To determine if it really was decomposers causing increased litter, scientists placed mesh bags with leaf litter collected from areas with no radiation in forests with varying amounts of radiation (and areas of no radiation as a control). They found that after a year, at least 70% of leaves were decomposed in areas of no radiation, but decomposed much less in areas with radiation due to inhibited microbial decomposition. Since less litter is being broken down in these areas, this could lead to the decreased tree growth they have observed there, as well.
Areas with more litter also have a higher fire danger, and could redistribute radiation particles.
http://www.smithsonianmag.com/science-nature/forests-around-chernobyl-arent-decaying-properly-180950075/?no-ist