How do lupins harm forestry on Iceland?

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?