Siberian pea shrub, or caragana, (Caragana arborescens Lam.), was intentionally introduced on the northern Great Plains of North America for erosion control and ornamental hedgerows, but has invaded the understory of native deciduous forests. Although this species is widespread, there is little information on the ecology of naturalized populations or impacts of invasion. Our objectives were to describe the demographic structure of an invasive population and changes in shrub community diversity along a gradient of caragana density in an upland aspen (Populus tremuloides Michx.) forest at Elk Island National Park, Alberta, Canada. This caragana population spread through the understory and increased from 50 to approximately 60,000 individuals over 75 years. More than 85% of individuals were immature (<10 yrs, <1 m tall, non-flowering), and the size class-frequency distribution best fit a negative power function curve. Several native shrub species, including the dominant beaked hazel (Corylus cornuta Marsh.), were negatively associated with caragana, and the relationship between diversity and caragana density was quadratic. Control efforts will require perseverance since immature pea shrubs are difficult to locate beneath the dense growth of beaked hazel and the immature phase can last a decade.