DECH, JEFFERY P.1* and PETER NOSKO2. 1Department of Plant Sciences, University of Western Ontario, London, ON, N6A 5B7, CANADA; 2Biology Department, Nipissing University, North Bay, ON, P1B 8L7, CANADA. - Phenotypic plasticity in Lythrum salicaria L. along a competition gradient in central Ontario wetlands.
Purple loosestrife (Lythrum salicaria L.) is an invasive
species of North American wetlands that competes vigourously with
native plants. Rapid growth, prolific reproduction, and high
phenotypic plasticity have all been suggested as factors contributing
to the competitive ability of this species. The relative importance of
these traits may depend on the nature of the competitive interaction
to which L. salicaria is exposed. The purpose of this study was
to determine the response of L. salicaria to a natural
competition gradient with cattails (Typha spp.) in central
Ontario wetlands. At three different sites, four plots (1x1 m) were
established along five transects extending from the peripheral to the
core habitat of the wetland. Within plots, we randomly selected up to
five L. salicaria plants and determined their growth rate,
height, and inflorescence production (number of inflorescences and
their total length). We also harvested plants from these plots to
determine total aboveground biomass. Gravimetric soil moisture,
Typha density, and L. salicaria density were determined
in each plot to quantify changes along the competition gradient from
the peripheral to the core habitat. Canonical correlation analysis of
height and inflorescence production data in L. salicaria,
showed a strong correlation for all three sites(canonical coefficient
> 0.6, p < 0.01) bewteen growth form and position on the competition
gradient. Plants at the peripheral end of the gradient were shorter,
with abundant lateral growth (more inflorescences),whereas those at
the core end of the gradient were taller, with much less lateral
growth (fewer inflorescences). Despite these differences, the total
production (length) of inflorescences (an indirect measure
reproductive potential) was not related to position on the gradient.
Furthermore, simple linear regression analyses indicated that growth
rates and total biomass showed non-significant or weak relationships
to this gradient. Under the conditions we studied, L. salicaria
responds to competition with Typha spp., by altering its growth
form in a trade-off between vertical and lateral growth, that does not
directly influence total growth or reproduction. This suggests that
phenotypic plasticity may play an important role in the competitive
ability of this species.
Key words: competition, invasive species, Lythrum salicaria, phenotypic plasticity, Typha spp., wetlands