The Ediacaran genus Globusphyton Wang et al., only including one species G. lineare Wang et al., is a eukaryotic macroalga in the Wenghui biota from black shale of the upper Doushantuo Formation (ca. 560–551 Ma) in northeastern Guizhou, South China. It was assigned as one of the significant fossils in the assemblage and biozone divisions in the middle-late Ediacaran Period. Morphologically, Globusphyton is composed of several structural components, displaying that it had tissue differentiation to serve various bio-functions. Its prostrate stolon, a long ribbon bundled by unbranching filaments, crept by holdfasts on the seafloor. Its pompon-like thalli, the circular to oval thallus-tuft composed of many filamentous dichotomies, may have served for photosynthesis. The fusiform ribbon-tubers, the caked and expanded segments of the ribbon, may have served to sustain the growth of the thalli and the possible holdfasts. The zigzag-shaped stolon and pompon-like thalli of Globusphyton, in a relatively low-energy environment, crept on the surface of the muddy sediments and were suspended in the water column, respectively. When water currents occurred occasionally, all or part of its body was probably suspended in the water column to be deformed in capricious patterns.
Introduction
Macroscopic algae have been reported in the Pre-Ediacaran by a number of authors (see Walter et al., 1976, 1990; Du and Tian, 1985; Hofmann, 1985, 1992; Du et al., 1986; Han and Runnegar, 1992; Kumar, 1995; Zhu and Chen, 1995; Yan and Liu, 1997; Sharma and Shukla, 2009; Singh et al., 2009; Babu and Singh, 2011; Zhu et al., 2016). However, more abundant and diverse macroscopic algal communities were found in the Ediacaran Chinese macro-biotas, including the Lantian flora from southern Anhui (Tang et al., 1997; Chen et al., 1994a; Yuan et al., 1995, 1999, 2011), the Wenghui biota from northeastern Guizhou (Wang et al., 2007, 2008, 2009, 2011, 2014, 2015a, b, 2016b; Wang and Wang, 2008, 2011; Tang et al., 2008a, 2009, 2011; Zhu et al., 2008), the Miaohe biota from western Hubei (Chen and Xiao, 1991; Chen et al., 1994b, 2000; Steiner, 1994; Ding et al., 1996; Xiao et al., 2002, 2013; Tang et al., 2008b), the Wulingshan biota from western Hunan (Steiner et al., 1992; Steiner, 1994; Chen et al., 1999; Chen and Wang, 2002), and the Jiangchuan biota of eastern Yunnan (Tang et al., 2007, 2009), in the Yangtze Block, South China (Figure 1A). The Ediacaran Wenghui biota, which has been found in black shales of the upper Doushantuo Formation in northeastern Guizhou (Figure 1B, C), is preserved as carbonaceous compressions (more than 31 genera and 33 species) (Wang et al., 2016b) and dominated by macroscopic algae (see Wang et al., 2007, 2009, 2011, 2014, 2015a, 2016b). The emergence of morphological and ecological diversities is a main characteristic of the Ediacaran macroalgae (Wang et al., 2011, 2015a).
The carbonaceous genus Globusphyton Wang et al., 2007 is one of many taxa in the Ediacaran Wenghui biota and has been treated as one of the significant fossils in the divisions of assemblages and biozones (Wang et al., 2011, 2014, 2015a, 2016b). When this genus was established by Wang et al. (2007), its morphology was briefly discussed and it was considered as a macroscopic alga that possibly crept on the seafloor. Later, the interpretation of Wang et al. (2007) was cited, without discussion (Wang et al., 2009, 2011, 2014, 2015a). Recently, numerous specimens of Globusphyton have been collected from black shales of the upper Doushantuo Formation at Wenghui Village, Jingkou, Guizhou (Figure 1B). This study reports more details of the morphological features and discusses the paleoecology and affinities of Globusphyton based on the newly found specimens. Moreover, creeping algae with prostrate stolon are living on the modern seafloor (see Felicini and Perrone, 1994; Chisholm et al., 1996; Shimada and Masuda, 2000; Chisholm and Moulin, 2003; Tronchin and Freshwater, 2007), although there have been few reports about the Pre-Cambrian macroalga with prostrate stolon. Therefore, the study on the Ediacaran macroalgae with prostrate stolon is of potential significance in understanding the evolution of macroscopic algae that crept on the seafloor.
Figure 1.
