Delayed Effects of Water-Limiting Conditions Influence the Ramet Demography of a Native Iterocarpic Thistle

Increasing temperatures and shifting precipitation patterns are major components of climate change. Yet, the demographic responses of plants to such changes remain poorly known. We used 20 years of demographic monitoring data (1990-2009) on native Cirsium undulatum (wavyleaf thistle) from two Sandhills prairie sites in the central Great Plains, USA, to assess long- and short-term effects of weather variation on ramet dynamics. C. undulatum is a deeply taprooted, short-lived perennial plant that can vegetatively reproduce rosette-like ramets. Since field evidence over the full length of the 20 years documented the recruitment and fate of rosettes, we evaluated ramet vital rates and dynamics. We first estimated annual recruitment, survival, and between-stage transitions. These vital rates were used to develop a matrix population model to calculate annual asymptotic population growth rates (λ _t ). We then fitted a functional linear model to explore the effect of temperature, precipitation, and standardized precipitation evapotranspiration index (SPEI), an integrated measure of drought intensity, on ramet demographic rates. Finally, we developed a population viability analysis (PVA) model to evaluate the persistence of the ramet population under worst- and best-case scenarios of future drought events. The results revealed that at the drier site, Arapaho, wetter than normal years over the previous 19 months increased five parameters and decreased one. Seedling recruitment, flowering single rosette sprouts, and λ _t increased with positive SPEI; and single rosette survival and multiple rosette stasis increased with precipitation. Retrogression from multiple rosette to single rosette was reduced when precipitation increased. Additionally, higher-than-normal temperatures between 12- and 18-months period before the population census significantly increased the flowering probability of single rosettes. At the second site, Niobrara, only seedling recruitment was significantly increased by positive SPEI. The PVA indicated C. undulatum ramet populations will likely persist even with an increase in drought frequency.