Conifer fine root and mycorrhizal root biomass within the forest floors of Douglas-fir stands of different ages and site productivities

Live and dead conifer fine root (≤2 mm) and mycorrhizal root biomasses were determined in the forest floors of 16 stands of Douglas-fir (Pseudotsugamenziesii (Dougl.) Forbes) of low (IV) or high (II) productivity. The maximum biomass of both live conifer and mycorrhizal root biomass occurred at canopy closure in site II and IV stands. After crown closure this biomass decreased significantly in site II but not in site IV stands. During and following crown closure, site IV stands generally had a significantly higher live conifer root biomass than site II stands. Only in the 65- to 75- and 150- to 163-year age groupings of stands was a significantly higher live mycorrhizal root biomass measured in site IV than in site II stands. The proportion of dead to total conifer fine root biomass fluctuated from 13 to 56% in site II stands and from 26 to 76% in site IV stands. The percentage of total mycorrhizal roots that were dead fluctuated from 25 to 57% in site II stands and from 19 to 76% in site IV stands. Generally the highest percentage of live root tips infected by mycorrhizal fungi occurred in the 45- and 46-year-old site II stands and in the 33-, 49-, 67-, and 69-year-old site IV stands.