The question cannot be answered without additional environmental info;
vis the area in which the trees are lying. If you have a suitable
microclimate - moderately damp on the underside of the logs, not too much
direct sun, modest amounts of organic matter in the topsoil, etc. - then
the use of enzymes such as cellulase would be practical. It would not be
prohibitively expensive if you use an organism that produces/uses the
enzyme in its quest for nutrients.
Commercial cellulase is produced in factories using the fungus
_Aspergillus niger_ (I believe other aspergillids are also used). The
product "BEANO" is the first thing to come to mind. It is not "cheap,"
but I am convinced that there must be a huge profit margin for the
producers! The notion of using bacteria or fungal fermentation to
produce this enzyme is not a new one.
So, if the microclimate id right, innoculate with a wood cellulose
digesting fungus, wait a while, then break up the log (mechanically) and
reinnoculate. I do this in my back yard and have reduced the cycle time of
rotting wood from 5 years (for firewood-sized chunks) to about 2 years.
I actually do not have to innoculate - I have plenty of fungi floating
around the yeard as it is! I also have a healthy population of
"wood lice;" the little isopods that love to chew on the wood and the