Diversity Relationships of Forest Layers in the Swan Valley, Montana

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Abstract
Species richness was greatest in the epiphyte and bryoid layers and decreased monotonically from the bryoid layer through higher vascular strata. Species equitability was approximately the same for all layers. Beta diversity paralleled species richness for all layers except for epiphytes, which had high alpha diversity but low beta diversity. Differences in species diversity between layers may be related to differences in the degree of environmental differentiation at the level of each structural group. The influence of forest structure on diversity through time was inferred from samples of 31-300+ yr old stands. Tree species richness is relatively stable through time while numbers of herb species are highly variable but tend to fall with canopy closure by shade tolerant trees and rise with gap formation. Epiphytes are slow to invade but maintain high diversity once established. Species richness in the shrub and bryoid layers remains relatively constant.