Wood-decay Polyporales reduce the volume of dead wood in the forest and are an important component of the carbon cycle. Wood is composed of primarily three types of tissue: lignin, cellulose, and hemicelluloses. White rot species of Polyporales are efficient degraders of the decay-resistant polymer lignin, leaving partially degraded cellulose as a residue. Brown rot species break down the cellulose fibres, leaving a brittle, brown lignin residue. Brown-rot residues such as humus can remain in the soil for hundreds of years, increasing aeration and water-holding capacity.