Recent research has challenged the long-standing hypothesis that forests in the Upper Midwest of the United States developed during wetter periods and retreated during dry periods. We explored this debate by examining patterns of tree establishment on an oak savanna in east-central Minnesota within the context of variable moisture availability and fire suppression. We used superposed epoch analyses (SEA) to evaluate the mean moisture conditions for a 21-year window surrounding tree establishment dates. Before effective fire suppression (1809–1939), 24 of 42 trees with pith dates (62%) grew to 30-cm height during dry years (Palmer Drought Severity Index < -1), versus only 5 of 42 (12%) that established in wet years (PDSI > 1). Significantly more trees established during dry periods (negative PDSI values) than would be expected with the proportion of wet-to-dry years (x²= 10.738, df = 1, p-value = 0.001). Twenty of the complete sample of 74 trees with pith dates (27%) established during drought in the 1930s. We hypothesize that dry conditions limited plant productivity, which in turn decreased competition between grasses and tree seedlings and reduced rates of accumulation of fine fuels, enabling seedlings to grow tall enough to resist subsequent fires. We recommend SEA as a methodological approach to compare historical climate conditions with the timing of regeneration success in other regions of forest expansion.

The increased soil organic phosphorus (P) mineralization observed under trees compared to pasture has been attributed to the contrasting mycorrhizal associations of the two systems but to date, little work has been conducted comparing P dynamics under different tree species with contrasting mycorrhizal associations. This study investigated rhizosphere P dynamics and P acquisition of three tree species with contrasting mycorrhizal associations (ectomycorrhizal Pinus radiata, arbuscular mycorrhizal Cupressus macrocarpa and tripartite Ecualyptus nitens) using a combination of field and controlled environment studies. Short-term field studies revealed greater acid phosphomonoesterase activity and therefore greater potential organic P mineralization under radiata pine and eucalypt compared with macrocarpa, which correlated with ectomycorrhizal colonization. Related field work revealed that the presence of pasture understory and litter had a significant impact on P dynamics. Pasture acts to increase phosphatase activity and microbial activity within the soil but with a litter layer, the increased activities are mainly above the soil in the overlying litter. A glasshouse study investigated the separate influence of mycorrhizal hyphae and roots and results showed that ectomycorrhizal fungi associated with radiata pine and eucalypt stimulated microbial activity and increased rhizosphere phosphatase activity with a concomitant decline in soil organic P. Macrocarpa and eucalypt roots exuded higher concentrations of low molecular weight organic anions compared with radiata pine which stimulated microbial activity, increased rhizosphere phosphatase activity and decreased soil organic P. A series of experiments were then conducted to further investigate the precise role of mycorrhizae in P dynamics and results showed that ectomycorrhizal colonization increased rhizosphere acid phosphomonoesterase to a greater extent than arbuscular mycorrhizae. Fluorescent staining revealed that the mycorrhizae associated with all three species had similar phosphatase production capacities. The ability of the three species to utilize organic P was investigated in two exhaustive pot experiments. Radiata pine was shown to be the best adapted to utilize organic P, especially in soils with high relative organic P contents. Eucalypt appeared to enhance soil organic P mineralization but did not take up the released inorganic P, while macrocarpa caused limited organic P mineralization compared with radiata pine and eucalypt. This work shows that ectomycorrhizal trees are best adapted to utilize organic P through increased rhizosphere phosphatase activity and stimulated microbial activity. Root exudation of LMWOAs is of higher relative importance to arbuscular mycorrhizal trees to increase microbial activity and consequently phosphatase activities when fewer hyphae are in the rhizosphere are present to produce them.

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