Tangential water transport facilitates crown water supply in mature Norway spruce

Global forests are increasingly impacted by repeated and long-term drought events. The survival of trees under such conditions critically depends on their ability to regulate water use and maintain transpiration even when soil water availability is limited. Internal water storage and distribution within tree stems may hereby play a key role in supporting these processes. However, the mechanisms governing water transport and distribution within mature tree stems remain poorly understood.

To investigate internal water transport, we injected deuterated and dyed water as tracers into three mature Picea abies (Norway spruce) trees located in Tharandt (Germany) on one side of the stem at 50 cm height. Deuterated water movement was monitored by repeated daily sampling of xylem water vapor at 1 m and 3 m above the injection point, from the sapwood on the injection side, the opposite side, and the central heartwood. Water vapor samples were hereby collected by drilling a 10 cm deep, 1 cm diameter hole, which was fitted with inlet and outlet tubes. Dry air was pumped into the hole through the inlet, and the air equilibrated with xylem water was collected from the outlet into glass vials. Water vapor samples were subsequentially analyzed in the lab for their water isotopic composition (²H, ¹⁸O) via cavity ring-down spectroscopy (Picarro 2130-i). Two weeks after injection, the trees were harvested, and stem discs were collected every 2–4 m along the stem to visualize dyed water distribution using image analysis. Additional xylem water samples were extracted from increment cores taken from each disc in the four cardinal directions for isotope analysis. This experimental setup enabled the examination of water transport dynamics along axial, radial, and tangential pathways within the stem.

We found that injected water remained on the side of the injection within the lower 5 m of the stem (detected via water isotope tracing) but started circulating around the stem higher up, completing approximately 1-1.5 helical turns along the trunk (detected via dye-tracing), likely reflecting the spiral growth pattern of spruce wood. Below the crown base, water movement was predominantly axial, whereas above the crown base, tangential distribution became more pronounced, allowing all upper sun crown branches across the four cardinal directions to receive the tracer water.

These findings highlight that tangential water mixing within the stem plays a critical role in supplying water to the entire crown of mature spruce trees. This may become even more important under drought conditions.