Alder plantation

Hi

As part of my dissertation i am looking at the tree species, canopy density and ground flora of two woodlands in the north west of England. One of the woodlands in question used to be an alder plantation, there is hardly any alder trees to be found in the wood now apart from one section. In this section Alder was the only tree species growing until the last 11 years where a disturbance has taken place ( the wood was made accessible to the public e.g. paths) this has allowed the growth of shade intolerant tree species to grow. But on the ground there is very little bare ground, and very high leaf litter and in regards to ground flora there is some Rubus fruticosus and less than 5 percent coverage of Pteridium aquilinum. So here is my problem i have to explain this, but all the research i do states that Alder leafs are beneficial to ground flora growth, and i have no way of explaining the lack of ground flora, i would be very grateful if you could help me out.

Thanks

Adam