Past Precipitation Stored in Deep Soils Sustains Greening of Dryland Tree Plantations in Northern China

Abstract Deep soil water ( θ d ), defined here as past precipitation stored in deep unsaturated soils and not replenished by precipitation in a single growing season, plays a vital role in helping trees withstand prolonged droughts in deep‐vadose‐zone regions. However, its contribution to total water use across tree growth stages and the effects of limited θ d access on tree transpiration and photosynthesis remain unclear. To address this, a process‐based model was parameterized using in situ root‐zone measurements (from the soil surface to the apparent maximum root depth) and then used to simulate root‐zone soil moisture, canopy transpiration and photosynthesis for two common species, apple ( Malus domestica fuji ) and black locust ( Robinia pseudoacacia ), in northern China. For trees aged 3–22 years, θ d below 200 cm ( θ d200 ) accounted for an average of 31.9% and 40.9% of total water use in apple and black locust trees, respectively. Restricted access to θ d200 led to decreases in annual transpiration rates and daily photosynthetic rates by 19.7% and 17.4% in apple, and by 26.2% and 20.2% in black locust. On a monthly scale, precipitation and transpiration greatly influenced θ d200 for both species, while tree age and diameter at breast height were key annual determinants. These findings highlight a trade‐off between physiological stability achieved via deeper rooting and the associated carbon costs of accessing θ d . The findings here provide insights into sustainability of planted trees in deep vadose‐zone regions.