High latitude Siberian boreal forests are underlain by permafrost, characterized by short growing seasons and low precipitation (< 300 mm per year), and are dominated by the tree genus Larix. Patterns in stomatal conductance and tree rings suggest that precipitation is a driver of Larix productivity. However, the influence of water availability across the rooting zone versus aboveground controls on plant water relations is not well described in these ecosystems. We evaluate the belowground vs aboveground drivers of transpiration in two high latitude Larix forests near Cherskiy, Russia in summer 2016. We focus on a closed canopy stand with shallow (<50 cm) permafrost thaw depths and an open canopy stand with deeper permafrost thaw depths (<100cm) to examine i.) how does canopy transpiration and stomatal conductance differ between the two stands? ii.) how do above- and belowground controls on transpiration vary across the growing season? Transpiration and stomatal conductance were measured using sap flow probes. Fine root biomass was measured throughout the active layer every two weeks. Soil moisture was collected with each root sampling and continuously collected with sensors across the growing season. We also measured plant water potential, leaf area, and sapwood in each stand.
The stand with shallow permafrost thaw (closed canopy) had higher average soil moisture than the stand with deeper permafrost thaw (open canopy). The closed canopy stand had a shallow root distribution limited to the upper 15 cm in early July by the permafrost table, and by mid-August roots reached as deep as 45 cm with 50 % of the root distribution in the upper 14 cm. The open canopy stand had thaw depths ranging from 64 cm (early July) to 96 cm (mid-August), and the depth with 50% of the root distribution shifted from 13 cm to 19 cm throughout the course of the growing season. Despite having deeper rooting zones, the open canopy stand did not have higher stomatal conductance or canopy transpiration. Patterns in stomatal conductance also showed a high sensitivity to moderate vapor pressure deficit and air temperature, particularly in the open canopy stand. The plant water potentials were also higher at the closed canopy stand compared to the open canopy site. These results suggest that precipitation and shallow soil water sources influence Larix plant-water relations more than water from deeper permafrost thaw.