Enhancing hydraulic trait visibility in Eucalyptus pauciflora subsp. niphophila tree rings Implications for snow-gum dieback analysis in Kosciuszko National Park

Abstract

Subalpine forests in south-eastern Australia are among the most vulnerable to rising temperatures, facing the threat of collapse, yet the mechanisms by which their xylem anatomy records and responds to climate variability remain poorly understood. This study addresses two key gaps in dendrochronological research: the absence of a standardised methodology for enhancing hydraulic traits without compromising those of other xylem structures, and the lack of published studies on vessel based climate proxies in Australia. Focusing on Eucalyptus pauciflora subsp. niphophila from Kosciuszko National Park, this research developed and tested methods to improve vessel and ring boundary visibility while additionally assessing the climatic sensitivity of hydraulic traits. Results showed that low concentrations of safranin, particularly at 1%, optimally enhance anatomical xylem traits without compromising measurement accuracy. The use of wet chalk as a vessel-filling medium, produced more consistent vessel measurements and simplifies the analytical workflow. Age-dependent increases in vessel lumen area reflected hydraulic adjustments to growth, emphasising the importance of standardisation procedures for removing age-related growth trends. Moreover, the study reveals that applying traditional standardisation methods and protocols associated with signal strength indices, such as the expressed population signal (EPS), may not be suitable for vessel-based chronologies compared to ring-width series. Anatomical anomalies coinciding with droughts revealed adaptive mechanisms towards hydraulic safety, which reduced vessel formation and increased parenchyma proportions. Climatic correlations revealed that especially, vessel traits respond positively to early-spring temperatures, while vessel development might be constrained by elevated summer temperatures. These elevated summer temperatures can potentially be buffered by precipitation and cloud cover, mitigating thermal stress. Overall, this study demonstrates that vessel-based traits in subalpine E. pauciflora subsp. niphophila are sensitive to intra-annual climate variability, confirming their value as complementary proxies to traditional ring-width chronologies. The findings provide a methodological framework for future multiproxy dendrochronological research in Australia and highlight the potential of vessel anatomy to improve climate reconstructions and early detection of hydraulic stress in subalpine forests.