An invasion’s progress: English holly (Ilex aquifolium) in a semi-natural Pacific Northwest forest

Background/Question/Methods

 English holly (Ilex aquifolium) is an increasingly prominent invader of Pacific Northwest forests, but little site-scale information exists about the pattern and processes of this invasion. We measured, mapped, aged, and removed all English holly plants in an 8.4 hectare area of invaded forest at St. Edward State Park, one of the few exemplars of relatively intact native forest in the Seattle metropolitan region. We used these data to characterize in detail the invading holly population, to describe and graphically depict the course of the invasion from earliest established tree in the study area to the present, and to project the future course of the invasion. Our goal was to improve scientific understanding of the invasion and to provide data needed to inform Park management of holly. In addition, time series maps of the course of holly spread produced by this study may be useful in conveying to the public the seriousness of holly invasion and invasion by non-native species generally. Public education may be important for managing this species, as it is likely that holly was, and continues to be, intentionally introduced.

Results/Conclusions

Holly in our sample (n = 466 known-age plants) ranged in age from 1 to 46 years, with the oldest trees exceeding 10 m in height and 20 cm basal stem diameter. Established trees (> 10 yrs old) showed very low mortality rates and rapid growth, as indicated by steepening height-age, stem diameter-age, and canopy diameter-age curves. Native vegetation was much sparser and less diverse under holly canopy than in adjacent areas. The oldest trees in our sample established shortly after much of the nearby residential development occurred, suggesting that the original source of the invasion may have been seed from trees planted by area residents. Our spatial and age data indicate that holly is spreading rapidly at two scales: contiguous, primarily vegetative, expansion of clumps of trees, and long-distance dispersal via animal-dispersed seed. Spread by both mechanisms appears to be accelerating, with new tree establishment by both means increasing exponentially. Projecting past spread patterns forward suggests that English holly has the potential to become a dominant species both in number and canopy extent within a few decades. This dominance would likely come at the expense of native plant diversity and forest structure.