INTRODUCTION
Invasive plants can threaten biodiversity (Dukes and
Mooney, 1999) and potentially affect both the structure
and function of ecosystems (Vitousek and Walker, 1989;
Mack et al., 2000). Biological invasion is estimated to
cause approximately $137 billion in global losses every
year (Pimentel et al., 2000). Therefore, increasing our
understanding of the physiological and environmental
factors that influence the invasive potential of plant species
is important to facilitate preventative and remediation
efforts (Nagel and Griffin, 2001). Previous studies have
indicated that specific physiological and morphological
characteristics may contribute to the success of invasive
species. These include high reproductive allocation,
rapid vegetative growth rates, and a high potential for
acclimation (Bazzaz, 1986; Rejamanek, 1996). Although
many attempts have been made to identify the properties of
species that predispose them to become invasive (Lodge,
1993; Carlton, 1996), few generalities have emerged
which would allow us to predict which introduced species
Botanical Studies (2007) 48: 331-338.
*
Corresponding author: E-mail: lsspsl@mail.sysu.edu.cn;
Tel: 86-20-84115356; Fax: 86-20-84115356.
will be so (Weltzin et al., 2003). However, plant growth
is always related to energy processes, and energy can thus
be considered a basic unit when comparing organisms
(Griffin, 1994). Furthermore, Nagel and Griffin (2001)
have proposed construction cost (CC) as a general
approach to evaluating invasive potential, reflecting
specific growth strategies while allowing for a more
general comparison of resource-use efficiency.
Construction cost is a quantifiable measure of energy
demand for biomass production (Griffin, 1994). It has
been defined as the amount of glucose required to provide
carbon skeletons, reductant, and energy for the synthesis
of organic compounds (Williams et al., 1987). In general,
low CC is associated with high relative growth rates (Lam-
bers and Poorter, 1992; Poorter and Villar, 1997). Even
small differences in CC can lead to substantial differences
in growth rate (Poorter and Villar, 1997). Previous stud-
ies have found lower leaf CC for the invasive vs. native
species (Baruch and Gomez, 1996; Baruch and Goldstein,
1999; Nagel and Griffin, 2001; McDowell, 2002), suggest-
ing that invasive species may require less energy and use
it more efficiently for biomass construction than co-occur-
ring noninvasive plant species (Nagel and Griffin, 2001).
eCOlOgy
energetic cost of leaf construction in the invasive weed
Mikania micrantha H.B.K. and its co-occurring species:
implications for invasiveness
Li-Ying SONG, Guang-Yan NI, Bao-Ming CHEN, and Shao-Lin PENG*
State Key Laboratory of Biocontrol, Zhongshan University, Guangzhou 510275, P. R. China
(Received September 14, 2006; Accepted February 16, 2007)
ABSTRACT.
Construction cost (CC) is a quantifiable measure of energy demand for biomass production and
is related to energy-use efficiency. Low construction cost was hypothesized to give invaders a growth advan-
tage by utilizing energy efficiently. The present study examines the energetic cost of leaf construction in the
invasive weed Mikania micrantha H.B.K. and its five common co-occurring species (Ageratum conyzoides L.,
Wedelia trilobata (L.) Hitchc, Lantana camara L., Urena lobata L. and Derris trifoliata Lour.), and provides
insight into the success of this invasive weed. Mikania micrantha had the lowest leaf construction cost both on
a mass basis (leaf CC
mass
, 1.32 g glucose g
-1
) and on an area basis (leaf CC
area
, 28.80 g glucose m
-2
). Mikania
micrantha dominated the studied community with 60% coverage. The low leaf CC associated with its great
abundance indicated that low energetic cost might benefit its spread. Additionally, a higher specific leaf area
(SLA) and lower C and N concentrations were found in M. micrantha, providing it with another competitive
advantage. All the six studied species could be grouped into either an invasive or a native species category.
