Research Article |
Corresponding author: Joshua Comrade Buru ( joshuacomradeburu@gmail.com ) Academic editor: Brad Murray
© 2019 Joshua Comrade Buru, Olusegun O. Osunkoya, Kunjithapatham Dhileepan, Jennifer Firn, Tanya Scharaschkin.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Buru JC, Osunkoya OO, Dhileepan K, Firn J, Scharaschkin T (2019) Eco-physiological performance may contribute to differential success of two forms of an invasive vine, Dolichandra unguis-cati, in Australia. NeoBiota 46: 23-50. https://doi.org/10.3897/neobiota.46.33917
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Invasive plant species are hypothesized as being more efficient at resource acquisition and use, resulting in faster growth than co-occurring non-invasive plant species. Nonetheless, some findings suggest that trait differences between invasive and non-invasive species are context dependent. In this study, two forms of an invasive vine species, Dolichandra unguis-cati, were used to test the context-dependent hypothesis. Dolichandra unguis-cati is a weed of national significance in Australia with two different forms: the ‘long pod’ (LP) and ‘short pod’ (SP). The two forms have different levels of distribution on the eastern Seaboard of the continent, with the SP form occurring extensively in both States of Queensland and New South Wales while the LP form is found only in isolated sites in South-East Queensland. This study examines whether differences in eco-physiological performance could be responsible for differential success of the two forms. A partially factorial experiment was set up in controlled conditions where potted plants of both forms were grown under two levels of light, water and nutrient resources (high and low) for 15 months. We measured several traits that are known to correlate with plant performance and resource use efficiency (RUE). The SP form exhibited higher values of carbon assimilation, RUE, number of subterranean tubers and leaf nitrogen than the LP form. However, the LP form produced greater biomass than the SP form, with the difference driven mainly by high resource conditions. The LP form displayed significantly higher phenotypic integration (number of traits significantly correlated) than the SP form in response to all treatments while the SP form exhibited higher phenotypic integration than the LP form in response to high resource conditions only. The SP form displayed traits that are well suited for successful colonization, possibly explaining its increased success in Australia, while the LP form possessed traits of opportunistic plants. Overall, we find that the two forms of the weedy vine deploy different carbon economies in response to resource conditions, which is evidence of the context-dependent trait hypothesis.
Cat’s claw creeper, disturbance, functional traits, resource use efficiency, invasiveness, Bignoniaceae
A key component of invasion ecology is to understand traits that enable introduced species to colonize and thrive in novel environments (
Invasive plants often display faster growth strategies than non-invasive species in the same environments (Davies et al. 2000;
Other studies have demonstrated that invasive species show higher trait integration than non-invasive ones (
Many studies have tried to identify the key traits that explain successful colonization and establishment by invasive plants, but results have shown that context matters more than any one trait and disturbance seems to frequently matter too (
Cat’s claw creeper, Dolichandra unguis-cati (L.) Lohmann (syn. Macfadyena unguis-cati (L.) Gentry) (Bignoniaceae) is an invasive vine in Australia. Two morphologically distinct forms of this species occur in Australia (‘long’ and ‘short’ pod – in reference to the average length of fruit pods produced by each form) (
To understand the eco-physiological mechanisms that underpin colonization success, it is vital to investigate potential links between plant growth and physiology, including photosynthetic traits (
Cat’s claw creeper, Dolichandra unguis-cati (L.) Lohmann (syn. Macfadyena unguis-cati (L.) Gentry) (Bignoniaceae) is a native of the Greater and Lesser Antilles, Mexico, South and Central America, Argentina and Trinidad and Tobago (Gentry, 1976). This species was introduced into Australia for ornamental purposes from South America in the 1800s (
This study was carried out in temperature-controlled glasshouse and shade-house facilities at the Queensland Department of Agriculture and Fisheries (DAF) in Brisbane, Australia. During the 15-month experiment, average temperature during the warmer months (October–April) ranged from 18 °C to 35 °C, and between 10 °C and 23 °C during the cooler months (May–September). Relative humidity ranged between 50–70% during this study.
