Demographic changes in Arisaema triphyllum populations caused by epidemics of a systemic rust pathogen

Arisaema triphyllum is a sequentially hermaphroditic perennial herb that is common to woodlands in Michigan. Most populations contain plants infected by the systemic rust pathogen, Uromyces ari-triphylli. The average disease incidence for populations in Michigan is 28%. Infected plants have increased rates of mortality and adult plant dormancy, and reduced rates of growth. Lefkovitch matrices were utilized to characterize population dynamics of A. triphyllum at six sites in Michigan. We also modeled hypothetical populations that were disease-free and completely diseased. Disease-free populations were projected to have a population growth rate (λ) of 0.97, while λ for a completely diseased population was projected to be equal to 0.65.  Projected stage structure for the diseased population was skewed towards plants of intermediate size relative to that of the disease-free population. Observed population dynamics were intermediate to that of the disease-free and diseased populations with an average observed λ = 0.91. Infected plants were more likely to flower than healthy plants, and were more likely to flower as females.  However, infected female plants only rarely set seed. Since A. triphyllum is found largely in small isolated woodlots, U. ari-triphylli epidemics alter plant population dynamics in a manner that may increase the likelihood of local extinction of the plant. Infected plants may be flowering in a maladaptive manner, since flowering as a male across a broader size range would maximize reproductive effort once a plant is diseased.