It is a rather interesting fact that Rhododendron, despite its common status as an 'invasive' species nowadays, and its undeniable deleterious environmental consequences, is in fact native to the British isles, having occurred there in the Eemian. 
https://twitter.com/IrishRainforest/status/1443807633195118593
How can this be explained? Few studies to my knowledge have been carried out explicitly on the subject, but it seems most probable that the key thing controlling R. ponticum was also the main thing now missing from European ecosystems - large, destructive megafauna.
https://twitter.com/IrishRainforest/status/1395969697330438144Likewise, though it's true that closed-canopy woods were once predominant across Western Europe, this was itself an unnatural result of human-induced megafauna extinctions. See for instance Sandom et al. (2014) comparing the Eemian to Early Holocene
I do not here mean to be merely contrarian - Daltun's points are not invalid. There *is* far, far less woodland today in Ireland than is natural - in the Eemian it constituted around 40% of land-cover. Truly dense, tangled ur-forests were only some 20%, however.
What is fascinating is that, although closed-canopy forest was far rarer in the Eemian than the Mesolithic, completely open, barren grassland was seemingly *entirely absent*. Most of the landscape, nearly 60%, was temperate savanna, much like what is seen in @KneppSafaris.
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