Herbaceous plant height is an early indicator of groundlayer response to an experimental manipulation of forest structure and deer pressure

•Variation in canopy conditions, deer pressure and abundance of downed woody material generated species-specific herb responses.•Herbaceous plant height was a more responsive metric to forest structure than density, cover or flowering.•Downed woody material has implications for the growth of multiple herbaceous species. There is growing interest in adapting forest management to increase diversity, in both composition and structure, to restore old-growth characteristics and to build resiliency against anthropogenic disturbances. In order to determine appropriate interventions, managers need to evaluate the quality of natural areas but efficient and effective measurements are elusive. Using a suite of herbaceous species and measuring their responses to experimental treatments, we aim to assess (1) the sensitivity of different metrics and (2) the current and likely future effects of gap-generating disturbances and abundant deer on the understory of a northern hardwood forest in Wisconsin, USA. More specifically, we investigated plant cover, density, height, aggregate height and the proportion of individuals producing flowers/fruits under unmanipulated canopies v. three gap sizes, with and without downed woody material (DWM) additions, and inside v. outside of white-tailed deer (Odocoileus virginianus) exclusion fences, approximately seven years after treatment. The effects were taxon-specific with Anemone quinquefolia, Arisaema triphyllum, Laportea canadensis and Trillium grandiflorum being most responsive to canopy structure, Impatiens spp. and T. grandiflorum to deer pressure, and A. triphyllum, Impatiens spp., L. canadensis and Sanguinaria canadensis to canopy structure in combination with DWM. Cover was least responsive to treatments, density and proportion of sexually reproducing individuals sometimes differed, but height was overall the best metric. Mean height and maximum height showed similar results so we suggest that managers focus on measuring maximum height to limit sampling time. We also draw attention to the importance of retaining and/or creating DWM which has long been recognized as an important structural feature promoting diversity in natural forests. In the present study, DWM additions appeared to play a role in mitigating microclimates and/or deer pressure for multiple herbaceous species.