The Ups and Downs of Life on the Edge: The Influence of Water Level Fluctuations on Biomass Allocation in two Contrasting Aquatic Plants

摘要

Plant communities at wetland edges typically exhibit strong zonation along water depth gradients, although community structure is not as simple as this common observation may suggest. Water levels fluctuate at many temporal scales and in varied patterns. Understanding of how water regime controls plant community structure requires information on how individual species are affected by water regime. The allocation of biomass to roots, tillers, stolons and inflorescences was measured over 16 weeks in two species of aquatic plants with contrasting life histories. Juncus articulatus L. produces large numbers of small seeds while Glyceria australis C. E. Hubb produces small numbers of large seeds. Both species also reproduce vegetatively. Changes in biomass allocation were measured in response to duration, frequency and amplitude of water level fluctuations. The fluctuations in water level were achieved by suspending plants in pots from chains and raising and lowering the pots in and out of water in replicate tanks. There were complex responses to water regime components. As is common to many emergents, both species increased allocation to above ground vegetative structures in response to increasing water depth. Differences between the species in overall patterns of allocation reflected differences in life history. Juncus articulatus showed greater overall biomass production and greater proportional resource allocation to inflorescences than Glyceria australis. There were other differences in response to components of water regime that distinguished the species, mainly in allocation to reproduction in response to the range of depths (amplitude) and extended dry periods. These different functional responses to aspects of water level fluctuations indicate it may be possible to manipulate species composition if water levels are controllable. The most important characteristics of water regime were threshold events such as extremes of depth and dry periods. These may not be captured in measures of water regime such as average water levels.