elven workers already have an increased work time... even on non-forested terrain.
if you remove the ability for elves to build under forest (that WILL develop into ancient forests if/when they go FoL), ljos and svart will become boring and WEAK civs.
(their über-cities are not due to being ljos... but only to the ability to work under forests).
so IMO, if you propose to do that, I expect that you will try to make a total conversion of the ljosalfars... to compensate.
A limited "nerf" would be to allow working under forest and ancient forest... save for farms if possible to make this limitation. (farms are the one that I always thought less credible as an improvement in forests).. pastures can be done in glads or under the trees, depending on what you are raising... plantations too (those are trees.. so why not), mines are ok, even windmills, watermills or workshops only need a small place or can be envisionned as morcelated working places.
however farms NEED big places and big fields... which kinda seems incompatible with forests.