Rain travels through the forest canopy to soils along two paths: throughfall (TF, water dripping through gaps/from surfaces) and stemflow (SF, water that runs down stems). TF and SF affects the quantity/quality of water available to humans & ecosystem processes across scales, including microscale patterns of fine root and soil microbial community structure, mesoscale stream discharge and chemistry, and continental-to-global variability in evapotranspiration. Canopy-rainfall interactions reduce system water inputs by 20-50%, can supply >100 kg of dissolved solutes ha-1 year-1 to soils, and affect global and regional mean temperatures by up to 2 C. Despite this importance, no work has coordinated a continental Macrosystem effort to measure, scale & predict the particulates traffic riding these hydrologic highways to the surface. Ignoring these highways and their particulate traffic introduces error in water & nutrient flux models at the first point where terrestrial biogeochemistry and hydrological cycles entwine (which may cascade those errors through all downgradient processes). Therefore, this project seeks to extend current Macrosystem biological understanding to include TF and SF particulate concentrations, fluxes and composition, specifically addressing 3 major objectives: (1) Estimate the net rainfall (TF+SF) water and particulate mass flux across forest types; (2) Characterize the particulate composition (C:N:P, including C components, like total C, organic C, black C, and microplastic C) of TF and SF; and (3) Identify major drivers of macrosystem variability in net rainfall particulate flux and composition.