Review on characteristics and main hypotheses of plant ecological stoichiometry

doi:10.17521/cjpe.2020.0331

Abstract

Abstract:

Plant ecological stoichiometry, as a branch of ecological stoichiometry, focuses on the study of elemental content, ratios and relationships within and across plant organs, and the underlying biotic and abiotic drivers. In the 19th century, chemists detected the elemental contents in plant organs via laboratory experiments, sprouting the exploration of plant stoichiometric characteristics. Nowadays, ecologists have explored plant ecological stoichiometric characteristics and their responses to global changes and relationships with plant functional traits, using both field investigation and manipulative experiments. These sustained efforts have largely enriched the knowledge and understanding of plant ecological stoichiometry. In this paper, we briefly introduced the history and reviewed the research progresses of plant stoichiometry since the 19th century. Firstly, we proposed the developmental history of plant ecological stoichiometry as three main periods: sprouting, hypothesis foundation, and theoretical construction periods, and introduced some representative works for each period. Secondly, we overviewed plant ecological stoichiometric characteristics across organs, life forms and environmental gradients. The geometric mean values of leaf nitrogen (N) and phosphorus (P) contents and N:P mass ratios in global terrestrial plants are 18.74 mg∙g^-1, 1.21 mg∙g^-1 and 15.55 (i.e. similar to the Redfield ratio of 16:1), respectively. Leaf N and P contents at either species or community level generally show a decreasing trend with increasing temperature and precipitation, and have large variations among life forms, with higher values in herbaceous than woody plants, and deciduous broad-leaved than evergreen broad-leaved and coniferous woody plants. Compared with leaves, the stoichiometric characteristics of fine roots and other organs in plants remain poorly documented. Thirdly, we reviewed the effects of nutrient addition on plant ecological stoichiometric characteristics. In general, N addition increases soil N availability, then the N content and N:P in plants, thus leading to an increase in plant productivity to some extents. P addition might alleviate the N and P imbalance induced by excessive N inputs, and then increase plant P content. However, long-term nutrient fertilization could perturb the intrinsic stoichiometric characteristics in plants, resulting in the deteriorated nutrient imbalance in tissues and then the subsequent decline in plant productivity. Fourthly, we introduced the main hypotheses of plant ecological stoichiometry. These hypotheses include function-associated hypotheses, environment-associated hypotheses and evolution-associated hypotheses, which delineate the relationships of stoichiometric characteristics with plant growth functions, environmental factors and plant evolutionary history, respectively. Finally, we made an outlook on future research in the area of plant ecological stoichiometry, and highlighted ten potential and important research themes.

Key words: stoichiometry, nitrogen, phosphorus, ecology, plant, hypothesis, brief history

TIAN Di, YAN Zheng-Bing, FANG Jing-Yun. Review on characteristics and main hypotheses of plant ecological stoichiometry[J]. Chin J Plant Ecol, 2021, 45(7): 682-713.

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Figures/Tables 14

Table 1 Leaf nitrogen (N), phosphorus (P) content and N:P mass ratio across different lifeforms of global terrestrial plants

功能群 Functional group	N (mg∙g^-1)	P (mg∙g^-1)	N:P (g∙g^-1)	n
所有生活型 All functional groups	18.74 (±8.33)	1.21 (±0.94)	15.55 (±9.95)	12 721
所有木本 All woody species	18.22 (±7.77)	1.10 (±0.81)	16.53 (±9.30)	9 098
常绿针叶 Evergreen conifers	12.79 (±4.25)	1.06 (±0.81)	12.07 (±8.39)	480
常绿阔叶 Evegreen broadleaved	15.29 (±6.66)	0.79 (±0.56)	19.29 (±10.60)	3 476
落叶阔叶 Deciduous broadleaved	21.29 (±7.53)	1.40 (±0.86)	15.24 (±7.66)	4 981
其他木本 Other woody species	18.63 (±8.32)	1.23 (±0.64)	16.58 (±8.77)	101
草本 Herbaceous species	20.56 (±9.33)	1.56 (±1.12)	13.17 (±11.03)	3 489
蕨 Ferns	11.46 (±4.75)	0.65 (±0.62)	17.75 (±13.06)	134

Fig. 1 Changes of carbon (C), nitrogen (N), and phosphorus (P) content in leaves, stems, and roots of terrestrial plants in China with mean annual air temperature (cited from Tang et al. (2018)).

