Photosynthesis, nitrogen allocation and specific leaf area in invasive Eupatorium adenophorum and native Eupatorium japonicum grown at different irradiances.

The mechanisms underlying biological invasions are still not well elucidated. In this study, ecophysiological traits of invasive Eupatorium adenophorum and native E. japonicum were compared at 10 irradiances in field. I hypothesized that the invader may allocate a higher fraction of leaf nitrogen (N) to photosynthesis and have higher light-saturated photosynthetic rate (P(max)) and specific leaf area (SLA) than E. japonicum. The invader had a significantly higher ability to acclimate to high irradiance than E. japonicum, while it showed a similar shade-tolerant ability. The invader indeed allocated a higher fraction of leaf N to photosynthesis than E. japonicum, which, with its high leaf N content (N(A)), resulted in a higher N content in photosynthesis (N(P)), contributing to its higher biochemical capacity for photosynthesis and P(max). However, the invader had a significantly lower SLA than E. japonicum, contributing to its higher P(max) but increasing its area-based leaf construction cost. The abilities to acclimate to a wider range of irradiance and to allocate a higher fraction of leaf N to photosynthesis, and the higher P(max), N(A), N(P) and leaf area ratio may contribute to the invasion of the invader. High SLA is not always necessary for invasive species.