Both the mean leaf CC
area
and CC
mass
for the invasive were lower than those for the native species though the
mean leaf CC
mass
was not significantly different. The result indicated that a low energetic cost of leaf construc -
tion might generally influence invasive potential. Using discriminant analysis, leaf CC
area
was identified to be
more powerful in distinguishing between invasive and native species. Therefore, leaf CC
area
might be a valu-
able index to predict invasiveness and has a meaningful management implication.
Keywords: Construction cost; Invasive species; Mikania micrantha H.B.K.; Specific leaf area.
pg_0002
332
Botanical Studies, Vol. 48, 2007
Mikania micrantha H.B.K. (Compositae), a perennial
vine native to tropical Central and South America, is one
of the worst weeds in the world (Holm et al., 1977). It
has been introduced into southern China since 1910 and
widely invades the disturbed forests and plantation crops.
In recent years, it has caused significant damage to many
native ecosystems (Zhang et al., 2004). The increasing
abundance of M. micrantha suggests a high competitive
advantage over its co-occurring native plants. Wide eco-
physiological tolerance (Wang et al., 2004), prolific seed
production, fast dispersal by its vegetative propagation (Hu
and But, 1994), and allelopathic effects (Shao et al., 2003)
have been hypothesized as factors facilitating the spread of
this weed.
Since energetic processes may influence plant growth
and interspecific competition, a lower CC would be ex-
pected to give invaders a growth advantage. However, the
number of studies examining construction cost and species
invasiveness remains small, and further studies would be
useful in determining if a statistical trend toward lower
construction costs among invaders is generalizable (Dae-
hler, 2003). The main aims of this study were to evaluate
the leaf CC of M. micrantha and its co-occurring species,
and to provide insight into the success of this invasive
weed. Furthermore, the specific leaf area (SLA), leaf ni-
trogen (N) and carbon (C) concentrations were examined
to assess invasive potential and their correlations with leaf
construction cost. The relation between construction cost
and invasive potential was also discussed.
MATeRIAlS AND MeTHODS
Study site and plant materials
Plant materials were collected on September 24, 2005
from Qi¡¦ao Island (113.39¡¦, 22.24¡¦), Zhuhai, China.
Mikania micrantha had seriously imperiled the banana
plantation and disturbed forests at the foot of hills on
the island. The study site selected was a shrub area of
approximately 250 ¡Ñ 10 m
2
, which M. micrantha had
seriously invaded and dominated with a 60% coverage.
Ageratum conyzoides L. (Compositae), Wedelia
trilobata (L.) Hitchc (Compositae), Lantana camara L.
(Verbenaceae), Urena lobata L. (Malvaceae) and Derris
trifoliata Lour. (Leguminosae) were the most abundant
species in this area. These species have been found always
co-occurring with M. micrantha (Wang et al., 2004) and
thus were selected as the experimental species. The five
species were smothered by M. micrantha to different
extents, and L. camara was especially endangered. Among
the six studied species, L. camara and U. lobata were
small shrubs about 1-2 m high while the other four species
were all herbs. Additionally, A. conyzoides, W. trilobata,
and L. camara were also recognized as invasive species (Li
and Xie, 2002), which had spread widely in open habitats
and endangered the native crops and plantations.
Within the studied shrub belt, three quadrats (5 m ¡Ñ
5 m) were established approximately 50 m apart, with
the first quadrat placed 50 m from the belt edge. In each
quadrat, the six selected species were identified. Leaves of
herbaceous species were collected from different positions
on the branches from 5-10 individuals. Since the amount
of shrubby species was limited, all the individuals in the
quadrat were collected from about 1-3 individuals per
quadrat. For each species, leaves from the same quadrat
were mixed into one sample. In total, there were three
samples for each species from three quadrats.
