Individual study: Effects of salinity and cutting on the development of common reed Phragmites australis, Saitama University, Kanto Region, Japan
Asaeda T., Manatunge J., Fujino T. & Sovira D. (2003) Effects of salinity and cutting on the development of Phragmites australis. Wetlands Ecology and Management, 11, 127-140
Common reed Phragmites australis is a clonal wetland species that has spread dramatically in parts of the United States and Japan, often replacing other freshwater, brackish and salt marsh vegetation, and often forming near-monocultures. Control of the dramatic expansion of P.australis has become a necessity for several reasons including the feared loss of biodiversity in these wetlands. The literature and data available on P. australis in North America revealed that its eradication in freshwater sites increased plant diversity in all cases. However, the management goal of control rather than eradication should be emphasized because in many areas this species provides numerous benefits to the plant community and may be important to other wetland wildlife. Here, the combined effect of salinity and cutting on the performance of P.australis was studied and the results of this study were used to suggest how cutting and increased salinity can produce better management and control of P. australis stands where its encroachment is considered a problem.
Study site: The experimental study of P.australis control in response to salinity and cutting was carried out in four outdoor tanks (300 cm х 100 cm х 70 cm deep each) within the premises of Saitama University (25 km north of Tokyo), Kanto Region on Honshū island, in the central part of Japan (35º51'N and 139º39'E).
Methods: Shoot emergence and plant growth was observed under unmanipulated conditions with an abundant supply of freshwater but without nutrient addition during the first year (in 1999). Growth, flowering and senescence were observed at the end of the growing season and these initial observations served as reference for the measurements carried out after the treatments in the following year.
Experimental treatments were applied during the growing season in 2000 and consisted in the factorial combination of high vs. low salinity and cutting vs. no cutting. Salt (NaCl) was added to two of the tanks when the plant height was about 100 cm to raise the salinity (NaCl concentration) of the overlying water to approximately 30‰. The water level was maintained at 20 cm above sand surface in all four tanks.
Measurements: Measurements were taken weekly in each tank for numbers of primary and secondary shoots, living and dead leaves, leaves on each primary and secondary shoot, the shoot basal diameter, primary and secondary shoot height and total number of panicles. Width and length of all live leaves were measured for both primary and secondary shoots. Water samples were collected every two weeks from overlying water and 'groundwater' for measurements of salinity and for nutrient analysis. The aboveground and belowground parts of surviving plants were sampled from each tank and sorted into plant parts to determine dry biomass and water content.
The number of live shoots in the freshwater-treated stand increased almost constantly at the beginning, while the number of shoots gradually decreased after adding saltwater in the saltwater tank. The rate of increase in secondary shoot number for saltwater-treated plants was much lower than for freshwater-treated plants. The height of freshwater-treated plants increased until inflorescence, while, stressed by salinity, the height of the saltwater-treated plants increased more slowly, but growth continued longer than for freshwater-treated plants due to the absence or delay of inflorescence. Shoots started to die from week 24 in both freshwater and saltwater tanks, but almost all freshwater-treated shoots were dead three weeks earlier than saltwater-treated shoots. It was noted that salinity alone had no significant effect on the number of leaves, but cutting decreased the number of leaves on a shoot significantly. The effect of cutting was further increased by salinity. Fifty-five percent of plants in the freshwater treatment formed panicles but in the saltwater-treated uncut stand, only 6% of shoots formed panicles.
Conclusions: The results of this study have important implications on better management of reed stands where its control is necessary. This study showed that cutting combined with increased salinity may have better results. Apart from negatively affecting growth of P.australis, formation of secondary shoots and panicles was also severely retarded at high salinity levels. All the growth parameters observed showed that the plants are severely affected by the combined stresses of cutting and increased salinity.
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