Population ecology of Cirsium pitcheri on Lake Huron sand dunes .2. Survivorship of plants

Cirsium pitcheri is a threatened plant species endemic to the shoreline sand dunes of the Greak Lakes. In this study, we attempted to identify some of the environmental factors that may limit the growth, survival, and reproduction of its populations. This species occurs as a minor associate among mixed populations of three dominant grass species, Ammophila breviligulata, Calamovilfa longifolia, and Andropogon scoparius in sandy soil of varying texture (fine, medium, coarse, or shingle). It occupies bare areas within stands of grasses on the first and second sand dune ridges and is prevalent in blowouts. The size class distribution based on the number of leaves per plant showed that the population was dominated by young plants (1-5 leaves) and the relative frequency of juveniles in each successive size class decreased from the smallest to the largest size class. Large juveniles were few in number, which suggests that there was high mortality during the juvenile phase. Seedling mortality in individual plots ranged from 11 to 90% in 1993 and 0 to 70% in 1994. Seed production of its populations was severely reduced because of consumption of whole capitula by white-tailed deer. Transition probabilities indicated that the probability of flowering was dependent on the size of the plant based on the longest leaf length.