Plant diversity under native and exotic forests: Implications for transitional forestry in Aotearoa New Zealand

Understanding native plant diversity in exotic forests is a globally important issue because exotic forestry plantations are expanding, and they may provide lower habitat value than native forests. Transitional forestry is an approach where exotic forests are transitioned to native forest which may help improve biodiversity outcomes in degraded areas. This study aimed to assess factors that influence plant diversity in native and exotic forests in Aotearoa New Zealand. It was predicted that native forest would support higher levels of total understorey plant diversity. To quantify understorey plant diversity, 18 plots under seven native and 11 exotic canopy species were surveyed at seven sites. To control for seed source availability, all exotic plots were within 300 m of native forest. The abundance and diversity of bryophytes and vascular plant species were sampled using quadrats within each 125 m x 60 m plot. From 2308 individual identifications, 254 species across 540 quadrats, within 18 plots were found, encompassing 132 vascular species and 122 bryophyte species. Vascular and bryophyte plant diversity were positively correlated. There was no difference in total species richness between the native and exotic canopy types, however there was consistently higher native diversity under both canopy types. Additionally, there was higher native plant diversity under native canopies than under exotic canopies, with no difference in exotic understorey species between canopy types. Three native vascular species were found to be more associated with native canopies than exotic canopies. The results indicate that in areas where native seed sources are available, native understory species can establish under exotic forest canopies. Given the increasing likelihood of exotic plantation abandonment in Aotearoa, the findings highlight the potential for traditional exotic forestry to improve native biodiversity, where there is a proximate native seed source. • Understory plant diversity was compared in native and exotic forests. • No difference in total plant diversity between forest types. • Higher native plant diversity under native forests. • Vascular and non-vascular diversity is correlated. • Highlights potential for transitional forestry in abandoned exotic plantations.