Relative contribution of ancient woodland indicator and non-indicator species to herb layer distinctiveness in ancient semi-natural, ancient replanted, and recent woodland

Questions: The floristic distinctiveness of ancient woodland relative to recent woodland is commonly measured by Ancient Woodland Indicator (AWI) species richness. However, focussing on a pre-defined subset of species means that wider community-level differences may be overlooked. Can ancient semi-natural, ancient replanted, and recent woodland herb layer communities be distinguished by alpha, beta, and gamma diversity? How are any differences partitioned across AWI and non-AWI species communities?. Location: Cotswolds, southwest UK. Methods: To quantify AWI and non-AWI responses to stand history in ancient semi-natural, ancient replanted, and recent woodland, we conducted floristic surveys of 45 sites. Using a modelling approach, we tested the relative and additive contribution of alpha-scale AWI and non-AWI species richness to woodland distinctiveness. Ordination was applied to analyse beta species composition distinctiveness, and multilevel pattern analysis was used to examine which species were significant contributors to gamma-scale richness differences. Results: AWI species richness models significantly distinguished ancient semi-natural woodland from both ancient replanted and recent woodland at the alpha scale. For the classification of ancient semi-natural woodland and recent woodland, the hierarchical inclusion of non-AWI alpha richness resulted in a superior and more significant model. AWI gamma richness was numerically similar for all three woodland categories, whereas non-AWI was more varied. AWI and non-AWI species composition showed significant beta diversity differences among all woodland types, with six species being significant drivers of differences. Conclusions: Our results have revealed previously undetected complexity in the contributions of AWI and non-AWI species to floristic distinctiveness of ancient woodland. In addition to traditional AWI species, the non-AWI assemblage also exhibited a sensitivity to habitat continuity that: (a) adds weight to the argument that ancient woodland is floristically distinct from recent woodland; and (b) provides a useful measure of success for ancient replanted woodland restoration.