Phenological differences between exotic and native shrub species in a Wisconsin forest

Background/Question/Methods

The propensity for exotic species to flush leaves early in the spring and to remain photosynthetically active late into the fall may contribute to their apparent competitive advantage over native species in temperate regions.   Mean annual air temperature and growing season length have increased in North America, possibly facilitating the invasion of exotic species that hold onto their leaves longer than natives. We explored differences in leaf phenology among three naturalized exotic shrubs (Lonicera tatarica L., Elaeagnus umbellate Thunb., Rhamnus cathartica L.) and three native shrubs (Rhus glabra L, Zanthoxylum americanum Mill., Cornus spp.) in a mixed hardwood forest in Southeastern Wisconsin. We predicted that exotic shrubs would senesce later and maintain green leaves longer in the fall and flush earlier in the spring in comparison to native shrubs. We randomly selected 10 individuals of each species and compared the change in foliage cover, leaf greenness, and leaf fall of each individual over a 12-week period.
Results/Conclusions

During the fall, all three exotic shrubs held their leaves between 3 and 5 weeks longer than all three native shrubs.  Also, the exotic shrubs maintained at least partial greenness up until leaf abscission, while native shrubs lost their greenness prior to abscission.  Greater leaf longevity of these exotic species allows them to benefit from a longer growing season and may partially explain their increasing abundance.