Invasive Acer negundo outperforms native species in non-limiting resource environments due to its higher phenotypic plasticity

Abstract

Background: To identify the determinants of invasiveness, comparisons of traits of invasive and native species are commonly performed. Invasiveness is generally linked to higher values of reproductive, physiological and growth-related traits of the invasives relative to the natives in the introduced range. Phenotypic plasticity of these traits has also been cited to increase the success of invasive species but has been little studied in invasive tree species. In a greenhouse experiment, we compared ecophysiological traits between an invasive species to Europe, Acer negundo, and early- and late-successional co-occurring native species, under different light, nutrient availability and disturbance regimes. We also compared species of the same species groups in situ, in riparian forests.

Results: Under non-limiting resources, A. negundo seedlings showed higher growth rates than the native species. However, A. negundo displayed equivalent or lower photosynthetic capacities and nitrogen content per unit leaf area compared to the native species; these findings were observed both on the seedlings in the greenhouse experiment and on adult trees in situ. These physiological traits were mostly conservative along the different light, nutrient and disturbance environments. Overall, under non-limiting light and nutrient conditions, specific leaf area and total leaf area of A. negundo were substantially larger. The invasive species presented a higher plasticity in allocation to foliage and therefore in growth with increasing nutrient and light availability relative to the native species.

Conclusions: The higher level of plasticity of the invasive species in foliage allocation in response to light and nutrient availability induced a better growth in non-limiting resource environments. These results give us more elements on the invasiveness of A. negundo and suggest that such behaviour could explain the ability of A. negundo to outperform native tree species, contributes to its spread in European resource-rich riparian forests and impedes its establishment under closed-canopy hardwood forests.

Figure 1

Relative height growth rates of the invasive and native species according to the environmental conditions. Relative height growth rates (RGR_h, mm. mm^-1. d^-1. 10^-3). Values are means of nine to twelve seedlings (± 1 SE of the mean) for the invasive species (A. negundo, full diamonds), late-successional native species (F. excelsior and F. angustifolia, grey triangles) and early-successional native species (S. alba and P. nigra, open squares) across the three shade levels (Full light C, Shade S, Deep shade SS), the two nutrient levels (nutrient supply N+ vs. no supply N-) and the two disturbance regimes (Disturbed D vs. Non-disturbed ND).

Figure 2

(A) Root/shoot ratio, (B) leaf weight ratio, (C) total leaf area and (D) specific leaf area of the invasive and native species according to the environmental conditions. Root/shoot ratio (RSR), total leaf area (TLA, m²), leaf weight ratio (LWR, g. g^-1), specific leaf area (SLA, m². kg^-1). Values are means of nine to twelve seedlings (± 1 SE of the mean) for the invasive species (A. negundo, full diamonds), late-successional native species (F. excelsior and F. angustifolia, grey triangles) and early-successional native species (S. alba and P. nigra, open squares) across the three shade levels (Full light C, Shade S, Deep shade SS), the two nutrient levels (nutrient supply N+ vs. no supply N-) and the two disturbance regimes (Disturbed D vs. Non-disturbed ND).

Figure 3

(A) Specific leaf area, (B) light- saturated assimilation rate, (C) leaf nitrogen content and (D) photosynthetic nitrogen use efficiency of the invasive and native species in the field. Specific leaf area (SLA, m². kg^-1), light-saturated assimilation rate (A_max, μmol CO₂. m^-2. s^-1), leaf nitrogen content (Na, g. m^-2 ) and photosynthetic nitrogen use efficiency (PNUE, μmol CO₂. g^-1 N. s^-1 ) of the invasive species (A. negundo, full bars), the late-successional native species (F. excelsior, grey bars) and the early-successional native species (A. glutinosa, light-grey bars) as measured in situ. Values are means of 25 to 34 adult trees (± 1 SE of the mean). ANOVA were highly significant for all variables, respectively: F = 18.51 p < 0.0001; F = 26.85 p < 0.0001; F = 19.6 p < 0.0001; F = 6.96 p = 0.0016. Means with the same letters are not significantly different (α = 0.05).

Figure 4

(A) Light-saturated assimilation rate, (B) photosynthetic nitrogen use efficiency and (C, D) leaf nitrogen contents of the invasive and native species according to the environmental conditions. Light-saturated assimilation rate (A_max, μmol CO₂. m^-2. s^-1), leaf nitrogen content (N_m %, N_a g. m^-2) and photosynthetic nitrogen use efficiency (PNUE, μmol CO₂. g^-1N. s^-1). Values are means of nine to twelve seedlings (± 1 SE of the mean) for the invasive species (A. negundo, full diamonds), late-successional native species (F. excelsior and F. angustifolia, grey triangles) and early-successional native species (S. alba and P. nigra, open squares) across the three shade levels (Full light C, Shade S, Deep shade SS), the two nutrient levels (nutrient supply N+ vs. no supply N-) and the two disturbance regimes (Disturbed D vs. Non-disturbed ND).