Arbuscular mycorrhizae (AM) and ectomycorrhizae (EM) are, globally, the most common mycorrhizal associations. During primary succession in temperate and boreal systems, the dominant mycorrhizal type may change from AM to EM. Read (1991) hypothesized that this shift is driven by the accumulation of soil organic matter as AM fungi use inorganic nutrients whereas EM fungi can mineralize organic material. The objective of this study was to test the strength of Read's hypothesis on a cottonwood (Populus trichocarpa) dominated floodplain chronosequnce ranging in age from 0 to 70 years. This system is useful in studying the shift of mycorrhizal groups, as well as the mechanisms, because cottonwoods may form both AM and EM, We measured EM and AM abundances, along with changes in edaphic properties across the chronosequence. AM fungal abundance (based on mycorrhizal inoculum potential and soil hyphal lengths) increased rapidly after disturbance, peaked around 10 years, and then declined. The number of EM colonized root tips increased rapidly to near 70 percent during the first five years. When the abundance of soil root tips were considered however, overall EM abundance increased linearly post disturbance. Significant changes occurred in soil pH, phosphorus, potassium over time. Most notably soil organic matter increased ten-fold during the first 28 years post disturbance. In summary, both AM and EM fungal abundances changed according to Read's hypothesis. However, the high number of EM colonized root tips in early succession, despite the low soil organic matter, was surprising and may suggest additional functions of EM in early succession.