Rubus persistence within silvicultural openings and its impact on regeneration: The influence of opening size and advance regeneration

We investigated the persistence and influence of Rubus spp. on tree regeneration in the Upper Peninsula of Michigan. This genus has been linked with delayed recruitment of trees following overstory removal in some regions but not others. Specifically, we examined the role of opening size and advance regeneration on subsequent regeneration dynamics 13 years post-harvest in 44 group-selection openings and at 15 reference sites, which had been treated with single-tree selection. We found that patches of Rubus were still a common feature in large group-selection openings 13 growing seasons post-harvest. Most of the patches observed appeared to have persisted since our initial survey (second growing season post-harvest), with approximately half of the plots containing Rubus in 2005 still containing it in 2016. Rubus cover was significantly associated with increasing opening size (P < 0.001). Successful recruitment of saplings (i.e. sapling abundance in 2016) was not negatively impacted by Rubus cover shortly after opening creation, but displayed a significantly positive relationship with the initial abundance seedlings (P < 0.001) suggesting that a lack of advance regeneration may be a stronger driver of regeneration failure or delay than competition from Rubus. Once established, Rubus patches may persist and resist re-invasion by trees for upwards of a decade, as evidenced by a negative relationship between 2016 seedling abundances and 2005 Rubus cover (P = 0.007). Consequently, while Rubus may delay subsequent re-colonization of trees, persistent patches of Rubus may be better conceptualized as a symptom rather than a driver of inadequate regeneration during the early stages of stand development. This differentiation may help reconcile regional differences in persistence of Rubus following overstory removal.