Overcompensation by Pitcher's thistle (Cirsium pitcheri) in response to damage.

Abstract

There has been evidence that plants exhibit overcompensation after damage through trampling or herbivory. Cirsium pitcheri is an endangered thistle found in the sandy dunes of northern Michigan. The purpose of our experiment was to determine if this monocarpic plant exhibits overcompensation by observing pollination and fitness measures. We recorded pollinator activity, head count, and seed output for both multi-stemmed plants, which are a result of damage, and single-stemmed plants at Sturgeon Bay in Emmet County, Michigan. We found that multi-stemmed plants have a higher number of pollinators as well as longer pollination time than single-stemmed plants. We also found that multi-stemmed plants produced about twice as many heads than single-stemmed. Lastly, we determined that although both groups of C. pitcheri contain about the same number of filled seeds per head, multi-stemmed plants produce much more seeds per plant, which is evidence for overcompensation. We believe that C. pitcheri responds positively to damage by exhibiting higher pollination activity and a greater seed output.