Unraveling vertical stem CO2 efflux variability in Norway spruce: A decadal perspective

Stem CO2 efflux varies in the vertical profile depending mostly on the variability in stem growth rate and the vertical pattern changes with tree age. Stem CO2 efflux (EA) from trees exhibits both temporal and vertical variability. EA is often measured only at the breast height, making it challenging to accurately upscale these measurements to represent the entire ecosystem. This study examines the potential drivers behind the EA vertical variability and evaluates the long-term changes in the EA ratio between two fixed heights. EA was monitored in two Norway spruce trees over eight growing seasons within a ten-year period (2005–2014). Measurements were carried out simultaneously at three heights (1.3, 3.2, and 4.5 m since 2005 till 2009 and 1.3, 4.5, and 7.5 m since 2011 till 2014) using an automated chamber system. Moreover, meteorological measurements and wood formation analyses were provided. Overall, EA generally increased with the height, though the pattern differed between trees. Assuming that EA at 1.3 m was representative of the whole stem resulted in underestimating the seasonal temperature-normalized EA by 5.3% in one tree and 16.6% in the other. Such variation makes it difficult to generalize the findings. A stem temperature gradient along the stem would contribute to a maximum increase in EA of 4.3%. Conversely, sap flow, which is understood to transport produced CO2 to the upper parts of the stem, had no apparent effect on the vertical variation of EA. Our findings suggest that the vertical variation in EA is primarily driven by the differences in stem growth, with the stem positions below the crown clearly distinguishable from those at the crown base and within the crown. Moreover, the differences in EA between 4.5 m and 1.3 m were not constant, showing a gradual decrease over time. This adds further uncertainty when generalizing observed vertical EA patterns over longer timescales.