Flashy, decoupled, or declining? Single theories fail to explain the diversity of drought mortality signals in tree rings.

Abstract

Growth patterns recorded in tree rings may predict drought 'winners' and 'losers'. Past studies of drought-killed trees have produced conflicting evidence. Some show killed trees were highly responsive to climate, while others suggest killed trees were climate-insensitive or became less sensitive over time. We leveraged ring width data from 2934 drought-killed and -surviving trees of seven species to compute growth sensitivity to seasonal climate variables via a Bayesian mixed effects model. Aided by clustering analyses, we evaluated how species conformed to three alternative hypotheses (theories): relative to surviving trees, killed trees (H1) have 'flashy' climate responses, (H2) are 'decoupled' from climate, or (H3) have 'declining' sensitivity over time. Differences in growth patterns were not consistent across species or status (surviving/killed). Drought-killed subalpine fir and Engelmann spruce exhibited 'flashy' growth - higher sensitivity of growth to climate over time - compared with survivors. Drought-killed aspen, Scots pine, and Norway spruce showed stable, climate-insensitive growth compared with survivors, suggesting 'decoupling' from climate. Most species showed nonstationary sensitivities, but rather than declining, some sensitivities increased, even in surviving trees. Our flashy-decoupled-declining framework links predictions for future drought-induced mortality to potential mechanisms, enhancing ecological and physiological understanding of growth-climate patterns preceding drought mortality events.