Fruits containing seeds of the LP and SP forms were collected from various sites around the greater Brisbane area in South East Queensland (SEQ) and parts of New South Wales (NSW), Australia. Seeds of both forms were germinated in plant growth chambers, (model ADAPTIS A1000; Conviron Ltd., USA). For further germination details see
A factorial experiment of light and moisture was set up in a shade-house. For each form of D. unguis-cati, growth and physiology under two light resource levels were investigated i.e., (a) High light (HL) in which plants received ~35–40% of full sun (870 -1100 µmol m-2 s-1); and (b) Low light (LL) in which plants received ~1–2% of full sun (25–50 µmol m-2 s-1). LL conditions were achieved by creating a shade using 2–3 layers of locally available Coolaroo 1.83m Charcoal 90% shade cloth. These two light levels were chosen as approximations of irradiance levels encountered in disturbed habitats and open spaces (HL), and in rainforest understories (LL) where D. unguis-cati usually occurs. Light levels were measured using a LICOR 6400 portable photosynthesis system (LICOR, Inc., Lincoln, NE). Both forms were grown under HL and LL conditions with plants in each light level receiving either of two nutrient regimes. High nutrient (HN) condition was obtained by adding granules of a slow-release all purpose fertiliser (Osmocote, NPK 21:2:6 plus trace elements) to the growth medium every two weeks. No additional fertilizer was added to the growth media to create the low nutrient (LN) condition. The two nutrient levels (HN, LN) were chosen to mimic habitats of high nutrients (e.g., following fertilizer discharge) and low nutrient pulses respectively. All plants were watered to pot capacity by the addition of ~ 300 ml of water every two days using an automated watering system. The combinations of treatments were as follows: HLHN, HLLN, LLHN and LLLN.
These experiments were set up in a temperature-controlled glasshouse facility with temperatures ranging from 22–28 °C during the experiment. The mid-day photosynthetically active radiation (PAR) in the glasshouse was 800–1500 µmol m-2 s-1. These light conditions are comparable to the HL conditions described above for the light x nutrient experiment. Two water regimes were applied, reflecting the riparian and non-riparian environments where D. unguis-cati occurs: (i) a well-watered or high water (HW) condition in which soil moisture level was maintained at 100% pot capacity by the addition of ~ 300 ml of water every two days, and (ii) a low water (LW) condition in which moisture was maintained at 5% pot capacity by adding ~ 15 ml of water once every two weeks. Pot water capacity was determined at the beginning of the experiment by filling four replicate 13.5 L plastic pots with the commercial potting mix (Osmocote Multi-Purpose). The potting mix was oven dried using a Thermolite Scientific + 6100 Model oven for 48 hours at 80 °C and weighed to determine dry weight (DW). The potting mix was then saturated with water and excess water allowed to drain freely for 2-3 hours until no more water drained out. The pots were weighed again to determine saturated weight (SW). Pot capacity was calculated as the difference between SW and DW (
The light and water treatments, at two nutrient levels, were replicated seven (7) times for both the LP and SP forms. Thus, there were 112 total number of plants for this experiment. Physiological and growth traits were measured at the end of the experiment after 15 months of plant growth.
Assimilation rates were measured using an open-path portable gas exchange system (LI-6400; LICOR, Inc., Lincoln, NE, USA). For each treatment, five replicates (plants) of each form were randomly selected, and for each plant, two recently matured leaflets were identified and tagged for measurements. Photosynthetic rate (A, µmol m-2 s-1), transpiration rate (E, mol m-2 s-1) and stomatal conductance (gs, mol m-2 s-1) were measured at a constant CO2 concentration of 400 µL L-1. The relative humidity within the leaf chamber ranged between 50–65% while the temperature was kept at 23–25 °C. To investigate the response of the leaflets to changes in PAR, instantaneous assimilation A and transpiration measurements were taken at 50, 500, 1500 and 2500 µmol m-2 s-1. Leaves were kept at the respective PAR levels for ~ 10 minutes until they were stabilised. From the primary physiological data collected, the instantaneous water-use efficiency (WUE) was calculated as follows:
WUE = Asat/E, where Asat was assimilation rate at 2500 µmol m-2 s-1
Leaf chlorophyll content (measured in SPAD units) was estimated using a chlorophyll meter (Konica-Minolta SPAD-502, Spectrum Technologies, IL, USA). The same leaves tagged for physiological measurements were used to determine chlorophyll content, taking three random measurements from each leaf. Physiological data could not be obtained for HWLN and LWLN combinations because the plants under these treatments developed very few leaves.
