Some balsam fir existed as an upper canopy tree, individually spaced and with crowns limited to the upper portions of the stems.
Linear regression was used to test the first hypothesis, that stand structural features in mixed aspen forests and aspen monocultures are related to overstory composition, by using the following structure variables as dependent variables: (1) balsam fir DBH ratio, (2) average shrub height, (3) average % cover of shrubs, calculated using the midpoint of each cover class, (4) average number of point interceptions of plant material and (5) modified FHD.
The third hypothesis, that herbaceous plant species diversity is related to stand structural features, was tested by using linear regression of herbaceous plant diversity as the dependent variable on the following independent variables: (1) balsam fir DBH ratio, (2) average shrub height, (3) average % cover of shrubs, (4) average number of point interceptions (0-3 m height profile) of plant material on vegetation plots and (5) modified FHD (0-7 m height profile).
Mean shrub height and mean % shrub cover values were not related to the proportion of balsam fir or hardwood (other than aspen) basal area.
Visual assessment of residual plots revealed that variance of diversity (H') decreased as the proportion of balsam fir increased.
In the Aspen-Conifer cover-type group mean shrub height and the balsam fir DBH ratio were significant and positively related to H' (e.g, [[r.sup.2].sub.adj].
Interaction of overstory and stand structure.--Models were constructed using interaction between overstory basal area proportions and the structure variables highlighted in the previous analysis, i.e., average number of interceptions of plant material for the Aspen-Hardwood cover-type group and balsam fir DBH ratio for the Aspen-Conifer cover-type group.
In contrast to aspen, which is limited to the overstory layer, balsam fir can exist in the subcanopy as a densely packed cohort or can form a more scattered pattern within the stand, extending into the upper canopy with aspen.