English holly (Ilex aquifolium) is an increasingly common invader of west-side Pacific Northwest forests, but little sitescale information exists about the pattern and processes of this invasion. We comprehensively surveyed English holly in an 8.4 ha area of invaded forest at St. Edward State Park (WA), a largely native forest in the Seattle metropolitan area. We measured, mapped, aged, and removed all holly ≥ 1 cm basal diameter or > 1 m from the nearest sampled holly, and used these data to characterize the invading population and the course of the invasion. Holly in our sample (n = 466 known-age plants; 55.5 stems ha-1) ranged in age from 1 to 46 years. Trees ≥10 years old appeared to have very low mortality rates and exhibited accelerating rates of size increase and biomass accumulation with age. Native vegetation was greatly reduced under holly canopy. Our spatial and age data indicate that holly is proliferating and spreading rapidly at two scales: contiguous, primarily vegetative, expansion of tree clumps, and long distance dispersal via seed. Spread by both mechanisms appears to be accelerating, with population and canopy area both increasing approximately exponentially, having doubling times of approximately 6 and 5 years respectively. Projecting past spread patterns forward suggests that holly has the potential to soon become a prominent species both in number and canopy extent, likely at the expense of native plant diversity and forest structure. Based on these results, we offer recommendations for holly management in forested areas in the region.
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Vol. 88 • No. 2