Download Leaf-level nitrogen-use efficiency of canopy and understorey

Functional
Ecology 2002
16, 826 – 834
Leaf-level nitrogen-use efficiency of canopy and
understorey species in a beech forest
7Blackwell Science, Ltd
Y. YASUMURA,*† K. HIKOSAKA,‡ K. MATSUI§ and T. HIROSE‡
*Biological Institute, Graduate School of Science, and ‡Graduate School of Life Sciences, Tohoku University, Aoba,
Sendai 980-8578, Japan, and §Biological Laboratory, Nara University of Education, Takabatake-cho, Nara
630-8528, Japan
Summary
1. In a forest stand, canopy and understorey species grow at completely different irradiances and consequently with different carbon and nitrogen availability ratios. We
studied how the difference in growth irradiance influenced plant N use in a mature
beech forest.
2. We defined leaf-level nitrogen-use efficiency (NUEL) as the amount of the leaf dry
mass produced per unit N taken up by leaves. NUEL was similar between the canopy
species (Fagus crenata) and the understorey species (Lindera umbellata and Magnolia
salicifolia).
3. NUEL was analysed further as the product of two components: leaf-level N productivity (NPL) and mean residence time of leaf N (MRTL). The canopy species had
significantly larger NPL and significantly shorter MRTL than the understorey species.
4. As the photosynthetic capacity was similar among the species, different NPL
between the species was attributable largely to the difference in light conditions to
which their leaves were exposed.
5. The difference in MRTL was not attributable to potential efficiency of N resorption
(REFF) determined at leaf senescence, but to actual REFF, which depended on the
amount of green leaf lost before full senescence. The canopy species had significantly
smaller actual REFF because of strong wind actions in the canopy.
6. Although the canopy species realized higher NPL by virtue of high irradiance, it had
shorter MRTL due to wind damage to pre-senescent leaves. On the other hand, the canopy species had shorter NPL under shady conditions, but had longer MRTL with little
wind damage. Interplay of local environmental factors such as light and wind strongly
influenced N use by plants in the beech forest.
Key-words: Light, mean residence time of nitrogen, nitrogen productivity, nitrogen-use efficiency, wind
Functional Ecology (2002) 16, 826 – 834
Introduction
The availability of mineral nitrogen (N) does not meet
the demand of plants in most natural environments.
The N limitation commonly found in terrestrial ecosystems may have selected plants to utilize N more efficiently. Nitrogen-use efficiency (NUE) can be defined
as the total net primary production per unit N
absorbed or lost (Hirose 1975; Vitousek 1982). It considers combined effects of a suite of physiological
processes involved in the N economy of the plant.
NUE often increases with decreasing soil fertility (e.g.
Birk & Vitousek 1986; Boerner 1984; Chapin & Shaver
1989; Hirose 1978), but not always (Aerts 1990; Aerts
& De Caluwe 1994). Therefore plants may not have
© 2002 British
Ecological Society
†Author to whom correspondence should be addressed. Email: [email protected]
adapted to N-poor environments simply by enhancing
their NUE.
Berendse & Aerts (1987) defined NUE as the product of the N productivity (NP, growth rate per unit N
in the plant; Ingestad 1979) and the mean residence
time of N in the plant (MRT). Subsequent studies have
shown that selection in N-poor habitats is not necessarily for high NUE, but for a long MRT. In contrast,
in N-rich habitats a high NP is favoured (Aerts 1990;
Eckstein & Karlsson 1997).
Many studies have aimed to clarify the relationship
between NUE and N availability by comparing the
NUE of plants growing at different soil fertilities
(Gray & Schlesinger 1983; Lajtha & Klein 1988;
Vázquez de Aldana & Berendse 1997). However, factors
other than N availability may also determine N use,
for example light. Plants growing at different irradiances
can have widely different traits such as photosynthetic
826
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Nitrogen use of
coexisting species
in a forest
© 2002 British
Ecological Society,
Functional Ecology,
16, 826–834
capacity, growth rates and leaf N content per leaf area.
Recently, Hikosaka & Hirose (2001) studied the N
economy of differently sized individuals growing at
different irradiances in a monospecific stand of Xanthium canadense. They demonstrated that larger individuals used N more efficiently (greater above-ground
NUE) with higher above-ground NP and longer
above-ground MRT than smaller plants. Although
Hikosaka & Hirose (2001) showed that light influenced N use, their scope was confined to conspecific
individuals in a monospecific stand. In most natural
environments, stands are composed of many species
that coexist and whose canopies form several strata.
