Erigeron decumbens spp. decumbens (Willamette daisy): Population monitoring and evaluation of mowing and burning at Oxbow West (West Eugene Wetlands)
There are three main objectives of this project. The first is to determine yearly population size and reproduction of the endangered Willamette daisy, E. decumbens ssp. decumbens at Oxbow West. The
There are three main objectives of this project. The first is to determine yearly population size and reproduction of the endangered Willamette daisy, E. decumbens ssp. decumbens at Oxbow West. The second objective is to determine if there are long-term trends in population size and reproduction. The final objective of this project is to assess the effects of mowing and prescribed burning on E. decumbens ssp. decumbens.
Growth and reproduction
In 2007 there were a total of 3,944 E. decumbens ssp. decumbens plants recorded within the monitoring plots at Oxbow West. This is the highest number recorded and almost twice the number of plants compared to when monitoring began in 1999. The proportion of reproductive plants in the population has been relatively stable, at 83% of the total population.
Average plant height and average crown cover remain relatively low compared to the initial years of monitoring at this site. There was a strong negative correlation between measurements of plant size and the number of plants. Similar to average height and cover, the average number of capitula per plant has decreased as the population has increased.
The effects of mowing and burning
In 2006, we reported that average elliptical crown cover of E. decumbens ssp. decumbens in the burned plots was marginally greater than that in the mowed plots and that there was no effect of treatment on any of the other variables. Similarly, there was little effect of mowing measured the year following either the first or second mowing event.
In 2007 there was a strong trend for there to be a fewer plants in plots that were mowed and burned compared to control plots. Due to the negative correlation between size and the number of plants, the burned and mowed plots tended to have less of a reduction in average crown cover. There was no effect of treatment on average height. Finally, there tended to be a negative effect of mowing on the average number of capitula, while there was almost no change in the average number of capitula in the control and burned plots. Mowing every-other-year appears to be having mild negative effects on E. decumbens ssp. decumbens.
We recommend altering the mowing treatment to a four year interval. We also recommend that there be no further treatments to the burn plots for at least another two years. Burning can stimulate the seed bank of some species and this type of effect would take several years to become apparent. In order to determine if this is occurring, future analyses should look for an increased proportion of vegetative plants in the burn plots relative to the mowed and control plots.