My understanding is that the Mayans, who lived in a very thick forested area, used slash-and-burn to clear land for farming, which was a terribly unsustainable way to do it. What is the most sustainable way to clear temperate forested land (such as the forested land in the pacific northwest), and prepare it for farming of plant crops?
An organic ranch in Virginia would clear forest for pasture by first penning goats into the area for a year or so. The goats would clear out all of the underbrush while producing milk (turned into cheese) and meat. Timber could then be much more easily harvested using traditional techniques - companies would pay slightly higher rates to come in because there was virtually no understory after the goats. The stumps will be problematic, but if you've got time, you could consider inoculating with mushroom spawn and then harvesting mushrooms for a few years while the fungus breaks down the stumps. Stumps will then be much easier to remove. If time is of the essence, you'll have to dig them out, but that will be the case no matter how you go about getting to that point.
I would say a good sustainable way is to move to Agroforestry.
What is Agroforestry?
It is a farming technique of mixing trees, shrubs, herbs and plants on the same farming land. It is most probably the firs form of farming that appeared for forest dwelling humanity: if you stay in the same place generation after generation, select the plants that are useful, edible, and favor there development against others.
Why is it sustainable?
That way you won't need to remove all the trees that are here, and you'll keep a fair part of the underground life system safe.
Moreover, You'll keep a higher biodiversity, and it's quite obvious to me how this is sustainable. But don't hesitate to ask if you need explanation on that point.
Of course, this can't work if you want to use extensive mechanization to plant and harvest in the future... but that's not what "sustainable" is to me.