The uppermost layer is occupied by canopy species,
many of whose leaves will be in full sunlight, while the
lowest layer is occupied by understorey species that may
be well adapted to shade (e.g. Hirose & Werger 1995).
In this study we measured NUE, NP and MRT at
the leaf level in deciduous woody species belonging to
the canopy or the understorey of a beech forest. Leaflevel NUE (NUEL) was defined as the amount of
leaves produced per unit N allocated to leaf; leaf-level
NP (NPL) as the rate of leaf dry mass production per
unit leaf N; and MRTL as the mean residence time of
N allocated to leaf (see Materials and methods for
more formal definitions and assumptions). NUEL,
NPL and MRTL are closely related to NUE, NP and
MRT at the whole-plant level, because leaf litter production is a major component of primary production
(Bray & Gorham 1964) and a large portion of N is
invested in leaves, and thus differences in whole-plant
NUE could be explained largely by leaf-level attributes
(Escudero et al. 1992; Garnier, Gobin & Poorter
1995). Furthermore, NUEL and its components can be
estimated with minimum damage to plants. The forest
we studied was composed of deciduous woody species,
enabling us to compare canopy and understorey species with little effect of different leaf habits on plant
N use (Eckstein & Karlsson 1997). We aimed to study
N use in species growing on the same ground soil, but
developing leaves at completely different irradiances in
a stand.
Here we test two alternative hypotheses. First, the
canopy species have higher NPL and thus higher NUEL
compared with the understorey species. Greater NPL is
expected in the canopy species because photosynthetic
rate per unit N in the plant increases with irradiance
(Hirose & Werger 1987). MRTL, which is strongly
affected by leaf life spans and N-resorption efficiency
(REFF) (Eckstein, Karlsson & Weih, 1999; Escudero
et al. 1992) is presumed to be similar between the species because all the species considered here are deciduous woody plants with similar leaf life spans, and
because REFF is not very variable among such species
(Aerts 1996). Alternatively, canopy and understorey
species have similar NUEL because coexisting species
have similar resource-use efficiencies (Hirose & Werger
1994; Hirose & Werger 1995; Hikosaka & Hirose 2000;
Hikosaka et al. 2002). Similar NUEL may result from
a trade-off between NPL and MRTL as it does at the
whole-plant level (Berendse & Aerts 1987).
In addition to NUEL and its components, we measured light-saturated photosynthetic rates per unit leaf
N (PNUEmax) and REFF to clarify their importance in
determining NPL and MRTL, respectively.
Materials and methods
    
The study site was a climax beech forest located
in Hakkoda Mountains, Aomori Prefecture, Japan
(40°38′ N, 140°51′ E, 800 m elevation). The soils
underlying the study site were brown forest soils with
pH values around 4·5. The availability of mineral N
was less in this forest (4 mg NH+4 l−1 soil and 34 mg
NO3− l−1 soil) than in others in this area (Morita 1935).
This area has a cool temperate climate with annual
mean temperature of 4·8 °C and annual precipitation
of 1000–1600 mm, and is characterized by a long
winter with heavy snowfall. Snow accumulated on the
forest floor remains until a week or two after the onset
of canopy closure in mid-May. The canopy layer is
dominated by Fagus crenata Blume (Japanese beech),
and the understorey is occupied by clumps of Sasa
kurilensis (Rupr.) Makino et Shibata (dwarf bamboo)
and several shrub species such as Lindera umbellata
var. membranacea (Maxim.) Momiyama, Viburnum
furcatum Blume et Maxim., and Magnolia salicifolia
(Sieb. et Zucc.) Maxim.
F. crenata was selected as the dominant canopy species and L. umbellata and M. salicifolia as understorey
species. Hereafter, we use the species’ generic names.
All three species are deciduous woody plants. Fagus
started to expand leaves in mid-May and completed
leaf abscission in mid-November; Lindera and Magnolia started leaf expansion later than Fagus (mid-June in
Lindera and late-June in Magnolia), and finished leaf
fall earlier than or simultaneously with Fagus (by the
end of October in Lindera and by mid-November in
Magnolia). Thus Fagus had a slightly longer leaf life
span (about 6 months) than the understorey species
(about 5 months). Fagus trees were approximately
16 m tall, whereas the understorey shrubs were 1–2 m.
