Fast-growing hybrids do not decrease understorey plant diversity compared to naturally regenerated forests and native plantations

Plantations of fast-growing hybrid trees, such as hybrid poplars and hybrid larch, are increasingly used for wood and timber production, but they are also believed to impair forest biodiversity. Most studies that have assessed how such plantations may alter the diversity and composition of understorey plants were established in agricultural landscapes or have compared tree plantations with old-growth natural forests. Moreover, many important aspects of biodiversity have been overlooked in previous studies, such as functional and beta-diversity. Here, we present results from a study that was aimed at quantifying alpha- and beta-diversity of understorey plant species and functional groups in hybrid poplar (9–10 years) and hybrid larch plantations (16 years) located within a forested landscape of Quebec, Canada. These hybrid plantations were compared to naturally regenerated secondary forests and to native plantations of black spruce of the same origin (clear cut) and similar age. Our results indicate that fast-growing hybrid plantations do not present lower taxonomic and functional alpha-biodiversity indices, but may harbour more diverse communities, in part through the introduction of plant species that are associated with open habitats. We provide further evidence that planted forests may be as heterogeneous as naturally regenerated forests in terms of understorey plant composition. Plant species and functional composition differed slightly between stand types (naturally regenerated forests, native and fast-growing hybrid plantations), with plantations offering a greater potential for colonisation by ruderal species, while being detrimental to species of closed forest habitats. Lastly, plantations of fast-growing hybrids do not induce greater changes in understorey vegetation relative to native plantations of black spruce, at least during the first stand rotation.