COS 14-5 - Plant migration along freeways: Results from seed bank and seed trapping studies in Phoenix, Arizona

Monday, August 8, 2011: 2:50 PM
18C, Austin Convention Center
Kristin J. Gade, School of Life Sciences, Arizona State University, Phoenix, AZ

General ecological thought pertaining to plant biology, conservation, and urban areas has rested on two potentially contradictory underlying assumptions.  The first is that non-native plants can spread easily from human developments to “pristine” areas.  The second is that native plants cannot disperse through developed area.  Both assume anthropogenic changes to ecosystems create conditions that favor non-native plants and hinder native species.  However, it is just as likely that anthropogenic alterations of habitats will favor certain groups of plant species with similar functional traits, whether native or not.

The function of corridors as conduit for plant movement has long been suggested, but the actual mechanisms at work in the process have only recently begun to be studies.  Functional traits of species determine which are the most successful at each of the stages of invasion or range enlargement.  I studied the traits that allow both native and non-native plant species to disperse into freeway corridors, germinate, establish, reproduce, and then disperse along those corridors in Phoenix, Arizona.  Sampling sites were selected along freeways throughout the Phoenix metro area, on both gravel-landscaped and non-landscaped road verges.  Field methods included seed bank sample collection and germination, vegetation surveys, and seed trapping. 


While many plant species found on the freeway verges are either landscape varieties or typical weedy species, some uncommon native species and unexpected non-native species were also encountered.  The seed trapping effort resulted in more seeds trapped at the gravel-landscaped sites, which had greater average daily traffic loads than the non-landscaped sites.  However, the seed bank germination study showed the opposite pattern, with higher numbers of seeds germinating in the samples from the non-landscaped sites than the gravel-landscaped sites.  During the spring and summer seed trapping periods, the majority of the seeds recovered in traps at the gravel-landscaped sites and the non-landscaped sites adjacent to low density development had adaptations for wind dispersal; the proportion was much smaller at the non-landscaped sites adjacent to Sonoran desert habitat.  The seed trapping data confirm that wind plays a large role in seed dispersal along the highways in developed areas.  Wildlife do not appear to use the freeway verges near the urbanized area to a significant degree, but seed dispersal syndrome proportions at the desert sites suggest that animals are playing a role in determining the plant community composition there.

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