Plant measurements
Fifteen to twenty leaves per species were placed in
moistened paper towels to prevent leaf curling, and leaf
blade area was determined with a leaf area meter (Li-
Cor 3100A, Li-Cor, USA). These leaf subsamples were
weighted after being dried at 60¢XC for 72 h to determine
SLA. All dried leaves were ground into a fine powder, ho-
mogenized, and then stored with a desiccant to maintain
dryness for the subsequent analysis. Leaf carbon (C) and
nitrogen (N) concentrations were determined with an el-
emental analyzer (Vario, Elmentar, Germany). To calculate
C and N concentrations per unit area, these values were di-
vided by SLA. Ash content (ASH) was measured by burn-
ing preweighted leaf powder samples in a 500¢XC muffle
furnace (Vulcan A-550, Vulcan, UK) for 6 h and weighing
the remaining mass. To obtain ash-free heat of combustion
(.Hc), three 0.5 g pellets of leaf powder from each sample
were pressed and combusted using a calorimeter (HWR-
15E, Shanghai, China) with correction for nitric acid for-
mation and ignition wire. The .Hc values obtained for the
triplicate pellets of each sample were then averaged.
Leaf construction cost per unit of mass (CC
mass
, equiva-
lent to grams glucose per gram dry mass) was calculated
according to the methods described by Williams et al.
(1987) as: CC
mass
¡×
[(0.06968.Hc-0.065)(1-ASH)
¡Ï
7.5(k
N/14.0067)]/E
G
.
Where k was the oxidation state of the N
substrate (+5 for nitrate or -3 for ammonium) and E
G
was
the growth efficiency. E
G
has been estimated to be 0.87
across species (Penning de Vries et al., 1974). Because
the relative proportions of these forms of N utilized by the
plants were unknown, we estimated leaf CC as the mean
of CC values calculated with each NH
4
+
and NO
3
-
oxida-
tion state as k for all species. To calculate leaf CC per unit
leaf area (CC
area
, equivalent to grams glucose per square
meter), these values were divided by SLA.
Statistical analysis
All calculations and statistical analysis were
performed with SPSS 11.5. A one-way model analysis
of variance (ANOVA) was performed to compare means
between species of leaf CC and other leaf chemical and
morphological variables. Mean values were considered
to be significantly different if P.0.05. Species were
compared to determine if means of the dependent variable
were significant at the 0.05 probability level with Student-
Newman-Keuls (S-N-K) post hoc analysis. Linear
regression analysis was used to determine the degree
pg_0003
SONG et al. ¡X Relation between construction cost and invasiveness
333
of association between leaf CC and other leaf variables
for all species. An additional discriminant analysis was
used to determine whether the measured SLA and leaf
CC
area
could be used to distinguish between invasive and
noninvasive species according to McDowell (2002). This
analysis was performed for these data by grouping each
case of the six species into either an invasive or a native
species category. An approximate F test was calculated
from a transformation of Wilks¡¦ lambda to test the
equality of group centroids and the distinctness of groups.
Standardized canonical discriminant function coefficients
were used to determine the relative importance of the
input variables of the classification function for predicting
group membership. To examine this discriminant analysis
graphically, Mahalanobis distances from the category
centroid were calculated for each case. The pair of these
distances was then plotted for each case, where similar
data points would have a similar pair of distances and thus
be plotted together as a group.
ReSUlTS
leaf construction cost
Leaf CC
mass
was significantly different between species
(F=140.671, P<0.001; Figure 1A) with values ranging
from 1.32 g glucose g
-1
to 1.59 g glucose g
-1
. Lantana
camara showed the highest leaf CC
mass
among all species,
followed by D. trifoliata, and then A. conyzoides and U.
lobata. Wedelia trilobata and M. micrantha had the lowest
CC
mass
. When calculated on an area basis, leaf construction
costs also showed significant differences between species
(F=13.698, P<0.001; Figure 1B). The rank of leaf CC
area
was changed as compared with that of leaf CC
mass
. Derris
trifoliata showed the highest leaf CC
area
. Overall, it
appeared that leaves from M. micrantha were the least
expensive to construct both per unit mass (1.32 g glucose
g
-1
) and per unit area (28.80 g glucose m
-2
) among all
studied species.
leaf structural and biochemical characteristics
SLA, N and C concentrations of M. micrantha and its
co-occurring species were examined in this study, and
significant differences were found between species (Table
1). In detail, M. micrantha exhibited the highest SLA, but
the lowest N and C concentrations, especially when the
data were expressed as per unit of area. The other three co-
occurring invasive species showed a similar trend in SLA,
leaf N, and C concentrations.
