I saw an unfamiliar (for me) maple in the pine forest near to Tallinn. I took a photo of the tree and another of the underside of a leaf, merged those together and attached to this message. Could it be Acer heldreichii subsp. trautvetteri?
Thanks! I studied my Dendrology books before I asked, but found that redish (carmine) underside was only mentioned in case of Acer heldreichii subsp. trautvetteri, that should have otherwise very similar leaves to the A. pleudoplatanus (except maybe slightly deeper cuts between lobes). My book says, that A. pseudoplatanus leaves have greyish-bluish-green underside. But as A. heldreichii subsp. trautvetteri should be too rare and too tender to grow in the forest, I agree with you.
There's several cultivars with leaves like this; their seedlings also often inherit the purple colour, so random seedlings away from gardens can show it too. From Acer pseudoplatanus at Trees and Shrubs Online:
Purple undersides are dominant here, both in cultivation and where the tree has seeded out into local woodlands etc. Even the variegated New Zealand origin introduction 'Esk Sunset' - the most recently circulated commercial offering - has them.
I skipped the cultivars part of my Dendrology book, because the tree was obviously not cultivated there. That was a mistake. Tallinn Botanic Garden is only ca 1 km away from the site, so I consider this to be most probable source for seed. There are following Acer pseudoplatanus cultivars and forms in their collection list: A. p. 'Atropurpurea', A. p. f. erythrocarpum, A. p. 'Leopoldii', A. p. 'Oru', A. p. f. purpureum and A. p. 'Simon Louis Freres'. They don't have Acer heldreichii subsp. trautvetteri in their list.
