The effect of English holly (Ilex aquifolium) and English ivy (Hedera helix) invasion on local soil pH and understory species diversity in coniferous and deciduous forests

Invasive plant species pose a serious threat to biodiversity and ecosystem function, though their indirect impacts on abiotic factors such as soil pH remain understudied. This study aims to investigate the effects of English Holly (Ilex aquifolium) and English Ivy (Hedera helix) on understory plant diversity and soil pH between the coniferous and deciduous forests of Pacific Spirit Regional Park in Vancouver, BC. A total of 16 plots—8 invaded and 8 paired uninvaded—were established across the two forest types. Soil samples were collected at three distances per plot, alongside visual estimation of understory species composition and cover. Mixed effects and linear models were used to assess whether invasive species and forest type interact to affect properties like soil pH and Shannon diversity. It was found that I. aquifolium significantly lowered soil pH in coniferous forests only, whereas H. helix showed no significant effect on soil pH in either forest type. Neither invader alone significantly affected diversity, and forest type did not appear to influence the strength of observed outcomes. These trends may reflect species-specific effects on soil conditions, the intermediate disturbance hypothesis, or a shift toward species better adapted to acidic soils. Overall, these findings emphasize the importance of context and invader identity in shaping ecological outcomes, highlighting that not all invasive species cause similar effects. Furthermore, they support the need for targeted, species-specific management practices and suggest that even subtle changes in soil properties can drive long-term shifts in community composition.