Table 1. Average specific leaf area (SLA), leaf carbon (C) and nitrogen (N) concentrations of invasive Mikania micrantha and its
five abundant co-occurring species within the study site.
Leaf variable Mikania micrantha Ageratum
conyzoides Wedelia trilobata Lantana camara Urena lobata Derris trifoliata
SLA (m
2
kg
-1
) 46.30¡Ó2.96a 43.54¡Ó2.61a 23.78¡Ó0.72b 22.77¡Ó2.57b 22.27¡Ó4.34b 14.82¡Ó1.67b
C (%)
38.89¡Ó0.29c 42.99¡Ó1.29b 39.06¡Ó0.37c 44.67¡Ó0.47b 44.34¡Ó1.15b 48.67¡Ó0.60a
C (g m
-2
)
8.48¡Ó0.59c
9.95¡Ó0.73c 16.46¡Ó0.58bc 20.14¡Ó2.33b 21.43¡Ó3.93b 33.23¡Ó3.33a
N (%)
2.48¡Ó0.10b
2.40¡Ó0.03b 1.95¡Ó0.21c 2.44¡Ó0.04b 2.50¡Ó0.16b 3.94¡Ó0.03a
N (g m
-2
)
0.54¡Ó0.01c
0.55¡Ó0.04c 0.82¡Ó0.08bc 1.10¡Ó0.14b 1.18¡Ó0.15b 2.70¡Ó0.32a
Values are means¡Ó1 SE. For each leaf variable, means labeled with the same letter are not significantly different according to S-N-K
post hoc analysis at P=0.05 level.
Figure 1. Mean leaf construction cost in per unit mass (leaf CC
mass
) (A) and per unit area (leaf CC
area
) (B) of Mikania micrantha and
its five abundant co-occurring species within the study site. A=Mikania micrantha; B=Ageratum conyzoides; C=Wedelia trilobata;
D=Urena lobata; E=Lantana camara; F=Derris trifoliata. Error bars represent 1 SE. Means with a common letter do not differ from
each other based on S-N-K post hoc analysis at P=0.05 level.
pg_0004
334
Botanical Studies, Vol. 48, 2007
Correlation between leaf CC and leaf traits
In the present data set, a strong negative correlation (R
2
¡×
0.863, P=0.007; Figure 2A) emerged between SLA and
leaf CC
area
across species, but no significant correlation
between SLA and leaf CC
mass
(R
2
¡×
0.305, P=0.256) was
observed. Leaf CC
area
was positively correlated with both
leaf C concentration per unit area (R
2
¡×
0.990, P<0.0001;
Figure 2B) and leaf N concentration per unit area (R
2
¡×
0.881, P=0.006; Figure 2C).
Comparison of leaf CC and SlA between
invasive and native species
Taking four invasive species as a group, the mean leaf
construction cost was lower than that for the native spe-
cies. Mean leaf CC
mass
of the native species was 3.5%
higher than that of the invasive species (1.47 g glucose g
-1
vs. 1.42 g glucose g
-1
; P=0.338; Figure 3A) although it was
not significantly different. However, mean leaf CC
area
of
the native species was significantly higher than that of the
invasive species (83.05 g glucose m
-2
vs. 47.29 g glucose
m
-2
; P=0.007; Figure 3B). In addition, the mean SLA was
significantly lower for the native than for the invasive spe-
cies (19.29 m
2
kg
-1
vs. 34.10 m
2
kg
-1
; P=0.021; Figure 3C).
The discriminant analysis result indicated a good
discrimination between invasive and native species
when using these variables of leaf SLA and CC
area
(F=7.163, P=0.028). For D. trifoliata, only two samples
were available. From a total of 17 cases, only two were
misclassified, one native and one invasive case. The data
fell into relatively distinct groups (Figure 4). The variable
CC
area
was identified as more powerful for discriminating
between invasive and native species because its
standardized canonical discriminant function coefficient
was larger than that of SLA (1.220 vs. 0.258).
DISCUSSION
Energetic prosperity is likely to influence plant growth.
As a quantifiable measure of energy demand for biomass
production, the low leaf CC suggests that these species
Figu re 2. The correlation between leaf cons truction cost per
unit area (leaf CC
area
) and specific leaf area (SLA) (A), leaf C
concentration (B), leaf N concentration (C), respectively.
¡»
=Mikania micrantha; ¡½ =Age ratum conyzoides; ¡¶=Wedelia
trilobata; ¡¼=Urena lobata;
¡º
= Lantana camara;
¡µ
=Derris
trifoliata. Error bars represent 1 SE.
Figure 3. Mean leaf construction cost in per unit mass (leaf CC
mass
) (A) and per unit area (leaf CC
area
) (B), specific leaf area (SLA) (C)
of the native (Urena lobata, Derris trifoliata) (filled bars) and the invasive species (Mikania micrantha, Ageratum conyzoides, Wedelia
trilobata, Lantana camara) (open bars) in the study site. Error bars represent 1 SE. * P < 0.05; ** P < 0.01.
pg_0005
SONG et al. ¡X Relation between construction cost and invasiveness
335
utilize carbon resources more efficiently and expend saved
energy on other competitive strategies, such as seed pro-
duction, biomass productivity, and relative growth rate
(Nagel et al., 2004). In the present study, both leaf CC
mass
and CC
area
for M. micrantha were the lowest among the six
species (Figure 1). Meanwhile, M. micrantha dominated
the studied community with 60% coverage. These results
suggest that low leaf CC might have facilitated the suc-
cessful establishment of M. micrantha. At the study site,
L. camara was damaged most seriously by M. micrantha,
and this might be attributable to its higher leaf CC (1.59 g
g
-1
) than M. micrantha (1.32 g g
-1
). The great difference in
leaf CC led to substantial differences in competitive abil-
ity. Mikania micrantha also showed a lower leaf CC than
the indigenous close congener Mikania cordata (Deng et
al., 2004). These results suggest that low leaf CC might
be intrinsic to M. micrantha, reflect its advantages in ener-
getic cost, and contribute to its successful invasion.
Besides lower leaf construction cost, M. micrantha ex-
hibited a higher SLA than its co-occurring species (Table
1). SLA is a plant trait extremely important in the regula-
tion and controlling of plant functions such as carbon as-
similation and carbon allocation (Lambers and Poorter,
1992; Reich et al., 1997). With higher SLA, M. micrantha
could produce larger assimilatory surfaces for a given
amount of carbon fixed carbon, indicative of a higher
capacity for light interception, which was consisted with
its high photosynthetic rates of 21.56 £gmol m
-2
s
-1
(Wen
et al., 2000). Additionally, leaf N and C concentrations of
M. micrantha were lower (Table 1). Plants with low leaf
nutrient concentrations generally tend to use nutrition ef-
ficiently (Chapin, 1980). Therefore, these characteristics
might provide M. micrantha another potential competitive
advantage. For M. micrantha, though C concentration in
leaf was lower (38.89%), more carbon had accumulated in
the stems (42.01%, unpublished data), which was expected
to facilitate its climbing and spreading.
The present results demonstrate that leaf construction
cost was correlated with SLA and leaf N and C concentra-
tions (Figure 2), which suggests that such costs might be
inherently determined by chemical composition. Plants
characterized by high SLA have thinner leaves and in-
vest less carbon in structural carbohydrates such as cel-
lulose, hemicellulose, and pectin. Additionally, nitrogen
and carbon are the respective constituents of protein and
secondary compounds (e.g. lignin and tannin) (Coley et
al., 1985; Lambers and Poorter, 1992). These compounds
are relatively expensive to synthesize (Poorter and Villar,
1997). Therefore, species with high SLA and low N and C
concentrations require less energetic cost for construction.
