Growth and mortality of adult invasive Rhamnus cathartica (European buckthorn) in west central Minnesota

Background/Question/Methods Previous research reveals a paucity of recruitment for native tree species at several forest sites along the prairie-forest ecotone in western Minnesota.  Recruitment of invasive Rhamnus cathartica (European buckthorn), on the other hand, is quite robust.  Recently we reported preliminary results for an ongoing seedling transplant experiment which suggest that while R. cathartica is less sensitive to deer browsing than native species in our ecotonal forests, growth of R. cathartica seedlings and saplings is highly sensitive to summer drought.  Here we extend that work into larger size classes via an extensive survey of R. cathartica population distribution and size structure at one site and tree ring analysis of naturally established R. cathartica at three sites along a climate gradient.  We sacrificed naturally recruited adult R. cathartica individuals in order to establish, via tree ring analysis, both the relationship between climate and growth and the relationship between growth and mortality.  This work is part of a larger study in which we seek insight into the impact climate change may have on tree populations at the northern U.S. prairie-forest border.

Results/Conclusions R. cathartica growth is strongly correlated with summer drought (r = 0.6 with July PDSI), despite little canopy exposure.  Severe drought in the mid-1970s and late-1980s are notably reflected in reduced ring widths.  Understory trees are usually avoided in tree-ring climate studies because of dampened climate response, but adult R. cathartica showed more drought sensitivity than either canopy Quercus macropcarpa or Acer saccharum for our study sites.  Size distributions for established R. cathartica populations at our extensively studied site, along with counts of standing dead individuals, suggest declining risk of mortality with increasing DBH.  We note an interesting pattern in the relationship between growth and mortality: dead R. cathartica individuals > 5 cm DBH exhibited a strong growth decline prior to death lasting 7-8 years and corresponding to > 60% reduced ring width compared to surviving R. cathartica, but dead individuals <5 cm DBH exhibited a much less pronounced growth decline prior to death and actually grew substantially faster in their first 10 years of growth (>50%) than survivors. Thus, fast growing small R. cathartica are seemingly at a higher risk of mortality, but the pattern reverses for those individuals reaching higher size classes.