The photon flux density (PFD) that the understorey
species received was approximately 5% of that above
the canopy on a cloudy summer day.
 ,   
  
Above-ground litter was collected with litter traps during the growing seasons in 1999 and 2000. The litter
fall of the canopy species was collected by 0·81 m2 litter traps (n = 10 in 1999; n = 8 in 2000) located below
the closed canopy. Litter traps for the understorey species (n = 5 for each species) consisted of an iron-pipe
frame and polyester mesh, and were placed around
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Y. Yasumura et al.
each plant so as not to damage or shade the plant. Litter
was collected monthly from early July to early October,
and then irregularly until leaf fall was completed in
each species. In the laboratory, litter samples were sorted
into leaf, reproductive and other parts, and weighed
after oven-drying at 70 °C for at least 72 h. Total N
concentration was measured with an NC analyser
(Sumigraph NC-80, Shimadzu Ltd, Japan).
Green leaves of each species were sampled on 6
August 1999, when the total N pool of a leaf was at its
seasonal maximum. Senesced leaves were sampled on
28 October 1999 by random collection from the forest
floor. In Fagus, green leaves were sampled separately
according to their crown positions because they had
been exposed to different irradiances. Leaf samples
other than green leaves of Fagus were not distinguished by the crown position. Leaf area was determined with a leaf area meter (AAM-8, Hayashi
Denkoh, Japan) immediately after sampling. The samples were dried and weighed, and then total N concentration was determined as described above.
On 6 August 2000, light-saturated photosynthetic
rates (Pmax) of each species were measured at the same
site using sunlit individuals growing at the forest edge
and shaded individuals within the forest. We randomly
selected plants that were different from the individuals
used for litter collection. Measurements were done on
at least 13 leaves per species at ambient CO2 concentration and a saturating PFD (1000 µmol m−2 s−1) with a
portable photosynthesis system (LCA-4 ADC, Shimadzu
Ltd, Japan). The same leaves were then cut at the petiole for determination of leaf area, dry mass and total
N concentration. Photosynthetic NUE (PNUEmax ),
which indicates the potential to photosynthesize per
unit leaf N, was calculated as Pmax/Narea, where Narea is
leaf N content per leaf area (g N m−2).
  
Calculation of NUEL, NPL and MRTL was based on
the assumption that the forest stand was at a steady
state: matter inflow equalled matter outflow in the
stand on an annual basis. We considered that our study
site met this criterion because it was a mature climax
beech forest. The dynamics of leaf N pool (NPOOL) were
modelled as shown in Fig. 1 on the following definitions/assumptions. (1) NPOOL is the N to be used for leaf
growth in the current year or to be stored for leaf
growth in the next year. It originates from new absorption of N from the soil and from N translocation from
storage organs. (2) Leaves import all their N at the
beginning of the growing season, with no additional
import thereafter. (3) Reduction in NPOOL occurs only
when leaves fall; first gradually with occasional leaf
falls, and sharply with simultaneous leaf senescence at
the end of the growing season. (4) Nitrogen that is lost
to the environment (dNL) is replenished by the new N
taken up from the soil in the next year. (5) Nitrogen
that is not lost to the environment is resorbed into the
storage organs where it is kept until the following
spring. Therefore NPOOL corresponds to N in the leaves
during the growing season, and N stored in other
organs during the dormant season. NPOOL was estimated in g N m−2 for Fagus, and in g N plant−1 for Lindera and Magnolia, as follows:
NPOOL = Alitter × Narea of green leaf
(eqn 1)
where Alitter is leaf area of litter (Fagus in m2 m−2, Lindera
and Magnolia in m2 plant−1), and Narea is leaf N content
per unit leaf area (g N m−2). Alitter was determined as:
Alitter = dWL/LMA
(eqn 2)
where dWL is total dry mass of leaf litter (g) and LMA
is leaf mass per area (g m−2). To estimate Alitter, the
mean LMA of green leaves was used for litter in the
earlier season (up to early September) and the mean
LMA of senesced leaves for litter in the later season
(from early September to the end of the growing season).
