Long-term nitrogen addition can make young and mature trees more vulnerable to drought, but through different physiological routes.

Nitrogen is often thought of as a fertilizer for forests. In temperate ecosystems, where tree growth is frequently limited by nitrogen, more nitrogen can seem like an obvious benefit.

But a new study published in Tree Physiology led by Professor Tao Yan at the Institute of Applied Ecology, Chinese Academy of Sciences, suggests a more complicated picture. Long-term nitrogen addition may quietly weaken trees’ water-transport system, increasing their vulnerability to drought.

The work was based on a long-running nitrogen-addition experiment in Larix principis-rupprechtii plantations at Saihanba in northern China. The experiment, established in 2010, spans young, intermediate and mature stands, allowing researchers to examine how trees at different developmental stages respond to chronic nitrogen enrichment.

The team found that young and mature trees did not respond in the same way. Under high nitrogen addition, young trees appeared to shift carbon belowground: soluble sugars declined in leaves and branches but increased in roots. At first glance, this looks like an adaptive strategy. By investing more carbon in roots, young trees may strengthen their ability to acquire limiting resources such as water and phosphorus.

But that strategy came with a cost. Young trees also became more vulnerable to xylem embolism, a failure in the water-conducting tissue that can disrupt the movement of water from roots to leaves. In practical terms, young trees may be gaining resource-foraging capacity while losing hydraulic safety.

Mature trees followed a different path. They did not show the same clear pattern of carbon redistribution. Instead, high nitrogen addition reduced several key indicators of hydraulic function, including predawn water potential, Huber value and leaf-specific hydraulic conductivity. These changes suggest that the entire water-transport continuum — from root water uptake to stem transport and leaf water supply — may be weakened in older trees.

The risks also differ by age. Young trees may be more exposed to sudden hydraulic failure during abrupt drought events. Mature trees, by contrast, may face a slower, cumulative decline in hydraulic function. Different strategies, the study suggests, can lead to the same outcome: reduced drought resilience under long-term nitrogen enrichment.

The findings challenge the traditional assumption that nitrogen deposition generally benefits temperate forest growth and carbon sequestration. Instead, they show that nitrogen can alter the coupling between carbon allocation and water transport in ways that depend strongly on stand age.

For plantation forests facing both rising nitrogen deposition and more frequent drought, the implication is direct: more nitrogen does not necessarily mean stronger trees. Forest management strategies may need to account for stand age when assessing drought risk and long-term plantation stability.