Annals of Botany 86: 833±839, 2000
doi:10.1006/anbo.2000.1243, available online at http://www.idealibrary.com on
Is Partitioning of Dry Weight and Leaf Area Within Dactylis glomerata A€ected
by N and CO2 Enrichment?
H . H A R ME N S *{} , C . M . ST I R L IN G{ {, C . M A R S H A L L } and J . F. FA R R A R }
{Centre for Ecology and Hydrology Bangor, University of Wales, Bangor, Gwynedd LL57 2UP, UK and
}School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK
Received: 5 April 2000 Returned for revision: 15 May 2000 Accepted: 22 June 2000 Published electronically: 14 August 2000
We examined changes in dry weight and leaf area within Dactylis glomerata L. plants using allometric analysis to
determine whether observed patterns were truly a€ected by [CO2] and N supply or merely re¯ect ontogenetic drift.
Plants were grown hydroponically at four concentrations of NO3ÿ in controlled environment cabinets at ambient
(360 ml l ÿ1) or elevated (680 ml l ÿ1) atmospheric [CO2]. Both CO2 and N enrichment stimulated net dry matter
production. Allometric analyses revealed that [CO2] did not a€ect partitioning of dry matter between shoot and root
at high N supply. However, at low N supply there was a transient increase in dry matter partitioning into the shoot at
elevated compared to ambient [CO2] during early stages of growth, which is inconsistent with predictions based on
optimal partitioning theory. In contrast, dry matter partitioning was a€ected by N supply throughout ontogeny, such
that at low N supply dry matter was preferentially allocated to roots, which is in agreement with optimal partitioning
theory. Independent of N supply, atmospheric CO2 enrichment resulted in a reduction in leaf area ratio (LAR), solely
due to a decrease in speci®c leaf area (SLA), when plants of the same age were compared. However, [CO2] did not
a€ect allometric coecients relating dry weight and leaf area, and e€ects of elevated [CO2] on LAR and SLA were the
result of an early, transient stimulation of whole plant and leaf dry weight, compared to leaf area production. We
conclude that elevated [CO2], in contrast to N supply, changes allocation patterns only transiently during early stages
# 2000 Annals of Botany Company
of growth, if at all.
Key words: Allometric growth, carbon dioxide enrichment, Cocksfoot, Dactylis glomerata L., dry weight
partitioning, leaf area ratio, nitrogen supply, shoot : root ratio, speci®c leaf area.
I N T RO D U C T I O N
Doubling atmospheric [CO2] stimulates the growth of
C3 species by an average of 41 % (Poorter, 1993). The
magnitude of this increase varies with the individual species
(Hunt et al., 1991; Poorter, 1993), experimental duration
and the availability of other resources such as N (Bazzaz,
1990). When N limits growth, root dry weight increases
relatively more than shoot dry weight, perhaps maintaining
a functional equilibrium that results in the balanced acquisition of carbon and nitrogen (Davidson, 1969; Reynolds
and Thornley, 1982; Brouwer, 1983; Wilson, 1988).
Atmospheric [CO2] might also a€ect the allocation of dry
matter between shoots and roots, although contrasting
results have been reported for the shoot to root ratio (S : R
ratio, de®ned as the dry weight of shoot divided by the dry
weight of root): CO2 enrichment may either increase,
decrease or not a€ect S : R ratio (Hunt et al., 1991; Stulen
and Den Hertog, 1993; Rogers et al., 1996). Although [CO2]
and N supply would be expected to interact regarding dry
matter production and partitioning (i.e. the distribution of
dry weight within the plant), the outcome cannot be
predicted (Lloyd and Farquhar, 1996).
uk
* For correspondence. Fax ‡44 (0)1248 355365, e-mail [email protected].
{ Present address: School of Agricultural and Forest Sciences,
University of Wales, Bangor, Gwynedd LL57 2UW, UK.