Location and stratigraphic positions of Globusphyton in the Ediacaran Dushantuo Formation, northeastern Guizhou, South China. A, Ediacaran paleogeographical configuration of the Yangtze region (modified from Liu and Xu, 1994 and Jiang et al., 2011) and the distributions of the Ediacaran Chinese macro-biotas in South China; B, location of the measured section near Wenghui, Jiangkou, Guizhou, China; C, D, measured sections through the Doushantuo Formation at Wenghui (C) and Miaohe (D), showing the stratigraphic positions of the Globusphyton-bearing intervals.
Stratigraphical and environmental settings
In the Wenghui section (27°50′07″N, 109°01′20″E), the Ediacaran Doushantuo Formation (>71 m thick) can be divided into four members (Figure 1C). The lowest Member I, overlying the tillites of the Nantuo Formation, is composed of dolostones (cap carbonates). The overlying Member II consists of muddy dolostones and black shales. The Member III is composed of dolostones and muddy dolostones, with black shales. The uppermost Member IV, underlying the bedded cherts of the Liuchapo Formation, is characterized by black shales with abundant and diverse compression fossils (i.e., the Wenghui biota). In the fossiliferous black shales, not only macroscopic fossils but also filamentous rhizoids of macroalgae are well preserved, so that previous researchers believed that this biota lived in a relatively low-energy, calm environment and was preserved in situ or nearby their growth position (Wang et al., 2007, 2009, 2010, 2011, 2014, 2015a; Wang and Wang, 2006, 2011; Cheng et al., 2013).
In the Zigui section (Figure 1D), West Hubei, about 380 km north of Wenghui, a zircon U-Pb thermal ionization mass spectrometry age from the top of the Doushantuo Formation is 551.1 ± 0.7 Ma (Condon et al., 2005) and a Re-Os age of the basal black shale of the Doushantuo Member IV is 591.1 ± 5.3 Ma (Zhu et al., 2013). However, Kendall et al. (2015) suggested that the Re-Os age of Zhu et al. (2013) may reflect post-depositional alteration; and they estimated the age of the lowest Member IV as ca. 560 Ma (Kendall et al., 2015) for the beginning of extensive ocean oxygenation, after the ca. 580 Ma Gaskiers glaciation (Canfield et al., 2007; Li et al., 2010; Och and Shields-Zhou, 2012; Johnston et al., 2012; Tahata et al., 2013). Lithologically, many researchers considered that the Doushantuo successions can be correlated in both Wenghui and Zigui sections (Qin et al., 1984; Wang et al., 1987; Liu and Xu, 1994; Zhu et al., 2007; Jiang et al., 2011; Wang et al., 2012). In addition, the Miaohe biota has also been found in the upper Doushantuo black shales (the entire Member IV) at Miaohe, Zigui (see Chen and Xiao, 1991; Chen et al., 1994b, 2000; Steiner, 1994; Ding et al., 1996; Xiao et al., 2002, 2013), coinciding with oxygenated middle-late Ediacaran oceans (Kendall et al., 2015; Li et al., 2015). Both Miaohe and Wenghui biotas can be paleontologically correlated (Wang et al., 2007, 2011, 2012, 2014, 2016b; Tang et al., 2009; Wang and Wang, 2011), sharing many of the same species (e.g. Baculiphyca taeniata, Beltanelliformis brunsae, Enteromorphites siniansis, Eoandromeda octobrachiata, Liulingjitaenia alloplecta, Longifuniculum dissolutum, Miaohephyton bifurcatum, Protoconites minor, and Zhongbaodaophyton crassa).
Systematic paleontology
All studied specimens preserved as carbonized compression have been collected from the lower Member IV black shales of the Ediacaran Doushantuo Formation in the Wenghui section, northeast Guizhou, and are housed in Guizhou University, Guiyang, China.
Genus Globusphyton Wang et al., 2007, emend.
Type species.—Globusphyton lineare Wang et al., 2007.
Original diagnosis.—Carbonaceous compression, like a string of beads, consisting of many thallus-tufts on a ribbon. The ribbon is tightly twisted by unbranching filaments; and the circular to oval thallus-tuft consists of radiately scattered dichotomous filaments from a ribbontuber on the ribbon. The ribbon is distinctly knee-bent at the ribbon-tubers. (Translated from Wang et al., 2007.)
