or (2) with an improved photosyn-

thesis per unit of foliar N (i.e. NUE). In this study,

the lower understorey subalpine �r trees appear to be

using the �rst strategy to survive the low light condi-

tion, as NUE did not differ by tree stratum (Wang et

al. unpubl. data).

The change in foliar N could be an important

determinant of the acclimation of photosynthesis to

light intensity, because changes in the amount of

photosynthetic pigments and protein are accompa-

nied by changes in foliar N (Field & Mooney 1986).

If changes in foliar N concentration in response to

increased irradiance are an important factor in deter-

mining the process of acclimation to changes in light

intensity, then the addition of N may partly offset the

negative effects of shading on photosynthesis. The

interaction of light and N availability on photosyn-

thesis is particularly important for forest practices

such as thinning and fertilization.

There have been no previous studies concerning

foliar morphological and nutritional responses of un-

derstorey conifers to thinning overstorey deciduous

trees. The present results suggest that there is a

linkage between foliar morphological and nutritional

characteristics. Combined with growth response data,

it will be possible to link leaf-level responses to tree

growth (height and diameter increments) level re-

sponses. Preliminary analysis showed that A. larsio-

carpa responded signi�cantly in diameter growth 4

yrs after the treatment (Wang et al. unpubl. data). In

practice, it may be possible to evaluate the effective-

ness of thinning by measuring foliar responses 1 yr

after thinning instead of measuring tree growth re-

sponses several years after thinning.

In conclusion, the 600 birch stems ha¼1 treatment

had the best needle responses among all thinning

treatments. This information suggests that when

managing a paper birch–conifer mixed-wood forest it

may be of bene�t to understorey conifers to leave a

birch canopy as a nursing crop. However, as the

paper birch canopy expands, light levels will decrease

for the understorey conifers. In response to birch

canopy expansion, appropriate measures have to be

taken to reduce the density of the deciduous canopy

until conifers become dominant.

ACKNOWLEDGMENTS

The authors gratefully acknowledge �nancial support

for the study from Forest Renewal BC through pro-

ject HQ96423-RE. We thank Ngaio Beausoleil for

�eld sampling and laboratory measurements. Com-

ments from two anonymous reviewers and Dr Anders

Ericsson, Scienti�c Editor, Scand. J. For. Res., are

gratefully acknowledged.

Fig. 6. Foliar responses to thinning in terms of changes in

relative foliar dry mass per 100 needles, foliar nutrientconcentrations and contents.

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J. R. Wang et al.620 Scand. J. For. Res. 15 (2000)

REFERENCES

Anon. 1982. Canadian Climate Normals 1951–1980: Tem-

perature and Precipitation, British Columbia. Environ-

ment Canada, Ministry of Supplies and Services,

Ottawa, Ontario.

Ballard, T. M. & Carter, R. E. 1985. Evaluating forest

stand nutrient status. Land Management Report No. 20,

Ministry of Forests, British Columbia. ISSN 0702-9861.

Brække, F. H. 1996. Needle analyses and graphic vector

analyses of Norway spruce and Scots pine stands. Trees

11: 23–33.

Brix, H. 1981. Effects of thinning and nitrogen fertilization

on branch and foliage production in Douglas-�r Pseu-

dotsuga menziesii. Can J. For. Res. 11: 502–511.

Donner, S. L. & Running, S. W. 1986. Water stress re-

sponse after thinning Pinus contorta stands in Montana.

For. Sci. 32: 614–625.

Field, C. & Mooney, H. A. 1986. The photosynthesis–ni-

trogen relationship in wild plants. In Givinish, T. J.

(ed.). On the Economy of Plant Form and Function,.

Cambridge University Press. Cambridge. ISBN 0-521-

26296-8: 25–55.

Ginn, S. E., Seller, J. R., Cazell, B. H. & Kreh, R. E. 1991.

Physiological and growth responses of eight-year-old

loblolly pine stands to thinning. For. Sci. 37: 1030–

1040.

Gower, S. T., Vogt, K. A. & Grier, C. C. 1992. Carbon

dynamics of Rocky Mountain Douglas-�r: in�uence of

water and nutrient availability. Ecol. Monogr. 62: 43–

65.

Harrington, C. A. & Reukema, D. L. 1983. Initial shock

and long-term stand development following thinning in

a Douglas-�r plantation. For. Sci. 29: 33–46.

Hokka, H., Penttila, T. & HaÊnell, B. 1996. Effects of

thinning on foliar nutrient status of Scots pine stands on

drained boreal peatlands. Can. J. For. Res. 26: 1577–

1584.

Hollinger, D. Y. 1996. Optimality and nitrogen allocation

in a tree canopy. Tree Physiol. 16: 627–634.

Kimmins, J. P. 1997. Forest Ecology – A Foundation for

Sustainable Management. 2nd edn,. Prentice Hall, En-

glewood Cliffs. NJ. ISBN 0-02-364071-5: 119–120.

Lavigne, M. B. 1988. Growth and net assimilation rates in

thinned and unthinned stands of balsam �r. Can. J. For.

Res. 18: 1205–1210.

Meidinger, D. & Pojar, J. 1991. Ecosystems of British

Columbia,. BC Ministry of Forests, Victoria. BC. ISSN

0843-6452: 209–222.

Mugasha, A. G., Pluth, D. J., Higginbotham, K. O. &

Takyi, S. K. 1991. Foliar response of black spruce to

thinning and fertilization on a drained shallow peat.

Can. J. For. Res. 21: 152–163.

Ninemets, U. 1997. Acclimation to low irradiance in Picea

abies : in�uences of past and present light climate on

foliage structure and function. Tree Physiol. 17: 723–

732.

O’Hara, K. L. 1988. Stand structure and growing space

ef�ciency following thinning in an even-aged Douglas-�r

stand. Can. J. For. Res. 18: 859–866.

Pang, P. C., Barclay, H. J. & McCullough, K. 1987.

Aboveground nutrient distribution within tree and

stands in thinned and fertilized Douglas-�r. Can. J. For.

Res. 17: 1379–1384.

Pothier, D. & Margolis, H. A. 1990. Changes in the water

relations of balsam �r and white birch saplings after

thinning. Tree Physiol. 3: 371–380.

Pothier, D. & Margolis, H. A. 1991. Analysis of growth

and light interception of balsam �r and white birch

saplings following precommercial thinning. Ann. Sci.

For. 48: 123–132.

Smith, D. M., Larson, B. C., Kelty, M. J. & Ashton, P. M.

S. 1997. The Practice of Silviculture – Applied Forest

Ecology,. John Wiley & Sons. New York. ISBN 0-471-

10941-X: 99–129.

Sokal, R. R. & Rohlf, F. J. 1995. Biometry. 3rd edn,.

Freeman and Co. New York. ISBN 0-7167-2411-1: 229–

240.

Staebler, G. R. 1956. Evidence of shock following thinning

of young Douglas-�r. J. For. 54: 339: 567–568.

Timmer, V. R. & Stone, E. L. 1978. Comparative foliar

analysis of young balsam �r fertilized with nitrogen,

phosphorus and potassium and lime. Soil Sci. Soc. Am.

J. 42: 125–130.

Wang, J. R., Simard, S. W. & Kimmins, J. P. 1995.

Physiological responses of paper birch to thinning in

British Columbia. For. Ecol. Manage. 73: 177–184.

Wright, El. F., Coats, K. D., Canham, C. D. & Bartemucci,

P. 1998. Species variability in growth response to light

across climatic regions in northwestern British Colum-

bia. Can. J. For. Res. 28: 871–886.

.

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