0305-7364/00/100833+07 $35.00/00
Comparisons of the e€ects of treatments on plant growth
are often based on ratios such as S : R with plants of the
same age. However, patterns of allocation between plant
parts change during growth and development independent
of resource availability (Pearsall, 1927; Bowler and Press,
1993; Farrar and Gunn, 1996). Treatments may a€ect
growth and development and alter S : R ratio and morphological characteristics compared to the control either
because of true treatment e€ects or ontogenetic drift,
i.e. phenotypic traits of plants change over the course of
plant growth and development (Evans, 1972; Coleman et
al., 1994; Farrar and Gunn, 1998; McConnaughay and
Coleman, 1999). Di€erentiation between true e€ects of
treatment and of ontogenetic drift is possible using
allometry, i.e. the study of the growth and development
of one part of the plant in relation to another (Pearsall,
1927; Troughton, 1955). A few studies have made
allometric comparisons between CO2 treatments, usually
restricted to the allometric coecient relating the net dry
matter production of shoot and root (Bowler and Press,
1993; Baxter et al., 1994; Farrar and Gunn, 1996; Hibberd
et al., 1996). More recently, some studies have included the
allometric relationship of dry matter and leaf area (Farnsworth et al., 1996; Gebauer et al., 1996; Stirling et al., 1998;
Gunn et al., 1999). In general, allometric coecients were
not a€ected by [CO2]. In contrast, allocation of dry matter
to roots decreased with increasing N supply (Gebauer et al.,
1996; McConnaughay and Coleman, 1999).
# 2000 Annals of Botany Company
834
Harmens et al.ÐPartitioning at Elevated CO2 and N
We suspect that many reports claiming that [CO2] alters
net dry matter partitioning might be confounding e€ects
of [CO2] with those of uncontrolled variables, especially
when plants are grown in a solid substrate. For example,
limited access to nutrients due to either growth limiting
concentrations or limited ¯ux from a solid substrate to the
roots, can mean that plants grown at elevated [CO2] become
more nutrient de®cient than controls due to increased dry
weight (Stitt and Krapp, 1999), resulting in a correspondingly lower S : R ratio. Recently it has become clear that
soil water content can be another confounding variable
(Samarakoon and Gi€ord, 1995; Knapp et al., 1996), and
one that is known to a€ect dry matter partitioning. Plants
rooted in a solid substrate with a ®nite supply of water may
have more favourable water status at elevated [CO2] due to
a lower stomatal conductance for water vapour at elevated
than ambient [CO2] (Drake et al., 1997). Accordingly, good
control of nutrient and water supply is needed to distinguish between direct e€ects of [CO2] and indirect e€ects
through nutrient and water status of soils.
Here we examine the e€ects of elevated [CO2] and the
interaction with N availability on growth, partitioning of
dry weight, and dry weight±leaf area relationships in
Dactylis glomerata L. (Cocksfoot). Plants were grown in
controlled environments at either ambient (360 ml l ÿ1) or
elevated (680 ml l ÿ1) [CO2] and four N concentrations (0.15,
0.6, 1.5 and 6.0 mM NO3ÿ ), ranging from growth limiting to
optimal. Confounding e€ects of soil water status were
minimized by growing plants in hydroponics. Comparisons
between treatments were made both as a function of plant
age and by applying allometry. It was hypothesized that
elevated [CO2] a€ects net allocation of dry matter and
distribution of dry matter and leaf area only through
accelerated growth, but that N supply has a direct e€ect
independent of changes in ontogeny.
et al., 2000). In order to reduce cabinet e€ects and e€ects
of environmental heterogeneity within the cabinets, CO2
treatments were swapped between the two cabinets twice a
week and troughs were randomized within each cabinet.
