<b>Contrasting strategies, shared risk: 12-yr nitrogen addition compromises hydraulic safety in young and mature trees via distinct pathways</b>

Nitrogen (N) deposition is widely assumed to stimulate growth in N-limited temperate forests, yet how N deposition interacts with tree ontogeny to regulate the carbon–water processes that shape tree resilience, decline, and mortality under environmental stress remains unclear. Here, we used a 12-year anthropogenic N addition experiment (control: no N addition; N20: 20 kg N ha−1 year−1; and N50: 50 kg N ha−1 year−1) spanning young, intermediate, and mature Larix principis-rupprechtii plantations in northern China to test age-dependent effects on hydraulics and nonstructural carbohydrates (NSCs) reserves. We found that, in young trees, N50 decreased soluble sugars in leaves, twigs, and branches, but increased in roots, suggesting a preferential belowground allocation, whereas N20 responses were limited. However, this apparent adaptive response was accompanied by a significant increase in xylem vulnerability to embolism as reflected by the stimulated percentage loss of hydraulic conductivity, indicating a potential trade-off between greater root carbon investment and hydraulic safety. In contrast, mature trees exhibited a systemic impairment of hydraulic function, manifested as the synchronized decline in predawn water potential, Huber value, and leaf-specific hydraulic conductivity under N50, but with no signs of NSC reallocation. For intermediate trees, sapwood-specific hydraulic conductivity and leaf soluble sugars decreased in N50. Our results demonstrate that decadal N addition disrupts carbon-water balance in an age-dependent manner: young trees adopt an active but risky strategy of carbon reallocation that compromises hydraulic safety, mature trees exhibit a passive hydraulic deterioration, while intermediate trees demonstrate a transitional strategy between these two states. Our findings challenge the traditional belief that N deposition universally benefits temperate forests and highlight the need for age-specific forest management. Incorporating these ontogenetic shifts into models is essential for accurately predicting tree performance and dynamics under the combined stress from N deposition and intensifying environmental stress.