After measurement of physiological data, the tagged leaflets were harvested, weighed (fresh weight) and photographed against a graduated background using an IPAD camera (Apple Inc., CA, USA) for leaf area estimation. The open access software, Image J 1.47v (www.imagej.nih.gov/ij) was used to calculate the leaf area (cm2) from the images. The harvested leaflets were thereafter dried at 65 °C for 72 hours, and their dry weight measured. The data collected were used to estimate specific leaf area (SLA = leaf area/leaf dry mass) and leaf dry matter content (LDMC = leaf dry mass/leaf fresh mass).
The dried leaf samples were analysed for total carbon (C) and nitrogen (N) concentrations using Plant CN Dumas combustion method (
Photosynthetic nitrogen use efficiency (PNUE) =
= Amax/leaf N, where assimilation rate was at 2500 µmol m-2 s-1
Leaf Carbon to leaf Nitrogen ratio (C: N) = Leaf C / leaf N
Leaf Chlorophyll to leaf Nitrogen ratio (Chl: N) = Leaf Chl / leaf N
At the end of the experiment (i.e. at 15 months growth), all plants were harvested and separated into above-ground (shoots) and below-ground (roots and tubers) tissues. The number of subterranean tubers per plant was recorded per treatment and per plant form. All plant tissues were dried at 65 °C for three weeks before weighing to determine total dry mass (g) and shoot/root ratio.
All data were tested for normality and homoscedasticity using the Shapiro-Wilks test. Data that violated the ANOVA assumptions of normality and homogeneity of variance were either log10 transformed (Amax, WUE), square-root transformed (shoot/root ratio) or Box-Cox power transformed (basal stem density [BSD], number of tubers, root and shoot dry mass, total dry mass, and all leaf traits). Values presented in this paper were back-transformed data, unless otherwise stated.
Mean differences for all traits were analysed using a two-way analysis of variance (ANOVA + an error structure of replicate/leaf number/or treatment) with treatments (HLHN, HLLN, LLHN, LLLN, HWHN, HWLN, LWHN, LWLN) and plant form (LP or SP) as fixed effects. When significant differences were detected, a Tukey LSD post-hoc test was performed to check differences between specific means. Pearson correlation coefficients were generated to determine the linear association among traits and how they compare between the LP and SP forms, and they were also used to test for the extent of trait integration within each of the two forms. A multivariate method of principal components analysis (PCA) based on Euclidian distances was used to explore how the two forms were separated by traits on an ordination space. Principal components smaller than 15% were discarded. All statistical tests were conducted using R version 3.1.0 (
Biomass production and allocation traits were not consistent within each form in response to high and low resources as shown by significant interactions between form and treatment for many of the traits examined (F1, 7 = 3.184, P<0.005; Table
The SP form developed more tubers than the LP form in high light (HLHN and HLLN) and high nutrient (HWHN and LWHN) conditions (F1, 7 = 46.459, P<0.001; Fig.
Trait response of the LP and SP forms to varying levels of light, water and nutrient conditions. Total biomass accumulated (a, b); Average number of tubers per plant (c, d); Shoot/root ratio (e, f). The legend in the graph (b) applies to all graphs. Graphs on the left (a, c, e) show traits responses to light x nutrient experiments and those on the right (b, d, f) show trait responses to water x nutrients experiments. Bars represent standard error of the mean (SEM). Differences across treatments are denoted by letters.