Growth analysis
Seven plants per treatment (three±four plants per trough)
were harvested at 23, 28, 34 and 38 d after sowing. Plants
were separated into leaf blades, leaf sheath ‡ stem, and
roots. The area of leaf blades was determined using a digital
leaf area meter (Delta T Ltd, Cambridge, UK) and plant
parts were dried for at least 48 h at 65 8C and weighed. The
following parameters were calculated: shoot : root ratio
(dry weight), leaf area ratio (LAR; leaf area per plant dry
weight), speci®c leaf area (SLA; leaf area per leaf dry
weight) and leaf weight ratio (LWR; leaf dry weight per
plant dry weight). Allometric relationships were determined
using means per trough for each harvest and an ordinary
linear regression equation (Pearsall, 1927; Troughton,
1955):
ln y ˆ ln a ‡ k ln x
where ln a is the y-intercept, k is the slope, and y and x are
respectively: shoot and root dry weight; leaf dry weight and
total plant dry weight; leaf area and total plant dry weight;
and leaf area and leaf dry weight. In the majority of cases,
the regression was linear as determined by linear and
sequential polynomial regression. Subsequently, the slope
(v) of the geometric mean regression was determined for all
relationships (Ricker, 1984; Farrar and Gunn, 1996), as
both y and x are dependent variables:
v ˆ k=r
M AT E R I A L S A N D M E T H O D S
Plant growth
Seeds of Dactylis glomerata L., `Sylvan', were sown in propagators on moistened ®lter paper in controlled environment cabinets (Sanyo Gallenkamp, model SGC660/C/HQI,
Loughborough, UK) at either ambient (360 ml l ÿ1) or
elevated (680 ml l ÿ1) CO2 concentrations. Nine days after
sowing, uniform-sized seedlings with one leaf were transferred to 3.5 l troughs, containing half-strength Long
Ashton solution (Hewitt, 1966) with 10 mg l ÿ1 of sodium
metasilicate. Supply of NO3ÿ was modi®ed to give four
concentrations: 0.15, 0.6, 1.5 and 6.0 mM. At the smaller N
supplies, the potassium and calcium concentrations were
made up by adding K2SO4 and CaCl2 . The pH of the
aerated solutions was controlled between 5.6±6.4 using
2 mM MES and KOH. Twenty seedlings were planted in
each trough (two troughs per treatment) and after 1 week
the nutrient solutions were replaced and the number of
plants per trough was reduced to 15. Thereafter, nutrient
solutions were replaced twice weekly and the number of
plants per trough was reduced at every harvest. The conditions of growth were as described previously (Harmens
where r is the correlation coecient of the ordinary linear
regression and v is the allometric coecient calculated by
geometric mean regression. Where there was no signi®cant
di€erence between the slopes due to [CO2] a comparison of
the elevations (as opposed to the y-intercepts) of the
regressions (i.e. comparison of the vertical position of the
lines) was carried out (Zar, 1996). Regression lines with the
same slope and elevation coincide, whereas regression lines
with the same slope but di€erent elevation are parallel.
Statistical analysis
Data were analysed by analysis of variance (ANOVA) of
the mean values of each trough (n ˆ 2 per treatment per
harvest) using the Genstat statistical package (Lawes
Agricultural Trust, Rothamsted, UK). Where indicated,
data were ln-transformed prior to analysis to obtain homogeneity of variances. E€ects of CO2 and N supply on the
allometric coecient (v) and elevations of the regression
were analysed by pairwise comparison using Student's t-test
with 12 degrees of freedom. Unless indicated otherwise,
signi®cant treatment e€ects refer to P 4 0.05.
Shoot:root ratio
Total dry wt (g)
Harmens et al.ÐPartitioning at Elevated CO2 and N
3
0.15 mM NO3−
0.6 mM NO3−
835
1.5 mM NO3−
6.0 mM NO3−
A
B
C
D
E
F
G
H
2
1
0
4
3
2
1
0
20
30
40
20
30
40
20
30
40
20
30
40
Time (d)
F I G . 1. Total dry weight (A±D) and shoot : root ratio (E±H) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying
[NO3ÿ ]. Each data point represents the mean of two troughs, three±four plants per trough (+s.e.).
R E S U LT S
Growth and ratios at the same plant age
Total plant dry weight was greater at elevated than ambient
[CO2] (Fig. 1A±D; Table 1) and although the magnitude of
the increase varied, no signi®cant interactions between
[CO2] and time, or [CO2] and N supply were found. From
28 d onwards, total plant dry weight increased with
increasing [NO3ÿ ]. Responses of shoot and root dry weight
to elevated [CO2] and [NO3ÿ ] were similar to that of total
plant dry weight (data not shown), except that N supply
did not a€ect root dry weight until 34 d. The S : R ratio
was generally not a€ected by [CO2] and increased with
increasing N supply (Fig. 1E±H; Table 1). However, at
T A B L E 1. Summary of statistical signi®cance of e€ects
of date of harvest, [CO2] and N supply on growth and
development parameters of D. glomerata
Source of variation
Parameter
Time
N
CO2
Time N
Wtotal
Wshoot
Wroot
Wshoot : Wroot
Leaf area
Leaf area ratio
Speci®c leaf area
Leaf weight ratio
***
***
***
***
***
***
***
***
***
***
**
***
***
**
***
***
***
***
***
n.s.
