Tree species physically and chemically engineer their local environments, shaping forest floor and topsoil carbon stocks

: Tree species are increasingly recognized as key drivers of soil biogeochemical processes, yet their combined chemical and physical effects on forest floor dynamics, microclimate, and mineral topsoil carbon stocks remain insufficiently understood. Using the Geographical Arboretum of Tervuren, Belgium, a 120-year-old common garden established on loess-derived silty Retisols, we quantified how tree species identity and mycorrhizal association shape litter traits, humus formation, microclimate buffering, and carbon distribution between the forest floor and the 0–20 cm mineral soil across 24 monospecific stands. Tree species generated distinct humus forms ranging from mull to moder and mor systems, resulting in strong contrasts in forest floor development and carbon stocks. Total carbon stocks, including the forest floor and topsoil, ranged from 7.49 kg·m −2 under arbuscular mycorrhizal (AM)-associated angiosperm stands to 12.95 kg·m −2 under ectomycorrhizal (EcM)-associated gymnosperm stands. EcM-associated gymnosperms stored a significantly larger proportion of the carbon stocks in the forest floor, around 14%, compared with 4%–6% in the other functional groups. Fresh leaf base cation concentrations were strongly associated with forest floor mass and topsoil pH, suggesting that tree species modified carbon cycling mainly through litter-induced changes in soil chemistry near a pedogenetic threshold. Forest canopies strongly buffered temperature extremes relative to the adjacent meadow, with summer maxima up to approximately 10 °C cooler inside the forest, but these physical effects varied mainly with canopy and understory characteristics and did not directly explain soil carbon stocks. Exploratory piecewise structural equation models and targeted carbon models indicated that carbon stocks were more closely linked to litter chemistry and soil acidity than to measured microclimatic conditions. These findings highlight tree species selection as a key driver of long-term soil carbon storage and vertical carbon distribution, which should therefore be explicitly considered in climate-smart forestry, restoration, and assisted migration strategies.