Overstory influences on herb and shrub communities in mature forests of western Washington, USA

Understanding the relationships between forest overstory and understory communities is essential for predicting changes in the abundance and distribution of understory plants through successional time and in response to forest management. We used correlation analysis, multiple regression, and nonparametric models to explore the relationships between overstory characteristics (canopy cover, stand density, and tree-size distributions) and the abundance of species in the herb and shrub layers in mature forests of western Washington. Overstory variables explained > 50% of the variation in the mean response of total shrub cover and ca. 50% of the variation in cover of Acer circinatum Pursh (the most common shrub species) and late-seral herbs (species reaching their greatest abundance in late-successional forests). Stronger relationships (80-90% variance explained) were found between overstory variables and the maximum cover of total shrubs, A. circinatum, total herbs, and each of three functional groups of herbaceous species. These empirical relationships represent both direct resource limitations and time-dependent responses for which overstory characteristics may be surrogates. Models of maximum abundance yielded the most consistent results, suggesting the relative importance of different overstory variables as limiting factors for understory response, although these limiting factors have different effects on plants with different life-history strategies.