The relationship of herbaceous plant diversity to overstory composition and stand structure in the mixed aspen forest of northern Minnesota was investigated on 23 study sites that contained aspen in monoculture or in mixture with boreal conifers or northern hardwood species. On each site overstory species were placed into species groups: conifers, aspen and hardwoods other than aspen. Each site was then placed in one of three cover-type groups based on proportion of the overstory species groups: Aspen (>0.9 basal area in aspen), Aspen-Conifer (>0.15 basal area in conifer species) and Aspen-Hardwood (>0.15 basal area in hardwood species other than aspen). The relationships between diversity of herbaceous vegetation and the following factors were tested: (1) overstory composition, defined as the proportion of basal area by species group in the overstory and (2) stand structure. Stand structure was described by the vertical position and horizontal arrangement of balsam fir within the stand. In addition, relationships were tested with respect to other stand structural features such as shrub height and cover, average amount of plant material intercepted within the vertical profile and an index of plant occupancy within the vertical profile (modified Foliage Height Diversity Index-FDH).

Understory herbaceous diversity (H′) and proportion of aspen basal area were significantly positively related whereas understory herbaceous diversity was significantly negatively related to proportion of hardwood basal area and not related to proportion of conifer basal area. Mixtures of overstory tree species provided a range of stand structures that can be represented by shrubs, subcanopy trees or the overstory trees. In the three cover-type groups different structural components were related to herbaceous diversity indicating that (1) overstory composition and stand structure interactively influence understory diversity patterns and (2) it is difficult to characterize stand structure for the range of stand conditions with a single measure. Increased diversity of structure (modified FHD) in the Aspen-Conifer group is negatively related with diversity (H′) of the herb layer. Composition of herbaceous species varied depending on presence or absence of conifers in the overstory. These patterns may be influenced by the interaction of a variety of resource levels and climate conditions, which, in turn, are controlled by factors such as tree architecture or shade tolerance of overstory trees.