Sourcing seeds and tissue cuttings Because the common garden design depends on knowledge of mycorrhizal fungi community composition from a previously conducted observational study, definitions of southern, middle, and northern range plants and fungi likewise depend on the sampled area from that study. The study’s 17 sites spanned from a southernmost latitude of 34.5389 (Buck’s Pocket State Park, Alabama) to a northernmost latitude of 46.6054 (Tahquamenon Falls State Park, Michigan) and from a westernmost latitude of -88.2884 (Natchez Trace State Forest, Tennessee) to an easternmost latitude of -80.5249 (Mountain Lake Biological Station, Virginia). Each of the ten target tree species has an individualized collection area within that range based on the sites at which it was found. Latitudinally dividing the rectangle formed from the most south, north, west, and east latitudes and longitudes at which I collected a given species’ mycorrhizal fungi into thirds yields that species’ collection area’s north, middle, and south. I additionally constructed a model using the NicheToolBox package in R (Osorio-Olvera et al., 2020) and GBIF observations within a target species’ collection area that models the top three CHELSA biovariables predictive of the species’ range within each region (north, south, middle) of its collection area. If seed or cutting sources from outside of a species’ collection area have climates that fall within the range of at least 2/3 of the model’s top three variables for a region and a latitude that aligns with the same region, I accepted the seed or cutting source as representative of those from within the region’s collection area. Seeds were either gathered from Georgia, Tennessee, Indiana, or Michigan in the fall of 2023 or sourced from commercial or state government suppliers. Cuttings of Tsuga canadensis and Acer rubrum will be taken in summer of 2024 from sites in Georgia, Indiana, and Michigan. Propagating seeds and cuttings All seeds will be surface sterilized in .5% bleach and 70% ethanol, following a protocol from the Hawkes Lab (Hawkes, 2009). Depending on species-specific recommendations, seeds will either then be sown directly into soil or scarified in sterile water and stratified in moist sand in the fridge before sowing. Sand will be autoclaved for three days in a row prior to contact with seeds to remove risk of mycorrhizal colonization. Cuttings of Acer rubrum and Tsuga canadensis will be kept on ice in the field and transported back to the University of Tennessee, Knoxville greenhouses. Each cutting will be treated with Hormodin 3 and rooted in sterile sand before being transplanted to soil. Soil was gathered from the two common garden sites, ORNL and UMBS, and likewise sterilized via three consecutive days of autoclaving. The propagation medium will consist of a mixture of 50% sterile soil and 50% sterile sand. Seeds and cuttings destined for the southern common garden will only be planted in ORNL soil, and seeds and cuttings destined for the northern common garden will only be planted in UMBS soil to reflect the reality that seeds and spores will move but soil will not. Plants will remain in the greenhouse until transported to the common gardens for planting, likely in the spring of 2025 and the spring of 2026. Inoculating plants with mycorrhizal fungi Soil with mycorrhizal inoculum will be collected from southern and northern sites whose fungi were previously characterized (Segal and Kivlin, in prep). Soil samples will be collected as they were previously – from under the canopy of 5 individuals of each target species using a soil corer, pooled, and kept on ice in the field. Spore extractions will be conducted at the University of Tennessee, Knoxville following a protocol adapted from Allen et al., 1979. Extracted spores will be suspended in sterile water at 4ºC until spore extractions have been conducted for soils associated with all target tree species from northern and southern sources. Plants will factorially receive 25ml of spore extract from a northern or southern source of the same species, such that half of each species’ plants grown from northern, middle, or southern seed stock receive northern fungal inoculum and the other half receive southern inoculum. Establishing common gardens The two gardens at ORNL and UMBS will have identical design (3 seed sources x 2 mycorrhizal fungal sources x 10 tree species x 5 replicates = 300 plants per common garden). Both gardens will be located in open areas to fulfill young plants’ light requirements. Plants will be transplanted into larger containers with local soil both to allow the plants more space and mimic interactions between migrating soil taxa and established soil microbiomes, while also keeping plant biomass contained for ease of processing once the experiment ends. Containers will be buried such that their tops are level with the soil surface to insulate the plant. Plant height and number of leaves will be recorded at the beginning of the experiment and at weekly intervals during the growing season. Aboveground and belowground biomass will be measured at the conclusion of the experiment.