Soil and Xylem Water Ages in Two Pinus radiata Forests With Contrasting Precipitation Regimes

ABSTRACT Understanding how long‐term precipitation regimes shape the temporal origin of water in soils and vegetation is critical for predicting forest responses to climate variability. We compared two Pinus radiata plantations on New Zealand's South Island that share similar stand characteristics but differ markedly in precipitation: Ashley Forest (735 mm year −1 ) and Mawhera Forest (2934 mm year −1 ). We combined 2 years of soil moisture and stable isotope observations from xylem and soils with isotope‐enabled Hydrus 1‐D modelling to assess differences in soil water dynamics, root water uptake and water age. Soil water at Ashley Forest was consistently older and more variable in age (84–99 days, maxima > 300 days) than at Mawhera (61–66 days, maxima ~180 days), reflecting more limited replenishment under irregular rainfall. Xylem water ages only partially mirrored these differences, averaging 50 days at Ashley and 44 days at Mawhera. At Ashley, episodic precipitation mainly replenished shallow soil, which was rapidly depleted by transpiration, while deeper layers remained isolated and aged. During prolonged dry spells, trees relied more heavily on deeper, older and isotopically depleted water sources. At Mawhera, root water uptake depths remained comparatively stable. Our findings demonstrate that precipitation regimes strongly control soil and xylem water ages, shaping both vadose zone water storage dynamics and plant water uptake strategies. Forests under low and irregular rainfall depend more on episodic events and older storage, increasing drought vulnerability, whereas wetter systems sustain younger water ages that buffer them against climatic variability.