Do Higher Resource Capture Ability and Utilization Efficiency Facilitate the Successful Invasion of Exotic Plant ? A Case Study of Alternanthera philoxeroides

We tested the hypothesis that introduced populations may have higher resource capture ability and utilization efficiency than native ones of invasive plants. We compared ecophysiological traits including maximum photosynthetic rate (Pmax), apparent quantum yield (Q), specific leaf area (SLA), photosynthetic energy use efficiency (PEUE), photosynthetic nitrogen use efficiency (PNUE), water use efficiency (WUE), mass-based and area-based leaf construction cost (CCmass and CCarea), and mass-based and area-based leaf nitrogen concentration (Nmass and Narea) between native (Argentina) and introduced (USA) populations of two varieties (North Apa and South Apo) of Alternanthera philoxeroides under common garden conditions in China. For Apo and Apa, Pmax, Q, Nmass and WUE were not significantly different between native and introduced populations; introduced populations had significantly lower SLA and lower CCmass but significantly higher Narea and CCarea than native ones. For Apa, the introduced populations showed significantly lower PEUE and lower PNUE while for Apo, PEUE and PNUE were not significantly different between native and introduced populations. The results indicated that introduced populations of A. philoxeroides do not show higher resource capture ability and resource utilization efficiency than their native ones in the common garden experiment, suggesting that these traits may not necessarily contribute to successful invasion of invasive plants.


Introduction
One mechanism which has been frequently mentioned and investigated empirically in the context of plant invasions is that higher resource capture ability and utilization efficiency may facilitate successful invasion [1][2][3].Studies comparing leaf traits of invasive plants and native plants have shown that invasive species have higher specific leaf area (SLA) [4,5], lower mass-based leaf construction cost (CC mass ) [4,[6][7][8], higher photosynthetic nitrogen use efficiency (PNUE) [3,4,9], higher photosynthetic energy use efficiency (PEUE) [8,10] and higher water use efficiency (WUE) [9,11] than native species.However, some studies have detected that there are no significant differences in CC mass [12], PNUE [8] and WUE [6,8] between invaders and natives.A recent study also suggests that the populations in the introduced range of an invasive species have evolved a higher PEUE and a shorter payback time but not lower CC mass than those in the native range [13].
Do exotic invasive plants have generally evolved higher resource capture ability and utilization efficiency in their introduced ranges?To answer this question, we compared leaf-level physiological traits that are related to resource capture and utilization efficiency, e.g., maximum photosynthetic rate (P max ), apparent quantum yield, SLA, PNUE, PEUE, WUE, area-based leaf construction cost (CC area ) and CC mass , area-based leaf nitrogen contention (N area ) and mass-based nitrogen contention (N mass ), between native (Argentina) and introduced (USA) populations of two varieties of Alternanthera philoxeroides (alligator weed) under common garden conditions in Shanghai, China.

Plant Species
Alternanthera philoxeroides (alligator weed), a clonal herbaceous perennial native to South America, is distributed from Buenos Aires province to southern Brazil (18˚S -39˚S) [14].It has been widely introduced to warm temperate and subtropical humid areas in North America, Australia and China [15].At present, A. philoxeroides is one of the most noxious weeds in wetlands and agro ecosystems [16].Many attributes have contributed to the invasion success of A. philoxeroides in China, such as rapid vegetative growth and reproduction [15], higher phenotypic plasticity than its native congener A. sessilis [16,17] and adaptation to physical disturbances [18].
A. philoxeroides typically emerges from belowground buds (on storage roots) in spring and then spreads vegetatively throughout a growing season, consequently forming dense monospecific stands.It overwinters with storage roots and rhizomes [18].Although A. philoxeroides may produce viable seeds, sexual reproduction contributes little to population regeneration due to extremely low seed outputs and low germination rates [14,15].In contrast, vegetative propagation (with storage roots and stems) is its primary regeneration strategy in the field [14].
At least two varieties of A. philoxeroides are recognized in Argentina [19]: the northern A. p. var.acutifolia (Apa) and the southern A. p. var.obtusifolia (Apo) [20].The two varieties are distributed in different geographic areas and habitat types (Figure 1(a); Flooding Pampa grasslands vs. wetlands along the Middle Parana River), and have evolved different leaf and stem morphologies [19,20].Both Apa and Apo have been introduced into USA (Figure 1(b)) [21].
In 2003 and 2004 we sampled stem fragments of A. philoxeroides from 16 populations: eight Argentina populations and eight USA populations (Figure 1).Both Argentina and USA populations were sampled widely across their distribution range.We collected 10 -20 stems for each population (more than 20 m apart from each other to ensure that different clones/genets might be sampled).All collected stems were cloned in a greenhouse for more than 3 years to reduce environmental maternal effects.

