Larger direct than indirect effects of multiple environmental changes on leaf nitrogen of forest herbs

Aims: Disentangling direct and indirect effects of global change drivers on plant nitrogen (N) uptake in leaves is important for understanding species and community responses in a changing world. Methods: We created understorey herb communities on forest soils with and without recent agricultural history. We traced pulse additions of ¹⁵NH₄ ¹⁵NO₃ within these mesocosms while applying two-level factorial treatments of N enrichment, warming and illumination. We modelled direct and indirect effects of treatments on leaf N content and ¹⁵N uptake in leaves. Results: Warming and illumination had three times larger direct negative effects on leaf N content per dry mass and percentage leaf N derived from label (Ndfl%) than their indirect negative effects via an increasing community cover. These results imply a tissue dilution of N with increasing growth, in response to environmental change directly and indirectly exacerbated by community cover. We additionally found that interspecific differences in Ndfl% correlated with a species’ colonisation capacity and resource acquisition strategy. Conclusions: Global change can directly affect allocation of N into foliage, with simultaneous indirect effects via altered community properties that influence individual plant responses. Predicting the future of plant communities in a changing world requires accounting for such understudied pathways.