Collaborative Research: MRA: Particulates in canopy flowpaths: A missing mass flux at the macrosystem scale?

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Forests cover one-third of the land on Earth. For rainfall to pass through the forest canopy, it must drain along two “hydrologic highways”: throughfall (water that drips through gaps and from leaves or bark); and stemflow (water that runs down stems). As throughfall and stemflow drain, they wash particles from leaves and bark. Although tiny, particles washed from the canopy by these hydrologic highways can constitute a significant chemical input to the soil, and represent a wide range of materials, from nutrients to pollutants. Despite this, no large-scale effort has sought to measure, scale, and predict the amount and quality of particles descending down these hydrologic highways. These particles are generally “missing” from current ecological theory of how forests cycle elements. This study seeks to fill this gap by monitoring storm conditions, throughfall, stemflow, and the particles in these hydrologic highways across sites representing major forest types in North America. Results will link throughfall and stemflow to common models used to inform freshwater and forest management. Outcomes will inform outreach efforts, including science comics and illustration exhibits, open-access articles written for (and reviewed by) primary and secondary school children with Frontiers for Young Minds, and YouTube videos with MinuteEarth, a channel with an international viewership of millions. The project will also provide research experiences to members of underrepresented groups to broaden participation in science.For 40% of the North American continent and one-third of global land surface, rainfall must pass through forests to reach the soil surface. This rainfall is partitioned by the forest canopy into two net rainfall fluxes: a drip flux called throughfall (TF), and a flow of water down stems, called stemflow (SF). How much rain travels along these hydrologic highways can alter water supply by 20-50%, and what they carry from the canopy can supply 100 kg per hectare of various materials to the soil surface each year. These canopy ecohydrological processes are on the front line of climate and land use change, being that the forest-rainfall interactions that initiate terrestrial hydrological pathways and supply nutrients/pollutants to the surface are the first ecosystem elements impacted by hydrologic intensification. Ignoring these fluxes, and their particulate traffic, introduces error in water and nutrient flux models at the first point where terrestrial biogeochemistry and hydrological cycles entwine, and which may cascade those errors through downgradient processes. This project aims to extend current macrosystem biological understanding to include throughfall and stemflow 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. Field monitoring of the above variables across 11 sites of the National Ecological Observatory Network representing the major US forest domains allows links to be tested between existing functional characteristics and the practical integration of throughfall and stemflow dynamics into continental-to-global scale biogeophysical models. The project will also support research training of a postdoctoral researcher, masters and doctoral students, and a technician.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.