Inter-annual and inter-species tree growth explained by phenology of xylogenesis.

Wood formation determines major long-term carbon (C) accumulation in trees and therefore provides a crucial ecosystem service in mitigating climate change. Nevertheless, we lack understanding of how species with contrasting wood anatomical types differ with respect to phenology and environmental controls on wood formation. In this study, we investigated the seasonality and rates of radial growth and their relationships with climatic factors, and the seasonal variations of stem nonstructural carbohydrates (NSC) in three species with contrasting wood anatomical types (red oak: ring-porous; red maple: diffuse-porous; white pine: coniferous) in a temperate mixed forest during 2017-2019. We found that the high ring width variability observed in both red oak and red maple was caused more by changes in growth duration than growth rate. Seasonal radial growth patterns did not vary following transient environmental factors for all three species. Both angiosperm species showed higher concentrations and lower inter-annual fluctuations of NSC than the coniferous species. Inter-annual variability of ring width varied by species with contrasting wood anatomical types. Due to the high dependence of annual ring width on growth duration, our study highlights the critical importance of xylem formation phenology for understanding and modelling the dynamics of wood formation.

Chilling and forcing temperatures interact to predict the onset of wood formation in Northern Hemisphere conifers.

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.

Climatic and physiological regulation of the bimodal xylem formation pattern in Pinus pinaster saplings.

Seasonality in tree cambial activity and xylem formation encompass large variation in environmental conditions. Abiotic stressors such as warming or drought also modulate plant behavior at species and individual level. Despite xylem formation susceptibility to carbon (C) and water availability, it is still unknown which are the key physiological variables that regulate xylogenesis, and to what extent plant performance contributes to further explain the number of cells in the different phases of xylem development. Xylogenesis and physiological behavior was monitored in saplings of Pinus pinaster Aiton, a bimodal growth pattern species, distributed in different irrigation regimes. Xylogenesis and plant physiological behavior were compared between treatments and the relationship between climate, physiology and the number of cells in the cambium, enlargement and cell-wall thickening phases was evaluated. Xylogenesis regulation shifted from physiological to climatic control as cell differentiation advanced to mature tracheids. The number of cells in the cambium increased with assimilation rates and decreased with the water potential gradient through the plant. Enlargement was the most susceptible phase to plant relative water content, whereas no physiological variable contributed to explain the number of cells in the wall thickening phase, which declined as temperatures increased. All treatments showed a bimodal growth pattern with a second growth period starting when primary growth was completed and after plants had experienced the highest summer hydraulic losses. Our study demonstrates the importance of including physiological responses and not only climate to fully understand xylogenesis, with special attention to the enlargement phase. This is critical when studying species with a bimodal growth pattern because the second growth peak responds to internal shifts of C allocation and may strongly depend on plant hydraulic responses and not on a fine tuning of cambial activity with soil water availability.