Periodic wet years and deer herbivory are tied to success of invasive Rhamnus cathartica (European buckthorn) in western Minnesota

Background/Question/Methods Previous research revealed a paucity of recruitment of native tree species at several forest sites along the prairie-forest ecotone in western Minnesota.  Seedlings of invasive Rhamnus cathartica (European buckthorn), on the other hand, are quite abundant.  To determine the role that deer herbivory may play in the relative success of R. cathartica, we established a seedling transplant experiment composed of ten pairs of control and protected plots across a light gradient in an ecotonal forest in west-central Minnesota.  Into each plot we transplanted fifteen seedlings each of R. cathartica and two regionally dominant native species: Fraxinus pennsylvanica (green ash) and Quercus macrocarpa(bur oak).  Protection from herbivory are provided by a series of small (1.5m x 2m) wire-mesh exclosures.  We have monitored growth and mortality of the transplanted seedlings for five growing seasons.  We have also periodically measured light and soil moisture available to each experimental seedling.

Results/Conclusions When exposed to deer herbivory, seedlings of R. cathartica consistently outperformed the two native species in both growth and survival.  Inside protective exclosures, however, survival for all species has been high and the native species outgrew the invasive R. cathartica for the first two growing seasons of our experiment.  Then, in the third and fourth seasons—both abnormally wet years (avg PDSI = 3.5)-- the rank order of growth changed dramatically.  Over that two-season period, R. cathartica outgrew the two native species by a 6:1 ratio.  Finally, in our fifth season (2012) a drought struck (avg PDSI = -2.4) and, once again the two native species substantially outgrew the invasive. Over the five seasons of our experiment, R. cathartica growth is tied closely to drought (R² = 0.93), and AIC-based mixed-model selection suggests that species, protection from herbivory, soil moisture, and light all substantially influence growth and survival.   Our results indicate that R. cathartica invasion at the prairie-forest ecotone is aided by both deer herbivory and by bursts of high growth during wet years.  Our results also show why it is important that seedling studies last for several years and include a range of climatic conditions.