Ultimately nothing is sustainable. Entropy increases. Eventually the sun goes out.
Ecologists refer to the 'carrying capacity' This is the amount of a resource in an ecological cycle that can be extracted without making a significant difference to the overall system.
The trick is that word 'significant'
So, for example, if you plant a forest on new land, it will initially not sequester carbon very fast. The trees are small. Then it will sequester rapidly, tying up carbon in wood mass. When the forest reaches climax, it no longer increases it's carbon mass. -- wood and leaves are rotting/being eaten as fast as they are growing on a year round scale.
I did the calcuations for my local poplar forest, estimating the total biomass at around 100 tons per acre. The maturation time of an aspen poplar forest is around 60 years in our climate. That gives me a 1.6 ton/year gross production over the life cycle of the forest.
So by one measure, I could extract something like 1 ton of biomass from the forest per acre per year, and do this indefinitely.
So that could be considered sustainable.
But wait: I take that wood away, and burn it or make chip board or fence posts (bad idea, rots to fast) and with the celluose I'm taking small quantities of Phosphorus and potassium.
Indeed: places where they have done intensive forestry, after about 3 generations of trees being removed, they have problems with mineral deficiencies. (That's removing something like half the biomass each time.) (Source Forestry Practices course 40 years ago in college.)
So if I'm burning wood for heat and return the ashes is it sustainable.
Maybe. That wood ash has calcium converted to CaO which reacts with water to make Ca(OH)2 which is quite alkaline. While sustainable in the sense that I've closed the cycle, it may make some radical changes in the soil chemistry.
It is no longer the same ecosystem.
Perhaps it would be better to estimate a 'sustainability' factor. How many times can this operation be repeated (or years) before the measurable impact occurs.