Note This answer makes the presumption that the crotch in the maple's trunk is indeed a breach through its bark, and does reach the inner woody tissues.
The heartwood of most trees consists of the dead connective tissues, xylem, which surrounded the cells that once formed the living layer of phloem under the protective bark. That connective tissue itself mostly cellulose, and is a polymeric chain of glucose i.e. sugar molecules bonded and linked in such a way as to make them indigestible to most lifeforms.
Certain bacteria and fungi, among others, are able to do so, and they are among the first responders in the decomposition scene. When these microbes, often present in the gut of various insects, are able to break down the woody fibers into forms that permit their re-use by other plants, the detritus is comprised mostly of carbohydrates, alcohols, carbonic acid, and water. Phosphates and various other trace elements which were present in the cells of the woody tissues are released also.
Later, visiting birds and mosses will further augment the ecosystem, adding nitrates and various salts — potassium, sodium, chlorides, and the like.
Another thing that needs to be considered is the drainage. 874
Where does the excess water go? Pressure on the capillaries inside the xylem of the maple should be able to channel some water downwards, but not much will exit through the roots. I've never heard of such a thing. The hole in the maple will widen and exacerbate, deepening the rotted area, but it will be acidic and somewhat saline due to the lack of drainage.
Can it completely sustain another tree? As the ecosystem flourishes, probably; unless the new tree has special requirements for the pH or salinity of its soil which are not maintained by the rotten wood, then it should be able to gain enough nutrition in the form of nitrates, phosphates, and water, but not all of that will come from the other tree directly.
The branches of a tree will only grow so large as can be sustained by their root system, of course. The limited space in which this little sapling has taken root will result in the less than maximal form that you see in the photographs.
So, in conclusion, that spruce will probably not thrive by any means, but it will manage to survive. Spruces do favor more acidic soils — lower pH — than many other trees, so it has that going in its favor.
That much is obvious, I suppose: ten years, and it no larger than a four–year shrub.
There is a problem if all this occurs inside a tree which has not fallen, however. Usually, the outer bark of a tree protects it from such decompositional actions, because the inner woody tissues of most trees do not contain adequate chemicals to deter insects and fungi from chomping or ‘mycelizing’ their way through the heartwood.
Eventually, the maple will either need to be cut down or will be weakened enough that it will collapse. 874