Abstract: We present a 966-year (1050–2015 CE) ring-width chronology of Pinus californiarum D.K. Bailey from Baja California, Mexico, representing the longest published dendrochronological record for any pine species in the country. Despite pronounced anatomical and ecological challenges, including frequent missing rings and growth irregularities, the final chronology exhibits strong signal strength and robust cross-dating, enabling the assessment of hydroclimatic variability across multiple temporal scales. Radial growth is primarily controlled by winter precipitation, with significant positive responses to both instrumental precipitation and precipitation associated with atmospheric river activity during November–March. Growth is negatively related to maximum temperatures, indicating enhanced drought stress under warmer conditions. Correlations with reconstructed drought indices confirm a strong sensitivity to regional moisture availability. At larger scales, P. californiarum growth is significantly influenced by Pacific climate variability. Positive correlations with the Multivariate ENSO Index and two independent ENSO reconstructions reveal a nonstationary ENSO–growth relationship over the last millennium, characterized by a prolonged interval of weakened teleconnections between approximately 1300 and 1600 CE, followed by strengthened coupling after ~ 1700 CE. Spectral and wavelet analyses further indicate intermittent expression of ENSO-scale variability and lower-frequency modulation consistent with Pacific decadal processes. Together, these results demonstrate that P. californiarum integrates both high-frequency hydroclimatic variability and long-term shifts in Pacific climate forcing, highlighting its value as a sensitive paleoclimatic proxy for the northeastern Pacific region and underscoring the importance of conserving old-growth stands in extreme water-limited environments.

Key words: Dendroclimatology, Tree longevity, Atmospheric rivers, ENSO, Mediterranean-type arid climate