Photoperiod and temperature as dominant environmental drivers triggering secondary growth resumption in Northern Hemisphere conifers

Wood formation consumes around 15% of the anthropogenic CO2emissions per year and plays a critical role in long-term sequestration of carbon on Earth. However, the exogenous factors drivingwood formation onset and the underlying cellular mechanisms arestill poorly understood and quantified, and this hampers an effective assessment of terrestrial forest productivity and carbon budget under global warming. Here, we used an extensive collectionof unique datasets of weekly xylem tissue formation (wood formation) from 21 coniferous species across the Northern Hemisphere (latitudes 23 to 67°N) to present a quantitative demonstrationthat the onset of wood formation in Northern Hemisphere conifers isprimarily driven by photoperiod and mean annual temperature (MAT),and only secondarily by spring forcing, winter chilling, and moistureavailability. Photoperiod interacts with MAT and plays the dominantrole in regulating the onset of secondary meristem growth, contrary toits as-yet-unquantified role in affecting the springtime phenology ofprimary meristems. The unique relationships between exogenous factors and wood formation could help to predict how forest ecosystemsrespond and adapt to climate warming and could provide a betterunderstanding of the feedback occurring between vegetation and climate that is mediated by phenology. Our study quantifies the role ofmajor environmental drivers for incorporation into state-of-the-artEarth system models (ESMs), thereby providing an improved assessment of long-term and high-resolution observations of biogeochemicalcycles across terrestrial biomes.