**
***
***
n.s.
***
***
***
***
***
***
***
***
Number of f.e.l. main stem
Total number of leaves
Total number of tillers
***
***
***
***
***
***
n.s.
n.s.
n.s.
*
***
***
Data for dry weight (W) and leaf area were ln-transformed to obtain
homogeneity of variance. A general linear ANOVA model was ®tted
to the mean value of each trough, so n ˆ 2 per treatment at each
harvest. n.s., Not signi®cant, *P 4 0.05, **P 4 0.01, ***P 4 0.001.
`Time CO2', `N CO2' and `Time N CO2' interactions were
not signi®cant; f.e.l., fully expanded leaves.
[NO3ÿ ] above 0.15 mM there was a tendency towards an
increase in S : R ratio due to CO2 enrichment. Developmental parameters such as number of tillers, leaves and
fully expanded leaves on the main stem were not a€ected by
[CO2], but increased with increasing N availability
(Table 1).
LAR was signi®cantly higher at ambient than elevated
[CO2] at all [NO3ÿ ] and it decreased with time (Fig. 2A±D;
Table 1). SLA was lower in plants exposed to elevated than
ambient [CO2], independent of [NO3ÿ ] (Fig. 2E±H;
Table 1). At 0.15 mM NO3ÿ the SLA increased with time,
but it decreased with time at 1.5 and 6.0 mM NO3ÿ ,
resulting in a higher SLA at the ®nal harvest at 0.15 mM
NO3ÿ compared to 1.5 and 6.0 mM NO3ÿ . LWR was not
a€ected by CO2 (Fig. 2I±L; Table 1), thus the decrease in
LAR at elevated [CO2] was due solely to a decrease in SLA.
At low N supply LWR decreased with time, whilst at high
N supply it was nearly constant, resulting in a higher LWR
at the ®nal harvest at high compared to low N availability.
Clearly, most ratios changed with time and therefore with
increasing plant dry weight.
Allometric relationships
Elevated [CO2] had no signi®cant e€ect on any of the
allometric coecients (v) and therefore did not a€ect net
dry matter partitioning and dry weight±leaf area relationships within the plants between the ®rst and ®nal harvest
(Figs 3, 4; Table 2). However, CO2 enrichment had a
signi®cant e€ect on some of the elevations of regression
lines, indicating a transient change in the partitioning of dry
matter and leaf area during the initial stages of growth
(before the ®rst harvest). At low N supply the allocation of
dry matter to the shoot or leaf blades was increased early
on at elevated compared to ambient [CO2]. At high N
supply CO2 enrichment decreased the production of leaf
area relative to whole plant dry weight early in growth.
Independent of N supply, CO2 enrichment reduced the
836
Harmens et al.ÐPartitioning at Elevated CO2 and N
LAR (cm2 g−1)
0.6 mM NO3−
0.15 mM NO3−
200
1.5 mM NO3−
6.0 mM NO3−
A
B
C
D
E
F
G
H
I
J
K
L
100
0
SLA (cm2 g−1)
500
250
0
LWR
0.8
0.4
0.0
20
30
40
20
30
40
20
30
40
20
30
40
Time (d)
F I G . 2. LAR (A±D), SLA (E±H) and LWR (I±L) of D. glomerata grown at 360 (ÐdÐ) or 680 (- - -s - - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each
data point represents the mean of two troughs, three±four plants per trough (+s.e.).
ln DW shoot (mg)
0.6 mM NO3−
0.15 mM NO3−
8
A
1.5 mM NO3−
B
6.0 mM NO3−
C
D
6
4
2
2
ln DW leaf (mg)
8
4
6
8
2
E
4
6
8
2
ln DW root (mg)
F
4
6
8
2
G
4
6
8
H
6
4
2
2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
2
4
6
8
10
ln DW total (mg)
F I G . 3. The allometric relationship between shoot and root dry weight (A±D) and leaf and plant dry weight (E±H) of D. glomerata grown at 360
(ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
production of leaf area relative to leaf dry weight early in
ontogeny.