NUEL is defined as the amount of leaves produced
per unit N absorbed from the soil annually. At steady
state, the amount of leaf production can be substituted
by dWL and the amount of N absorbed from the soil
annually by dNL (Garnier & Aronson 1998; Vitousek
1982). Therefore:
NUEL = dWL/dNL
(eqn 3)
Thus NUEL can be measured as the inverse of the N
concentration of leaf litter. Introducing the annual
mean of NPOOL (NL) and the time period (dt) into the
right-hand side of eqn 3, we obtain:
NUEL = [(1/NL) (dWL/dt)] × [(NL/dNL)dt]
Fig. 1. Dynamics of the leaf N pool (NPOOL). NPOOL is constant in storage organs during
the dormant period and starts to increase with the start of the growing season. After
© 2002 British
reaching
its peak, NPOOL decreases gradually as leaves are lost through litter fall. The
Ecological
Society,
total
amount
of N loss (dNL) is the same as the amount of N initially taken up from
Functional
Ecology, part of the N is resorbed into storage organs. NL is the mean NPOOL
the
soil. Meanwhile,
over
the whole year.
16, 826–834
(eqn 4)
The first term of the right-hand side, indicating the rate
of leaf mass production per unit NPOOL, defines the
leaf-level N productivity (NPL). The second term gives
the mean time unit N remains in NPOOL and thus
defines the mean residence time of leaf N (Garnier &
Aronson 1998). Thus eqn 4 can be rewritten as:
829
Nitrogen use of
coexisting species
in a forest
NUEL = NPL × MRTL
(eqn 5)
Plants with larger NPL can grow more rapidly with
the same amount of N, while those with larger MRTL
can use the same N for longer (Berendse & Aerts
1987).
When leaves senesce, part of leaf N is resorbed into
storage organs. Two measures of resorption efficiency
(REFF) were defined: REFF realized in the forest (actual
REFF); and that which is genetically determined (potential REFF). Actual REFF collectively measures the proportion by which a plant has resorbed N from its
leaves. It decreases when leaves are lost before being
fully senesced, that is, before they have exported all
their translocatable N to the storage organs.
Actual REFF = (max NPOOL − total N in leaf litter)/max
NPOOL
(eqn 6)
where max NPOOL denotes NPOOL at the seasonal maximum. Potential REFF measures the efficiency with
which N has been resorbed from a leaf that is senesced
thoroughly. It indicates the inherent ability of a species
to resorb N from senescing leaves.
Potential REFF = (Narea of green leaf − Narea of senesced
leaf )/Narea of green leaf
(eqn 7)
To estimate the variances in potential REFF, data from
green and senesced leaves were randomly paired.
 
Statistical tests were performed using SV version
5·0 (SAS Institute, Inc., Cary, IN). Actual and potential REFF data were arcsin-transformed, and PNUEmax
data were logarithmically transformed before analyses.
All data sets had a variance that was not significantly different from the normal distribution (P > 0·05;
Kolmogorov–Smirnov test). The effects of species and
status (whether green or senesced) on LMA, leaf N
content per unit mass (Nmass) and Narea, and those of
species and year on NUEL, NPL, MRTL and actual
REFF, were tested by . Difference between two
variables was tested by Student's t-test, and that
among three or more variables was tested by the
Tukey–Kramer test.
Results
 
© 2002 British
Ecological Society,
Functional Ecology,
16, 826–834
Above-ground litter collected by litter traps contained
leaves, reproductive parts (flowers and fruits) and
other parts (branches and bark). Of the two years,
2000 was a mast year for Fagus: the percentage dry
mass of reproductive parts rose from 1% in 1999 to
35% in 2000 and that of leaf decreased from 85 to 55%.
In Lindera and Magnolia, almost all the above-ground
litter was leaf, and there was only a negligible amount
of reproductive and other parts (1·3% at most, 0·7% on
the average, of total above-ground litter).
Most leaf litter was produced during autumn
(October to mid-November) in all three species
(Fig. 2). Because the size and timing of major litter fall
differed among individuals, the understorey species
had large variances in leaf-litter dry mass. Fagus lost
many leaves prior to autumn senescence, probably
caused by strong wind action in the canopy layer.
