Stand-age variation in leaf N:P stoichiometry and soil nutrient associations at the upper treeline and lower forest limit in arid mountains

The upper treeline and lower forest limit of montane forests mark the leading edges of altitudinal forest distribution and are ecological boundaries that are highly sensitive to environmental change. However, it remains unclear whether trees at these two boundaries experience different types or intensities of nutrient limitation, and whether such differences vary across stand developmental stages. To address this question, we investigated pure stands of Qinghai spruce ( Picea crassifolia ) in the Qilian Mountains on the northeastern margin of the Qinghai–Tibet Plateau. Four stand developmental stages—young, middle-aged, near-mature, and mature—were selected at both the upper treeline (3200 m) and the lower forest limit (2700 m). We examined the altitudinal and stand-age patterns of leaf N:P stoichiometry and explored the environmental pathways underlying these patterns. Leaf N:P increased significantly with stand age at the lower forest limit, whereas no significant difference among age classes was detected at the upper treeline. However, the elevation × age-class interaction was not statistically significant. These are therefore boundary-specific age patterns within the sampled plots, not a statistically confirmed difference in age effects between boundaries. Soil alkali-hydrolyzable nitrogen was the dominant correlate of leaf N:P, and its positive association strengthened progressively with stand age. Soil available phosphorus exerted a significant negative effect only in near-mature stands, possibly indicating a transient window of phosphorus limitation. Near-mature stands also showed significantly stronger interactions between stand age and both available nitrogen and available phosphorus, suggesting that this stage represents the most active life-history phase for nitrogen–phosphorus interactions. Our findings demonstrate that stand age plays a critical role in shaping nutrient association patterns along elevational gradients and provide a basis for developing stage-specific treeline management strategies.