Emended diagnosis.—Carbonaceous compression with several structural components: a long ribbon, thallus-tufts, ribbon-tubers, and nodules (Figure 2). The circular to oval thallus-tuft is composed of many dichotomously branching filaments that emanate radially from a ribbon-tuber. The ribbon-tuber is a caked and expanded segment of the ribbon and is irregularly distributed in the ribbon. The ribbon, a bundle of unbranching filaments, is distinctly knee-bent near the ribbon-tubers to show a zigzag pattern in the well preserved specimens, or irregularly curved to form a varied preservation. The small nodule is observed near the center of the ribbon-tuber.
Discussion.—When the Ediacaran genus Globusphyton was established, Wang et al. (2007) considered that the ribbon was tightly twisted by unbranching filaments and covered up by the ribbon-tubers. After examining all specimens from the Wenghui biota, including the newly collected specimens and the collections of Wang et al. (2007, 2009, 2010, 2011, 2014, 2015a), it appears that the ribbon is a bundle of unbranching filaments, not twisted by unbranching filaments; and the ribbon-tuber, a three-dimensionally carbonaceous mass, is a caked and expanded segment of the ribbon, not a covering on the ribbon.
The genus Globusphyton is similar to the Ediacaran genus Anhuiphyton (Chen et al., 1994a, emend. Yuan et al., 1999), having unbranching filaments but without ribbon-tuber and ribbon. It is characterized by the circular to oval thallus-tufts that have many filamentous dichotomies scattered radially from a ribbon-tuber distributed irregularly in a long bundle of unbranching filaments, in distinction from other Pre-Cambrian macroalgae.
Occurrence.—The upper Doushantuo of the Ediacaran at Wenghui, Guizhou, South China.
Globusphyton lineare
Wang et al., 2007, emend.
Figures 3, 4
Globusphyton lineare Wang et al., 2007, p. 832, pl. I, figs. 22, 23; Wang et al., 2009, no description, fig. 2.13; Wang et al., 2010, no description, fig. 2J; Wang et al., 2011, no description, fig. 3K; Wang et al., 2014, no description, fig. 3m, n; Wang et al., 2015a, no description, fig. 3L, M.
Longifuniculum dissolutum Steiner et al., 1992; Wang et al., 2007, p. 833, pl. II, fig. 6 (on the upper-left); Wang et al., 2015a, no description, fig. 40 (on the bottom-left).
Examined material.—Holotype (MH-40-0248; Figure 3A, B) assigned by Wang et al. (2007) and another 47 specimens housed in the School of Resources and Environments, Guizhou University, Guiyang, China.
Emended diagnosis.—As per genus.
Description.—Carbonaceous compression with a long ribbon, thallus-tufts, ribbon-tubers, and nodules on bedding planes of black shales (Figure 2). The long ribbon, a bundle of unbranching filaments (Figures 3A, 4C, E, I, J), curled or sinuous or nearly straight, is commonly a zigzag pattern knee-bent near the ribbon-tubers in well preserved specimens (Figures 3A–I, 4E, F), or an irregularly variable preservation (Figure 4A–D, G–J). The fusiform ribbon-tuber, a carbonaceous mass, is a caked and expanded segment of the ribbon and is irregularly distributed in the ribbon (Figures 3A, B, I, 4A, E, I, J). Near the center of the ribbon-tuber, a small nodule is vaguely observed (Figures 3D, H, 4B). The ribbon-tubers and nodules are usually three-dimensional preservations (Figures 3A–E, H, 4A–C). The thallus-tuft is composed of many dichotomously branching filaments that emanate radially from a ribbon-tuber (Figures 3A–I, 4A–J); and their filaments branch up to four times with dichotomies (Figures 3B, D). In the circular to oval thallus-tufts, the density of these filamentous dichotomies gradually reduces toward the periphery (Figures 3A–E, G, H, 4A–C, E–J). In a few specimens, an expanded end of the ribbon, with some short spiked-up filaments on its surface, can be observed (Figure 4C, D).
Measurements.—The thallus-tuft is 8.7–19.2 mm in diameter. The ribbon-tuber is 0.3–2.3 mm in diameter and 2.4 mm to 12.8 mm in length, respectively. The ribbon varies from 0.1 mm to 0.7 mm in width. The length of the ribbon segment between two ribbon-tubers is changeable, varying from 3.2–46.7 mm. The nodule in the center of the ribbon-tuber is about 0.3 mm in diameter.