Although we did not measure the wind velocity, the
meteorological data obtained near the study site
(<2 km away) revealed that pre-senescent leaf fall of
Fagus coincided with particularly windy months (June,
September and October in 1999; July and September
in 2000). Lindera lost all its leaves by the end of October in both years, half a month earlier than the other
two species. Magnolia lost only a small amount of
leaves early in the season. Leaf-litter N concentration
decreased gradually through the growing seasons in all
species (Fig. 2).
   
LMA was significantly different among species and
between leaf status (Table 1). Fagus had higher LMA
than Lindera and Magnolia (P < 0·0001; Table 2).
Nmass and Narea differed significantly among species and
between leaves of different status (Table 1). Nmass was
lower in Fagus (P < 0·0001; Table 2), but Narea in Fagus
exceeded that in the understorey species (P < 0·0001;
Table 2). In all species, green leaves had significantly
higher Narea than the senesced leaves (P < 0·0001).
LMA and Narea of green leaves of Fagus decreased,
while Nmass increased with decreasing depth in the canopy (Table 3).
Table 1.  for the effects of species and leaf status
(whether green or senesced) on leaf mass per area (LMA,
g m−2) and leaf N content per mass (Nmass, percentage dry
mass) and per area (Narea, g N m−2)
Source of variation
df
MS
F
LMA
Species
Status
Species × status
Error
2
1
2
134
14521·2
842·1
74·8
107·8
134·7****
7·8*
0·7 NS
Nmass
Species
Status
Species × status
Error
2
1
2
84
Narea
Species
Status
Species × status
Error
2
1
2
84
9·20
53·33
0·37
0·10
4·682
1·249
1·439
0·034
*P < 0·05; ****P < 0·0001; NS, not significant.
91·4****
529·6****
3·7*
136·8****
36·5****
42·0****
830
Y. Yasumura et al.
Table 3. Vertical variation in leaf mass per area (LMA) and
N content per mass (Nmass) and per area (Narea) of green Fagus
leaves taken from different canopy positions. The canopy was
roughly partitioned into three layers (upper, middle and
lower); means ± SD; n = 10
Canopy
position
LMA
(g m−2)
Nmass
(% dry mass)
Narea
(g N m−2)
Upper
Middle
Lower
69·7 ± 11·9a
47·1 ± 13·0b
31·0 ± 4·5c
1·90 ± 0·18a
2·21 ± 0·40b
2·72 ± 0·24c
1·34 ± 0·34a
1·00 ± 0·14b
0·84 ± 0·10b
Different letters indicate a statistical difference at the 5% level
(Tukey–Kramer test).
Fig. 3. Leaf-level N-use efficiency (NUEL) of Fagus, Lindera and
Magnolia in 1999 (open bars) and 2000 (solid bars). Means
with SD. Sample numbers are given in Fig. 2. Differences
among all pairs of values were compared using the Tukey–
Kramer test. Different letters indicate a statistical difference
at P = 0·05.
Fig. 2. Seasonal variation in dry mass (histogram) and N concentration (line) of leaf
litter of Fagus, Lindera and Magnolia. Means with SD, n = 10 for Fagus in 1999; n = 8
for Fagus in 2000; and n = 5 for Lindera and Magnolia in 1999 and 2000.
Table 2. Leaf mass per area (LMA) and leaf N content per
mass (Nmass) and per area (Narea) of green and senesced leaves
(means ± SD)
Species
© 2002 British
Ecological Society,
Functional Ecology,
16, 826–834
LMA
(g m−2)
Nmass
Narea
(% dry mass) (g N m−2)
Fagus
Green leaf
49·3 ± 19·0a
Senesced leaf 41·6 ± 11·0a
2·28 ± 0·44a
0·85 ± 0·17b
1·06 ± 0·30a
0·35 ± 0·12b
Lindera
Green leaf
18·5 ± 1·8a
Senesced leaf 14·3 ± 1·8b
3·48 ± 0·26a
1·70 ± 0·22b
0·66 ± 0·06a
0·25 ± 0·03b
Magnolia
Green leaf
17·7 ± 2·7a
Senesced leaf 14·7 ± 2·7b
3·24 ± 0·29a
1·39 ± 0·23b
0·56 ± 0·04a
0·22 ± 0·03b
Different letters within the same species indicate statistical
difference at 5% level (Student’s t-test).
