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Biogeosciences, 15, 2033–2053, 2018 https://doi.org/10.5194/bg-15-2033-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Shrub type dominates the vertical distribution of leaf C : N : P stoichiometry across an extensive altitudinal gradient Wenqiang Zhao1, Peter B. Reich2, Qiannan Yu1,3, Ning Zhao4, Chunying Yin1, Chunzhang Zhao1, Dandan Li1, Jun Hu1, Ting Li1, Huajun Yin1, and Qing Liu1 1CAS Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization & Ecological Restoration Biodiversity Conservation Key Laboratory of Sichuan Province, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China 2Department of Forest Resources and Institute on the Environment, University of Minnesota, Minnesota 55108, USA 3Southwest Jiaotong University & Faculty of Geosciences and Environmental Engineering, Chengdu 611756, China 4Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China Correspondence: Qing Liu ([email protected]) and Huajun Yin ([email protected]) Received: 14 November 2017 – Discussion started: 14 December 2017 Revised: 26 February 2018 – Accepted: 7 March 2018 – Published: 9 April 2018 Abstract. Understanding leaf stoichiometric patterns is cru- deciduous, broadleaf, and conifer), their leaf element con- cial for improving predictions of plant responses to environ- tents and responses to environments were remarkably differ- mental changes. Leaf stoichiometry of terrestrial ecosystems ent. Shrub type was the largest contributor to the total varia- has been widely investigated along latitudinal and longitudi- tions in leaf stoichiometry, while climate indirectly affected nal gradients. However, very little is known about the vertical the leaf C : N : P via its interactive effects on shrub type or distribution of leaf C : N : P and the relative effects of envi- soil. Collectively, the large heterogeneity in shrub type was ronmental parameters, especially for shrubs. Here, we ana- the most important factor explaining the overall leaf C : N : P lyzed the shrub leaf C, N and P patterns in 125 mountain- variations, despite the broad climate gradient on the plateau. ous sites over an extensive altitudinal gradient (523–4685 m) Temperature and drought induced shifts in shrub type distri- on the Tibetan Plateau. Results showed that the shrub leaf bution will influence the nutrient accumulation in mountain- C and C : N were 7.3–47.5 % higher than those of other re- ous shrubs. gional and global flora, whereas the leaf N and N : P were 10.2–75.8 % lower. Leaf C increased with rising altitude and decreasing temperature, supporting the physiological accli- mation mechanism that high leaf C (e.g., alpine or evergreen 1 Introduction shrub) could balance the cell osmotic pressure and resist freezing. The largest leaf N and high leaf P occurred in val- Ecological stoichiometry examines the interactions among ley region (altitude 1500 m), likely due to the large nutrient organisms’ element composition and their environments, leaching from higher elevations, faster litter decomposition which provides an effective way to enhance our understand- and nutrient resorption ability of deciduous broadleaf shrub. ing of ecosystem function and nutrient cycling (Allen and Leaf N : P ratio further indicated increasing N limitation at Gillooly, 2009; Venterink and Güsewell, 2010). Over the past higher altitudes. Interestingly, drought severity was the only decades, great attention has been paid to the leaf stoichiom- climatic factor positively correlated with leaf N and P, which etry of terrestrial plants at regional (Townsend et al., 2007; was more appropriate for evaluating the impact of water sta- Matzek and Vitousek, 2009), national (Han et al., 2011; Sar- tus than precipitation. Among the shrub ecosystem and func- dans et al., 2016), and global scales (Elser et al., 2000; Re- tional types (alpine, subalpine, montane, valley, evergreen, ich and Oleksyn, 2004). The leaf macroelements (carbon, nitrogen and phosphorus) were widely explored to indicate Published by Copernicus Publications on behalf of the European Geosciences Union. 