Chilling and forcing temperatures interact to predict the onset of wood formation in Northern Hemisphere conifers
Abstract
The phenology of wood formation is a critical process to consider for predicting how trees from the temperate and boreal zones may react to climate change. Compared to leaf phenology, however, the determinism of wood phenology is still poorly known. Here, we compared for the first time three alternative ecophysiological model classes (threshold models, heat‐sum models and chilling‐influenced heat‐sum models) and an empirical model in their ability to predict the starting date of xylem cell enlargement in spring, for four major Northern Hemisphere conifers (Larix decidua, Pinus sylvestris, Picea abies and Picea mariana). We fitted models with Bayesian inference to wood phenological data collected for 220 site‐years over Europe and Canada. The chilling‐influenced heat‐sum model received most support for all the four studied species, predicting validation data with a 7.7‐day error, which is within one day of the observed data resolution. We conclude that both chilling and forcing temperatures determine the onset of wood formation in Northern Hemisphere conifers. Importantly, the chilling‐influenced heat‐sum model showed virtually no spatial bias whichever the species, despite the large environmental gradients considered. This suggests that the spring onset of wood formation is far less affected by local adaptation than by environmentally driven plasticity. In a context of climate change, we therefore expect rising winter–spring temperature to exert ambivalent effects on the spring onset of wood formation, tending to hasten it through the accumulation of forcing temperature, but imposing a higher forcing temperature requirement through the lower accumulation of chilling.
What the paper shows and why it matters
(AI-generated)
Terrestrial ecosystem models mostly tie wood formation to leaf phenology or to generic temperature functions, because the timing of wood growth is far less understood than that of leaves. Earlier studies asked when the cambium, the wood-forming cell layer, resumes activity in spring, and proposed three temperature rules: crossing a threshold; accumulating heat above a base temperature (a heat sum); or a heat sum whose requirement depends on accumulated chilling, shown to work for Douglas fir (Ford et al. 2016). Each was tested separately, and according to the paper they had not been compared on the same data. Heat-sum models capture that spring warmth restarts growth, but they assume that dormancy ends on a fixed date, so that heat alone sets the timing. The paper asks which temperature rule best explains when wood formation starts.
The study finds that the spring onset of wood formation in four Northern Hemisphere conifers is best predicted by a model in which both chilling and forcing (warm) temperature act, not by a threshold or by heat accumulation alone. It compared the three model types, plus an empirical regression, by Bayesian inference on wood-formation records from 220 site-years in Europe and Canada, for Larix decidua, Pinus sylvestris, Picea abies and Picea mariana; onset was the start of xylem (wood) cell enlargement. The chilling-influenced heat-sum model, in which more chilling lowers the heat needed, received most support for all four species and predicted validation data with a 7.7-day error on average. Its predictions of site means showed virtually no spatial bias despite large environmental gradients, which the study reads as environmentally driven plasticity (trees adjusting to local conditions) outweighing local adaptation.
The study concludes that warmer winters and springs act in two directions: more forcing heat hastens onset, but less chilling raises the forcing requirement. It therefore questions predictions of a continuing advance of onset under warming, and proposes the chilling-influenced model as a building block for wood-phenology modules in ecosystem models. The study notes that direct evidence for a chilling effect on xylem onset is lacking and that some fitted chilling thresholds look low, so the mechanism needs experimental tests. Tumajer et al. (2021), who share one co-author with the paper, follow its heat-sum delimitation of active cambium in their wood-growth model.
References
- Ford et al. (2016). Will changes in phenology track climate change? A study of growth initiation timing in coast Douglas‐fir. Global Change Biology 22, 3712–3723. doi:10.1111/gcb.13328
- Tumajer et al. (2021). Forward Modeling Reveals Multidecadal Trends in Cambial Kinetics and Phenology at Treeline. Frontiers in Plant Science 12, 613643. doi:10.3389/fpls.2021.613643