Environmental preferences and constraints of Daphne laureola, an invasive shrub in western Canada

Abstract

Daphne laureola L. is an evergreen forest understory shrub native to the Mediterranean regions of Europe and North Africa that has invaded parts of western North America, including coastal British Columbia (BC) and the states of Washington and Oregon. It can form dense thickets that are likely to prevent the establishment and growth of native plants. Despite its expanding range in the west coast regions, not much is known about its environmental preferences and the ecophysiological attributes associated with its presence and distribution. A 2-year study conducted in Victoria, BC, found that D. laureola attained higher densities in forests with moderate shade at 12%–15% canopy gap opening, whereas densities decreased at higher and lower levels of canopy openness. Specifically, variation in patch density was significantly associated with the level of diffuse light (UOC, uniform overcast sky) and sunfleck duration in the summer, but many leaf-level properties such as photosynthetic rate, leaf dry mass per unit area (LMA), and long-term water use efficiency (as indicated by δ13C) were similar between patches. Taken together, highest plant densities were achieved in forest understory that received less direct but more diffuse sunlight in summer, suggesting that the best growing condition is a compromise between reduced drought stress through lower sunfleck exposure and increased carbon gain under brighter canopies. In future, the high fruit output combined with the readily available seed dispersers seems to ensure that D. laureola will continue to spread, particularly into mainland areas where milder summers may offer a wider range of potential sites for occupation.