2034 W. Zhao et al.: Shrub type dominates the vertical distribution of leaf C : N : P stoichiometry nutrient limitation and its response to environmental change site for examining the altitudinal patterns and environmental (Elser et al., 2010). Investigating the interactions among leaf variables influencing shrub stoichiometry. stoichiometry and the environment along geographic gradi- The objectives of this study were to (i) analyze the leaf ents is critical to understand the nutrient cycling process and C : N : P stoichiometric patterns of various shrub types and the development of biogeochemical models. (ii) clarify the significant factors affecting shrub stoichiome- Nowadays, it is increasingly rare to localize and work on try across an extensive altitudinal gradient. In this work, we extensive and natural altitudinal gradients varying from low measured the leaf C, N and P concentrations of 48 shrub to high-altitude mountaintops (Nogués-Bravo et al., 2008). species on the Tibetan Plateau. The geographic, climatic and A few studies have investigated the variations of leaf N and soil data of sampling sites were recorded. Given that the Ti- P at several elevations (Soethe et al., 2008; van de Weg et betan Plateau encompasses a singular region of high spatial al., 2009; Fisher et al., 2013; Zhao et al., 2014); however, the heterogeneity and complex climatic conditions (H. Chen et scientists reported different altitudinal trends for leaf N and al., 2013) that may greatly affect shrub nutrient accumula- P. For instance, Soethe et al. (2008) found that the foliar N tion, we hypothesized that (i) the overall leaf C : N : P vari- and P concentrations of trees, herbs and shrubs were larger at ations would be dominated by climate, and (ii) the shrub 1900 than 2400 and 3000 m in an Ecuadorian montane forest. leaf element contents would be different from other terres- Van de Weg et al. (2009) observed that the foliar P along an trial ecosystems. In addition, plant types and species can altitudinal transect (220, 1000, 1500, 1855, 2350, 2990 and greatly affect the leaf element concentrations (McGroddy et 3600 m) from lowland to montane cloud forest showed no al- al., 2004). To reveal this effect, all shrubs were classified titudinal trend. Fisher et al. (2013) reported that leaf N and P into four ecosystem types located in different vertical veg- firstly increased and then declined with increasing altitudes etation belts (alpine, subalpine, montane and valley shrub), (200, 1000, 1500 and 3000 m) in the Peruvian Andes. Zhao et or three functional types based on different leaf traits (ever- al. (2014) observed that the leaf N and P of 175 plant species green broadleaf, evergreen conifer, and deciduous broadleaf decreased with elevation on Changbai Mountain (540, 753, shrubs). Four dominant shrub species (Rhododendron tel- 1286, 1812, 2008 and 2357 m). This discrepancy of previous mateium – alpine, Quercus monimotricha – subalpine, Co- results indicates that the leaf N and P can vary along differ- riaria sinica – montane, and Bauhinia brachycarpa – valley) ent altitudinal ranges at regional scale. Moreover, much less were also chosen to assess the leaf patterns at species level. information was available on the vertical distribution of leaf C. Hence, the more general patterns of leaf C : N : P along an extensive altitudinal gradient need to be further understood. 2 Materials and Methods While comprehensive investigations of plant stoichiome- try in forestland, grassland, wetland, and macrophyte ecosys- 2.1 Description of the study area tems have emerged (Güsewell and Koerselman, 2002; He et al., 2006; Townsend et al., 2007; Sardans et al., 2012; Shrub is defined as a small or medium-sized woody plant, Xia et al., 2014), much fewer studies focused on moun- which is distinguished from a tree by its multiple stems tainous shrubs. In China, shrubland is a widely distributed and shorter height (below 5 m). Since shrub ecosystems biome type, covering ∼ 20 % of the country. However, in- are mainly distributed in the southeastern margin of the formation on the element concentrations of shrubs is very plateau (Fig. B1), we chose 108 mountainous sites of this scarce (Piao et al., 2009). Thus, intensive investigation of region to examine the leaf stoichiometry of shrubs that in- shrub stoichiometry can provide detailed information for cluded alpine, subalpine and valley areas. Additionally, the the growing global database of plant stoichiometry. As the 17 neighboring mountainous sites on the east of the Tibetan earth’s highest plateau, the Tibetan Plateau exhibits one of Plateau were selected to provide a representation of low- the few extensive elevational vegetation gradients remain- altitude montane region. In these areas, shrub is one of the ing in the world (H. Chen et al., 2013) (Figs. 1 and B1 most important growth forms. Evergreen broadleaf and de- in Appendix B). Large precipitation and temperature gra- ciduous broadleaf shrubs are the primary functional types. dients along the steep mountains bordering the plateau to The mean annual temperature (MAT) and mean annual pre- the east lead to a heterogeneous environment. This plateau cipitation (MAP) vary from −4.67 to 22.16 ◦C and from is also considered as one of China’s hotspot ecoregions for 366.3 to 1696.3 mm, respectively. biodiversity (Tang