Discussion.—Globusphyton lineare is different from Longifuniculum dissolutum (Steiner et al., 1992) in that the latter has many loosened segments, irregularly distributed in a twisted bundle of filaments. Its expanded end of the ribbon is similar to Gemmaphyton taoyingensis (Wang et al., 2016b) and all species of Longfengshania (Du, 1982), but the former has many short spiked-up filaments on its surface and is expanded by a bundle of unbranching filaments, not a stipe or axis. The ribbon of G. lineare is different from Liulingjitaenia alloplecta (Chen and Xiao, 1991, emend. Steiner, 1994) in that the latter was a wider cylindrical tube with helical folds.
Locality and horizon.—The lower part of the upper Doushantuo black shales at Wenghui, Jiangkou, Guizhou, China.
Morphological features and paleoecological reconstruction
In the Wenghui section in northeastern Guizhou, the Ediacaran compression Globusphyton has been found in the lower part of the upper Doushantuo black shales (Figure 1C), so that it was treated as one of the significant fossils in the Protoconites-Linbotulitaenia-Eoandromeda-Anomalophyton assemblage biozone (Wang et al., 2016b) and the Globusphyton assemblage (Wang et al., 2011, 2014, 2015a, 2016b). Based on the brief discussions of its characteristics, Wang et al. (2007) considered that Globusphyton probably was, in terms of morphology, a macroscopic alga that crept on the sediment surface, and this view was followed frequently thereafter (Wang et al., 2009, 2010, 2011, 2014, 2015a).
The thallus-tuft of Globusphyton lineare, which is a circular to oval pattern in outline on the bedding plane of black shales (Figures 3A–E, G, I, 4A–C, E–J), shows a gradual decrease in the density of the dichotomous filaments toward its periphery (Figures 3A–E, G, H, 4A–C, E–J). In the thallus-tufts, the dichotomous filaments emanate radially from a ribbon-tuber (Figures 3A–I, 4A–J), and generally are displayed such that the shorter dichotomies cover the longer dichotomies (Figure 3A–E). The fusiform ribbon-tuber in three dimensions consists of a caked and expanded segment of the ribbon (Figures 3A–E, 4A–C, E); and the small nodule also is three-dimensionally preserved near the center of the ribbon-tuber (Figures 3D, H, 4B). Although there is no evidence as to whether the dichotomous filaments of the thallus-tufts are linked together with the small nodule, it is clearly observed that some dichotomous filaments grow up from the fusiform ribbon-tuber (Figures 3A, D, F, 4E). In addition, these dichotomous filaments branch up to four times (Figures 3B, D, F, 4F), indicating that they were closely dependent on sunlight. Therefore, the circular to oval thallustuft with many filamentous dichotomies can be regarded, in life position, as a pompon-like thallus growing on an expanded ribbon-tuber, to serve for photosynthesis by the dichotomous filaments (Figure 5A).
Figure 3.
Globusphyton lineare from black shales of the Ediacaran upper Doushantuo Formation, Wenghui, Guizhou, China. A, B, specimen MH-40-0248 (holotype); A, complete specimen; B, magnified view of A, showing the thallus-tufts grown on the three-dimensional ribbon-tubers; C, D, specimen WH-P-01019; C, complete specimen; D, magnified view of C, showing the dichotomous filaments grown on the fusiform ribbon-tuber; arrow shows the nodule; E, F, specimen WH-P-01005; E, complete specimen; F, magnified view of E, showing the dichotomous filaments grown on the ribbon-tuber; G, H, specimen MH-41-0730; G, complete specimen; H, magnified view of G, showing a three-dimensional nodule near the center of the ribbon-tuber; I, specimen WH-P-01004, showing a zigzag-shaped ribbon and the thallus-tufts on the ribbon-tubers.
Figure 4.
Globusphyton lineare from the upper Doushantuo black shales of the Ediacaran, Wenghui, Guizhou, China. A, B, specimen MH-50-0004; A, complete specimen; B, magnified view of A, showing a three-dimensional nodule near the center of the ribbon-tuber; C, D, specimen WH-P-01022; C, complete specimen; D, magnified view of C, showing the expanded end of the ribbon, with short spiked-up filaments; E, specimen MH-40-0248; F, specimen MH-41-0713; G, specimen MH-48-0004; H, specimen MH-50-0015; I, specimen WH-P02095; J, specimen WH-P-03035.
