High-resolution insights into the seasonal physiological responses of Great Basin bristlecone pine (Pinus longaeva) through in-situ δ13C LA-IRMS

Predicting the physiological responses of tree species under future hydroclimate scenarios is essential for understanding and mitigating the impacts of anthropogenic climate change. Here, we present our work on reconstructing the intraseasonal physiological responses of one of the longest living tree species on Earth – Great Basin bristlecone pine (Pinus longaeva, Pinaceae). This species is infamous for its tree ring chronologies that can extend beyond 5,000 years, yet the key physiological traits that will determine its ability to tolerate warmer, drier conditions in the future remain to be characterized. Moreover, the extent to which localized changes in topoclimate and seasonal water availability will impact overall growth performance and survival is also uncertain. To address this, we collected needle samples from trees along an elevation gradient near Great Basin National Park, NV, USA. Using the unique phyllotaxy of this species, we isolated annual needle samples corresponding to five distinct growth years (2018 – 2022). We then developed a novel method for quantifying intraseasonal variation in carbon isotope discrimination and intrinsic water use efficiency by using fine-scale, sequential measurements of needle δ¹³C in-situ via laser ablation isotope ratio mass spectrometry. We obtained an average of 25 individual δ¹³C measurements along the lengths of each needle sample, which were all consistent with whole-needle δ¹³C values measured via traditional elemental analyzer isotope ratio mass spectrometry. However, these sequential δ¹³C values varied in excess of 1 ‰ (VPDB) along the lengths of each needle sample, likely reflecting intraseasonal changes in water availability through the time in which individual needles were being constructed. Paired with our previous measurements of annual ring width, stomatal density, and needle length from trees at this site, we discuss how this new method provides a more comprehensive understanding of the role of intraseasonal variation in water availability on the overall physiological performance of this species in the past as well as under future hydroclimate scenarios.