Phenological Shifts in Wood Formation Tracked by Frost Rings Across Two Centuries.

Accelerated warming, particularly in mountain regions, is altering plant phenology across ecosystems. However, the extent of these changes varies among species and regions. While phenophases in plant compartments such as leaves or flowers are relatively easy to observe, monitoring xylem phenology remains challenging due to the labour-intensive methods required to capture intra-annual growth dynamics. Here, we adopted an indirect retrospective approach to infer cambial phenology by analysing the timing and occurrence of frost damage in the growth rings of three Alpine conifer species. Increment cores were collected from 4481 individuals (1897 Larix decidua Mill., 980 Picea abies L. Karst., and 1604 Pinus cembra L.) at two high-elevation sites in the Eastern Alps. Frost rings were identified, dated, and compared with long-term (1774-2020) daily temperature records to determine their timing, using the 1.7°C threshold for cambial activity onset and subsequent episodes when minimum temperatures dropped below 0°C. We found that the cold spells responsible for frost ring formation remained consistent, typically involving temperatures dropping below freezing for an average of 4-5 consecutive days. However, the timing of frost ring formation, and thus cambial onset, has shifted over the past 200 years with no significant differences across taxa. This shift corresponds to about 7 days earlier per century or per °C of warming and is notably smaller than phenological shifts reported for other plant compartments in the same region. Given the critical role of cambial activity in forest carbon dynamics, these findings can help refine global vegetation models and improve predictions of ecosystem responses to climate change.