Sample numbers: LMA, n = 30 for Fagus and n = 20 for
Lindera and Magnolia; Nmass and Narea, n = 30 for Fagus green
leaf, n = 20 for Fagus senesced leaf, and n = 10 for Lindera
and Magnolia.
-  - 
(   )
NUEL of Fagus ranged from 66 to 83 g g−1 N; that of
Lindera from 54 to 70 g g −1 N; and of Magnolia
from 56 to 87 g g−1 N (Fig. 3).  showed that
NUEL differed significantly among species, but not
between years (Table 4). Interspecific differences
were significant between Lindera and other species
in 1999, but not between any pair of species in 2000
(Fig. 3).
NUE can differ depending on the level (leaf, aboveground or whole plant) at which it is evaluated (Aerts
1990; Aerts & Chapin 2000). We calculated aboveground NUE (an inverse of N concentration of aboveground litter; Vitousek 1982) of the three species
and compared these to the NUEL values. In Fagus,
above-ground NUE (77·7 ± 5·5 g g−1 N in 1999;
72·4 ± 5·5 g g−1 N in 2000) was slightly higher than
NUEL due to low N concentrations of branch and
twigs. Above-ground NUE was similar between the
two years, regardless of differential amounts of reproduction. The N concentration of the total reproductive
parts was similar to that of the leaf litter: the high
N concentration of the seed part was offset by the low
N concentration of its shell. Above-ground NUE
831
Nitrogen use of
coexisting species
in a forest
Table 4.  for the effects of species and year on leaf-level
N-use efficiency (NUEL, g g−1 N); leaf-level N productivity
(NPL, g g−1 N year−1); mean residence time of leaf N (MRTL,
year); and actual resorption efficiency (actual REFF, %)
Source of variation
df
MS
NUEL
Species
Year
Species × year
Error
2
1
2
32
446·2
10·8
148·5
38·6
11·6***
10·3 NS
3·8*
NPL
Species
Year
Species × year
Error
2
1
2
32
3827·4
4·9
24·8
3·9
987·2****
1·3 NS
6·4**
MRTL
Species
Year
Species × year
Error
2
1
2
32
Actual REFF
Species
Year
Species × year
Error
2
1
2
32
3·613
0·018
0·229
0·064
1196·1
2·4
48·5
F
Fig. 4. Relationships between leaf-level N productivity
(NPL) and mean residence time of leaf N (MRTL) of Fagus
(squares), Lindera (triangles) and Magnolia (circles). Open
symbols, 1999; closed symbols, 2000. Means ± SD.
56·8****
0·3 NS
3·6*
80·2****
0·2 NS
3·3 NS
14·9
*P < 0·05; **P < 0·01; ***P < 0·001; ****P < 0·0001; NS,
not significant.
for Lindera (59·1 ± 2·9 g g−1 N in 1999; 60·7 ± 6·4 g g−1
N in 2000) and Magnolia (71·6 ± 10·2 g g−1 N in 1999;
64·0 ± 7·3 g g−1 N in 2000) were almost the same as
NUEL. Again, there was no significant difference at
P = 0·05 between Fagus and Magnolia in each year.
Lindera had a significantly lower above-ground NUE
than Fagus and Magnolia in 1999, but not in 2000.
Therefore above-ground NUE showed the same trend
as NUEL. This suggests that above-ground NUE and
its components can be substituted by leaf-level values
in these species.
-    (N   )
      
   (   )
© 2002 British
Ecological Society,
Functional Ecology,
16, 826–834
NPL and MRTL differed significantly among species,
but not between years (Table 4). The NPL of Fagus
was about twice as high as that of the understorey
species, with a highly significant difference in both
years (P < 0·0001; Fig. 4). There was also a significant
difference in NPL between Lindera and Magnolia
(P < 0·01). MRT L was shorter in Fagus than in
Lindera and Magnolia. The differences between the
canopy and understorey species were significant
(P < 0·0001 in 1999 for both understorey species;
P < 0·001 for Lindera and P < 0·01 for Magnolia in
2000). MRTL differed significantly between the two
understorey species in 1999 (P < 0·05), but not in
2000.
