Treelines are an ecotone structured by the interactions of exogenous gradients and endogenous feedbacks. Endogenous feedbacks operate through adult trees that modify their microclimates and impact seedling dynamics. These impacts are assumed to be positive, but the mechanisms remain poorly understood. This study investigated (1) how the distribution of adult trees impacted snowmelt and in turn how snowmelt impacted seedling distribution, survivorship and growth. (2) how adult trees modify thermal regimes and how these regimes influence seedling distribution. The study was conducted at a 60x150m area of interest of a diffuse treeline on Pikes Peak, CO from 2019-2021. 419 seedlings were mapped in 2019 and their survivorship and growth was measured in 2021. Aerial images of snowmelt and thermal regimes were collected in May and August 2021, respectively. A qualitative analysis was used to determine snow distribution and temperature regimes. The relationship between seedling establishment and snow distribution was measured with a chi-squared test and an R analysis calculated seedling density in relation to thermal regimes. Snowpack was heterogeneously distributed throughout the field site, with complete cover in the densely forested area initially and patches of snow cover in the tundra. The snow in the forest melted quickly, and the last patches of snow were distributed on the leeward side of adult trees. There was a significant relationship between seedling establishment and snowpack, indicating that seedlings preferentially established in snow. There was no significant relationship between snowpack and survivorship or between snowpack and the most recent year of growth (mm). In the predawn aerial images of temperature, the trees were the warmest areas, the tundra was the coldest and the interstitial spaces of the forest were intermediate temperatures. Seedlings were absent in the areas of extreme high (2.5-3.5°C) and minimal in areas of low temperatures (-4.5°C) in the predawn thermal imaging and seedling density was bimodal within the middle of the temperature range. The midday aerial images showed that the treetops had the coldest temperatures and the interstitial spaces of the densely forest zones were often the warmest. Seedling density was also bimodal and seedlings were absent in the areas of the highest temperatures (27.5°C-31.5°C). The bimodal aspect of seedling density in both the predawn and midday datasets suggests that seedling density is influenced by temperature through multiple pathways. These results indicate a significant impact of adult trees on seedling microclimates that influence seedling distribution and establishment.