Table 2 Root nitrogen (N), phosphorus (P) concentration and N:P ratios from previous published researches*

Fig. 2 Natural log response ratios of plant carbon (C), nitrogen (N), phosphorus (P) and their stoichiometric ratios to N and P additions, respectively (replotted based on Yuan & Chen (2015)). Circles are for results in natural environments with grey and green representing insignificant (p > 0.05) and significant (p ≤ 0.05) difference between the log response ratio and zero, respectively. Triangles are for results in controlled environments with grey and red representing insignificant (p > 0.05) and significant (p ≤ 0.05) difference between the log response ratio and zero, respectively. Error bars are the 95% confidence intervals for the mean. The numbers out- and inside parentheses represent the numbers of observations for experiments in natural and controlled environments, respectively.

Fig. 3 Main theories and hypotheses in plant stoichiometry. Three categaries are included: (1) function-associated hypotheses (annotated in blue), (2) environment-associated hypotheses (annotated in brown), and (3) evolution-associated hypotheses (annotated in green). The cartoon graphs and phylogenetic tree graph are cited from the google image website (https://www.google.com/imghp?hl=en) in December 10, 2020.

Fig. 4 Bivariate plots of lg-transformed data for growth rates vs leaf phosphorus (P) content and N:P, respectively, of Eranthis hyemalis and Arabidopsis thaliana. A and B are replotted based on Niklas & Cobb (2005). C and D are plotted based on data from Yan’s nutrient enrichment manipulation experiment on A. thaliana (Yan, 2017).

Fig. 5 Original proposition and re-evaluation of the N:P Threshold Hypothesis. A, The N:P threshold values of 14 and 16 as the judgement standard for N and P limitations proposed by Koerselman & Meuleman (1996), redrawn from Koerselman & Meuleman (1996). B, Re-evaluation of the N:P Threshold Hypothesis by Yan et al. (2017) with a larger dataset following the same statistic method as Koerselman & Meuleman (1996), redrawn from Yan et al. (2017). Nutrient limitation here is judged by results of field nutrient fertilization experiments. Each point indicates a field experiment site.

Fig. 6 Leaf N-P scaling relationship at global level, site level and nutrient fertilization experimental level. A, Replotted based on Tian et al. (2018b), showing the significant variation of leaf N-P scaling relationship at site levels. B, C, Replotted based on Yan et al. (2018), indicating the effects of N and P fertilizations on leaf N-P scaling relationship of Arabidopsis thaliana.

Fig. 7 Relationship between ecosystem gross primary productivity (GPP) and leaf nitrogen (N) and phosphorus (P) content, respectively. This figure is replotted using data from Tang et al. (2018).

Fig. 8 Schematic diagram of Stoichiometric Homeostasis Theory. A shows the coordinated variation of organism vs environment nutrient ratios (i.e. no stoichiometric homeostasis). B shows that organism nutrient ratios keep stable with the changing environment nutrient ratios (i.e. strict stoichiometric homeostasis). This figure is replotted from Sterner & Elser (2002).

Fig. 9 Re-analysis of the original data of the Stability of Limiting Elements Hypothesis. This figure is replotted based on Han et al. (2011).

Fig. 10 Scaling relationships of nitrogen (N) and phosphorus (P) content among different organs from different biomes in China. The scaling exponents from leaf vs stem and leaf nutrient vs root nutrient relationships are all below 1, suggesting that nutrient content in leaves relative to stems and roots is more stable and less sensitive to environmental changes. These results support the Stable Leaf Nutrient Content Hypothesis. This figure is replotted based on Tang et al. (2018).

Fig. 11 Diagram of Relative Resorption Hypothesis, replotted based on Han et al. (2013). The y-axis shows the relative resorption efficiency of nitrogen (N) and phosphorus (P) (NRE - PRE). ① desert; ② tropical forest; ③ subtropical forest; ④ wetland; ⑤ China temperate forest; ⑥ temperate forest; ⑦ China boreal forest; ⑧ meadow; ⑨ boreal forest; ⑩ heathland; ? tallgrass prairie.

Fig. 12 Diagram of Biogeochemical Niche Hypothesis, modified from Peñuelas et al. (2019). A shows biogeochemical niche segregation of multi-nutrients among coexisting plant species based on the principal component analysis. B shows the shift in biogeochemical niche among coexisting plant species in a drought experiment. Ellipses in different colours show specific biogeochemical niches.

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