Canopy closure exerts weak controls on understory dynamics: a 30-year study of overstory-understory interactions

Stem exclusion and understory re-initiation are commonly described, but poorly understood stages of forest development. It is assumed that overstory trees exert strong controls on understory herbs and shrubs during the transition from open to closed-canopy forests, but long-term observations of this process are rare. We use long-term data from 188 plots to explore patterns and correlates of variation in understory richness and abundance 15-45 yr after clear-cut logging and burning of two experimental watersheds in western Oregon, USA. We test whether variation in the temporal dynamics of plots can be explained by topoedaphic factors that influence resource availability (insolation and soil moisture), variation in the pace and intensity of overstory development, or characteristics of the vegetation prior to canopy closure. Changes in forest structure were substantial over the study period; canopy cover increased four-fold, stem density by 75%, and bole biomass by two orders of magnitude,...