Invasive English holly (Ilex aquifolium L.) in Clear-Cut and Forest Units in a Western Washington Managed Forest
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English holly (Ilex aquifolium L.) is a dioecious, shade-tolerant evergreen shrub or small tree native to Europe, Western Asia and North Africa that is becoming a prominent invasive in west-side Pacific Northwest (PNW) forests, with potentially negative impacts on native biodiversity and forest succession. Little is known about the susceptibility of managed forests and the potential influence timber harvest practice has on English holly invasion (e.g., through edge creation). In a managed forest in western Washington State, I surveyed for English holly in belt transects running from recent clear-cuts (harvested 2000-2005) into adjacent mature forest units (established 1925-1935). For all holly in transects measuring ≥ 1 cm basal diameter, I recorded location relative to the forest edge, size (basal diameter and height), and presence or absence of berries. For smaller English holly, only total number was recorded. Additional English holly samples were collected for a comparison of pre- and post-harvest growth rate (annual Basal Area Increment; BAI). A total of 286 holly were present in two out of four transects sampled, with 158 appearing in one forested unit and 128 appearing across two clear-cut units. The oldest holly sampled established in 1976, and total number of holly steadily increased in the study area up to the time of sampling. Proportion of fruit producing trees was more than 7 times greater in clear-cuts compared with forest. Growth rates of holly (calculated as basal diam. ÷ age) were higher on average, and more variable, in recently harvested areas compared to forest and decreased with distance into the forested unit. After-harvest growth rate (estimated with BAI) was larger on average compared to before harvest in both environments. The largest after-to before-harvest growth ratios were exhibited by holly located in clear-cuts. These results suggest that managed forests are susceptible to holly invasion, and that timber harvest without holly control could result in an increase in holly growth and fruit production, possibly accelerating holly spread.
- Forestry