Dense understories have limited influence on evaporative interception loss in temperate deciduous forests

In temperate forests, changes in understory vegetation density and composition are reshaping stand structure, with potential consequences for the forest water balance. In eastern North America, American beech (Fagus grandifolia) sapling proliferation is forming dense understories in sugar maple (Acer saccharum) stands, which could increase evaporative interception losses. However, the contribution of understory vegetation to interception remains poorly understood. This study assessed how understory density and composition, specifically beech sapling proliferation in sugar maple stands, influence rainfall partitioning into throughfall, interception and stemflow. In Québec (Canada), we selected three plots with beech proliferation (beech sapling basal area > 2.0 m² ha⁻¹) and three control plots (< 0.5 m² ha⁻¹). For 31 rainfall events, incoming rainfall was measured with rain gauges in open areas and throughfall was measured using ground-based collectors arranged in a grid. Stemflow was monitored on a limited number of saplings (one sugar maple and two beech). Interception losses ranged from 15% to 47% of incoming rainfall and did not differ significantly between proliferation and control plots, despite the leaf area index (LAI) being on average 1.33 times higher in proliferation plots. Interception was instead controlled by meteorological conditions and decreased with rainfall intensity. Given a stemflow funneling ratio up to 20 times higher in beech than in maple, beech proliferation is likely to shift rainfall partitioning toward stemflow and away from throughfall. These findings suggest that understory changes can alter hydrological pathways to the forest floor (stemflow vs throughfall) but have limited influence on evaporative interception losses.