Population biology of a rare hemiparasite, Aureolaria patula (Chapman) Pennell (Scrophulariaceae): Demographic trends, host interactions, and reproductive biology

Aureolaria patula, spreading false-foxglove, is a rare, herbaceous, perennial hemiparasite restricted to calcareous slopes and bluffs along waterways, primarily along the Tennessee River and its tributaries. Root hemiparasites attach to the roots of hosts to access water and solutes in the vascular system of the host. Demographic structure and processes of four populations of A. patula on public lands in east Tennessee were evaluated using a stage-based matrix projection model. Plants have the ability to be dormant for a growing season, then return the following year. Probability of dormancy and mortality decreased with increasing plant size. Reproductive effort increased with increasing plant size. Dormant stages were incorporated in the population model and were shown to be important influences on population growth rate (λ) at some sites. Population growth rates varied both spatially and temporally and ranged from 0.35 to 1.6. The relative importance of fecundity, growth, and survival in the dynamics of the populations varied by site and year.;Greenhouse experiments on host usage showed host tree had a significant effect on biomass of A. patula and A. virginica, a common relative. Aureolaria patula grew larger than A. virginica on all hosts and grew as well as A. virginica without a host in nutrient-poor soil. Number of haustoria was positively correlated with shoot biomass of the hemiparasites. Survival of hemiparasite seedlings was significantly affected by host species, and growth of both hemiparasites was significantly suppressed by some hosts. Both hemiparasites grew larger and formed more haustoria on the host when fertilized.;Aureolaria patula was found to be self-incompatible. Seedlings from open-pollinated flowers grew significantly larger in a greenhouse than seedlings from flowers hand-outcrossed with plants 30-m distant. There was no evidence of inbreeding depression in seedling growth in the small population studied.;Aureolaria patula can be viewed as having a more opportimistic life history strategy than A. virginica, which has the strategy of a generalist. The conditions under which A. patula thrives include plant communities with a high density of the most beneficial hosts, lack of herbaceous competitors that block light, proximity to a river or stream, and calcareous substrate.