An 1800-year record of Austrocedrus chilensis growth reveals an unprecedented recent decline in the Patagonian forest–steppe ecotone

Recent decades brought persistent warming and declining precipitation to the North Patagonian Andes, raising concerns about the capacity of temperate mountain forests to withstand climate stress at the semiarid forest–steppe ecotone. We assess whether current growth declines in Austrocedrus chilensis are unprecedented in a late Holocene context and whether intensified drought has contributed to a weakening of the relationship between climate and tree growth.

Increment cores from living trees were combined with subfossil cross-sections to develop cross-dated, millennial-length ring-width chronologies at three ecotonal sites in northern Patagonia. Site chronologies were standardized using multiple detrending approaches and integrated into a regional chronology. Recent growth trends were evaluated against long-term variability using chronology comparisons, climate–growth regression models, observed-minus-predicted residual analyses, and drought legacy approaches.

Sustained growing-season warming since mid-1970s, together with more frequent severe droughts after 1997, has induced persistent multi-year moisture deficits since 2009. Across all sites, recent decades show a pronounced, persistent decline in A. chilensis growth that remains evident after accounting for age-related trends and differences in intrinsic growth rates among trees. Millennial records were integrated into a composite regional chronology extending back to CE 178, with a robust common signal from CE 520 onward. For 1917–2022, warm-season temperature and annual precipitation explain 42% of tree-growth variance, increasing to 46% when long-memory hydroclimatic variables capturing multi-year moisture deficits are included. Consistently, model performance deteriorates from the mid-1990s onward, overestimating observed growth and suggesting a loss of stationarity in climate–growth relationships. Residual analyses further show that growth reductions intensify under higher antecedent composite stress and persist for several years after droughts, consistent with drought legacy effects.

Our results reveal an unprecedented, persistent low-growth regime in ecotonal A. chilensis forests over the past two to three decades. This decline falls outside the range of natural variability observed in the millennial-length records and is not captured by long-term climate–growth models, pointing to a weakening of the relationship between climate and radial growth under ongoing warming and recurrent drought. These findings identify antecedent stress and drought legacy effects as key drivers of current forest performance in Patagonian forest–steppe ecotone.