Continuous increase in atmospheric nitrogen deposition caused by fossil fuel and fertilizer use has profoundly affected forest ecosystems. However, previous studies have mostly focused on trait changes at the species level, leaving a gap in our understanding of how functional traits at the community level respond to long-term nitrogen deposition, and the respective roles played by intraspecific and interspecific variations.

An eight-year nitrogen addition experiment now suggests that nitrogen treatment had little impact on the traits of the upper understory, yet it altered leaf growth and defense traits with strong intraspecific variation in the lower layer, positioning intraspecific trait variation as a vital indicator for monitoring forest responses to nitrogen deposition.

The experiment was carried out by the restoration ecology team at the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS). The relevant findings were published in Forest Ecology and Management on June 20.

The team assessed the response of understory plant growth and defensive traits to nitrogen deposition at the community level. The results showed that after eight years of nitrogen addition treatment, the functional traits of the upper understory community (mainly composed of small trees and large shrubs) remained stable overall and were not significantly affected. In contrast, the lower understory community (mainly composed of shrubs and herbaceous plants) exhibited clearer responses.

Among them, growth traits such as leaf phosphorus concentration and mineral concentration, as well as defensive traits such as leaf lignin concentration, showed significant changes, indicating that the impact of long-term nitrogen deposition on the understory community is mainly reflected in key functional traits in the lower understory.

Further analysis revealed that the key mechanism driving the aforementioned changes is intraspecific trait variation. Notably, in the lower layer of forests, the contribution rate of intraspecific trait variation to changes in key functional traits such as leaf mineral concentration and lignin concentration exceeds 50%, which is significantly higher than that of species turnover.

These findings indicate that in the face of environmental changes brought about by long-term nitrogen deposition, understory plants respond more through trait differences and adjustments among different individuals within the same species, rather than solely relying on changes in species composition within the community.

The study also found that the biomass production of understory plant communities is significantly correlated with a few key functional traits. Among them, a larger specific leaf area and higher leaf phosphorus concentration (growth traits) are beneficial for enhancing understory productivity, while higher leaf lignin concentration (defensive trait) is associated with lower biomass.

More notably, these relationships are primarily driven by intraspecific trait variation, suggesting that intraspecific trait changes not only participate in the community's response to nitrogen deposition but also play a significant role in regulating forest understory productivity.

This study shows that under the long-term background of nitrogen deposition, the overall function of the understory community in evergreen broadleaf forests exhibits strong stability. However, intraspecific trait variation can reveal the important ecological regulatory processes behind the changes in community average traits.

According to the researchers, this study emphasizes the importance of intraspecific trait variation in understanding the response of forest ecosystems to global change, providing a new scientific perspective for long-term monitoring and conservation management of forest ecosystems.

JI Lingbo, a graduate student from the SCBG, is the first author of the paper, with contributions from SCBG's Profs. LIU Nan and JIAN Shuguang. Prof. REN Hai and Dr. WEI Liping from the SCBG are corresponding authors.

This research was funded by Science and Technology Program of Guangzhou and the National Natural Science Foundation of China.