Fig. 5. Relationships between light-saturated photosynthetic
rates (Pmax) and leaf N content per leaf area (Narea) of Fagus
(squares), Lindera (triangles), and Magnolia (circles). Solid
line, linear regression fitted to Fagus ( y = 5·43x + 0·04;
r 2 = 0·79); dashed line, linear regression fitted to Lindera
( y = 4·30x + 0·08; r 2 = 0·70); dotted line, linear regression
fitted to Magnolia ( y = 5·70x − 0·76; r 2 = 0·81).
   
 (R  )
Pmax was correlated strongly with Narea in all species
(Fig. 5). PNUEmax (= Pmax/Narea) was significantly
higher in Fagus (5·4 ± 1·0 µmol CO2 g−1 N s−1) than in
Lindera (4·4 ± 0·7 µmol CO2 g−1 N s−1) (P < 0·05). There
was no significant difference in PNUEmax between
Fagus and Magnolia (4·7 ± 0·9 µmol CO2 g−1 N s−1).
Potential REFF, which was determined only in 1999,
did not differ significantly among species (Fig. 6).
There was a significant difference in actual REFF
among species, but not between years (Table 4). Actual
REFF was significantly smaller in Fagus than in Lindera
and Magnolia in both years (Fig. 6). While the two
understorey species had actual REFF that was not significantly different from potential REFF, Fagus had significantly lower actual REFF than potential REFF.
Discussion
The hypothesis that NUEL and NPL are higher in the
canopy species, and that MRTL is similar among
832
Y. Yasumura et al.
Fig. 6. Potential and actual N-resorption efficiency
(potential REFF and actual REFF) of Fagus, Lindera and
Magnolia. Potential REFF was measured only in 1999; actual
REFF was measured in 1999 and 2000. Means with SD.
Sample numbers: potential REFF, n = 20 for Fagus and n = 10
for Lindera and Magnolia; actual REFF, the same as in Fig. 2.
Differences between all pairs were tested using the Tukey–
Kramer test after arcsin transformation. Different letters
indicate a statistical difference at P = 0·05.
© 2002 British
Ecological Society,
Functional Ecology,
16, 826–834
species, was only partly supported. Fagus, the canopy
species, had a significantly higher NPL but lower
MRTL than the understorey species Lindera and
Magnolia (Fig. 4). Consequently, NUEL did not differ
significantly between Fagus and Magnolia in 1999,
and among all species in 2000 (Fig. 3). These results
were closer to the prediction in the alternative hypothesis that coexisting species have similar resource-use
efficiencies.
The initial hypothesis that NPL of canopy species
exceeds that of the understorey species was derived
from Hirose & Werger (1987), who showed that the
CO2 exchange rate per Narea (PNUE) increased with
increasing irradiance in a Solidago altissima stand.
Because PNUE is a major determinant of NPL (Garnier,
Gobin & Poorter 1995), NPL should also increase with
increasing irradiance. The light incident on top of the
canopy was about 20-fold higher than that on the
understorey layer. Therefore light availability was
undoubtedly one of the principal causes of the NPL
pattern across the canopy and the understorey species.
In their theoretical paper, Hikosaka & Terashima
(1995) demonstrated that PNUE can be enhanced by
increasing Narea under higher irradiance, although
excess N decreases PNUE. It is possible that Fagus
reinforced NPL by Narea that was higher at a higher leaf
position (Table 3). A similar trend has been reported in
Narea of Acer saccharum (sugar maple) and attributed
to the vertical distribution in LMA (Ellsworth & Reich
1993), which in turn is attributable to shading
(Chabot, Jurik & Chabot 1979). PNUEmax was higher
in Fagus than in Lindera and Magnolia, but the difference was small (20%) compared to that in NPL (70–
110%) (Fig. 4). These results suggest that most of the
difference in NPL was derived from the difference in
irradiance between the canopy and understorey layers.
There are two main ways to increase MRTL: (1) hav-
ing leaves with long life spans; and (2) resorbing N
from senescing leaves with a high efficiency (Eckstein
et al. 1999; Escudero et al. 1992). Previous studies have
suggested that leaf life spans are more important in
explaining interspecific variation in MRT (Eckstein
et al. 1999; Escudero et al. 1992). However, we
observed that the understorey species had shorter leaf
life spans (5 months) than the canopy species
(6 months) and thus the significantly longer MRTL in
the understorey species should not be ascribed to leaf
life spans. Understorey species commonly start leaf
expansion before the forest canopy closes. In our study
site, however, snow accumulation delays leaf expansion of understorey plants. Potential REFF was not significantly different among species, while actual REFF
was significantly smaller in Fagus than the understorey
species (Fig. 6). Thus shorter MRTL in Fagus is
explained by its low actual REFF, which was probably
caused by wind damage to pre-senescent leaves. These
results suggest the importance of distinguishing two
measures of REFF when assessing N resorption in a natural stand.