Figure 5.
Reconstructions of the carbonaceous compression Globusphyton. A, showing that Globusphyton crept on muddy sediments in a low-energy environment. B, showing that part of the body of Globusphyton was suspended in the water column when water currents occurred occasionally. Arrow shows the direction of flow.
Although the holdfast of Globusphyton lineare has not been discovered, it is not possible to exclude that its holdfast was not able to separate from the filaments of the thallus-tufts, that it is by the ribbon-tuber or that only its top part is preserved as the nodule. In the thin sections, it is difficult to distinguish whether the carbonaceous fibers in black shales are parts of G. lineare or other macrofossils, because the macrofossil preservation in the Wenghui biota commonly is dense in the millimeter-level columnar sections (see Wang et al., 2014). By the reasons of the ribbon-tuber caked and expanded by the ribbon and the small nodule near the center of the ribbon-tuber, the nodule can be better interpreted as a remnant top part of the holdfast of G. lineare. In all studied specimens from the Wenghui section in northeast Guizhou, the ribbon is either curved irregularly in the disorderly preserved specimens (Figure 4A, C, G–J), or zigzagged regularly near the ribbon-tubers in the well preserved specimens (Figures 3A–C, G, I, 4E, F). In addition, the ribbon segment between two adjacent ribbon-tubers is more regular and straighter in the zigzag-shaped ribbon than in the disordered ribbon; and the disorder ribbon is more inflexibly bent in the ribbon-tuber segments than in other segments (Figure 4A, C, H–J). Wang and Wang (2006) and Wang et al. (2011, 2014, 2015a, b, 2016a) suggested that the Wenghui biota lived in a relatively low-energy environment, with occasional currents. Thus, the zigzag-shaped ribbon of G. lineare was probably its original nature when it lived; and the disordered ribbon can be interpreted as a deformed pattern after it had been transformed by current. However, this originally zigzag-shaped nature means that the ribbon of G. lineare was fixed by some possible holdfasts close to the sediment surface rather than being suspended in the water column. The benthic organisms of the middle-late Ediacaran Wenghui biota generally have holdfasts to anchor themselves to soft muddy sediments (see Wang et al., 2007, 2011, 2015b, 2016a, b). The Ediacaran macroalga Anhuiphytom lineatum (Chen et al., 1994a, emend. Yuan et al., 1999), with many unbranching filaments growing radially out from an unseen center, without ribbon-tuber and holdfast, was considered to have had a possible holdfast (Yuan et al., 1999; Wang et al., 2016b). The ribbon-like compression Grypania spiralis (Walter et al., 1976, emend. Walter et al., 1990) from the Paleoproterozoic to the Ediacaran (see Walter et al., 1976, 1990; Hofmann, 1985; Du et al., 1986; Han and Runnegar, 1992; Kumar, 1995; Sharama and Shukla, 2009; Wang et al., 2016a), for which no holdfast has been discovered, also was interpreted as a benthic alga fixed to the seafloor by the innermost end of its coiled ribbon (Walter et al., 1990; Wang et al., 2016a). By comparison, G. lineare, with the three-dimensional ribbon-tubers and nodules and the zigzag-shaped ribbon, can be interpreted as a benthic macroalga with holdfasts.
Moreover, the interpretations of Globusphyton lineare as originally zigzag-shaped as well as the holdfasts support each other, rather than being mutually exclusive. Namely, the zigzag-shaped ribbon, in life position, may have served as a long prostrate stolon to creep by the possible holdfasts on the sediment surface. During the Ediacaran times, ocean currents occasionally prevailed in the Guizhou Sea (Wang and Wang, 2006; Wang et al., 2011, 2014, 2015a, b); and Wang and Wang (2006) estimated that the upper Doushantuo black shales were hydrous muds with a water content of 78% during deposition. Thus, the holdfasts of G. lineare were easily pulled out from the hydrous muds when an ocean current occurred. After removal of the anchorages of the holdfasts, all or part of G. lineare's body was suspended in the water column so that the zigzag-shaped ribbon and the fusiform ribbon-tubers were deformed and preserved in capricious patterns (Figure 5B).
At last, the expanded end of the ribbon (Figure 4C, D), with short spiked-up filaments, can also be interpreted as an embryo ribbon-tuber, a next ribbon-tuber, preparing for the growth of the thallus-tuft (pompon-like thallus) and the possible holdfast.
