Phenological Shifts and Photosynthetic Trade-Offs in Phragmites australis Under Experimental Warming: A Seasonal Perspective

Although climate warming affects photosynthetic carbon sequestration in coastal wetland plants, the seasonality of this effect has not been assessed. We investigated the growth traits and photosynthetic properties of Phragmites australis by using open-top chambers (OTCs) to conduct a warming experiment in the coastal wetlands of the Yellow River Delta during a single growing season. The OTCs significantly elevated temperatures by ~1 °C across the growing season, and the effects of warming on stem diameter, net photosynthetic rate (Pn), and water use efficiency (WUE) were characterized by a significant month × warming interaction. Early-season carboxylation efficiency (φ) increased by 71%, but a significant late-season decline of Pn by 49% accompanied by a rise of intercellular CO2 concentrations (Ci) and decline of stomatal limitation (Ls) led to a seasonal shift from stomatal to non-stomatal (biochemical) limitation of growth. A consistent increase in plant height and Ci across all months and concomitant decrease in Ls indicated that the additive effects of warming were independent of phenological stage. The results revealed that the phenological mediation of warming responses is trait specific. Carbon cycle models should therefore adopt trait-specific parameterizations to accurately project the impact of the wetland carbon sink under future warming.