Abstract Detail


Mohl, Emily [1], McCall, Andrew [2], Scanga, Sara [3], Saunders, Patricia [4], Garneau, Danielle [5], Nuzzo, Jacqueline [6].

Geographic variation in growth and compensation in common milkweed, Asclepias syriaca.

Plants defend themselves by resisting, avoiding, or tolerating damage by herbivores.  The geographic distribution of defense traits may be explained either by top-down factors, including the risk of herbivory, or bottom-up factors, such as resource availability. Typically, high levels of resistance (i.e., traits that reduce damage) are expected in geographic regions with a high risk of herbivory; however, bottom-up factors also affect plant growth traits and tolerance of herbivory (i.e., ability to recover after damage). Specifically, plants from resource-rich environments are thought to grow faster and have higher tolerance. Previous research (Woods et al. 2012) demonstrated a surprising cline in common milkweed plants: northern populations were more resistant to herbivores even though herbivory rates were highest in the center of the range. They hypothesized that slower-growing northern populations have lower tolerance to herbivory and consequently have evolved resistance. We investigated part of this hypothesis in a greenhouse experiment that used genotypes from 14 populations spanning across the distribution range of common milkweed. Although we did not measure fitness, we tested the hypothesis that variation in compensatory growth after damage (a component of tolerance) could be explained by latitude, resource availability (a composite measure of temperature, precipitation, and soil cation exchange capacity), and distance from range center (a proxy for herbivory). We found no clear predictors of variation in compensatory growth but weak evidence consistent with reduced compensation in northern latitudes.  Two measures of plant size showed a quadratic relationship with latitude of origin.  Furthermore, resource availability at a plant’s site of origin was positively associated with relative investment in stems, but surprisingly negatively correlated with plant size.  These patterns contradict the idea that plants from resource-rich environments should grow faster and thus better tolerate damage. Distance from range center did not predict any growth traits.  Our results suggest that if southern plants are more tolerant of herbivory, it may be because they store resources in stems that can be reallocated after damage, or because they have a longer growing season with more time to recover from attack. Alternatively, the distribution of plant resistance traits may relate less to the distribution of growth and compensation and more to the distribution of generalist and specialist herbivores that select for different types of tolerance and resistance across space.

1 - St. Olaf College, Biology, 1520 St. Olaf Ave, Northfield, MN, 55057, United States
2 - Denison University, 100 West College St, Granville, OH, 43023, United States
3 - Utica College, Department of Biology, 1600 Burrstone Road, Utica, NY, 13502, USA
4 - Ashland University , Biology and Environmental Science, 401 College Ave., Ashland, OH, 44805, USA
5 - SUNY Plattsburgh, Center for Earth and Environmental Science, 101 Broad St, Plattsburgh, NY, 12901, USA
6 - Simplicity Works, 615 W Lake Street, Minneapolis, MN, 55408, USA

Plant Defense
Asclepias syriaca
geographic cline.

Presentation Type: Oral Paper
Number: ECO5003
Abstract ID:828
Candidate for Awards:None

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