SPATIAL PATTERNS OF MYCORRHIZAL INFECTIVENESS OF SOILS LONG A SUCCESSIONAL CHRONOSEQUENCE

This study quantified intersite variation and spatial pattern in arbus cular mycorrhizal (AM) and ectomycorrhizal (ECM) infectivity of soils among six sites constituting a successional chronosequence in south-we stern Ohio, USA. The study sites included an active agricultural field (chronic disturbance), a site which had been stripped of its surface soil (pseudo-stripmine, acute disturbance), 5- and 10-year-old fields, a 25- to 30-year-old prairie restoration, and an undisturbed, mature forest. AM infectivity was lower in the agricultural field, succession al fields, and prairie than in the mature forest, but there was no cle ar correlation between time since disturbance and the overall level of AM infectivity. Spatial structure in AM infectivity de creased with t ime since disturbance. In the pseudo-stripmine site and active soybean field, semivariance analysis attributed 44-50% of the total variance in AM infectivity among samples to spatial structure, whereas spatial dependancy accounted for only 18% of total variance in the mature fore st. Kriging of AM infectiveness demonstrated small, isolated areas in the disturbed plots that were devoid of AM infectiveness, whereas the kriged AM maps of the other four sites showed AM infectiveness to beco me progressively more homogeneous. ECM infectiveness was lacking from 35-50% of the samples from the disturbed sites, and both overall ECM i nfectiveness and ECM diversity increased with time since disturbance. Approximately 44% of the variance in ECM infectiveness was related to spatial structure in the two disturbed sites, and large areas entirely devoid of ECM infectivity were present on the kriged ECM maps for the se sites. There was less spatial structure in ECM in the old fields an d prairie and very little in the mature forest. The results of this st udy emphasize the need to explicitly evaluate spatial heterogeneity in mycorrhizal infectivity in studies of the role of mycorrhizae in succ ession.