Does this mean that this is positively affecting the radioactive halife of the area because the fungi are essentially living off the excesses radiation present and will help contribute to reduce the radiation levels over time?
However, every gamma it adsorbs, is, well, adsorbed. Its probably not a very good shield compared to sandwiches of iron and water, but its probably better than empty air. Perhaps in the future you could grow a reactor core using a tree. This would make a good minecraft mod.
If it were very aggressive about growth, maybe it would physically block rainwater from hitting the contaminants and running off contaminating downstream. Rainwater hitting harmless fungus and flowing away would help, a little.
To take your analogy the torch will loose battery at the same rate but with the plants leaves receiving more light it would be less light that goes in the face of the person standing behind the plant. Do the plants absorbing radiation lead to less radiation being around to be absorbed by humans and animals instead? Maybe I am fundamentally misunderstanding something here.
It would be like building a firewall that acts like any other firewall, except under DDOS conditions it uses a whole network of diodes and stuff to store up the electrical energy of each attack packet. This doesn't work with routers because they take maybe "hundreds of watts" to run but the received laser power might only be "hundredths of watts". For a fungi that grows slow enough and has no competition in its niche, gammas are apparently enough.