Recently, I have been reading more and more articles saying that Alaskan lupines (Lupinus nootkatensis) are hindering the growth of trees in Iceland. Unfortunately, however, I could not find any valid reasons why this is so? The story goes that in 1945, the then director of the forest service brought two spoonfuls of seeds from Alaska to amfortify the nutrient-poor Icelandic soil, stop erosion in Iceland and prevent sandstorms. After all I know, lupines draw nitrogen from the air and store it in the soil. So why is that not good for native tree species, such as birch or larch?
I won’t speak to the specific plants because the other answer was good but I work in invasive weeds and to simplify why a nonnative might be bad for trees is that, usually, the “weed” will prevent germination or climb over/shade and choke out juveniles.
Basically, they take the “niche” from the native tree. Without a niche to get started in, they don’t start.
And usually the reason the weed wins the race is they don’t face the same endemic pressures a native tree does; be it either fungus, bacteria, insect, environment etc.