In contrast with [CO2], nitrogen supply signi®cantly
a€ected most allometric coecients (Figs 3, 4; Table 2). At
0.15 and 0.6 mM NO3ÿ , dry matter was preferentially
partitioned to the root (v 5 1), whereas at 1.5 and
6.0 mM NO3ÿ it was allocated equally to shoot and root
(v was not signi®cantly di€erent from 1). Consequently, the
increase in leaf weight per unit of plant dry weight was
signi®cantly lower at low compared to high N supply. At
Harmens et al.ÐPartitioning at Elevated CO2 and N
0.6 mM NO3−
0.15 mM NO3−
6
A
837
1.5 mM NO3−
B
6.0 mM NO3−
C
D
4
ln leaf area (cm2)
2
0
0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
0
2
4
6
8
10
ln DW total (mg)
6
E
F
G
H
4
2
0
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
0
2
4
6
8
ln DW leaf (mg)
F I G . 4. The allometric relationship between leaf area and plant dry weight (A±D) and leaf area and leaf dry weight (E±H) of D. glomerata grown
at 360 (ÐdÐ) or 680 (- - -s- - -) ml l ÿ1 CO2 and varying [NO3ÿ ]. Each data point represents the mean three±four plants per trough.
T A B L E 2. E€ects of [CO2] and N supply on the allometric coecient (v) relating dry weight (W) and leaf area in
D. glomerata
CO2 concentration
(ml l ÿ1)
v [NO3ÿ concentration (mM)]
0.15
0.6
1.5
6.0
0.79b (q***)
0.78b
0.99c
0.99c
1.07c
1.05c
0.65a (q**)
0.59a
0.84b (q**)
0.83b
0.97c
0.95c
0.99c
0.98c
360
680
0.74a
0.77a
0.77a
0.80a
0.85b (Q***)
0.82a
0.90c (Q***)
0.88b
360
680
1.13a (Q**)
1.32a
0.91b (Q***)
0.96b
0.88b (Q***)
0.86c
0.91b (Q***)
0.91bc
x
y
Wroot
Wshoot
360
680
0.56a (q**)
0.52a
Wtotal
Wleaf blade
360
680
Wtotal
Leaf area
Wleaf
Leaf area
The natural logarithm of y was plotted against the natural logarithm of x for all harvests and an ordinary linear regression was ®tted. The
allometric coecient v was then calculated from the slope of the regression (k) and the correlation coecient (r): v ˆ k=r (r ˆ 0.98±1.00). E€ects
of [CO2] and N supply on v were tested by pairwise comparison using Student's t-test (degrees of freedom ˆ 12). Di€erent superscripts within rows
indicate signi®cant di€erences (P 4 0.05) between v at di€erent N treatments. Although there were no signi®cant e€ects of CO2 on v, signi®cant
e€ects of CO2 on the elevation of regressions were found as indicated within parentheses: Q or q indicate a signi®cant higher or lower elevation at
360 compared to 680 ml l ÿ1 CO2 , respectively; **P 4 0.01, ***P 4 0.001.
ambient [CO2] the increase in leaf area per unit of plant dry
weight was signi®cantly less at 0.15 and 0.6 mM NO3ÿ than
at higher [NO3ÿ ]. At 0.15 mM NO3ÿ , leaf area increased
relatively more than leaf dry weight compared to higher
[NO3].
DISCUSSION
When growth was analysed allometrically, it was evident
that elevated [CO2] in comparison with N supply had
minimal e€ects on the partitioning of dry matter within
D. glomerata. An increase in N availability a€ected partitioning throughout ontogeny, such that enhanced N supply
reduced allocation of dry matter to roots. Dependent on N
availability, elevated [CO2] changed the partitioning of dry
weight and dry weight±leaf area relationships within
D. glomerata only during the initial stages of growth.
Hence, e€ects of CO2 enrichment on S : R ratio, LAR and
SLA in plants of the same age were sometimes still present
when ontogenetic drift was taken into account.
Ratios indicate the state of the plant at an instant in time,
are the product of the plant's history and will be an
insensitive measure of changes in partitioning throughout
ontogeny (Farrar and Gunn, 1998). We applied allometry
to identify e€ects of treatment independent of ontogeny
over a period of growth; allometry has a clear biological
838
Harmens et al.ÐPartitioning at Elevated CO2 and N
meaning and proved to be statistically more robust than
comparing plants on the basis of similar total dry weights
(Gunn et al., 1999). We found that dry weight and leaf area
to dry weight ratios were subject to ontogenetic drift. Had
we not accounted for this, we would have misjudged, in
some treatments, the adjustments in allocation patterns.
