Fig. 2: Changes in plant δ15N and N stress index over time.
From: Progressive nitrogen limitation across the Tibetan alpine permafrost region
a–c Frequency distributions of plant δ15N during the two sampling periods, changes in plant δ15N derived from the large-scale resampling investigations and changes in plant N stress index derived from the DeNitrification-DeComposition (DNDC) model, respectively. Plant N stress index refers to the supply of N relative to plant demand, with a lower value meaning a greater limitation of N on plant growth38,39,40. Temporal dynamics of both indicators were examined with linear mixed-effects models, in which the fixed effect was year and the random effect was sampling site. Points in b denote mean values and error bars represent 95% confidence intervals. The shade accompanying with the solid fitted line in c represents 95% confidential interval. N, the number of sites used for analyzing plant δ15N; median, median value of plant δ15N; df.resid, residual degrees of freedom.
