The goals of the study were to assess the relative importance of wind disturbance, European earthworm invasion and deer grazing on plant species diversity and successional trajectory in mesic, sugar maple-dominated forests of southeastern Minnesota, USA. A 15 x 15 m plot was installed in each of 28 stands on hilly topography at the edge of the driftless area. Each plot was surveyed for deer grazing impacts, stage of earthworm invasion, and presence and abundance of herbaceous plant species, tree seedlings and woody shrub species in 2013. Five years prior to the plot surveys, the study area was hit by a major storm system with straightline downburst winds (160 km/hr) and accompanying tornado touchdowns, creating a large magnitude of variability in remaining basal area of canopy trees. Plant diversity was measured as species richness, Shannon’s diversity and Simpson’s diversity. Successional trajectories were analyzed via time-series ordination using non-metric multidimensional scaling of the tree canopy layer (representing current composition) and regeneration layer (representing future canopy composition).
Results/Conclusions
Sugar maple, American basswood, northern red oak, bitternut hickory and ironwood were the main tree species on these mesic sites. Coarse woody debris volume ranged from 4 to 298 m3 per ha, reflecting variability in severity of wind damage, with essentially no tree canopy in the most severely damaged plots. Species richness of herbs ranged from 13-38 species per plot. Most measures of plant diversity were positively related to wind-damage severity. Shade-tolerant herb species persisted after the windstorm and less tolerant species entered the community, accounting for the high diversity of wind-damaged stands. Successional trends were complex. Some sugar maple stands are stable while others are succeeding to hackberry or bitternut hickory. Basswood stands are succeeding to sugar maple, hackberry, ironwood or bitternut hickory. Variability in wind damage among plots was more important as a regulator of plant diversity and successional trajectories than variability in stage of earthworm invasion or deer grazing intensity. Wind damage is a mechanism that maintains herbaceous plant diversity at the landscape scale; it also promotes multiple successional pathways by releasing advanced regeneration, thus it also maintains canopy tree diversity.