For example, the decrease in LAR due to CO2 enrichment
at low N supply when plants of the same age were compared was simply the result of accelerated growth. On the
other hand, a transient increase in allocation of dry matter
to the shoot at elevated [CO2] and very low N supply during
early stages of growth would not have been detected when
only S : R ratios of plants of the same age were compared.
Although other studies have shown that elevated [CO2]
has little e€ect on allometric coecients (Bowler and Press,
1993; Baxter et al., 1994; Farnsworth et al., 1996; Gebauer
et al., 1996; Stirling et al., 1998; Gunn et al., 1999), little
attention has been paid to changes in the elevation of
regressions (Gunn et al., 1999; Marriott, 1999). Whilst
previously values for ln a have been used to quantify
treatment e€ects on allometry early in growth, extreme care
is required since the results of analyses will be highly
dependent on the units used, so ln a has little biological
signi®cance (Baxter et al., 1994; Stirling et al., 1998). This
study clearly shows that both allometric coecients and
elevations of regressions should be analysed to determine
whether changes in partitioning occur throughout ontogeny
or are restricted to a de®ned period in the plant's
development. Whereas CO2 enrichment did not a€ect any
of the allometric coecients (v), it did a€ect elevations of
the regressions in most cases, indicating a change in
partitioning during the initial stages of growth of
D. glomerata. These early changes in partitioning are hard
to detect, mainly due to the diculties associated with
measuring very small plants and the large number of
replicates that would be required to pick up small changes
in v, which appear to occur in a short period of time.
Optimal partitioning models and theory suggest that
plants respond to variation in the environment by partitioning dry weight among plant organs to optimize the
capture of resources in a manner that maximizes plant
growth rate (Reynolds and Thornley, 1982; Brouwer, 1983;
Wilson, 1988; McConnaughay and Coleman, 1999). In
D. glomerata, optimal partitioning did apply to the range of
N availability such that at limited N supply dry matter was
preferentially allocated to roots. Whereas optimal partitioning models predict increased allocation of dry matter to
roots at elevated CO2 , the partitioning of dry matter
between shoots and roots in D. glomerata was little a€ected
by CO2 enrichment. In contrast, a transient increase in the
partitioning of dry matter to the shoot (and leaves) was
observed at elevated [CO2] at low N supply at an early stage
of plant development. Clearly, if the basic assumptions of
optimal partitioning (i.e. that plants do alter dry weight
distribution in response to changes in availability of
resources to maximize growth rate) are incorrect, models
attempting to predict the ecological outcome of environmental changes based on the optimal partitioning theory
should be re-evaluated (McConnaughay and Coleman,
1999).
Reductions in SLA, and consequently in LAR, have been
reported at elevated CO2 (Bazzaz, 1990; Den Hertog et al.,
1993; Poorter, 1993; Baxter et al., 1994), and are often
correlated with an increase in non-structural carbohydrates.
Although the non-structural carbohydrate content was
increased in the youngest fully expanded leaf of
D. glomerata (Harmens et al., 2000), reduction in its SLA
at elevated compared to ambient [CO2] was still evident
when expressed per unit structural dry weight (Harmens,
unpubl. res.). The current study indicates that decreases in
SLA cannot simply be explained by accelerated growth at
elevated [CO2], but are due to changes in the distribution of
leaf dry weight and area during early stages of growth.
In conclusion this work has shown the importance of
assessing the e€ects of treatment on dry weight distribution
and leaf area development using the dynamic allometric
approach rather than by using ratios of plant components
which change as a consequence of ontogeny. There is
increasing evidence that CO2 enrichment does not a€ect
allometric coecients during ontogeny, in contrast to
resources such as N and light (McConnaughay and Coleman, 1999). Therefore, sequential harvesting in combination
with allometry is a useful tool to distinguish between e€ects
primarily due to enhanced [CO2] and confounding e€ects
such as water and nutrient status of soils.
AC K N OW L E D G E M E N T S
This work was completed whilst H. H. was in receipt of a
NERC Post-doctoral Fellowship. We thank Tim Sparks
(ITE) for his help on statistical analyses and Ray Rafarel
(ITE) for technical support.
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