@Elber Thank you for this post, it has made me think.
I think we will have to disagree on the meaning of 'under control'. To my mind it means the dog isn't running around causing damage, chasing wildlife or being a nuisance, and it will come back to you immediately if you ask it to. I agree you need an awareness of where it is and what it's doing, but I don't think that that necessarily means you have to have eyes on every single moment - 'he's just gone behind that tree' for example. If you are saying that it means you have to be able to pick up its poo then you might as well say it has to be on lead, surely?
I hear what you say about even one poo changing the local ecosystem and I'm wondering why, if that's the only thing to take into account, Forestry England don't ask everyone to keep their dogs on leads all the time. They do ask you to stay on the paths where possible, so it makes sense to me to think that the damage from searching for and retrieving sparsely spread poo may outweigh the damage done by leaving it where it is. If you can throw any light on this I'd be glad to see it. I hope that answers your question.
More broadly, it has got me thinking about what's 'right' when it comes to conservation, and whether there's any single right answer. Many years ago I was one of the last people to see the Lascaux cave paintings before they were sealed off for their protection. Now no-one can see them. Is that right? If no-one can see them, what's the point of keeping them at all? Nevertheless my gut feeling is that that's probably the right thing to do.
How does that apply to a landscape? It would be possible to seal it off completely to protect it, but somehow that doesn't feel equally right. Partly because it would ignore any rights that people may have to enjoy it, but also because unlike a painting, a forest is in a perpetual state of change in any case. So where do you draw the line between what's best for the landscape and what's best for the people? That's not clear cut.
As with the vegan/vegetarian/organic/any old thing spectrum, people will have different views and unless these are in some way perverse, they are equally valid. My personal view of the forest is that people and animals should be allowed to enjoy it responsibly. In deciding what that looks like I am guided by the views of Forestry England as I understand them, because I don't have the detailed knowledge necessary to decide entirely for myself. I accept that there will be some people who think that people should be permanently restricted to pathways in the forest or that dogs should only ever be allowed if they're on a lead. That's a valid view, but it's no more right or wrong than mine. I don't hold it because I think that it doesn't give sufficient weight to the rights of people and animals and to the fact that over time, it's in the nature of a forest to change. Equally there are people who think they should be allowed to ride their dirt bikes anywhere. I don't hold that view either but I accept that it's just a different point on the spectrum.
At the end of the day I am open to changing my behaviour, but that would have to be based on new evidence/information, and also be congruent with my position on the preserve it for ever just as it is/let everyone do anything they want spectrum. For the time being I will carry on following Forestry England's advice (which is why I no longer stick and flick).
Thanks for helping me think more broadly about this.