Forest nutrients have received extensive attention in ecology, forestry and global change research due to their crucial roles in controlling plant growth and metabolism. Strictly speaking, all the services provided by forests, including carbon sequestration, oxygen releasing, biodiversity supporting, etc., rely on mineral nutrients. For instance, forest productivity is often limited by nitrogen (N) and/or phosphorus (P), and to some extent by potassium (K), calcium (Ca), magnesium (Mg), etc.  

During the past several decades, especially since the International Biological Programme established in 1964, numerous studies have been conducted to quantify the pools of nutrients in forest ecosystems. The data obtained in various individual studies provided essential information for understanding patterns of nutrients stored in main types of forests. Yet, the global-scale patterns of nutrient density and partitioning in forests have not been well characterized. 

Since 2007, Dr. ZHANG Kerong and his colleagues from Wuhan Botanical Garden have been focusing on the scientific questions. They compiled a dataset including 372 forest stands and explored the global-scale patterns of nutrient density and partitioning.  

The research found that temperature and precipitation were key factors driving the nutrients stored in living biomass of forests at global-scale. The N, K, and Mg stored in living biomass tended to be greater in increasingly warm climates. For tropical, subtropical, temperate, and boreal forests, the mean biomass N density was 577.0, 530.4, 513.2, and 336.7 kg/ha respectively. 

Climate, stand age, and biomass density significantly affected nutrients partitioning at forest community level. The mean Root: Shoot ratios (R:S): R:S_N, R:S_P, R:S_K, R:S_Ca, R:S_Mg of all sites were 0.30, 0.36, 0.32, 0.27, and 0.35, respectively.  

The fractional distribution of nutrients to roots decreased significantly with temperature, suggesting that forests in cold climates allocated greater nutrients to roots. The angiosperm forests and gymnosperm forests showed different patterns of nutrient partitioning. Gymnosperm forests tended to allocate more nutrients to leaves as compared with angiosperm forests, whereas the angiosperm forests distributed more nutrients in stems. The scaling exponents of the relationships describing root nutrients as a function of shoot nutrients were >1.0, indicating that as nutrient allocated to shoot increases, nutrient allocated to roots increases faster than linearly with nutrient in shoot. 

"To our knowledge, our study provided the first global scale nutrients-based Root: Shoot ratios and their relationships with climatic factors”, the corresponding author of this paper Prof. ZHANG Quanfa said. The anonymous reviewers of Global Change Biology commented: “your work is challenging, the research has done properly” and “The paper will make a useful new contribution in this field”. 

The findings have been published in Global Change Biology entitled “Global-scale patterns of nutrient density and partitioning in forests in relation to climate”. It was funded by the National Natural Science Foundation of China and the Strategic Priority Research Program A of the Chinese Academy of Sciences.  

Nutrients in standing biomass in tropical (T), subtropical (S), temperate (E), and boreal (B) forests across five age classes (Image by ZHANG Kerong)