Is photosynthetic performance of a common plant invader linked to leaf nitrogen allocation patterns? A test of the pre-adaptation hypothesis

Robert J. Griffin-Nolan, Department of Biology, Syracuse University, Syracuse, NY, Julie LeVonne, Department of Biology, Syracuse University, Thomas Kichey, Université de Picardie Jules Verne, Masako Mishio, Tohoku University, Kouki Hikosaka, Graduate School of Life Sciences, Tohoku University, Sendai, Japan, Guillaume Decocq, University of Picardie Jules Verne, Amiens, France and Jason Fridley, Biology, Syracuse University, Syracuse, NY

Background/Question/Methods:
Non-native locally dominant (i.e., invasive) species are a major cause of biodiversity loss and alter the structure and functioning of ecosystems worldwide. The competitive advantage of invasive plant species is in part due to their higher photosynthetic rates compared to co-occurring native plants. This physiological advantage is potentially driven by greater allocation of leaf nitrogen (N) to photosynthesis rather than structure and/or defense. However, it remains unresolved whether invasive species are pre-adapted with such physiological advantages in their native range or if such characteristics evolved de novo in the invaded range. To determine home vs. away range differences in invasive plant physiology, we measured photosynthetic performance of populations of Fallopia japonica (Japanese knotweed) in its native range of Japan and invasive ranges of France and eastern North America. Gas exchange measurements were paired with extractions of leaf fiber and proteins associated with light capture (chlorophyll), CO2 fixation (Rubisco), defense against herbivory (secondary metabolites), and structural support (cell wall proteins). The global scale of this project allowed us to test hypotheses related to pre-adaptation, if home and away populations exhibit comparable physiology, or evolution of increased competitive ability (EICA), if invaders shift allocation patterns in their invaded range.
Results/Conclusions:
We characterized the leaf N allocation strategies of native and invasive populations of F. japonica. Using principle component analysis, we identified two primary axes of trait variation that explained regional differences/similarities in physiology. We observed no differences between invasive and native populations along a primary trait axis related to fiber, specific leaf area, and total protein content, which supports the hypothesis that this species is pre-adapted from its native range. However, we observed clear separation between invasive and native populations along a second axis of variation that was associated with chlorophyll content and photosynthetic performance. While a portion of trait variation (17%) along this axis can be explained by site characteristics (e.g., elevation and soil pH), our findings provide support for the EICA hypothesis. Specifically, invasive populations of F. japonica in France and North America had higher photosynthetic rates and chlorophyll content than native populations in Japan. The apparent shift of N allocation towards greater light acquisition in the away range was surprising given that F. japonica is a high-light adapted species and photosynthesis is more likely carboxylation limited. Our results suggest certain traits can be pre-adapted while others can shift in the invaded range with consequences for physiology.