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