Basically, radiation is effecting the ability of natural decomposition processes in the exclusion zone. A study was conducted with natural leaves in panty hose bags within the exclusion zone and outside it. 60% percent of the weight of the bags was still retained in the exclusion zone after one year.

This is a major concern because the exclusion zone (particularly the Red Forest) has 27 years of litter (leaves and other plant material) which is a fire hazard. A large fire could redistribute the radioactive material outside of the exclusion zone.

Hope that helps.