Flowering patterns related to insect herbivory and resource conditions in Platte thistle (Cirsium canescens Nutt)

Insect floral herbivores can have drastic effects on the population dynamics and densities of thistle populations (Cirsium sp.). However, little is known about how these plants might respond to loss of reproductive potential through damage, especially within the context of a varying environment. Thistles flower sequentially from top to base, meaning new flowers open at different times throughout the season. Therefore, one mechanism for plant response to damage might be to reshuffle resources among later flowers, in response to the fate of earlier flowers. To begin addressing this hypothesis, we used a 15-year dataset on Platte Thistle (Cirsium canescens Nutt.) to examine patterns of flower development throughout a growing season. Observations of flower state and fate taken over the growing season each year were used to construct a Markov transition matrix of probabilities reflecting the fate of individual flowers over time. This matrix was then coupled with other variables, such as insect damage, to identify parameters contributing to variation in flowering patterns. Preliminary results suggest the most common transition was to remain in the initial developmental (bud) state. However, beyond the early bud state, there was a greater chance of transitions being forward (transitioning to a later state) rather than static (remaining in the same state). Timing and location of insect damage, and flower position were key parameters influencing transitions. These insights into individual flowering patterns provide a first step toward examining strategies thistles use to navigate their complex ecological interactions.