Soil Impacts Due to the Invasion of Ilex aquifolium (English Holly) into Second Growth Forests of the Pacific Northwest
Pacific Northwest temperate rainforests are generally distinctive in their resistance to invasive species. Nevertheless, a novel evergreen tree species, Ilex aquifolium, also known as English Holly, is increasingly recognized as a successful invasive tree in undisturbed Pacific Northwest forests. One of the characteristics that makes this invasive plant so unique is its ability to readily establish itself in otherwise undisturbed forests. Nevertheless, no studies have examined the ecosystem impacts of this invasive species. Because I. aquifolium has tough, recalcitrant leaves, the species may also the potential to significantly alter ecosystem soil properties in some of the most productive forests in the northern hemisphere. We investigated soil characteristics underneath I. aquifolium canopies, and in adjacent paired locations without the invasive species, to determine the long term physical and chemical effects on the soil. Our chemical analysis focused on pH, carbon-nitrogen elemental analysis, mineralized nitrogen and phosphorus, extractable cations (K, Mg, Ca, Na), sulfate, soil respiration, and cation ex- change capacity. We also investigated changes in soil moisture, organic matter content and bulk density. Our findings suggest heavy depositions of sulfur and organic matter associated with the presence of these trees (p<0.001; p<0.031 respectively) while soil moisture and pH were correlated with age of the trees (p> 0.0074; p>0.0031 respectively). These results cumulatively suggest the potential ecosystem impacts of a novel invasive species in a temperate rainforest ecosystem.