To me, the benefits of these cards have always seemed limited for several reasons:
Initial outlay. You need to invest a number of wood into the card to begin with.
Delayed gratification. The first harvest after you sow the forester/copse, you simply get back your initial investment, therefore it is two harvests before you get any benefit at all. This is particularly negative when we are talking about wood, which is most useful early in the game.
Needing to sow. Sow spaces are often at a premium, particularly in games with 4/5 players. You aren't going to want to waste a whole action just to sow your copse, so it's only really valuable if you are going to be sowing grain anyway.
For these reasons, an occupation like the Wood Collector seems preferable. On the surface it might seem like a poor alternative, since the total amount of wood that you get is potentially less: 5 versus 12 if you were to use the Forester to its full potential. But compare them over time, if both were played as early as possible:
On round four, the Wood Collector is already up by 3 wood, whereas the Forester is DOWN by three wood.
On round six, the Wood Collector is up by 5 wood and the Forester is drawing even.
It's not till round 10 that the Forester is actually ahead by 1 wood over the Wood Collector (6 vs 5).
By round 14, the Forester has gained 9 wood compared to the WC's 5. There is an extra three to come, but it is only useful if you have the Joinery/Sawmill.
And that's all if the Forester is used perfectly. If you don't get the chance to sow before the first harvest (which requires having picked up three wood and used the "Sow/bake bread" spot) then your return is much poorer.
Being able to expand rooms as well as building fences to get that first pool of sheep seems pretty powerful.
From this, I wonder if you have misinterpreted the cards in some way. You should be taking one wood from the top of each stack during the field phase of each harvest and putting it into your supply. The wood on the card is not available for you to use to pay for anything until you have harvested it. For that reason, you don't get ANY benefit from the Forester/Copse until Round 8 at the earliest (the first harvest is just recouping your initial investment), so to use it to get the first pool of sheep seems implausible.
