FORMATION of the mor type of forest litter, characterized by slow decomposition, has been attributed to a number of factors1–3, but the effect of mycorrhizal fungi, though postulated4,5, has not been studied experimentally. We have conducted an experiment in which root activity and thus mycorrhizal activity were reduced in small areas of an unthinned Pinus radiata stand. The following treatments (three replicates in a randomized block design) were applied to 1 × 1 m plots sited between the trees.