Given the above, Globusphyton lineare was a benthic macroalga, with a long prostrate stolon that crept by holdfasts on the Ediacaran seafloor, with the pompon-like thalli for photosynthesis, and ribbon-tubers that sustained the growth of the pompon-like thallus and the possible holdfast.
Systematic affinity of Globusphyton
Globusphyton in the Wenghui biota, a centimeterscale compression fossil, is almost entirely composed of carbonaceous filaments (e.g. the thallus-tuft composed of filamentous dichotomies and the ribbon bundled by unbranching filaments). Thus, it was regarded and described as a macroscopic alga, when Wang et al. (2007) established the Ediacaran genus Globusphyton. Generally, tissue differentiation was considered as a key trait of eukaryotic alga (see Du and Tian, 1985; Yuan et al., 1995, 2011; Ding et al., 1996; Chen et al., 2000; Xiao et al., 2002; Wang and Wang, 2006; Wang et al., 2015b, 2016a). Some Ediacaran macroalgae with a dichotomously branching thallus (e.g. Doushantuophyton, Enteromorphites, Miaohephyton, and Zhongbaodaophyton) were generally regarded as eukaryotic algae (see Chen and Xiao, 1991; Chen et al., 1994b, 2000; Steiner, 1994; Yuan et al., 1999, 2011; Xiao et al., 1998, 2002; Wang et al., 2007, 2014, 2015a, b). Similarly, Globusphyton had tissue differentiation to serve various bio-functions (see above discussions) and the filaments in the thallus-tuft branch dichotomously many times (Figures 3B, D, F, 4F). In addition, the carbonaceous ribbon-tubers were three-dimensionally preserved and inflexibly bent, meaning that they were probably filled by cells. Therefore, Globusphyton, with dichotomously branching thalli and a long prostrate stolon, was a high eukaryotic macroalga.
Comparatively, some modern macroscopic algae with prostrate stolon, thallus, and holdfast, such as chlorophytes Caulerpa (Chapman and Chapman, 1976; Boudouresque et al., 1995; Chisholm et al., 1996; Chisholm and Moulin, 2003) and rhodophytes Pterocladiella (Santelices, 1978; Felicini and Perrone, 1994; Shimada and Masuda, 2000; Tronchin and Freshwater, 2007), creep as they live on the seafloor. The Ediacaran macroalga Globusphyton has an ecological similarity with modem algae with stolon, although further examination is necessary to discuss any phylogenetic affinity.
Conclusions
Numerous specimens of the carbonaceous compression Globusphyton, one of the Ediacaran algae genera, have been collected from the upper Doushantuo black shale (ca. 560–551 Ma) in northeastern Guizhou, South China. Globusphyton has several structural components: thallus-tufts, ribbon-tubers, nodules, and a long ribbon, indicating that it had emerged tissue differentiation to serve various bio-functions. The circular to oval thallus-tuft, which is composed of many filamentous dichotomies clustered on the ribbon-tuber, may have served as a pompon-like thallus for photosynthesis. The three-dimensionally preserved ribbon-tuber, a caked and expanded segment of the ribbon, may have served to sustain the growth of the thallus and the possible holdfast. The ribbon, a bundle of unbranching filaments, may have served as a long prostrate stolon to stay close to the muddy seafloor. The small nodule near the center of the ribbon-tuber may be the top part of a possible holdfast to fix the ribbon-tuber and anchor the prostrate stolon. The Ediacaran Globusphyton, a eukaryotic macroalga, lived in a low-energy environment, and in life position its zigzag-shaped stolon and pompon-like thalli crept on the surface of the muddy sediments and were suspended in the water column, respectively. When water currents occurred occasionally, all or part of its body was probably suspended in the water column to be deformed and preserved in capricious patterns. Nevertheless, its phylogenetic affinity requires further research owing to the compression and homogenization of the carbonaceous specimens.
Acknowledgments
We thank the villagers of Wenghui, Jiangkou, Guizhou, for assistance in the field; and the reviewer A. Kano and another anonymous reviewer for their constructive criticism and comments that improved this manuscript greatly. This research was supported by grants from the National Science Foundation of China (No. 41762001, No. 41663005, No. 41763006 and No. 41572024) and CAGS Research Fund of China (No. YYWF201602).
