Continuous in situ monitoring of the vertical and horizontal passage of a labeled-water pulse through a boreal Scots pine forest

Abstract. Water labeled with stable isotopes provides a conservative tracer, being neither produced nor consumed, for water flowpaths within soils and root systems. We added a strong, evenly distributed ²HHO label to one m² of soil surface and continuously monitored its passage downward into the soil and upward into the stems of surrounding trees, with the objective of illuminating spatiotemporal lateral root water uptake and overlap. The study was conducted during the historic drought of 2018 in a mature Scots pine (Pinus sylvestris) forest growing on sandy soil in northern Sweden. Continuous in situ isotopic measurements of tree xylem water evidenced root system overlap of six trees within the labeled square meter. This result is consistent with previous estimates from labelled nutrient uptake measurements at this site. However, label uptake differed sharply among trees, even within the same radius; 90 % of the label was taken up by one of the two trees closest to the labelled plot. Horizontal transport rates in tree roots averaged 0.17 ± 0.05 m d^-1, meaning that the arrival of label pulse in tree stems was delayed by 6–33 days from first tree to last. Root water uptake by trees appeared restricted to the upper 60 cm of mineral soil, even at the peak of the drought. Label intensity of the mineral soil weakened throughout the drought, consistent with the notion that the label was being dispersed or diluted. Labeled water recovery was low (3–4 %), and we hypothesize that this was due to a significant upward flux of water into the organic surface horizons. Our data provide a daily and three-dimensional description of the passage of a labeled water pulse, highlighting the heterogeneity in horizontal water transport flowpaths and the uneven partitioning of label among individual trees in a boreal forest.