Time-Dependent Competitive Displacement of Typha angustifolia by Lythrum salicaria

We conducted a four-year field competition experiment involving Lythrum salicaria and the North American wetland species that it occasionally displaces, Typha angustifolia. The design placed Lythrum and Typha in a modified replacement series experiment having four starting densities (64, 36, 16, and 1 plant(s) per quadrat) and four relative proportions of each species (i.e., 0.25, 0.50, 0.75, or 1, [for densities 64, 36 and 16 only]). Results of a repeated measures ANOVA indicate that overall rate of ramet production (ORP) differed significantly between species, between density treatments and between years. We also found evidence of significant intra-specific competition in both species. In the first year of the experiment, ORP in Typha was greater than that in Lythrum. However, from the second year onward, this situation was reversed and ORP in Typha was much lower than that in Lythrum. Analysis of covariance indicates a significant effect of species proportion on ORP, suggesting that both intra-, and inter-specific competition were occurring. Log-transformed ratios of input/output (i.e. proportions of Lythrum/Typha established at the beginning of the experiment, divided by the proportions found at the end of each growing season) demonstrated that in the first year Typha gained advantage in the mixtures, while by the second and third years the output ratio line crossed the 45° threshold, suggesting the two species could coexist. However, by the fourth year, the output ratio line was entirely above the 45° line, suggesting an overall advantage to Lythrum in the mixtures. The experiment clearly demonstrates temporal development in competitive behaviour, and suggests that Lythrum may take at least four years before becoming competitively dominant in a wetland community. The results also suggest that short-term experiments should be interpreted with care, if they are to be used to forecast the long-term outcome of competition in nature.