by oldclaypaws » Fri Jul 11, 2014 11:11 am
I suspect the issue is you're changing the balance and light conditions which enables other more light-suited species to out-compete the bluebells. You've said where there's more shade under Sweet Chestnut and Sycamore (which are very densely shady) the bluebells do well, here it'll be partial shade and too dark for the Dogs Mercury. Ash trees have quite a light and late leafing canopy, less than other trees, and the increased light from thinning will enable more photophilic species to get an edge on shade tolerant ones. Each plant establishes defensive root systems and may steal the nutrients and light to fight off other plants, so the winner will be the plant optimally suited to the particular localised environment.
If you stop thinning and plant (or allow natural spreading) of more shady trees to give a dappled shade, the bluebells may well be the winners, provided you don't have too much shade-then it'll benefit ivy.
Its a delicate balance, and a mixture of different conditions (and perhaps a coppicing regime) will ensure greater biodiversity. Nature will adapt to what you do, but it won't always be exactly what you want, it has a mind of its own !