Effect of minor water depth treatments on competitive effect and response of eight wetland plants

Two facets of plant competition, competitive effect (CE) and competitive response (CR), can be used to explain plant community composition but our understanding of abiotic factors that may differentially affect species’ competitive ability is incomplete. We tested whether water-depth affected CE (ability to suppress neighbour) and CR (avoid suppression from neighbour), and if so whether there was consistence in the rank order of both measures of competition under different water depth treatments. CE and CR were measured and compared for eight wetland plant species (Carex lurida, Carex tribuloides, Elymus virginicus, Juncus tenuis, Lythrum salicaria, Phalaris arundinacea, Rumex orbiculatus and Verbesina alternifolia) at five different water-depth treatments (+2, 0, −2, −4 and −6 cm relative to the substrate). Overall, we found that mean CE was at its lowest value at +2 cm water depth, while mean CR was highest at +2 and −6 cm compared to the other water treatments. There was a significant variation of CE between species, with a defined hierarchical order. Pairwise CE rank order correlations between water depth treatments were significant but CR correlations were generally not. There was no significant correlation between CE and CR. CE was significantly correlated with biomass of species grown alone but CR was not. These findings indicate that CE may be used as a general measure to predict wetland species performance, and thus community assemblage, across a range of water depths. CR does not seem to demonstrate predicable patterns between species and water depth treatments. Our results suggest that competition intensity may be reduced in a non-resource-stressed flooded environment by a reduction in CE, but the corresponding increase in CR could dampen this effect on overall competitive ability.