How much edible biomass does a forest generate?
This highly depends on what portion of the foliage is edible to a dinosaur. Since we don't really know how efficient a dinosaur's metabolism is, and we don't know how good they were at eating things like sticks, it is hard to say. But we can do some estimates.
First, Kloeppel, et al., 2007 as some estimates of net primary productivity for forest-dominated ecosystems. Here are some minimum and maximum calculated values from Table 1 of that paper:
Annual Net Primary Productivity (g/m^2)
Forest Type Minimum Maximum
Boreal evergreen 120 439
Boreal deciduous 169 635
Temperate evergreen 60 1555
Temperate deciduous 230 555
Tropical evergreen 140 1505
So there is wide variety in the mount of green biomass generated by such forests. The percentage of what a forest generates that a sauropod could eat is also variable. In an evergreen forest of juniper and pinyon pine, in the foothills of the Rockies, 100% of the biomass would be within the distance that a Sauropod could reach. But in an old growth Virginia hardwood forest, the trees would all be 100 feet+ and maybe only 10% of the biomass would be accessible to a sauropod. Also consider that a sauropod isn't the only creature eating in a forest. Bugs will get theirs, squirrels or other similar creature will eat acorns and seeds, etc.
Let us use a range of assumptions then. The worst case scenario would be something like 10 g/m$^2$ of edible biomass, the best case something like 200 g/m$^2$.
How much does a sauropod need to eat?
The problem with a bulk biomass calculation is that there is no accounting for nutrition. You need to eat a lot less mass in leaves if you can get good nutrition from acorns.
But, ignoring that problem, let us assume that a sauropod scales up linearly from an elephant. An elephant in the wild eats something like 250 kg of biomass each day. Now, lets assume a sauropod is 8 times as massive as an elephant (so elephants average about 3 tons, and apatosaurus about 24 tons). Then an apatosaurus needs to eat 2000 kg of biomass each day.
For our worst case forest, that is 20 hectares; for the best case, 1 hectare. That is the total per dinosaur, so a group of dinosaurs would have to multiply by group size....a 'family' of 10 sauropods would need 10-200 hectares. Then we also have to multiply by some 'bad conditions' factor. If food supply is lower in a bad year, a dry summer, or what have you, then you would have to account for that in the area needed.
Overall, I think it is reasonable to expect something in the range of 5-100 hectares per single sauropod, depending on the fertility of the local soil, whether or not the local biomass is accessible to the dinosaurs, etc.