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Effects of Soil Fumigation and Seeding Regimes on Secondary Succession in a Semiarid Shrubland

In order to ascertain the degree to which disturbance and subsequent recovery of belowground biotic components (mycorrhizal and decomposer systems) influence successional dynamics, a portion of semiarid shrubland in northwest Colorado was mechanically disturbed in 1984. Two treatments, soil fumigation (fumigated with methyl bromide and not fumigated) and seeding (early-seral species, late-seral species, and not seeded) were imposed on 500 m2 plots in each of four replications. Aboveground annual production by species, and decomposition rates were measured in 1991, 7 years after the initial disturbance and treatments. Mycorrhizal infection potential had been previously measured at this site from 1984-1988. Seeding to late-seral species resulted in greater production rates (and greater community composition) of late-seral species, whereas seeding to early-seral species had only a minor effect when compared to unseeded plots 7 years after disturbance. Fumigation slowed the rate of succession on unseeded plots, primarily by decreased production of perennial grasses. Mycorrhizal infection potential, initially greatly reduced by the disturbance and by fumigation, recovered to near-background levels on unfumigated plots but not on fumigated plots after 4 years. Decomposition experiments suggest that seeding may have facilitated recovery of the decomposer system after initial disturbance and fumigation. Our results suggest that the redevelopment of soil biotic communities following disturbance has a significant impact on successional dynamics, particularly the replacement of early-seral annuals by mid-seral perennial grasses.