I know it's widely believed that a range of trees and other plants put toxins into the soil to reduce competition. But I'd love to see a scientific reference to this. Googling gives me references to the toxic effects of walnut but not much else.

I suspect the lack of growth in a conifer forest could be explained by deep shade and drying out of soil. Even in deciduous woods there isn't much growing in the summer - most of the understorey growth is in the spring when there is no leaf cover.

My own experience of 4 25ft leylandiis is that a bay tree, ivy and a Vibunum bodnantense are growing perfectly well amongst them, and that corydalis and daffodils managed for about 10 years. In other words, whay I am experiencing is much more like the effects of dryness and shade rather than soil toxicity.