Climate warming accelerates the musk thistle (Carduus nutans) lifecycle

Background/Question/Methods

            Climate warming may lengthen the growing season for fall-germinating seedlings in two ways: 1) by advancing seeds’ mothers’ phenology, resulting in earlier seed release and germination; and/or 2) by extending the occurrence of temperatures suitable for growth later into the year. For monocarpic perennials, which commonly exhibit size-dependent survival and flowering, increased fall growth is expected to have dramatic effects on population dynamics, increasing overwinter survival and reducing the mean age at which individuals reach size thresholds for flowering. Together, these changes can accelerate the population growth of important agricultural weeds, including musk thistle (Carduus nutans), an invasive pasture pest across much of North America. We conducted a field experiment to assess how temperature (with two levels: ambient and warmed via fiberglass open top chamber) and sowing time (with seven levels ranging from August 2 to October 20) affect vital rates (including growth, survival and fecundity) of C. nutans. Data from two cohorts were used to inform an integral projection model for the population growth of this weed.

Results/Conclusions

            We observed significantly larger end-of-season size for C. nutans individuals planted earlier in the year. Germination rate did not vary significantly over the tested sowing dates. The proportion of individuals completing their lifecycle as annuals varied substantially between sowing dates: for seeds planted early in August, a one week advance in sowing date increased the proportion of individuals flowering as annuals by 25.8%. Individuals grown in fiber glass open top chambers were larger than those grown under ambient conditions, but this difference was not statistically significant. Warming also had no significant effect on seedlings’ overwinter survival. Warmed individuals did, however, produce significantly more capitula than individuals grown under ambient conditions. Taken together, larger size as a result of earlier sowing and increased fecundity due to warming are projected by our model to result in significantly more rapid local population growth for C. nutans under climate warming. Passage time from germination to flowering is also reduced. Additionally, the model suggests that warming will change the sensitivity of C. nutans’ population growth rate to management actions, highlighting targets for control of this costly weed in future climates.