Mean (± SE) growth, physiological and leaf chemical traits of both forms, long pod (LP) and short pod (SP) of D. unguis-cati. Summary ANOVA refers to F- and P-values of a two-way ANOVA (+ an error structure) of log transformed performance traits and physiological traits for both forms, with a fixed effects structure of form (LP and SP) and treatments (HLHN, HLLN, LLHN, LLLN, HWHN, LWHN). NS = not significant.<br/>
Traits | Form | Summary ANOVA | Direction of difference | ||
---|---|---|---|---|---|
LP | SP | F -value | P -value | ||
Total biomass (g) | 38.11 ± 0.25 | 35.75 ± 5.89 | 8.12 | 0.0060 | LP>SP |
Leaf area (cm2) | 26.27 ± 2.14 | 11.74 ± 0.80 | 54.52 | 0.0001 | LP>SP |
SLA (cm2 g-1) | 3.95 ± 0.19 | 4.08 ± 0.27 | 0.45 | 0.60 | NS |
LDMC (mg g-1) | 296.94 ± 14.11 | 293.04 ± 9.88 | 0.16 | 0.694 | NS |
No. of tubers plant-1 | 2.48 ± 0.24 | 5.61 ± 0.69 | 46.46 | 0.0001 | LP<SP |
Root dry mass (g) | 11.35 ± 2.13 | 15.23 ± 2.54 | 2.54 | 0.122 | NS |
Shoot dry mass (g) | 26.76 ± 6.76 | 20.90 ± 3.90 | 0.01 | 0.912 | NS |
*Shoot/root ratio (SRR) | 1.78 ± 0.19 | 1.31 ± 0.15 | 1.99 | 0.20 | NS |
A max (µmol m-2 s-1) | 3.48 ± 0.27 | 3.97 ± 0.34 | 4.07 | 0.05 | LP<SP |
WUE (µmol CO2mol-1 H2O) | 4.03 ± 0.21 | 4.53 ± 0.31 | 30.29 | 0.001 | LP<SP |
PNUE (µmol mol s-1) | 1.27 ± 0.15 | 1.21 ± 0.08 | 0.14 | 0.71 | NS |
C (g m-2) | 43.82 ± 0.26 | 43.28 ± 0.24 | 6.28 | 0.018 | LP>SP |
N (g m-2) | 3.45 ± 0.26 | 3.73 ± 0.19 | 5.31 | 0.03 | LP<SP |
C: N | 14.63 ± 1.38 | 12.37 ± 0.82 | 7.29 | 0.01 | LP>SP |
Chl. (SPAD units) | 41.00 ± 0.85 | 57.24 ± 0.03 | 58.52 | 0.0001 | LP<SP |
Chl: N | 13.29 ± 0.38 | 15.74 ± 0.77 | 1.60 | 0.22 | NS |
Overall, both the SP and LP forms differed in terms of SLA depending on treatments (F1, 4 = 257.845, P<0.0001; Table
Across treatments, the LP form accumulated significantly higher total leaf carbon (C) than the SP form (F1, 4 = 6.282, P<0.018). Conversely, the SP form showed higher area based total leaf nitrogen (N) (F1, 4 = 5.310, P<0.03) (Table
Mean trait performance and summary of ANOVA of the SP and LP forms in different light, water and nutrient treatments. *, P<0.05; **,P<0.02; ***, P<0.001; NS, not significant. Treatments: HL, high light; LL, low light; HN, high nutrient; LN, low nutrient; HW, high water; LW, low water.
