Losses in understory diversity over three decades in an old-growth cool-temperate forest in Michigan, USA

Diversity in temperate forests is concentrated in the understory, but understory dynamics are poorly understood for old-growth forests. We use repeated measurements of more than eight hundred 1 m2 plots over three decades to assess patterns of understory diversity in old-growth mesic and wet forests in northern Michigan, USA. We ask whether diversity changes systematically over time and whether dynamics are related to spatial scale. We find, for all habitats, significant understory diversity loss at square-metre scales but not at coarser scales. Total herbaceous cover, however, remained constant or increased in total and for nearly all frequent species, and no species were lost overall. We explore hypotheses about diversity regulation by exploring correlations with habitat, canopy composition, and properties of understory species. Nonindigenous plants are rare at the study site, earthworm invasion is not apparent, and deer browse is not intense. Diversity changes may be related to ecological guild membership. We suggest that the general loss of fine-scale diversity is driven by either changing canopy composition or competitive dynamics within the understory community. Management for diversity maintenance in temperate forests must address understory communities; if herbaceous diversity is scale dependent and unstable over decadal time frames, management approaches need to account for factors driving changes.