Intra-annual variations in leaf traits of Cynodon dactylon (L.) Pers. during exposure period in riparian zone of Three Gorges Reservoir Area

To study intra-annual variations in leaf traits of long-term submerged plants in the riparian zone of the Three Gorges Reservoir Area (TGA) during the dry period of the flood-dry-flood cycle, we investigated eight leaf functional traits (maximum net photosynthesis rate (A_area, A_mass), stomatal conductance (G_s), specific leaf area (SLA), leaf nitrogen concentration (N_area, N_mass), and leaf phosphorus concentration (P_area, P_mass)) of Cynodon dactylon ( L.) Pers. at four elevations (176, 170, 164, and 158 m) in June and August 2014. Results showed that: (1) Exposed period significantly contributed to variations in A_area, A_mass, G_s, SLA, and P_mass (more than 26% of total variation), similar to or even higher than submergence intensity. However, exposed days contributed little to variations in N_area, N_mass, and P_area (less than 15%). (2) Five bivariate relationships between leaf traits (A_area vs. G_s, A_mass vs. G_s, SLA vs. G_s, A_area vs. N_area, P_mass vs. N_mass) showed significant changes between two exposed periods. The intercept of A_mass vs. G_s changed significantly between June and August. (3) A_mass, N_mass, and P_mass showed no obvious trends in submergence intensity between June and August; A_area was significantly higher at 158 m than at 176 m regardless of month; SLA and P_area were significantly lower in the riparian zone (170, 164, and 158 m) than at 176 m; variation in N_area was in the order: 158 m > 164 m > 176 m > 170 m. These results suggest that annual exposed period can lead to significant variations in C. dactylon leaf traits, as well as the relationships between them. As N_area and P_area were stable traits (less affected by exposed days and submergence intensity), they may be candidate predictors for the structure and function of the TGA riparian ecosystem.