Many studies have attempted to test whether certain leaf traits are associated with invasive plants, resulting in discrepant conclusions that may be due to species-specificity. However, no effort has been made to test for effects of species identity on invasive-native comparisons. Here, we compared 20 leaf traits between 97 pairs of invasive and native plant species in seven disturbed sites along a southwest-to-northeast transect in China using phylogenetically controlled within-study meta-analyses. The invasive relative to the native species on average had significantly higher leaf nutrients concentrations, photosynthetic rates, photosynthetic nutrients- and energy-use efficiencies, leaf litter decomposition rates, and lower payback time and carbon-to-nitrogen ratios. However, these differences disappeared when comparing weakly invasive species with co-occurring natives and when comparing invasives with co-occurring widespread dominant natives. Furthermore, the magnitudes of the differences in some traits decreased or even reversed when a random subset of strongly to moderately invasive species was excluded from the species pool. Removing rare to common natives produced the same effect, while exclusion of weakly to moderately invasives and dominant to common natives enhanced the differences. Our study indicates that the results of invasive-native comparisons are species-specific, providing a possible explanation for discrepant results in previous studies, such that we may be unable to detect general patterns regarding traits promoting exotic plant invasions through multi-species comparisons.
Leaf-construction cost (CC, g glucose g-1) was calculated as (5.39 carbon concentration - 1191) / 1000, following Feng et al. (2007). Photosynthetic nitrogen-use efficiency (PNUE, μmol g-1 s-1), photosynthetic phosphorus-use efficiency (PPUE, μmol g-1 s-1), photosynthetic potassium-use efficiency (PKUE, μmol g-1 s-1), photosynthetic water-use efficiency (PWUE, μmol mol-1), and photosynthetic energy-use efficiencies (PEUE, μmol g-1 s-1) were calculated as the ratios of Pmax to Nm, Pm, Km, Gs, and CC, respectively. Payback time (PT, d) of leaf-construction costs was calculated as (CC 106) / [12 (Pmax / 2 12 3600 - Rd 12 3600) 180 / 72], following Feng et al. (2011).
Based on our phylogenetically controlled within-study meta-analyses, the invasive relative to the native species on average had significantly higher leaf-nitrogen concentrations (Nm), light-saturated photosynthetic rates (Pmax), photosynthetic energy- (PEUE), nitrogen- (PNUE), phosphorus- (PPUE), and potassium-use (PKUE) efficiencies, leaf carbon- (Loss-C) and nitrogen- (Loss-N) loss rates (Fig. 2). Leaf-phosphorus concentrations (Pm; 95% CI: -0.016 to 0.623) and stomatal conductance (Gs; 95% CI: -0.027 to 0.531) were marginally higher for the invasives. In contrast, the invasives had shorter payback times (PT) and lower carbon-to-nitrogen ratios (C:N) than co-occurring natives. The invasive and native species were not significantly different in leaf-construction costs (CC), leaf tissue density (Density), leaf-potassium concentrations (Km), leaf mass-loss rates (Loss-M), photosynthetic water-use efficiency (PWUE), dark respiration rates (Rd), specific leaf area (SLA), and leaf thickness (Thickness).
Our phylogenetically controlled within-study meta-analyses showed that the invasive relative to the co-occurring native species had significantly higher leaf nutrient concentrations, photosyntheses, photosynthetic nutrients- and energy-use efficiencies, and higher leaf litter decomposition rates, but lower carbon-to-nitrogen ratios and shorter payback time of leaf construction cost (Fig. 2). Our results regarding leaf nitrogen and phosphorus concentrations and leaf carbon to nitrogen ratio were consistent with those from all multi-species comparisons and reviews that compared the three traits (Suppl. material 2: Table S1; Baruch and Goldstein 1999; Leishman et al. 2007; Heberling and Fridley 2013; Huang et al. 2020). Our results regarding photosynthetic energy-use efficiency and payback time of leaf construction cost were also in line with those of Heberling and Fridley (2013), which was the only multi-species comparison of these traits between invasive and native species. These traits may contribute more to the invasion success of invasive species than the others (Liu et al. 2017; Huang et al. 2020).
Overall, the invasive plants had significantly higher leaf nutrient concentrations, photosyntheses, photosynthetic nutrients- and energy-use efficiencies, and higher leaf litter decomposition rates, but shorter payback time of leaf construction cost and lower carbon-to-nitrogen ratios than co-occurring natives. More importantly and interestingly, the differences were affected significantly by identities of both the invasive and the native species. Furthermore, the magnitudes of the differences in some traits decreased or even reversed when gradually excluding a random subset of strongly to moderately invasive species from the species pool. Removing rare to common natives produced the same effect, while exclusion of weakly to moderately invasive species and dominant to common natives enhanced the differences. Our results provide a possible explanation for the discrepant results between our current and previous studies, and indicate that it may be unlikely to obtain general leaf traits (if any) for invasives through multi-species comparisons, which are species-specific and environment-dependent. In the future, we should compare invasive and native species at both species and community levels in different habitats, and account for possible influencing factors. 153554b96e