Surviving a megadrought: shifts in climate sensitivity of an austral conifer in Chile due to persistent water shortage

• A. chilensis climate sensitivity increased due to recent warmer and drier conditions. • Tree growth resilience declined, while iWUE increased as the megadrought progressed. • Trees in mixed stands showed higher growth synchrony but lower drought stress. • Positive mixing effects faded with the persistence of the megadrought. Southern South America has experienced a warming and drying climate trend since the 1970s, culminating in an unprecedented megadrought (2010–2022). Despite reports of declines in forest productivity, there are still limited tree-level assessments of climate sensitivity and drought stress. Here, we analysed the radial growth and intrinsic water-use efficiency (iWUE) of the austral conifer Austrocedrus chilensis (Cupressaceae) across climatic and geographical gradients in central Chile. We aimed to unravel how recent climate shifts have influenced growth patterns (spatial synchrony, year-to-year variability), climate-growth relationships, and drought stress. We compared two contrasting ecological conditions: pure-xeric stands vs. mixed-mesic stands. Although no signs of growth decline or die-off, the increased spatial synchrony and interannual variability, along with stronger climate-growth relationships, indicate heightened climate sensitivity in the tree populations under study. Our findings also suggest rising drought stress, as evidenced by reduced resilience and increased iWUE following the onset of the megadrought. The effect of ecological conditions seems to operate differently at short and long temporal scales. Mixed-mesic stands exhibited greater climate sensitivity but also higher drought resilience and lower iWUE, though differences with pure-xeric stands diminished as the megadrought progressed. This study provides evidence of increased climate sensitivity in A. chilensis to warmer and drier climate conditions, as well as cumulative stress under prolonged drought. These indicators may signal future growth declines, the detection of which is essential for developing effective management and conservation strategies for this unique conifer under a rapidly changing climate.