(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 (Amax, μmol CO2. m-2. s-1), leaf nitrogen content (Nm %, Na 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).