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Marked_sdlg_site_EDI: The plots are 0.05 hectares or 500 m2 in size measured in a 12.62 m radius circular plot. In June-July 2019, sugar maple seedlings from the 2018 cohort were double marked and mapped along a 50 m by 1 m transect running first 25 m East and then 25 m West from the plot center, if necessary, until either 50 seedlings or 50 m had been reached (Figure S1). Seedlings were assessed for leaf number, leaf size and leaf damage. Leaf damage was on a five point scale (less than 5% damage, 6-25% damage, 26-50% damage, 51-75% damage and greater than 75% damage). Leaf size was standardized using a leaf sizer (sugar maple leaves drawn onto a transparency) and every leaf was assigned a size to within half size classes. These classes were then converted to leaf area (cm2) for each seedling, In 2019, seedlings were reassessed for leaf damage in August. Every June from 2020 to 2023, the seedlings were reassessed for status and leaf metrics.

Each plot was characterized for average slope and slope aspect. The slope in the four cardinal directions and the main slope of the plot were taken using a clinometer 5m on each side of plot center, and slope aspect was taken using a compass. Slope aspect was decomposed into two linear variables: Eastness and Southness, unitless and derived from the aspect using the sine and cosine, respectively. Soil cores (N=12 per plot, composited) were taken using a 2 cm diameter metal push corer every 5 m along N-S and E-W diameters of the plot. For each core, the length of the organic horizon was recorded and all 12 cores per plot were composited. Soils were air dried and analyzed for pH and NH4Cl extractable bases at Cornell Nutrient Analysis Lab in November 2019. Tree data: tree number and sugar maple basal area were from tagged tree inventories detailed in that data set.

Litter_coll_ForestSoc_2yr: On 6 plots per site, three crates (0.096 m2 catch area each) were placed in each plot: at plot center and 10 m N and S of center in August 2019. Collectors were lined with nylon screening to allow water to pass through. In 2020, collectors were raised to 1 m on fence posts. Collections of leaves and seeds were made after leaf fall (late Oct – early Nov) in 2019 and 2020 and all leaves and seeds were sorted to species and counted. Samples were separated into sugar maple litter and all other species and dried and weighed. Due to animal disturbances, collections from two of the sites, Sudrabin and Kauffmann, were often less than N = 18.

Sweep_ForestSoc: Plots were established in autumn of 2018. In general, 12 plot locations were established for each of the four forest stands. Plots are spatially-uniform and placed as close to a 100 m grid system as possible with the restriction that the plot had to include three canopy sugar maple trees. The plots are 0.05 hectares or 500 m2 in size measured in a 12.62 m radius circular plot. The understory tree composition of each plot was measured in a 2 m wide transect that spanned the north–south diameter of the plot. Tree saplings ≥2 cm dbh and <10 cm dbh were measured to the nearest 0.1 cm dbh, identified to species and noted as live or dead

Tagged_trees_ForestSoc: Plots were established in autumn of 2018. In general, 12 plot locations were established for each of the four forest stands. Plots are spatially-uniform and placed as close to a 100 m grid system as possible with the restriction that the plot had to include three canopy sugar maple trees. The plots are 0.05 hectares or 500 m2 in size measured in a 12.62 m radius circular plot. Within the plots, all live trees ≥10 cm diameter at breast height (dbh; 1.37 m above ground surface from upslope side of tree) were tagged, measured to the nearest 0.1 cm dbh, identified to species and assessed for vigor on a 4 point scale: 1) healthy, 2) unhealthy (< half canopy in good health) 3) dying (one branch or less of tree in good health) and 4) dead (reassessment surveys only). All tagged trees were reassessed for vigor in 2021 and 2023. In 2023, tree diameters were also remeasured for growth. In 2021 and 2023, any new tree to reach 10 cm in diameter were tagged and added to the survey.

Common_garden_sdlgs: Between 18 April - 9 May 2022, soil was sampled from each of the four sites in areas that were either directly under a mature sugar maple (abbreviated: ACSA; N=5) or underneath a different tree species and with no sugar maple litter visible (other; N=5). Leaf litter was cleared, and a gallon bag was filled with soil within 8 cm of the soil surface. Soil samples were refrigerated until the setup of the common garden experiment. In mid-May, soils from the same site and microsite (ACSA or other) were combined and passed through a hard cloth screen prior to potting.

To test the effects of soil origin both between and within sites on sugar maple seedling growth, a common garden experiment was conducted (5 replicate pots per treatment). Recently germinated sugar maple seedlings were collected from Bethlehem, NH on 16 May 2022. We used large pots (vol = 800 mL) to accommodate anticipated biomass gain below 1 g L-. In addition to four sites with two areas per site (N = 40), we also ran two controls. The first control was a sterilized (100°C for 48 hours) homogeneous mixture of soils from all four sites, a pathogen free control. The second control was soil from the seedling collection site, a control for potential benefit of seedling origin soil. The full experiment had 50 pots that had two seedlings each for 100 seedlings at the start of the experiment.

The experiment ran from 16 May - 9 August 2022. The seedlings were watered every two days. There were three rows of trays that were rotated weekly in a screen tent. At the end of the experiment, one seedling from each pot was randomly chosen. Seedlings were measured for leaf number and damage. Leaves were traced to obtain leaf area. Above and below ground biomass were obtained by cutting seedlings at the root collar and drying in the oven (60°C for 48 hours) and weighing.

ACSA_samaras_2019: The high production year allowed us to score the condition of seeds and compare pre-dispersal damage across sites. Pre-dispersal damage was greater at Yatsevitch a site dominated by sugar maple.

2020_germinant_counts: Germinants resulting from the 2019 mast year were counted at each site in mid-May 2020 and germination was notably higher at Kauffmann. This site had one seedling for every 38 seeds compared to Yatsevitch which had one seedling for every 128 seeds, 3.4 times more seeds needed to successfully produce a seedling.