图1. 氮(N)和磷(P)添加对树木生长和森林生产力影响的论文数量和实验数量。
A, 1970-2019年每年的论文数量及累积论文数量。B, 不同N、P添加处理在不同气候区森林的实验数量。N表示该研究中只有N添加; P表示该研究中只有P添加; N, P表示该研究中有单独的N和单独的P添加; N, P, NP表明该研究中包含单独的N、单独的P和N、P共同添加。

Fig. 1. Number of papers and experiments investigating the effects of nitrogen (N) and/or phosphorus (P) addition on tree growth and productivity in forest ecosystems.
A, The number of papers in each year and the accumulated number of papers between the year 1970 and 2019. B, The number of experiments with different treatment of nitrogen and phosphorus addition. N represents only N addition in the case study; P represents only P addition in the case study; N, P represent only N and only P addition in the case study; N, P, NP represents only N, only P and N plus P addition in the case study.