Estimating tree-ring growth using the radiocarbon bomb pulse

In the 1950s and 1960s, atmospheric nuclear bomb tests caused a significant and rapid increase of the atmospheric radiocarbon content, almost doubling it in 1963 (Hua et al. 2022). Known as the ¹⁴C bomb pulse, this provides a clear timestamp for materials formed during this period and afterwards. It has proven invaluable in tracking carbon cycle dynamics and environmental changes (Levin & Hesshaimer 2000). It can be used in any process which exchanges carbon with the atmosphere or incorporates carbon from the atmosphere such as plants.

Here we present a study using the rapid atmospheric radiocarbon fluctuations of the 1950s & 1960s to assess the tree-ring growth pattern and growing season length of five Scots pine trees from five Norwegian sites, from 63°15’ to 69°24’ N, over a period of 15 years (6 years using direct measurements and 9 years using indirect measurements). For each tree, rings within the period 1950-1965 were sliced into the largest practical number of subannual sections (up to 10), depending on the width of the ring in the sample. After cellulose extraction, the ¹⁴C content of each increment was measured to a high precision.

Cumulative wood formation usually follows a sigmoid shape, with slower growth during spring and early summer, faster growth in midsummer, and decreasing activity towards the end of the vegetation period (e.g. Schmitt et al. 2004). In European and North American conifers of cold environments, the onset of cambial activity can vary from the beginning of May to early June, depending on intra-annual weather-, snow-depth- and soil conditions (Vaganov et al. 1999; Deslauriers et al. 2003; Rossi et al. 2007; Hettonen et al. 2009). Despite these variations, maximum tree-ring growth rate seems to be limited to a short period, which in most European and North American conifer species is about the time of maximum day length (Rossi et al. 2006).

By adapting the sigmoid growth curves to match the ¹⁴C results of the cellulose increments to the atmospheric signal, we obtain a growth curve and growing season length which are independent from other assumptions. Comparison to the presumed growing season parameters (start, end, and length) derived from meteorological data, then, provides a valuable source of information to understand the connection between tree growth and environmental parameters. The ¹⁴C bomb pulse, acting as a magnifying lens, this research will help to understand the connection between atmospheric CO₂ isotopic values and that of the tree-rings formed under these conditions