Abstract:
Sierra Nevada forests have high understory species richness yet we do not know which site factors influence
herb and shrub distribution or abundance. We examined the understory of an old-growth mixed-conifer
Sierran forest and its distribution in relation to microsite conditions. The forest has high species richness
(98 species sampled), most of which are herbs with sparse cover and relatively equal abundance. Shrub
cover is highly concentrated in discrete patches. Using overstory tree cover and microsite environmental
conditions, four habitats were identified; tree cluster, partial canopy, gap, and rock/shallow soil. Herb and
shrub species were strongly linked with habitats. Soil moisture, litter depth and diffuse light were the most
significant environmental gradients influencing understory plant distribution. Herb cover was most strongly
influenced by soil moisture. Shrub cover is associated with more diffuse light, less direct light, and sites with
lower soil moisture. Herb richness is most affected by conditions which influence soil moisture. Richness is
positively correlated with litter depth, and negatively correlated with direct light and shrub cover. Disturbance
or management practices which change forest floor conditions, shallow soil moisture and direct
light are likely to have the strongest effect on Sierran understory abundance and richness.
