Effect of soil surface topography and litter cover on the germination, survival, and growth of musk thistle (Carduus nutans)

In natural populations of musk thistle (Carduus nutans L.) germination occurs over several months in the fall and spring. To better understand the physical environmental mechanisms that influence this pattern of germination, a greenhouse experiment was established consisting of ten treatments in which soil surface topography and the amount and consistency of litter cover was varied. Significant differences occurred among the treatments for germination percentage, levels of mortality and individual growth rates. Optimum levels ofgermination, survival and growth occurred in treatments that provided microhabitats with reduced evaporation. Treatments with cracks or moderately irregular surface topographies provided the best conditions. A light covering of litter benefited establishment and growth but thick layers of litter prevented seeds from reaching the soil surface and reduced germination and establishment. These results were consistent with what is known about the germination ecology of musk thistle in natural populations. IN MANY PLANT species the highest levels of mortality occur between the seed dispersal and the seedling establishment phases of the life cycle (Harper and White, 1974; Harper, 1977). Unfortunately, this is often the most poorly understood portion of the life cycle. Usual monitoring procedures are capable of quantifying seed production and seedling establishment but are generally incapable of describing interactions between seeds and the soil surface environment. Yet many biologists (e.g., Grubb, 1977; Grime, 1979) feel that these interactions are critical to the reproduction of populations and the composition of plant communities. Successful germination involves complex interactions between the seed's physical position, its physiological state, and its microenvironment. Some species have genetically determined, innate dormancy requirements (Harper, 1977) that prevent germination until after specific environmental keys have been experienced (e.g., Betula pubescens, Black and I Received for publication 13 February 1986; revision accepted 19 August 1986. We wish to express our gratitude to Karen J. Hamrick and Jenna R. Hamrick for their technical assistance. Steve Stewart provided valuable assistance during the data analyses. The comments of two anonymous reviewers greatly improved this paper. This work was supported by funds supplied by University of Kansas General Research Fund Grant #3675 and Environmental Protection Agency Grant R-805455030. 2 Current address: Departments of Botany and Genetics, University of Georgia, Athens, GA 30602. 3Current address: Department of Botany and Plant Sciences, University of Califomia, Riverside, CA 92521. Wareing, 1955; Avenafatua, Marshall and Jain, 1970; Naylor and Jana 1976; Ambrosia artemisifolia, Willemsen 1975a, b; Avena barbata, Marshall and Jain, 1970). Other species have less rigid dormancy (enforced, Harper, 1977) and can germinate whenever minimum environmental conditions are met (usually heat, light, moisture, and proper aeration). Adverse conditions such as drought, cold, or deep burial in soil or litter prevent or retard germination in these species (Werner 1975; Harper, 1977). Several investigators (Harper and Sager, 1953; Harper, Williams and Sager, 1965; Sheldon, 1974) have demonstrated that subtle features of the soil surface topography and the position of the seed in relation to this topography can have dramatic effects on germination and seedling establishment. Studies of the population ecology of musk thistle (Carduus nutans L.) indicate that germination occurs over periods of several months in both fall and spring (Lee and Hamrick, 1983). Subsequent survival, growth and fecundity of monitored individuals was influenced by the their time of germination. Generally, individuals germinating in the early fall had lower mortality, attained larger vegetative sizes and produced more seeds than those that germinated later in the fall or in the spring. Later studies (J. L. Hamrick, unpubl. data) indicate, however, that germination early in the fall can be followed by a period of high mortality if a period of drought immediately follows germination. It is clear, therefore, that the dynamics of musk thistle populations may be