Cutting time slices of tree rings —How intra-annual dynamics of wood formation help to decipher space for time conversion in tree-ring sciences

Despite a long-standing interest in retrieving intra-annual environmental information from tree-ring features, none of the approaches developed so far for accurately dating intra-ring sector has been validated on observations. Here, we investigated space-for-time association across regular intra-ring sectors for which we estimated the timing of formation. For this purpose, a unique dataset containing quantitative wood anatomy measurements and kinetics of tracheid differentiation was compiled for 45 trees grown in North-East France (three years of wood formation monitoring, for five trees, from three different conifer species). Tracheid dimensions were measured directly on the best anatomical sections at the end of the growing season, while the kinetics of xylem cell differentiation were provided at tree-level by an empirical model of wood formation dynamics. Our results confirmed that the time taken to form sectors of the same width increased from earlywood (composed of wide thin-walled tracheids) to latewood (composed of narrow thick-walled tracheids). This mainly reflected the increase of the duration of cell wall deposition through the growing season, and, to a lesser extent, the augmentation of the number of tracheids per sectors. However, our results also show that regular intra-ring sectors, which were well separated in space, overlapped in time. The overlapping culminated during the summer period, reaching 40 % for 10 sectors. It could be reduced to approx. 30 % by increasing the number of sectors (from 10 to 25, for example), but it cannot be removed. Therefore, successive intra-ring sectors could not be attributed to a succession of separated time intervals by simply using their relative position along the ring. However, the formation of sectors of equivalent ranks were noticeably synchronous between the different trees and years, reaching 80 % of synchronicity for the process of wall thickening. This suggest that data from regular intra-ring sectors could be reliably used to build mean chronologies expressing the common signal of tree populations. Our results show the limits that the xylogenesis process itself imposes on the dating of intra-ring features. They also argue for an in-depth understanding of the association between cell differentiation processes (enlargement, wall thickening and lignification) and wood characteristics (density, anatomy, stable isotope composition).