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Soil solarization effects on plant growth variables of field-grown tree saplings

Soil solarization has been used throughout the world, but its effect on plant growth variables in ornamental tree saplings in the Willamette Valley are not well documented. Solarization could be an alternative to chemical controls for soil pathogens and weeds, and it may also influence plant growth factors. This study examined three of these plant growth factors: biomass, shoot length, and arbuscular mycorrhizal fungi (AMF) colonization, which facilitate nutrient uptake. The species used were red oak (Quercus rubra), Mazzard cherry (Prunus avium), and hawthorn (Crataegus monogyna), three of the common tree species grown at J. Frank Schmidt & Son Co. nursery in Boring, Oregon. Solarization plastic was installed on site in early summer 2014 and left for nearly eight weeks, after which plants were seeded. Both a solarized soil and a nonsolarized control treatment were included. Plants were collected on two dates, and biomass, shoot length, and AMF colonization were measured. The significance of differences in observations was determined using a t-test. Red oak showed little difference between treatments. Mazzard cherry had greater shoot length in the solarized treatment early in the season, while hawthorn shoot length was greater in the nonsolarized treatment for both collection dates. Hawthorn root biomass was greater in the solarized treatment on both dates, while hawthorn shoot biomass was greater in the nonsolarized treatment. AMF colonized roots to a greater extent in the nonsolarized treatment for both Mazzard cherry and hawthorn; red oak is not a known host of AMF. All these results showed significant differences. Red oak was examined for ectomycorrhizal fungi (ECM), however too few instances of ECM were observed to make any conclusions. Solarization affected the three species differently. These observations suggest that solarization can reduce AMF colonization slightly, indicating the possibility of serious impact on plant growth depending on the plant species. It is unclear if hawthorn grew less under the solarized treatment due to AMF suppression by solarization. The reduced presence of pathogens in the solarized treatment of Mazzard cherry may explain why this treatment outgrew the nonsolarized. Further research should be done to examine soil solarization effects on soilborne pathogen inoculum and disease susceptibility of Mazzard cherry.