Field and Lab Experiments Examining the Effects of Elevated Soil Temperature on the Growth of Cirsium arvense (Canada Thistle)

Canada thistle (Cirsium arvense) is an invasive species of plant from Europe that reproduces using both seeds and rhizomes. This species spreads easily into disturbed patches in riparian forests and may alter ecosystem structure and function. Increased temperatures resulting from global climate change may increase the growth and reproduction of Canada thistle and other invasive plants, which could increase colonization rates in disturbed environments. Using lab- and field-based approaches, we examined if elevated soil temperature affected plant height and plant biomass of Canada thistle. Laboratory and field studies both examined individual Canada thistle plants growing from rhizomes in one gallon pots. Initial rhizome biomass and aboveground biomass of the plant the rhizome originated from were measured for use as covariables. Plants used for the lab-based experiment were grown in the greenhouse under ambient or warmed soil conditions. Soil temperature was increased about 5 degrees Celsius using soil warming mats. Plants used for the field-based experiment were grown outside at the Waterdale Environmental Education Center. Sixteen plants were randomly assigned to one of three treatments (48 plants total). Four pots were grouped together and covered with 1) black plastic sheeting (to increase temperature), 2) white plastic sheeting (to mimic ambient conditions), and 3) no plastic sheeting (a control to determine the effect of the plastic). The soil temperature of the pots were monitored during both experiments using soil temperature loggers, and air temperature was monitored during the field experiment. We measured plant mortality, plant above ground biomass, and plant height for the lab experiment and plant mortality, insect damage, above ground biomass, and belowground biomass for the field experiment. The lab experiment resulted in no effect of the treatment on aboveground biomass, but we did find a significant difference in plant height by the end of the study. Mortality, insect damage, and plant biomass varied among treatments for the field experiment, but did not represent a clear effect of temperature. The plastic sheeting, however, demonstrated an effect on plant growth. We also report on our secondary goal of developing a cost effective model system for examining the effects of increased soil temperature on plant growth