Gas exchange and growth responses to nutrient enrichment in invasive Glyceria maxima and native New Zealand Carex species

We compared photosynthetic gas exchange, the photosynthesis–leaf nitrogen (N) relationship, and growth response to nutrient enrichment in the invasive wetland grass Glyceria maxima (Hartman) Holmburg with two native New Zealand Carex sedges (C. virgata Boott and C. secta Boott), to explore the ecophysiological traits contributing to invasive behaviour. The photosynthesis–nitrogen relationship was uniform across all three species, and the maximum light-saturated rate of photosynthesis expressed on a leaf area basis (Amaxa) did not differ significantly between species. However, specific leaf area (SLA) in G. maxima (17 ± 6 m2 kg−1) was 1.3 times that of the sedges, leading to 1.4 times higher maximum rates of photosynthesis (350–400 nmol CO2 g−1 dry mass s−1) expressed on a leaf mass basis (Amaxm) when N supply was unlimited, compared to the sedges (<300 nmol CO2 g−1 dry mass s−1). Analysis of Covariance (ANCOVA) revealed significant positive relationships between leaf N content and chlorophyll a:b ratios, stomatal conductance (gs), dark respiration rate (Rd), and the photosynthetic light saturation point (Ik) in G. maxima, but not in the sedges. ANCOVA also identified that, compared to G. maxima, the sedges had 2.4 times higher intrinsic water use efficiency (A/gs: range 20–70 cf. 8–30 μmol CO2 mol−1 H2O) and 1.6 times higher nitrogen use efficiency (NUE: 25–30 cf. 20–23 g dry mass g−1 N) under excess N supply. Relative growth rates (RGR) were not significantly higher in G. maxima than the sedges, but correlations between leaf N, gas exchange parameters (Amaxa, Amaxm, Rd and gs) and RGR were all highly significant in G. maxima, whereas they were weak or absent in the sedges. Allocation of biomass (root:shoot ratio, leaf mass ratio, root mass ratio), plant N and P content, and allocation of N to leaves all showed significantly greater phenotypic plasticity and stronger correlation to final biomass in G. maxima than in the sedges. We therefore conclude that photosynthesis and growth rates are not intrinsically higher in this invader than in the native species with which it competes, but that its success under nutrient enrichment is a consequence of greater physiological responsiveness and growth plasticity, and stronger integration between gas exchange and growth, coupled with indifference to resource wastage (i.e. low WUE and NUE) at high nutrient supply. The poorer performance of G. maxima than the sedges under low nutrient supply supports the importance of nutrient management, especially N, as a strategy to minimise the invasive behaviour of fast-growing herbaceous species in wetlands.

► We compare gas exchange and growth in invasive Glyceria maxima with native sedges.
► G. maxima does not have inherently higher growth or photosynthesis rates.
► It has greater integration between nitrogen uptake, photosynthesis and growth.
► Its photosynthesis and growth respond more plastically to nitrogen than the sedges.
► Lower nitrogen supply will reduce the invasive behaviour of species like G. maxima.