A strengthening earlywood-to-latewood carry-over dominates latewood formation under additive summer VPD–precipitation co-limitation in Mediterranean maritime pine
Abstract
Introduction Mediterranean forests are undergoing rapid atmospheric aridification driven by rising temperatures and declining summer precipitation, and vapor pressure deficit (VPD) has emerged as an important co-limiting factor for tree physiological functioning and radial growth. The degree to which rising VPD and seasonal precipitation co-limit intra-annual growth coordination and modulate climate memory in maritime pine ( Pinus pinaster Ait.), a foundational Mediterranean species, remains poorly quantified. This study quantified the relative effects of summer VPD, seasonal precipitation, and the intra-annual earlywood-to-latewood carry-over on latewood width, testing whether increasing atmospheric aridity erodes lagged climatic legacies and weakens the carry-over. Methods We analyzed earlywood and latewood width chronologies from nine mature maritime pine stands in northern Portugal spanning 1965–2023. Hourly climate reanalysis datasets based (ERA5 and ERA5-land reanalysis) were used to derive seasonal VPD metrics and precipitation totals across ecologically relevant windows. Climate–growth relationships were examined with Generalized Least Squared (GLS) incorporating lagged climatic terms, comparing pre- and post-2000 regimes under two complementary detrending frameworks. Threshold-based VPD accumulative intensity and climate–growth coupling was evaluated across both periods. Results Summer VPD imposed a temporally stationary constraint on latewood width, statistically additive to seasonal precipitation, whereas pre-summer precipitation controls were non-stationary, reversing signs between the pre- and post-2000 periods. Summer precipitation remained the strongest predictor in the baseline framework, while VPD prevailed in decadal-trend and anomaly-based analyses, consistent with an additive co-limitation rather than a replacement of precipitation by VPD. After 2000, current-year VPD sensitivity remained stable while lagged climate signals weakened; most importantly, the intra-annual earlywood-to-latewood carry-over provided the strongest overall explanatory power ( R 2 = 0.30–0.40), remaining statistically independent of VPD and, contrary to our third hypothesis, strengthening rather than weakening in recent decades. Conclusion Our results do not support the displacement of precipitation by VPD or a direct destruction of biological legacies by atmospheric aridity but instead point to an additive co-limitation whose apparent balance depends on the analytical framework adopted. While the lagged (prior-year) climate signal has attenuated since 2000, the intra-annual earlywood-to-latewood carry-over strengthened rather than collapsed and remains the dominant structural driver of latewood formation.