Thermal Imaging of Conducting Sapwood Width Enhances Dendroecological Studies in Southwestern U.S. Conifers

Quantifying conducting sapwood width and area in trees facilitates studies of water and nutrient transport. Yet, distinguishing conducting from non-conducting xylem is not always straightforward due to the species-specific presence or absence of visible color differences. To improve field-based measurements, we present a novel method that uses a thermal infrared sensor to pinpoint the common temperature change at the transition between conducting and nonconducting xylem on increment cores. Sapwood width estimated this way matched the values obtained from the traditional "touch test" in Pinus ponderosa (PIPO; R² = 0.99) and Pinus strobiformis (PISF; R² = 0.98). A more rigorous evaluation against the visible color difference in Pseudotsuga menziesii (PSME) also resulted in an R² = 0.96. In a case study at the Mt. Bigelow flux-tower site (US-MtB, Arizona), we paired these thermal sapwood measurements with observations of tree size, radial growth, and its climate sensitivity. PIPO had a larger sapwood (mean width = 7.6 cm) and exhibited less growth decrease under extreme drought compared to PSME (mean width = 4.7 cm). Conversely, PSME was the most productive species, hinting at its competitive strategy but also drought susceptibility under continued aridification. Based on this successful study, we encourage broad application of this user-friendly and species-independent method of quantifying sapwood width towards research on water, growth, and carbon dynamics.