 Some Hydnellum species , including H. ferrugineum and H. scleropodium , form a tough mat of mycelia in the humus and upper soil of pine forests . This mycelial mat grows larger with old trees , and can cover an area of several square meters . These areas generally lack dwarf shrubs and promote the vigorous growth of mosses ; reindeer lichens often occur in the center of large mats . The presence of the fungus changes the nature of the soil , resulting in a thinner humus layer , decreased groundwater penetration , decreased soil pH , and increases in the level of root respiration as well as the quantity of roots . The fungus also decreases the organic carbon and nitrogen concentrations . Soil with the mycelium becomes more podzolized than the surrounding soil . 
