In the Conservation Land Management magazine there is an interesting article studying the effects of invasive Rhododendron on the ground flora. This study was done in the Atlantic oak woods of Scotland which also has Birch, Hazel and Rowan, as well as a rich ground flora.
Under Rhodo there is little that can survive the deep shade. When areas were cleared of this noxious shrub the original ground flora never reappeared, even after 30 years. However the Bryophytes did return and carpeted the ground with mosses, which produced a novel ground flora for which there is no comparison in natural ecosystems.
They found the popular idea that Rhodo ‘poisoned’ the soil was not supported by soil tests and is a bit of a myth. They found the only way to return a semblance of the original flora was to reintroduce seeds and scarify the moss covering.
I read elsewhere that we refer to it as Rhododendron ponticum but in reality this country has introduced multiple species from different parts of the world. These have hybridized to such an extent the resultant shrubs cannot be named with any certainty as it is a mish-mash. This has made it more invasive than R. ponticum.
Rhodo is a prolific spore distributor of the disease Phytophthora ramorum, an oomycete pathogen, which arrived in this country in about 2002. It has killed millions of Larch trees. It also affects Beech and the disease has been confirmed in the upper valley, where mature trees are dying and collapsing.
It is difficult to understand why there is not more effort to clear Rhododendron. It is easy to cut down and just needs a bit of effort for a year or two to strip off any regrowth and the stump is dead.