Arrested development? Investigating the role of bamboo in Araucaria Forest succession in Southern Brazil

Aims Assessing the role of a dominant native bamboo species on tree species diversity and structure in the medium term. Methods Over a 7-year period, we studied the natural regeneration of two dominant forest types in Southern Brazil (Araucaria Forest or AF; Bamboo Forest or BF) after a bamboo (Merostachys skvortzovii Send.) die-off between 2004 and 2006. The study was carried out in the Embrapa Research Station in Caçador, Santa Catarina State, Brazil. Important Findings The die-off provided ideal conditions for the establishment of several species and it kickstarted forest succession dynamics, which in turn affected regeneration diversity. Tree species richness was relatively stable with a transitory increase between 2007 and 2014 in both AF and BF. However, species richness rose in BF because of a relative increase in abundance of some species (especially late and secondary species) while a plunge in some pioneer species drove an increase in diversity. Overall, we found that BF has a lower diversity of recruits and that density declined over time, while AF is more diverse, with a more stable density. In BF, the bamboo die-off created optimal conditions for initial regeneration development (mainly fast-growing pioneer trees), which quickly transited to higher size classes. Yet, after this initial stage of pioneer recruitment, the number of recruits dropped followed by a virtual absence of growth regardless of the species group as a result of a quick bamboo reestablishment. As bamboo recreated a dense understory it reduced species diversity to original levels, suggesting a self-maintaining cycle that halts forest succession. On the other hand, the bamboo die-off had little impact on AF where a slow recruitment process typical of old-growth forests was observed. The results indicate that the die-off event had a temporary effect on species diversity i.e. restricted to forests where bamboos are dominant in a similar process described in other southern South American forests. As the first study to observe the medium-term forest dynamics related to bamboo die-off, we can conclude that when being dominant, native bamboos can hinder forest regeneration, maintaining lower levels of diversity and arresting forest succession that lasts well beyond the short-term, post-die-off effects. Many forest fragments in the region are dominated by bamboos, thus their potential for conservation is at risk and requires appropriate management.