- Seven sites of uniform topography and soil were selected within
a self perpetuating ponderosa pine forest found on the eastern
flank of the central Oregon Cascades. These plots were located along
a vegetational gradient caused mainly by an orographic rain shadow.
Data gathered included density, frequency, and cover for all understory
species, amount of direct overhead tree cover, and amount of
shade cast by the tree canopy. An attempt was made to relate the
distributional pattern of understory species to influences caused by
shade, overhead tree cover, and shrub cover within each pIot. The
Dfd index and Cole's index were statistical methods used to correlate
the herbaceous species pattern to overhead tree cover, shrub cover,
and shade. Of the two, Cole's index appeared to be a more sensitive tool for this study.
All the shrub species with the exception of Purshia tridentata
showed a consistent distributional pattern to insolation. The position
along the vegetational gradient appeared to be related with the
shrub's affinity for an insolation class. Those shrubs restricted to
the mesic end of the gradient showed an affinity for Iow insolation or
deep shade. Conversely, those shrubs occurring on the xeric end
showed an affinity for higher insolation. Those shrubs found in the
center of the gradient showed an affinity for moderate insolation.
Ten herbaceous species out of a total of 16 observed showed
apparent affinities for a certain microhabitat within the stand. Plot
location, shade, overhead vertical crown cover, and shrub cover
influenced the distribution of herbaceous species within the plot.
Species found within mesic plots showed a weak affinity for cover,
but those on xeric plots showed a strong affinity for cover. A weak
indication of the spatial ecological niche was gained for 15 herbaceous
species, but a strong indication was observed for Trientalis
latifolia. Similarly, some insight into the ecological performance
of the species was gained in regard to insolation. Usually the plot
location coincided with the species' affinity for an insolation class.
If the species occurred mainly on the mesic end of the gradient, it
showed an affinity, for deep shade (Lupinus caudatus, Fragaria
virginiana, Trientalis latifolia, and Pteridium aquilinum). If the species occurred mainly on the xeric end of the gradient, it showed
an affinity for high insolation (Viola purpurea, Carex inops, and
Madia minirna). Five species showed some affinity for moderate
insolation (Clarkia rhornboidea, Eriogonurn urnbellaturn, Kelloggia
galioides , Mimulus nanus, and Stipa occidentalis), but the correlation with plot location was less clear. Four species (Festuca
idahoensis, Sitanion hvstrix, Achillea rnillefoliurn, and Lathvrus
lanswertii) were not restricted to any part of the gradient. These
species showed preference to insolation with the plot, but this
preference was not usually the same throughout the gradient. Indications
of the limits of ecological potential were found for some of
these species, in particular Achillea millefolium which did not occur
on any plot where insolation was less than 10%.