Photoperiod drives cessation of wood formation in northern conifers

AimsShifts in xylem phenology directly determine the forest capacity for carbon sequestration. However, a systematic understanding of the spatial patterns and the underpinning drivers in determining the cessation of wood formation (C-cw) is lacking at a pan-continental scale. Here, we addressed this knowledge gap by compiling a new dataset of multiple xylem phenology timings for northern conifers. LocationsSixty-two study sites, Northern Hemisphere (25-55 degrees N). Time period2003-2018 (16 years). TaxaThirty-three conifer species. MethodsA generalized additive model was fitted to characterize the latitudinal pattern in C-cw. Structural equation modelling and a linear mixed-effects model were applied to determine the main drivers underlying the latitudinal pattern in C-cw. ResultsThe C-cw followed a flat S-shaped pattern with increasing latitude. Photoperiod was the dominant determinant of the latitudinal pattern of C-cw, and a longer photoperiod was associated with an earlier C-cw. Both mean growing-season temperature and total growing-season precipitation exhibited significantly positive relationships to the cessation of cell elongation and thus the C-cw across all study sites. In arid regions, the pre-growing-season temperature had a significantly negative effect on C-cw. In humid regions, C-cw was negatively affected by the mean growing-season temperature. The onset of wood formation showed significantly positive coupling with C-cw at arid sites but not at humid sites. Early successional species were sensitive to hydrothermal variations during the pre-growing season. Main conclusionsWe reveal the dominant role of photoperiod in determining the cessation of wood formation for northern conifers and highlight differentiated interactive effects between photoperiod and seasonal climatic factors and the preceding xylem phenophases in determining C-cw among ecoregions and tree species. These insights provide evidence to reduce uncertainty in prediction of the forest carbon uptake potential and the consequent biophysical feedbacks of northern forests.

Automated summary

This study examines the cessation of wood formation in northern conifers and reveals the dominant role of photoperiod in this process. It also highlights the interactive effects between photoperiod, seasonal climatic factors, and preceding xylem phenophases in different ecoregions and tree species.