Experiment Design
The study was carried in August 2012 at the Experimental Field Unit of Fudan University, Shanghai, China, which is an outdoor field previously used for other garden experiments.The climate is humid subtropical, with rainfall averaging 1160 mm per year and with mean monthly temperatures ranging from 27.9˚C in July to 4.2˚C in January.In July 2011, we vegetative cloned 16 populations in experiment garden.Two weeks later we selected four individuals (with 2 -3 internodes and four or six leaves) per populations, and planted them individually at a depth of 2 cm in round pots (diameter, 23 cm; depth, 18 cm).We used nutrient soil (Beilei, Beilei Organic Fertilizer Co., Ltd., Zhenjiang, China) with the content of N, P, K ≥ 2% (dry weight basis), organic matter ≥ 35% (dry weight basis), water ≤ 45%, and pH 5.5 -6.5.All pots were randomly arranged on a desk and re-randomized weekly to reduce position effects.Pots were watered by hand every other day to keep the substrate moist.We harvested all plant materials 8 weeks after planting.Each plant was separated into leaves, stems, and roots.We determined leaf area for each plant with a leaf area meter (LI-3100A; LI-COR, Lincoln, NB, USA).All materials were oven dried at 55˚C for 72 h and then weighed to the nearest 0.001 g.SLA (cm 2 •g -1 ) was calculated as the ratio of leaf area to leaf dry mass.And then we finely ground the dried leaves.
Mass-based carbon concentration (C) and N mass of the powdered leaves were determined with OEA analyzer (Organic Elemental Analysis, FlashEA1112, Thermo Finnigan, Italy).Ash concentration (Ash) was determined after combusting leaf sample in a muffle furnace at 550˚C for 6 h.Ash alkalinity (AA) was determined acidimetrically [22].The mineral concentration (Min) of each sample was calculated according to [22] as follows: CC mass can be calculated [23,24] N area and N mass ), with range (Argentina vs. USA) as a fixed factor, population nested within range as a random factor.The residuals for all tests were normally distributed and no transformations were necessary.All analyses were carried out using SPSS 13.0 (SPSS, Chicago, USA).

Results
For both Apo and Apa, P max (Figure 2 Nitrate concentration were negligible, thus we assumed that N org = N mass [25].

Determination of Physiological Traits
For Apa the introduced populations showed significantly lower PEUE and PNUE, while for Apo, PEUE and PNUE were not significantly different between native and introduced populations (Figures 3(a) and (b)).For both Apa and Apo, WUE were not significantly different between native and introduced populations (Figure 3(c)).
Net photosynthetic rate in relation to varying photosynthetic photon flux density (light response curves) was determined on the youngest fully-expanded leaves with a Li-6400 Portable Photosynthesis System (LI-6400; LI-COR, Lincoln, NB, USA).Measurements were made on 2 -3 representative leaves on 4 individuals of each varieties in a pair during continuously sunny days from 09:00 to 12:00 am in the field in August 2012.All leaves of the Argentina and USA populations in a pair were of similar age and position on the stems.PPFD decreased in a stepwise fashion from 2000 to 0 µmol photon m -2 •s -1 (at 2000, 1500, 1200, 1000, 800, 600, 400, 200, 150, 100, 70, 50, and 0).During the measurements, CO 2 concentration, temperature and relative humidity within the leaf chamber were similar to those of ambient conditions.Each leaf was acclimated for 10 -20 min to 2000 µmol photon m -2 •s -1 of PPFD prior to the measurement.
For both Apa and Apo, N mass were not significantly different between native and introduced populations (Figure 4(a)), but introduced populations had significantly higher N area than native ones (Figure 4).Introduced populations showed significantly lower CC mass (Figure 4(c)), but significantly higher CC area (Figure 4(d)) than native ones.

