A comparison of photosynthetic characteristics between an invasive plant Spartina alterniflora and an indigenous plant Phragmites australis

As a consequence of rapid globalization, greatly increased emphasis is being placed on research into the ecology of invasive plants. Since Spartina alterniflora was introduced into Chongming Dongtan in 1995, it has gradually occupied a large area of the Phragmites australis zone, and has became a threat to the intertidal vegetation. . In this study, the photosynthetic characteristics of an invasive plant, Spartina alterniflora, and an indigenous plant, Phragmites australis, were measured during the growing season of 2004 by a Li-6400 Portable Photosynthesis (LI-COR inc, USA.) at Chongming Dongtan Nature Reserve, Shanghai, between 31°25′-31°38′N and 121°50′-122°05′E. The results showed that: (1) Compared with the local plant Phragmites australis, the invasive plant Spartina alterniflora had a significantly higher apparent quantum yield (AQY) carboxylation efficiency (CE) and maximum net photosynthetic rate (Pn_max); (2) In the early growing season, the curves of net photosynthetic rate (Pn), leaf stomatal conductance (Gs) and transpiration rate (Tr) of Spartina alterniflora during midday displayed a "single-peak" pattern, which was generally consistent with the changes in light intensity and temperature. In contrast, Phragmites australis showed a distinct "noon-break" phenomenon after reaching the peak, which was not consistent with the changes in light intensity and temperature; (3) The net photosynthetic rate (Pn), leaf stomatal conductance (Gs) and transpiration rate (Tr) of Spartina alterniflora were higher than those of Phragmites australis during the measurement period. In particular, Spartina alterniflora had a stronger photosynthetic capability above PAR 1 600 μmol/(m²·s); (4) These two plants had their own distinctive photosynthetic characteristics at different stages of the growing season. The average Pn for these two plants reached the maximum value in September, while the average Gs and Tr were the highest in May. In general, the photosynthetic parameters of Spartina alterniflora measured in May and September were significantly higher than those of Phragmites australis, while Phragmites australis exceeded Spartina alterniflora later in the growing season (November). The results from this study have important ecological implications in terms of the fast growth rate, high productivity and a strong competitive ability of the invasive plant Spartina alterniflora, which could provide a sound basis for formulating effective control measures on this species.