Identifying the main drivers for the production and maturation of Scots pine tracheids along a temperature gradient

- We identified the most important factors for the inter-annual tracheid production.
- The rate of tracheid production was partly weather-independent at the warmest sites.
- Temperature influenced tracheid production in the middle of the temperature range.
- Photosynthesis influenced tracheid production in coldest sites.
- All successful models for the mature cell production included photosynthesis.

Even though studies monitoring the phenology and seasonal dynamics of the wood formation have accumulated for several conifer species across the Northern Hemisphere, the environmental control of tracheid production and differentiation is still fragmentary. With microcore and environmental data from six stands in Finland and France, we built auto-calibrated data-driven black box models for analyzing the most important factors controlling the tracheid production and maturation in Scots pine stem. In the best models, estimation was accurate to within a fraction of a tracheid per week. We compared the relative results of models built using different predictors, and found that the rate of tracheid production was partly regular but current and previous air temperature had influence on the sites in the middle of the temperature range and photosynthetic production in the coldest ones. The rate of mature cell production was more difficult to relate to the predictors but recent photosynthetic production was included in all successful models.