having worked a lot with the Forestry Commission with mtb access in Wales, if the trail says closed than treat it as such, The FC are famous for being over sensitive when it comes to health and safety, not least because the industry has one of the highest accident records.
If MTB riders are seen to make personal choices as to whether they take heed of notices that are in force and the likelihood is they will act by introducing more severe restrictions widening the area rather than saying to riders its ok to take pot luck and try your chance.
After the race fatality at Clocaenog a few years ago, FC Wales policy is usually suspend all harvesting operations in the vicinity of any bike races not least to ensure contractors vehicles will not then be needing to drive about the roads.