Tree physiology is essential for interpreting Miyake events

There are likely many smaller Miyake events still unfound, and recognizing those from normal variability requires a better understanding of tree physiology. During the past decade or so, the number of annually resolved tree-ring 14C measurements has increased drastically, as researchers have competed to find yet undiscovered events during the Holocene. Indeed, many such events have since been identified, as reviewed by Heaton et al. (2024). The majority of these rapid 14C increases are thought to originate from extreme solar particle events associated with flares or coronal mass ejections. Searching for such events is important as it helps us to understand the risks similar events could pose to modern society. A further and perhaps surprising benefit for archaeologists has been the ability to tie these peaks to historical events with high precision. If archaeological material contains the fingerprint of a 14C event whose occurrence year is precisely known that material can, in principle, be dated to a specific calendar year. This approach has found many successful applications, from precisely dating historical volcanic eruptions to resolving archaeological chronologies. Recent eye-catching examples include dating the onset of long-distance trade networks at Ribe, Denmark (790 ± 10 CE; Philippsen et al., 2022) and pinpointing Viking presence in the Americas (1021 CE; Kuitems et al., 2022). To fully appreciate Hessl et al.'s contribution, it helps to recognize that how the tree-ring 14C signal is shaped can be understood through two broad categories of processes: first, 14C production and atmospheric dynamics; second, tree physiology. The first begins with 14C being produced in the upper atmosphere either by galactic cosmic rays (GCRs) or, in the case of Miyake events, by energetic solar particles. For GCRs, the production of 14C is latitudinally more uniform, whereas for solar particles, the production happens almost exclusively within polar areas (Usoskin et al., 2023). Following production, the 14C quickly oxidizes into 14CO2 and becomes entangled with atmospheric dynamics. Large-scale transport of 14CO2 is governed by the Brewer-Dobson circulation, which pumps air upward in the tropics and moves it poleward. Another key dynamic is the isolation of air within the polar vortex and its breakdown during boreal spring, and the associated injection of stratospheric air into the troposphere. Full mixing is not instantaneous; an excess remains in the stratosphere for years, contributing to the long tail observed in tropospheric 14C concentrations. The production and atmospheric processes described above have been the main focus of the past decade in the study of Miyake events. In this author's view, however, there are still gaps in our understanding of these dynamics. Observations hint at complexities yet to be fully explained, including latitude-dependent intensities (Uusitalo et al., 2018), earlier signal onset at high latitudes (Miyake et al., 2022), and variations in peak shapes across locations (Zhang et al., 2022). The former two are plausible given that 14C is produced within polar areas, so higher latitude sites are closer to the source, while dilution may occur further from the origin. Current carbon cycle box models are not designed to account for such spatial and temporal heterogeneity. Fortunately, ongoing work is developing more advanced, dynamic 3D models that better capture these processes (Golubenko et al., 2025). This brings us to the second category: tree physiology. While the first has been the main focus of Miyake event research, the second has been largely neglected, and this is where Hessl et al. make their important contribution. When researchers compare tree-ring 14C records from around the world, it is often implicitly assumed that trees capture the same well-mixed tropospheric air through photosynthesis and are therefore fully comparable. Although few would claim this assumption is strictly true, and although the topic comes up at workshops and conferences, it is rarely discussed at length in scientific literature and ends up more often as a passing note. This makes the contribution by Hessl et al. even more timely, as it gives full focus to the issue and tackles it systematically, from photosynthetic capture, to wood growth, to carbon storage effects and utilization. Following the seasonal stratospheric injection of 14CO2 into the troposphere, two key factors come into play: photosynthetic capture and wood growth. Though controlled by many of the same variables, like temperature, species, and location, they do not always coincide; critically, earlywood growth can begin before photosynthesis, requiring trees to draw on stored carbon from previous years. The CO2 uptake period depends on tree type (deciduous or evergreen) and temperature, with higher latitudes having shorter growing seasons. This means the sharp seasonal 14C increase gets sampled differently. Trees with long photosynthetic periods smooth out the intra-annual peak, while those at higher latitudes may capture a steeper part. As Hessl et al. detail, wood growth is further affected by water and light availability, tree age, size, and canopy position. Taken together, even trees at the same location may record subtly different 14C signals. In addition to these factors, there is a third, and possibly most important factor concerning Miyake events: tree carbon storage. The carbon captured through photosynthesis by trees may not only be used for the current year's growth but may also be stored for later use to support metabolism or growth. Therefore, any given year's wood may incorporate previous years’ carbon, thereby potentially increasing or decreasing the current year's 14C concentrations depending on the age of the stored carbon. In their paper, Hessl et al. review this effect thoroughly, noting that stored carbon can range in age from days up to decades, with considerable variability depending on, among other things, species, tissue type, and individual stress factors. This effect is not new to the 14C community. It has been noted, for example, that for studying annual 14C variations in deciduous trees, latewood may be preferable as it appears less affected by internal carbon stores (e.g. McDonald et al., 2019). However, experimental work to systematically evaluate these effects across different tree types, species, and environmental conditions is still needed. To illustrate the potential magnitude of these effects, Hessl et al. create a simplified model where they simulate Miyake-like peaks while assuming different carbon storage properties. By varying those properties, such as storage turnover rate and the ratio of stored carbon used for earlywood growth, they find noticeable differences between the resulting peaks, with deciduous trees showing more dampened peaks due to their larger reliance on carbon stores. This observation is important, as it may help explain variations in peak shape across locations, either independently or, more likely, in combination with production and atmospheric effects. Despite these potential sources of variation, it is important to note that overall, the agreement between different 14C records is remarkably consistent for Miyake events (Büntgen et al., 2018). The subtler differences between records, however, could tell an important story of the finer dynamics affecting recorded concentrations. There are likely many smaller Miyake events still unfound, and recognizing those from normal variability requires a better understanding of tree physiology. This includes smaller events such as those c. 1052/1055 CE and 1279 CE (Terrasi et al., 2020; Brehm et al., 2021), and the transient offset between high- and mid-latitude tree-ring records following the Carrington solar storm of 1859 (Uusitalo et al., 2024). A better understanding of both atmospheric dynamics and tree-related processes would help determine whether those anomalies represent actual sudden 14C production events or arise from other sources. To sum up, the process by which 14C ends up in tree rings as measurable concentrations is multifaceted and complex, requiring understanding from the perspectives of production, transport, and tree physiology. The former two have gained more attention with respect to Miyake events, while tree physiology has received less. In their work, Hessl et al. have done an excellent job of systematically reviewing and concretely demonstrating the importance of photosynthetic timing, wood formation, and trees' internal carbon storage effects. Their work is an important step toward a more complete understanding of Miyake events. And it matters. The risks posed by future energetic solar events are neglected compared to other natural hazards, yet our modern society, with its reliance on electrical infrastructure and interconnected systems, is more vulnerable than ever. As with many natural phenomena, the question is not if, but when they occur. Improving our understanding is essential. This work has been supported by the Research Council of Finland project WELT (355266). The New Phytologist Foundation remains neutral with regard to jurisdictional claims in maps and in any institutional affiliations.