Yong Li, Dashuan Tian, Hao Yang and Shuli Niu
The pattern of nutrient limitation is fundamental to terrestrial productivity and its response to global changes. The traditional view is that plant growth is primarily limited by phosphorus (P) in tropical forest and by nitrogen (N) in cold temperate forest, while warm-temperate forest (a mid-latitude ecosystem) should be co-limited by N and P. However, this traditional paradigm lacks empirical evidence and its underlying mechanisms are far from clear. Hence, we conducted a coordinated experiment of N and P addition at three forest sites in China, a subtropical forest, a warm-temperate forest and a cold-temperate forest (Fig. 1). Through measuring relative growth rate (RGR) and leaf nutrient traits in small and large trees, we examined how tree sizes and plant nutrient traits regulate nutrient limitation patterns along latitude in China.
Our results demonstrated that the RGR of small trees, but not large trees, was primarily limited by P in subtropical and warm-temperate forests, but co-limited by N and P in cold-temperate forests. These results are inconsistent with the traditional view of nutrient limitation patterns along latitude. The sensitivity of small tree RGR to nutrient limitation is likely due to a resource allocation tradeoff between growth and leaf nutrient concentration. With increasing light limitation, small trees tended to first allocate nutrient to growth rather than raising leaf nutrient concentration. Overall, this size-dependent nutrient limitation highlights the importance of considering tree size classes when assessing nutrient limitation in forest.
Image caption: An overall view of the study sites and experiment design of N and P addition (Photos provided by authors). DHS, a subtropical forest of Dinghu Mountain (DHS, 23°9′41″N, 112°32′36″E); JGS, a warm-temperate forest of Jigong Mountain (JGS, 31°51’58”N, 114°5’12”E); CBS, a cold-temperate forest of Changbai Mountain (CBS, 42°24′2″N, 128°05′42″E).