Automated quantification of intra-annual density fluctuations using microdensity profiles of mature Pinus taeda in a replicated irrigation experiment

We developed a new automated method to use wood microdensity profiles to detect and quantify intra-annual wood density fluctuations in earlywood of pine trees. We developed a new, automated method to use X-ray wood microdensity profiles to detect and quantify intra-annual density fluctuations within the earlywood region (IADFe) of each annual growth ring. The method quantifies the number and area of IADFe by detecting variations in wood density beyond the limits of "normal" values observed during periods without environmental stress. As a case study, we examined the effect of water availability and water stress associated with the formation of IADFe in irrigated and non-irrigated 11-year-old loblolly pine trees. As expected, non-irrigated trees formed significantly more IADFe than irrigated trees. Strong relationships were observed between IADFe formation (as a proportion of total earlywood area or as the number of IADFe formed each year) and the minimum monthly Palmer Drought Severity Index for the earlywood growing season (February to July). When compared against visual detection, the number of IADFe detected optically was significantly fewer than the number detected with densitometry. This difference likely comes from the higher resolution and more objective criteria that confidently detected and counted more very small IADFe than was possible optically at 10x magnification. The method to detect and quantify IADFe described in this study can allow climate analysis in long-lived species prone to producing intra-annual growth zones and false rings.