Tree-Ring Widths, Wood Density, and Anatomical Measurements

We measured TRW on the radiographic images with CooRecorder v. 9.3.1 and visually and statistically crossdated the TRW series with CDendro (Cybis Elektronik and Data AB, Sweden). For each ring on the same image, we also measured the average ring density (AVD) with CooRecorder, which is the AVD over the whole ring, and the maximum latewood density (MXD), which is the highest density value of the latewood taken from the 20% darkest pixels within the ring (Björklund et al., 2019).

We used the software ROXAS v3.0 (von Arx and Carrer, 2014) to quantify xylem anatomical parameters and ring area and width in the anatomy-selected trees. The borders of the tree rings were drawn manually based on the cross-dating procedure previously performed for the same trees. For each annual ring, we quantified xylem anatomical parameters related to hydraulic efficiency, spatial vessel distribution and connectivity, wood density, and growth. Thus, we measured hydraulically weighted mean vessel diameter (Dh, calculated as Σd5/Σd4, where d is the cross-sectional mean lumen diameter of each vessel; Kolb and Sperry, 1999), theoretical hydraulic conductivity (Kh) based on Hagen-Poiseuille’s law (Tyree and Zimmerman, 2002) and theoretical xylem-specific hydraulic conductivity (Ks) calculated as Kh/ring area as parameters related to hydraulic efficiency. We studied spatial vessel distribution and connectivity with the parameters vessel density (CD, number of vessels per area) and vessel grouping index (RVGI, mean number of vessels per group; von Arx et al., 2013). We measured fiber cell wall thickness (CWT) in all fibers within the ring area analyzed as a parameter related to wood density and mechanical support (Ziemińska et al., 2013; Prendin et al., 2017). Finally, we measured the number of vessels per tree ring (CNo), mean TRW, and ring area (RA) as growth-related parameters. For all these parameters, we obtained a mean value for each annual ring.

We used anatomical data to calculate wood density profiles. For this, we divided each tree ring in the image into 50 μm-wide sectors (s) parallel to the ring border (corrected for the ring boundary curvature, Supplementary Figure 3). The size of the sectors was chosen so that they could contain at least one row of complete vessels. We calculated the mean density for each sector assuming a fixed density of wall material (y = 1.504 g cm-3; Kellogg and Wangaard, 1969) using the following equation (modified from Peters et al., 2020):

where the wall area of each cell (c, both for vessels and fibers) was calculated using CWT and cell lumen area (A, assuming an elliptical shape using α and β). We excluded the area of ray parenchyma from the equation, because our samples only showed scattered uniseriate rays and, within the 4.3 mm-wide sections, these were not representative. We excluded the area of axial parenchyma because it was also poorly representative within the sections analyzed. We built wood anatomical density profiles for all types of cells combined (wood anatomical density), and separately for the vessel tissue and the fiber tissue. See Peters et al. (2020) for more details on the method used.