Form | Treatment | N, g m-2 | C, g m-2 | C: N | Chl., spad units | Chl: N | Amax, µmol m-2 s-1 | WUE, µmol CO2mol-1 H2O | PNUE, µmol mol s-1 | SLA, cm2 g-1 | LDMC, mg g-1 | Total dry mass,g | No. of tubers | S/R ratio | BSD, cm |
SP | HLHN | 4.27 | 43.74 | 10.24 | 62.24 | 14.59 | 5.79 | 5.68 | 1.36 | 4.63 | 354.89 | 105.08 | 15.0 | 2.94 | 3.14 |
LP | HLHN | 4.14 | 44.76 | 10.87 | 42.01 | 10.19 | 5.41 | 4.36 | 1.33 | 3.83 | 341.39 | 140.41 | 6.00 | 3.91 | 5.53 |
SP | HLLN | 2.21 | 42.47 | 19.47 | 41.02 | 18.69 | 2.50 | 4.49 | 1.10 | 3.67 | 394.25 | 69.23 | 10.0 | 0.55 | 3.30 |
LP | HLLN | 1.87 | 42.72 | 22.96 | 33.51 | 17.82 | 3.67 | 4.83 | 1.94 | 7.35 | 458.48 | 60.95 | 2.20 | 0.95 | 5.25 |
SP | LLHN | 3.96 | 42.95 | 10.83 | 59.36 | 14.99 | 3.78 | 3.35 | 0.96 | 5.74 | 308.15 | 7.97 | 2.75 | 2.47 | 2.03 |
LP | LLHN | 4.08 | 42.66 | 10.46 | 55.04 | 13.49 | 2.44 | 2.94 | 0.59 | 4.67 | 332.58 | 4.55 | 2.33 | 1.91 | 2.79 |
SP | LLLN | 3.42 | 42.13 | 12.34 | 60.09 | 17.59 | 4.60 | 3.24 | 1.35 | – | – | 2.99 | 1.33 | 1.55 | 1.71 |
LP | LLLN | 2.64 | 44.05 | 19.38 | 52.42 | 23.00 | 3.16 | 3.20 | 1.38 | – | – | 1.33 | 1.00 | 0.86 | 1.77 |
SP | HWHN | 4.41 | 44.40 | 10.07 | 61.80 | 13.99 | 5.55 | 5.10 | 1.26 | 3.32 | 361.79 | 87.04 | 8.60 | 1.68 | 4.59 |
LP | HWHN | 4.27 | 44.56 | 10.64 | 34.47 | 7.86 | 3.75 | 3.88 | 0.89 | 3.85 | 363.97 | 41.74 | 4.20 | 3.16 | 3.99 |
SP | HWLN | – | – | – | – | – | – | – | – | – | – | 2.19 | 2.14 | 0.35 | 1.53 |
LP | HWLN | – | – | – | – | – | – | – | – | – | – | 4.40 | 1.43 | 0.70 | 1.78 |
SP | LWHN | 4.28 | 44.68 | 10.45 | 61.14 | 14.30 | 2.07 | 4.68 | 0.49 | 4.34 | 405.94 | 30.93 | 6.00 | 0.782 | 2.73 |
LP | LWHN | – | – | – | – | – | 2.45 | 4.52 | – | 4.54 | 334.41 | 6.93 | 2.00 | 1.82 | 2.43 |
SP | LWLN | – | – | – | – | – | – | – | – | – | – | 2.16 | 1.29 | 0.14 | 1.24 |
LP | LWLN | – | – | – | – | – | – | – | – | – | – | 0.97 | 1.14 | 1.58 | 0.97 |
Summary of ANOVA | |||||||||||||||
Form | * | * | NS | *** | * | * | *** | NS | NS | NS | ** | *** | NS | ** | |
Treatment | *** | ** | ** | * | *** | *** | *** | * | *** | *** | *** | *** | NS | *** | |
Form x Treatment | . | . | * | NS | NS | ** | * | * | *** | * | ** | * | NS | *** | |
Direction of form difference | LP<SP | LP>SP | LP=SP | LP<SP | LP<SP | LP<SP | LP<SP | LP=SP | LP=SP | LP=SP | LP>SP | LP>SP | LP=SP | LP>SP |
The SP form showed a significantly higher rate of carbon assimilation (A) than the LP when compared across all resource treatments (F1, 5 = 4.067, P<0.05) (Table
The SP form showed a significant shift in carbon assimilation in response to light levels as there was nearly a two-fold difference in A between low light (LLHN) and high light (HLHN) conditions (P<0.0001; Fig.
The SP form showed higher water use efficiency (WUE) than the LP form in response to different treatments (F1, 5 = 30.294, P<0.001; Table
Carbon assimilation rates and water use efficiency of the LP and SP forms in response to light, water and nutrient resources. Maximum carbon assimilation, Amax (a, b); water use efficiency, WUE (c, d). The legend in the graph b applies to all graphs. Graphs on the left (a, c) show traits responses to light x nutrient experiments and those on the right (b, d) show trait responses to water x nutrients experiments. Bars represent standard error of the mean (SEM). Differences across treatments are denoted by letters.