The high SLA and low N concentration may also decrease
energy loss via respiration (Reich et al., 1998). In the pres-
ent study, the coordination of the above-mentioned leaf
traits was observed in the invasive species. Species with
these characteristics always show a high photosynthetic
rate and short leaf lifetime, representing a specific growth
strategy with quick returns on investment of nutrients and
dry mass in leaves (Wright et al., 2004). Therefore, the
observed leaf traits could contribute to the success of the
invasive species.
The abundant species co-occurring with M. micrantha
¡XA. conyzoides, W. trilobata, and L. camara¡Xwere also
invasive. These species exhibited higher leaf construction
costs than M. micrantha, which suppressed their invasive
potentials. Therefore, these species showed less damage
in the studied community. The mean leaf CC was lower
for the invasive than for the native species (Figure 3A
and 3B), which agreed with previous studies (Baruch and
Gomez, 1996; Baruch and Goldstein, 1999; Nagel and
Griffin, 2001; McDowell, 2002). These results suggested
that low leaf CC might be one of the factors increasing
species ability to invade an environment. However, it
should be noted that two life forms existed in both the
native and invasive groups, and additional comparisons
among the same life form species were conducted. Among
the four herbs, the native D. trifoliata had a higher leaf CC
than any of the other three invasive herbs. Between the
two shrubs, the native U. lobata
showed inversely lower
leaf CC than the
invasive L. camara, a result which was
inconsistent with the previous study on shrubs (Baruch
and Goldstein, 1999). The ambiguous results concerning
shrubs were mainly due to the paucity of species and
further studies are necessary.
The variation of leaf construction cost was found
to be magnified when it was expressed in per unit area
rather than in per unit dry mass. In the present study, the
mean leaf CC of native species was only 3.5% higher
on a mass basis than that of invasive species, but it was
75.6% higher on an area basis. Poorter and Villar (1997)
concluded that variation in leaf construction costs on a
mass basis was small (within 10%) and due to a balance
between various energetic compounds (Chapin, 1989;
Poorter and Bergkotte, 1992). The large variations in leaf
CC
area
were due to the leaf area morphological trait. In our
study, significant differences in SLA appeared between
Figure 4. Mahalanobis distances calculated from a discriminant
analysis, in which all cases from the six species were classified
into either an "invasive" (file d diam ond) or "noninvas ive"
category (open diamond) using specific leaf area (SLA) and
leaf CC based on unit area (leaf CC
area
) as input variables. The
dashed line separates the two categories.
pg_0006
336
Botanical Studies, Vol. 48, 2007
invasive and noninvasive species (Figure 3C). High SLA
is of paramount importance if the invasive species is to
establish in disturbed habitats because it is well correlated
with the short leaf life-span and fast growth rate (Wright
and Westoby, 1999). Hamilton et al. (2005) reported that
high SLA was significantly and uniquely correlated with
invasion success on a continental scale.
Using leaf SLA and CC
area
as variables, the discriminant
analysis clearly differentiated between the groups of
invasive and native species, with 88.2% of original
grouped cases correctly classified. Therefore, the two
characteristics might be important factors contributing to
invasive success. In particular, the variable CC
area
emerged
as a powerful discrimination tool. Nagel and Griffin (2001)
proposed that construction cost was a general approach to
evaluate invasive potential. The present study suggests that
leaf CC
area
is more preferable for an evaluation of invasive
potential because it considers both the energetic cost of
biomass construction and leaf area morphology. It also
reflects the integrated competitive ability of plants. Energy
is a basic unit of comparison between organisms (Griffin,
1994). Therefore, leaf CC
area
is expected to be a general
index to predict invasive potential and has a meaningful
management implication. Given the substantial impact of
construction cost upon invasive potential, further studies
on energy assimilation, investment, and allocation patterns
will greatly aid our understanding of the underlying
physiological mechanisms of invasive success.
Acknowledgements. This research was founded by
the Key Program of the Chinese Ministry of Education
(704037), the National Natural Science Foundation, China
(30270282) and the Scientific Research Fund, Hongda
Zhang, Zhongshan University. We greatly appreciate the
invaluable assistance provided by Dr. You-Hua Ye, Dr. Jin-
Rong Wu, and Dr. Feng-Lan Li.
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