White-tailed deer (Odocoileus virginianus) as facilitators of a European buckthorn (Rhamnus cathartica) invasion into western Minnesota forests

Background/Question/Methods In a prior study, we documented that invasive European buckthorn (Rhamnus cathartica) is the only tree species with substantial regeneration at five forested sites along the prairie-forest border in western Minnesota.  Here we examine the role white-tailed deer (Odocoileus virginianus) may play in facilitating the success of R. cathartica. Deer populations have grown dramatically in Minnesota recently, with DNR August surveyed populations rising 500% since 1974.  In the spring of 2008, we constructed 10 small (1.5 m x 2 m) deer exclosures across a light gradient in a forest near Herman, Minnesota.  Into every exclosure we transplanted 45 seedlings—fifteen each of R. cathartica and two dominant native species, bur oak (Quercus macrocarpa) and green ash (Fraxinus pennsylvanica).  Equivalent exposed control plots were established adjacent to each of the ten exclosures.  Since establishment, growth, survival, light, soil moisture and leaf chlorophyll content have been periodically monitored for each of the 900 transplanted seedlings.

Results/Conclusions Growth for all three species was significantly decreased when exposed to deer browsing, but species were not equally impacted.  Inside the deer exclosures, Q. macocarpa grew fastest and R. cathartica grew most slowly.  Outside the exclosures, the order was reversed with R. cathartica exhibiting the most rapid growth.  Increases in light, soil moisture, and leaf chlorophyll content corresponded with increased growth for all three species, but these variables impacted seedling growth much less than did browsing.  Similarly dramatic results are seen with survival for the two native species.  Two-year seedling survival inside the exclosures was 93% for both Q. macrocarpa and F. pennsylvanica, but only 48% and 55% respectively in the control plots.  R. cathartica had similar two-year survival inside (87%) and outside (84%) the exclosures.  As with growth, the species survival rankings were reversed by protection from browsing.  When protected, Q. macrocarpa fared best and R. cathartica the worst.  When unprotected, R. cathartica had the highest survival among the three species by a large margin.  Our results suggest that deer may play a substantial role in facilitating the current R. cathartica invasion into the forests of western Minnesota.