Across treatments, all performance traits examined changed in response to changes in biomass accumulated, although the trends were not significant for the SP form in terms of assimilation rate (A) and leaf chlorophyll content. It is instructive to see that at a given plant biomass, higher trait values were obtained for the SP form relative to the LP form (Fig.
Trait relationships across light, water and nutrient regimes between total biomass accumulated versus Amax(a), WUE (b), SLA (c), number of tubers (d), chlorophyll content (e) and leaf N concentration (f). The LP form is represented by triangles (▲) and a solid line (―) while the SP form is represented by open circles (○) and dotted lines (---). Significant relations (P<0.05) are shown by underlined R2 values, **, P < 0.0001; *, P < 0.05.
Considering all possible bivariate relationships for the traits measured in the study (i.e. 45 pairwise comparisons), the number of significant correlations were higher for the LP form (23) than the SP form (17) (Table
In the high nutrient scenario, more traits (4) were correlated with SLA in the SP form (biomass gained (negative), Amax (positive), WUE (negative) and total leaf N (negative). In contrast, only two traits (leaf N and leaf Chl.) were positively associated with SLA in the LP form. In the same high nutrient condition regardless of light and moisture condition, a slightly higher number of traits (compared to the high light scenario) were linked to biomass gained for the LP form (5 out of 12), but more traits were significantly correlated with total biomass for the SP form (7 out of 12) in the high nutrient scenario (Suppl. material
Matrix of Pearsons correlation coefficients (r) for functional traits of the LP and SP forms (SP shown in brackets). Data have been pooled across light, water and nutrient treatments. Significant correlations (P<0.05) are shown by bold font and asterisks (**, P < 0.001; *, P < 0.05); n = 18-20.
SLA LP (SP) | Total biomass LP (SP) | No of tubers LP (SP) | WUE LP (SP) | PNUE LP (SP) | N LP (SP) | C: N LP (SP) | Amass LP (SP) | Amax LP (SP) | Chl. LP (SP) | |
SLA | 1 | |||||||||
Total biomass | 0.403(-0.663**) | 1 | ||||||||
No of tubers | -0.073(-0.519*) | 0.667**(0.893**) | 1 | |||||||
WUE | -0.383(-0.424) | 0.583**(0.652**) | 0.474*(0.515*) | 1 | ||||||
PNUE | -0.570*(-0.213) | 0.352(0.163) | 0.069(0.161) | 0.688**(-0.094) | 1 | |||||
N | 0.588*(0.684**) | -0.319(-0.471*) | 0.404(-0.383) | -0.472*(-0.120) | -0.736**(0.097) | 1 | ||||
C: N | -0.237(-0.083) | -0.018(0.039) | -0.519*(0.037) | 0.318(-0.183) | 0.657**(-0.293) | -0.924**(-0.782**) | 1 | |||
Amass | -0.107(0.392) | 0.426(-0.142) | 0.715**(-0.069) | 0.515*(-0.117) | 0.552*(0.659**) | 0.157(0.812**) | -0.215(-0.781**) | 1 | ||
A max | -0.533*(-0.099) | 0.482*(0.111) | 0.593**(0.109) | 0.595**(0.069) | 0.679**(0.821**) | -0.126(0.521*) | -0.108(-0.785**) | 0.898**(0.877**) | 1 | |
Chl. | 0.564*(0.027) | -0.498*(0.045) | -0.254(-0.097) | -0.380(0.421) | -0.401(0.314) | 0.568*(0.446) | -0.310(-0.569**) | 0.034(0.529*) | -0.239(0.545*) | 1 |
A graphical representation of a principal component analysis (PCA) of the LP and SP forms based on 10 traits under four resource treatments (HLHN, HLLN, HWHN and LLHN) is shown in Figure
Principal component analysis of LP and SP across four treatments (HLHN, HWHN, HLLN, and LLHN) based on 13 eco-physiological traits projected on the first two axes. The traits on each axis are the main drivers of the variation explained by that axis. The percentage of the variance explained by each principal component is shown in brackets. There was no determination of leaf chemistry for treatments that are not included in this PCA because of insufficient leaf materials. Dotted lines connect the LP and SP forms under similar treatments for the sake of comparison.
