Competition between Lythrum salicaria and a rare species: combining evidence from experiments and long-term monitoring

  • Published source details Denoth M. & Myers J.H. (2007) Competition between Lythrum salicaria and a rare species: combining evidence from experiments and long-term monitoring. Plant Ecology, 191, 153-161.


The rare endemic plant Henderson's checker-mallow Sidalcea hendersonii (Malvaceae), a tall-growing perennial, occurs in wet meadows and tidal marshes from southeastern Alaska to northwestern Oregon. Fewer than 100 scattered populations persist, and the species is considered vulnerable to extirpation across much of its range. A threat to its survival may be the presence of purple loosestrife Lythrum salicaria (native to Europe) which has become invasive in many of the localities where Sidalcea still occurs. The abundances of Lythrum and Sidalcea in a wetland in British Columbia (Canada) in 1999 was compared to those in 1979; Sidalcea frequency decreased by >50%, whilst Lythrum increased by almost 20%, however, there appeared no significant relationship between the changes of the two species. To help elucidate whether Lythrum is having a detrimental impact on this population of Sidalcea, a field experiment was undertaken to record the response of Sidalcea to Lythrum removal and also to that of native species, over a 2-year period.

Study area: Two large populations of Sidalcea hendersonii (>1,000 individuals) occur in southwestern British Columbia: one a tidal site on Vancouver Island, where Lythrum does not occur, and the other at Ladner Marsh (49º6'N, 123º5'W), a tidal wetland along the Fraser River, where this experiment was undertaken.

Experimental design: A total of 27 plots (1 m²; at least 5 m apart) were established that contained Lythrum and Sidalcea. To potentially maximize treatment effects, plots that contained a relatively high abundance of Lythrum (average 1,913 ± 238 cm shoot length/plot) were selected. One of three treatments were applied to the plots:

i) control (no manipulation);

ii) Lythrum removal - all Lythrum plants killed (leaves of shoots wiped with Round-up®, this kills the root system, thereby reducing both above- and below-ground competition);

iii) native-removal treatment - selected native plants, were killed (one shoot of a native plant wiped with Round-up® for every shoot of Lythrum present in the plot). To imitate Lythrum distribution, only shoots of native species (mostly herbs and grasses, and occasional Salix shrubs) adjacent to Lythrum shoots were treated.

Sidalcea performance: In July 2000, prior to treatment, the total shoot length of Sidalcea was recorded in each plot. In August 2001 and June of 2002, the following were recorded: total shoot length; average length of the inflorescences of the five tallest shoots; and total number of inflorescences. In 2002 at the end of the growing season, above-ground biomass of Lythrum in control and native-removal plots, and Sidalcea in all plots, was collected, dried and weighed.

A 95% reduction of Lythrum in 2001 (one year after treatment) and a 98.5% reduction in 2002 showed that it was effectively removed from the treated plots by wiping with Round-up. Lythrum did not recolonize over the duration of the experiment.

Removal of Lythrum significantly improved the vegetative performance (i.e. average shoot length) of Sidalcea (7.4 in) compared to the removal of selected native plants (7.2) and the control (7.1) treatments in 2001. In 2002, the performance of Sidealcea did not differ significantly with treatment (Lythrum removal 7.7; native plant removal 7.7; control 7). Removals did not influence the reproductive performance of Sidalcea in either year.

Conclusions and discussion: In North American wetlands, the non-native Lythrum salicaria can rapidly produce monospecific stands. This was not the case at Ladner Marsh where overall Lythrum cover was less than 10%, both in 1979 and 1999. This and the experimental results suggest that the competitive effect of Lythrum is not consistently stronger than that of native species. Other than producing a slight increase in Sidealcea average shoot length a year following removal, Sidealcea showed no other positive responses to its absence.

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