Just incase it has not been answered here, the plain fact is the sign is about restriction of open access for the stated resons this could be anything such as a mountain bike race rally or tree operations.
The Welsh Assembly on its creation became the owner of all state forests in Wales. in 2000 when open access was introduce as a dumb act of bravado to one up England and to entice other woodland owners to do the same they dedicated the state forests as being open access land like it is on open land mountains and commons ( ie for walkers only they can go where the **** they like), for cyclists and horseriders the principle is still of permissive access on forest roads only or dedicated trails which can be stopped as and when the forestry feel like it.. this is why you find the stupid bobbled hatted **** walking along the marin trail .. it’s because they can… and open access is largely useless in forestry for old dodderers as they need paths and for this purpose dedicated mtb trail is fine and there is nothing the forestry can do to legally stop them as they made the forests open access.
on occaissions there will be a need to stop Bill and Betty Bobble-Hat from roaming and the forestry by notice can suspend open access temporarily but not permenantly. They must do so by submitting a notice to the CCW, the Local Authority and placing a notice on site at all points of entry. The notice only suspends open access for walkers, unless they apply for a legal closure on any public right of way all rights of way will be open during the periods of notice. As far as cyclists and horseriders are concerned the forestry only need erect any kind of sign to stop use for it to have legal effect, as all they need do is withdraw permission.
Generally these notices are aimed to cover an area greater than the actual work taking into account clearly defined boundaries that make sense. It does not follow that roads do need closing unless they go into felling areas or are being used by timber extraction vehicles. You should take note of any advice the forestry give, if you ignore it the forestry will recal land use it in arguments when the lieks of the CTC etc try to get extensions to our own opportunities.