There are a variety of tools and methods for hand planting tree seedlings. Here in the Cascades, rock and roots are common, and the best way to get it done is with a planting hoe, aka hoedad. The hoe has a scalping, cutting edge on the back end, making it much more effective than a narrow bladed shovel, which is also popular.
Frankly, most planters prefer to work on anything but flat ground, Stig. The "work site" is closer to you, with less bending over involved, as you always plant with your body oriented to face uphill. Your daily numbers are about the same as what we see here too.
Seedlings are carried in a single or double bag belted around the waist, bags to the rear. Commonly a bag-up is 200 to 300 seedlings, which will weigh from 35 to 45 pounds. Of course, the more you've planted, the lighter it becomes...until you bag up again

. You scalp off the duff and slash, swing the hoe into the soil once or twice or thrice to get deep enough (about 12 inches), and open a wide slot, slide a single seedling into the hole keeping the roots straight down all the way to their tips, then pack the soil carefully with the hoe tip, bottom to top. Takes a good planter 20 to 30 seconds per tree, like Stig says.
It's hard on your hands, elbows, shoulders and back.
I should have a few pictures somewhere. I'll see if I can find 'em.