Understory communities of warm-dry, mixed-conifer forests in southwestern Colorado

Mixed conifer of southwestern Colorado is poorly understood in comparison to other common forests in this area, in part due to its compositional complexity. We identified four stand types in warm-dry, mixed-conifer forests; Abies concolor (white fir), Pinus ponderosa (ponderosa pine), Pseudotsuga menziesii (Douglas-fir), and Populus tremuloides (aspen). We examined how composition and abundance of understory varied across these four types, and assessed the correlation between biotic and abiotic variables and understory vegetation. Composition of understory community differed significantly among stand types, with aspen plots having the most distinct understory plant community. Total plant cover was significantly higher in aspen, and shrub richness was significantly higher in the Douglas-fir stand type. On average, for all stand types combined, shrubs dominated the understory cover (11.35%), followed by forbs (7.89%), and graminoids (4.34%). Multivariate multiple regression showed that several topographic site factors (distance to drainage, slope, and aspect) and characteristics of stands (white fir/ha, ponderosa pine/ha and basal area, and basal area of aspen) explained variability in the understory community. Univariate regression showed that variation in annual species richness and Simpson's diversity index were partially explained by stand type and site. Our findings illustrate the necessity to not simplify forest dynamics for all western forest types or even within one forest type (warm-dry mixed conifer) for a general region. Implementation of forest management should be based on site-specific knowledge within localized geographic regions to restore or preserve semi-natural communities within a range of natural variability.