Size classes of root diameter and mycorrhizal fungal colonization in two temperate grassland communities

Abstract

SUMMARYThis study addresses the hypothesis that the amount of fibrous roots (i.e., roots > 1.0 mm diameter) colonized by vesicular‐arbuscular mycorrhizal (VAM) fungi in grasslands is dependent upon root morphology. Two grassland communities with contrasting root system morphologies, a restored tallgrass prairie and an ungrazed pasture, were studied over a growing season. Although there was no difference in fibrous root biomass between the two communities, fibrous roots accounted for 99 % of the total root biomass in the pasture; but only 70 % of the total root biomass in the prairie. The length of fibrous roots was greater (P < 0.0001) in the pasture than in the prairie. Correspondingly, the mean diameter of roots in the pasture was 018 mm, but that in the prairie averaged 0.23 mm. Although the total length of fibrous roots colonized by mycorrhizal fungi did not differ between the two communities (P > 0.34), when the colonized lengths of roots within each of three size classes were compared, significant differences between the two communities were found. The ratio of the fraction of the root length colonized within a given diameter size class to the fraction of the total length of root with cortex occurring within that size class was used to compare the degree of association between roots and mycorrhizal colonization among the three diameter size classes. Changes in this ratio, termed the colonization index (CI), indicate that as the diameter size class of roots in a community increases, a greater proportion of the cortical root length within that size class is colonized by mycorrhizal fungi. Furthermore, despite community differences in the cortical and colonized root lengths within each diameter size class, the CI value for each root size class did not differ between communities. These findings suggest that in grasslands the degree of colonization by mycorrhizal fungi is more dependent upon the diameter size class distribution of the fibrous root systems than upon the species composition of the community.