Our results differ from those obtained by Hikosaka
& Hirose (2001), who found that above-ground NP
and MRT were higher in larger, dominant individuals
in a monospecific stand of Xanthium canadense. MRT
did not compensate for lower NP in smaller, shaded
individuals. Smaller individuals were inefficient in
resorbing N from leaves, nor did they prolong the leaf
life span. As a sun species, X. canadense might not be
able to modify the growth pattern to use N efficiently
under shady conditions. For example, sun species
growing in the shade have Narea in excess of the optimum, whereas shade species do not (Anten & Hirose
2001; Hikosaka & Terashima 1996). Different N use
among conspecific individuals represents plastic
adjustments. On the other hand, interspecific difference in N use represents genetic difference in growth
patterns. Lindera and Magnolia may be inherently
adapted to shade by, for example, having lower Narea
and/or having lower leaf respiration per unit N
(Noguchi et al. 2001). Through such mechanisms the
understorey species may be able to use N efficiently,
which helps them to maintain themselves under the
closed canopy.
Plant N use is affected by many environmental
factors other than light and N availability (Chapin,
Johnson & McKendrick 1980; Kost & Boerner 1985). For
example, herbivory removes green leaves, which in turn
reduces the time period for photosynthesis and the
actual REFF. Reproductive behaviour may also strongly
influence N use of masting species such as Fagus studied
here. In mast years these species invest more N in fruit
production than in intervening non-reproductive
years, which may reduce the amount of leaf N for
growth. This study, however, did not show a drastic
change in NUEL of Fagus in the mast year (2000) compared with the year before (1999, little fruiting). Several factors (such as wind damage and reproduction)
833
Nitrogen use of
coexisting species
in a forest
acted on N use of Fagus simultaneously, which made it
difficult to determine if similarity in NUEL between
the two years was due to self-regulation or was coincidental. For a better understanding of yearly variation
in NUEL, long-term observations are necessary.
While canopy species in a forest stand have an
advantage in capturing light, they can lose green leaves
easily. Foster (1988) found close relationships between
canopy position and susceptibility to wind damage in
forest stands in central New England, USA. The species occupying the uppermost position in the canopy
(such as Pinus strobus and Betula papyrifera) were
exposed to the strongest wind, whereas the species
occupying subordinate canopy positions (such as Acer
rubrum and Quercus alba) were protected from the
wind. This pattern is likely to be general for mixedspecies forests, for we observed the same phenomenon
in the beech forest. Additionally, canopy species may
suffer from more damage by herbivores because those
plants usually produce smaller amount of defence
metabolites (Coley, Bryant & Chapin 1985). These
losses can significantly suppress MRT. On the other
hand, being an understorey species may entail limited
light availability, but may allow successful retention of
leaves until natural senescence. Through these mechanisms, understorey species can maximize MRTL and
thus compensate for low NPL.
In this paper we investigated the N use of canopy
and understorey species by studying NUEL and its
components NPL and MRTL. Although the canopy
species had higher NPL by virtue of high growth irradiance, NUEL did not differ among the species: the
canopy species exposed to strong wind had shorter
MRTL than the understorey species. Leaf-level NUE is
relatively easy to measure, and thus becomes a useful
tool to understand plant N use and its underlying
mechanisms in forests.
Acknowledgements
We would like to thank K. Sato for his help in littertrap establishment, T. Aoki, S. Ishizaki, H. Kimura,
T. Kinugasa, R. Oguchi, Y. Onoda and A. Yamashiro
for their help with the fieldwork, and Dr N.P.R. Anten
for his valuable comments on the manuscript. Logistic
support of the Mt Hakkoda Botanical Laboratory of
Tohoku University is gratefully acknowledged. This
work was partly supported by Grants-in-Aid of the
Japan Ministry of Education, Science and Culture.
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Received 14 December 2001; revised 12 April 2002; accepted 5
June 2002