Resource Capture Ability
Our results showed that introduced populations of A. philoxeroides showed no significant difference in P max , apparent quantum yield, and N mass relative to the native populations, suggesting that invasive populations of A. philoxeroides do not have advantage in resource capture ability compared to their native populations.This result was inconsistent with previous studies.Previous studies have found that most invasive plants have higher P max [6,9,11] compared to their co-occurring natives.Ref [8] found that invasive species had significantly higher P max (mass-based) than their non-invasive alien congeners.In a study of comparing the functional traits between plants from invasive and native populations of alien plant, higher P max and higher SLA of invasive populations was also found [13].
We fitted entire light response curves using the nonrectangular hyperbola model according to [26] as shown in Equation ( 3), where Photo is the leaf net photosynthesis rate, PAR is the light intensity, P max is maximum photosynthetic rate, Q is apparent quantum yield, R is dark respiration rate and K is a constant.

Statistical Analysis
SLA can be envisaged as an indicator of thickness or density; leaves with a higher SLA are typically thin and have greater levels of herbivory [29].Our results showed that invasive plants of A. philoxeroides had significantly lower SLA than their corresponding natives, indicating Nested analysis of variance (ANOVA) was used to compare if there had differences between plants from invasive and native populations in parameters (P max , apparent quantum yield, SLA, PNUE, PEUE,WUE, CC area , CC mass ,  

Resource Utilization Efficiency
Our results showed that invasive Apa had significantly lower PEUE and PNUE than native Apa, and invasive with previous studies.Invasive plants have often higher PNUE [4,9,30], higher PEUE [8] or both higher PEUE and PNUE [6,10,12] than natives species.Also in their work comparing energy use strategy of an invasive species from populations of its native ranges and introduced ranges, Ref. [13] found that plants from invasive populations had a higher PEUE than native populations.
Our results showed that invasive populations of A. phi- loxeroides had significantly lower CC mass and the same levels of N mass compared with native populations.However, due largely to significantly lower SLA, invasive populations had significantly higher CC area and N area than native populations (Figure 4).Nitrogen allocation to the photosynthetic apparatus is suggested to be a major factor responsible for the interspecific variation in PNUE [31].Introduced Apa had the same leaf N mass but significantly lower PNUE than native ones, suggesting a decreased fraction of leaf nitrogen invested in the photosynthetic apparatus in introduced plants of Apa.
WUE did not differ between native and invasive populations of A. philoxeroides, which is accordant with previous results that invasive plants are not at an advantage over native species in WUE [6,8,9].
Our finding that introduced populations of A. philox- eroides do not have higher resource capture ability and resource utilization efficiency than their native ones in the common garden experiment, suggesting that these traits may not necessarily contribute to successful invasion of invasive plants.
The importance of resource use efficiency will vary across habitats and timescales [6].Future research will be needed to examine if there are differences in resource capture ability and utilization efficiency between introduced and native populations of invasive plants across resource gradients and across their whole growth seasons.

Figure 1 .
Figure 1.Sample sites of two varieties, Apa (black circle) and Apo (black triangle), of Alternanthera philoxeroides from native (Argentina) and introduced (USA) range.
(a)) and apparent quantum yield (Figure 2(b)) were not significantly different between native and introduced populations, and introduced populations had significantly lower SLA (Figure 2(c)).

Figure 2 .
Figure 2. Differences in maximum photosynthetic rate (P max (a), apparent quantum yield (b) and specific leaf area (SLA) (c) between native (Argentina) and introduced (USA) populations of two varieties (Apo and Apa) of Alternanthera philoxeroides.Dates are means ± SE (NS, no significant differences, *P < 0.05).introduced populations of A. philoxeroides may have evolved higher resistance to generalist herbivores than native ones (Pan et al. unpublished results).

Figure 3 .Figure 4 .
Figure 3. Differences in photosynthetic energy use efficiency (PEUE) (a), photosynthetic nitrogen use efficiency (PNUE) (b) and water use efficiency (WUE) (c) between native (Argentina) and introduced (USA) populations of two varieties (Apo and Apa) of Alternanthera philoxeroides.Dates are means ± SE (NS, no significant differences, *P < 0.05 and **P < 0.01).Apo had no significant difference in PEUE and PNUE compared to native Apo.These results indicate that introduced populations of A. philoxeroides do not have higher resource utilization efficiency than their native ones in the common garden experiment.This contrasts as follows: