Blue intensity parameters derived from Ponderosa pine tree rings characterize intra-annual density fluctuations and reveal seasonally divergent water limitations

A set of novel parameters extracted from fine-spatial resolution blue intensity profiles characterizes intra-annual density fluctuations in Ponderosa pine and complements information on climate sensitivity obtained from radial growth. Rapidly rising evaporative demand threatens forests in semi-arid areas around the world, but the timing of stem growth response to drought is often coarsely known. This is partly due to a shortage of sub-annual growth records, particularly outside the Mediterranean region where most intra-annual density fluctuation (IADF) chronologies are based. We anticipate that an automated, cost-effective, and easily implementable method to characterize IADFs could foster more widespread development of sub-annual chronologies. We applied a peak detection algorithm to fine-spatial resolution blue intensity (BI) profiles of Ponderosa pine tree rings from two sites in southern Arizona (similar to 300 m elevation difference). Out of seven BI parameters that characterize IADFs, peak height, width, and area showed satisfactory chronology statistics. We assessed the response of these BI and radial growth parameters to six monthly resolved climate variables and to the onset date of the North American summer monsoon. Radial growth at the lower-elevation site depended mainly on winter precipitation, whereas the higher site relied on spring and monsoon precipitation. A regular May-June drought period promoted IADFs in early ring portions at both sites. Yet, IADFs at the higher site were only formed, if spring was sufficiently humid to assume enough radial growth. Late-position IADFs were caused by a weak monsoon and additionally promoted by favorable conditions towards the end of the growing season. The contrast between sites is likely attributable to a three-week difference in the growing season onset, emphasizing the importance of growth phenology for drought impacts on forests in the US Southwest.