Long-term nitrogen addition can make young and mature trees more vulnerable to drought, but through different physiological routes, according to a study published in Tree Physiology by researchers led by Professor YAN Tao from the Institute of Applied Ecology of the Chinese Academy of Sciences.

Nitrogen is often considered a fertilizer for forests. In temperate ecosystems, where tree growth is often limited by nitrogen, additional nitrogen may seem beneficial. However, this new study suggests that long-term nitrogen addition quietly weakens trees' water transport systems, increasing their vulnerability to drought.

The study 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, enabling researchers to examine how trees at different developmental stages respond to chronic nitrogen enrichment.

The researchers 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 their roots, young trees may strengthen their ability to acquire limiting resources such as water and phosphorus.

However, this strategy came at a cost. Young trees also became more vulnerable to xylem embolism, which is the failure of the tissue that conducts water, and can disrupt the movement of water from the roots to the leaves. In practical terms, young trees may gain 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, the 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 susceptible to sudden hydraulic failure during abrupt drought events. Mature trees, by contrast, may experience a slower, cumulative decline in hydraulic function. The study suggests that different strategies can lead to the same outcome: reduced drought resilience under long-term nitrogen enrichment.

These findings challenge the traditional assumption that nitrogen deposition generally benefits temperate forest growth and carbon sequestration. Rather, the findings demonstrate that nitrogen can alter the relationship between carbon allocation and water transport depending strongly on stand age.

For plantation forests facing both rising nitrogen deposition and more frequent droughts, the implication is clear: 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.