Spruce up your climate analysis: Dendroclimatology of Picea engelmannii and Picea pungens

Warming and more variable climates threaten to upend historical tree ranges, climatic sensitivity, and vigor. In western North America, the species Picea engelmannii var. engelmannii Parry ex Engelmann (Engelmann spruce) and Picea pungens Engelmann (Colorado blue spruce) are widespread spruce that act as foundational species in their montane to subalpine habitats. However, there is currently a lack of knowledge on P. pungens climatic responses, and how it differs from P. engelmannii. To address this gap, we assessed the climatic sensitivity and correlates of tree growth in a co-occurring old-growth stand of P. engelmannii and P. pungens, at high elevation in southern Utah, USA. We report the putative oldest cross-dated P. pungens, with 457 rings, and sampled multiple P. pungens >400 years old. Both Picea populations had strongly positive growth responses to May–July precipitation and negative responses to maximum May–July temperature. Notably, October in the previous year had the strongest correlation with growth for both Picea species. Neither population exhibited signs of directional changes in climate–growth responses. Spectral analysis identified peaks associated with El Niño–Southern Oscillation at 3–4 years and quasi-decadal oscillations (18–20 years) in both species. Cumulatively, our results highlight the growth–climate relationships of two frequently understudied subalpine tree species. In particular, P. pungens may warrant further study across its range and identification of other ancient populations.