Northwest Science
Published by: Northwest Scientific Association
Northwest Science 88(2):75-93. 2014
doi: http://dx.doi.org/10.3955/046.088.0204
doi: http://dx.doi.org/10.3955/046.088.0204
David L. Stokes1, Elliott D. Church, David M. Cronkright and Santiago Lopez
University of Washington, Bothell, 18115 Campus Way NE, Box 358530, Bothell, Washington 98011-8246
Author to whom correspondence should be addressed. Email: dstokes@u.
Abstract
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.
Received: March 28, 2013; Accepted: December 04, 2013
