Competition and Facilitation in Three Marsh Plants in Response to a Water-Level Gradient

Abstract

Biomass accumulation and neighbor effects (measured with a relative neighbor effect index, NEI) were investigated in three marsh plant species (Carex lasiocarpa, Glyceria spiculosa, and Deyeuxia angustifolia) grown at three water levels (0, 20, and 40 cm relative to the soil surface). The three species occur naturally along a water-level gradient in the Sanjiang Plain, the largest freshwater marsh in China. Both intraspecific and interspecific competition were measured using a target-neighbor design, which subjected the three species to different intensities of inter- and intra-specific competition. Biomass accumulation and NEI differences among water levels and competition treatments suggested that intra- and inter-specific competition was strongest at 0 cm inundation. In contrast, neighboring plants stimulated the growth of C. lasiocarpa and D. angustifolia at the 40 cm water level. These results indicate that the strength of intra- and inter-specific competition decreases and the strength of facilitation increases with increasing water levels. Our results support the prediction of the stress-gradient hypothesis (SGH) that plant-plant interactions should switch from competition to facilitation along an increasing water-level stress gradient.