Root-gap dynamics in a lodgepole pine forest: ectomycorrhizal and nonmycorrhizal fine root activity after experimental gap formation

Belowground responses to aboveground disturbance were studied in experimental gaps created in a 95-year-old Pinus contorta ssp. latifolia (Engelm. ex Wats.) Critchfield stand, southeastern Wyoming. We hypothesized that active fine root densities within the canopy gaps would remain comparable with densities in undisturbed forest and would not decline abruptly until a threshold number of trees had been killed, after which the root gap would be recolonized gradually and centripetally. One-, 5-, 15-, and 30-tree clusters were felled to create a series of gaps in the root mat of lodgepole pine forest soil. For 2 years, densities of ectomycorrhizal (ECM) root tips were determined by intensive soil coring in the experimental gaps and in a nearby 8-year-old clearcut. Data on fine-root activity were obtained using mesh screens inserted into the soil and incubated for up to 5 years. The results were consistent with the original hypothesis, but no clear threshold gap size was observed for ECM root tips. Densities of active ECM root tips measured along transects through one of the 30-tree gaps decreased quickly within 5-6 m of the forest edge, and did not significantly differ from zero at the center of the gap; in contrast to the 2-year-old gap, active ECM densities were fairly constant across the 8-year-old clearcut. Moreover, root-tip densities declined gradually with increasing gap size, while fine root penetrations of the mesh screens were significantly lower in the 30-tree gaps than in the smaller gaps. During the first 3 years, fine roots intercepting the screens were nonmycorrhizal; at the final removal, root screens that had been incubated for 1837 days in the gaps, including the 30-tree gaps, contained ectomycorrhizal root tips. The root screen removal data suggested that belowground gaps persisted 5 years after initial clear-felling of the 30-tree clusters, but the screens also provided evidence for closure from the edges of these gaps