Plant morphological and physiological traits are stable in a nitrogen-saturated tropical forest after 18-year nitrogen additions

Background and aims

While nitrogen (N) deposition often alters plant functional traits in N-limited ecosystems, its long-term effects in N-saturated forests across various plant types remain unclear.

Method

We examined the responses of 15 leaf and 22 root traits in eight species of trees, shrubs, and herbs after 18 years of N addition (control: 0; N50: 50; N100: 100; and N150: 150 kg N ha⁻¹ yr⁻¹) in an N-saturated tropical forest in southern China.

Results

The results indicated that N addition had negligible effects on leaf and root traits in trees, shrubs, and herbs. The leaf economics spectrum more effectively differentiated among plant types than the root economics spectrum; however, significant shifts in nutrient acquisition strategies were not observed under the N treatments. Although total leaf phenotypic plasticity remained stable across all plant types, herbs uniquely demonstrated increases and decreases in the integration of leaf and root traits, respectively, at higher N levels. Significant negative correlations between phenotypic plasticity and integration were only observed for the root functional traits of herbs.

Conclusion

The results highlight stabilization of above- and underground plant traits under prolonged N addition in tropical N-saturated forests. These findings are essential for improving trait-based predictions of vegetation dynamics in increasingly N-rich ecosystems.