The two forms of D. unguis-cati in Australia were found to display significant differences in traits suggesting they likely occupy different positions in the LES (also see
The LP form accumulated more biomass when grown under high light and high nutrient resource conditions while the SP form did so under low resource conditions. This suggests that the LP form exhibits traits of an opportunistic invader that effectively exploits extra resources in the environment while the SP form does not. The theory of fluctuating resource availability holds that species that can exploit excess resources have a higher chance to successfully colonize disturbed habitats (
This study supports the context-dependent hypothesis of trait differences (
Interestingly, another study showed that SP accumulated more biomass under low nutrient scenarios (
The SP form developed a significantly higher number of tubers than the LP form (Fig.
Although the LP form is known to have broader leaves than the SP form (
Developing thicker leaves by the SP form could be a strategy to compensate for less surface area by way of increasing photosynthetic apparatus (palisade parenchyma). Indeed, the SP form has significantly thicker palisade mesophyll tissue than the LP form (JC Buru, unpublished data). This trend also follows suit with our other findings that this form accumulates more biomass in undisturbed conditions where resources are lower. Thicker mesophyll tissues are known adaptations to low light conditions (
We found differences in carbon economy between the two forms with the SP form exhibiting higher assimilation rates (A) and WUE than the LP form. In the low nutrient scenario, carbon assimilation was two-fold higher under low light than high light for the SP form. This was accompanied by a greater leaf N concentration at the low light level. The leaf economic spectrum suggests that high A needs more leaf N to drive rapid growth (
The LP and SP forms were found to use resources in similar ways, at least under same and/or fluctuating light, water and nutrient resources, as no significant difference was found in their photosynthetic nitrogen use (PNUE). When light conditions were considered separately, a negative relationship between PNUE and biomass gained was obtained for the LP form, an indication of less RUE in this form. However, considering nutrient conditions separately, both forms show a positive relationship between PNUE and biomass gained. Correlation coefficient (r) values were greater in the SP form suggesting that at a given PNUE, a higher biomass was always attained for the SP form relative to the LP form, also indicating less RUE in the LP form.
Previous studies have found non-native invasive species to have higher RUE than native non-invasive congeners (
The traits measured in this study were correlated for each form to assess the extent of covariance among them, which gives an indication of phenotypic integration (
There was a significant shift of trait integration in favour of the SP form in response to high light and nutrients resources, a result similar to findings of
As both forms of D. unguis-cati were found to thrive in high resource environments, care must be taken to ensure that disturbances are minimised, especially along sensitive habitats like riparian corridors that the weed invades. Effluent discharge into creeks and riparian habitats of QLD should be monitored and minimised as this might encourage proliferation of this species, especially the opportunistic LP form (see
The biological control agents that have been released to control this weedy vine include a leaf mining beetle, Hylaeogena jureceki and leaf sucking tingid, Carvalhotingis visenda (Dhileepan et al. 2010; Dhileepan et al. 2013). These agents have shown evidence of success in controlling the populations of both the LP and SP forms (Dhileepan et al. 2013). Their feeding behaviour significantly reduce foliage and thus minimise photosynthetic capacity of the weed. Thus, concerted efforts must be prioritised to continually release these agents in large numbers to reduce the rate of shoot growth for both forms, thus keeping their populations within acceptable limits. We suggest that biological control agents that attack tubers in combination with the agents currently in use would be appropriate for this species, especially for the SP form which was found to produce significantly higher number of tubers than the LP form in this study (also see Raghu et al. 2006).
Overall, the results provide support for the context-dependent hypothesis (
We would like to thank Queensland University of Technology, Biosecurity Queensland, Department of Agriculture and Fisheries (Australia) and the Government of Botswana for jointly funding the work. JCB would like to thank Elizabeth (Liz) Snow for assistance with fieldwork. We would like to thank two anonymous reviewers for helpful feedback on an earlier version of the manuscript.
Supplementary tables
Data type: occurrence
Explanation note: The Supplementary Material for this article consists of three Tables showing traits coordination under high light and high nutrients (Table S1 and S2) and PC loadings for traits used in the PCA that is shown in Figure