Vegetation types and climate conditions reflected by the modern phytolith assemblages in the subalpine Dalaoling Forest Reserve, central China

Abstract

This research describes modern phytolith records and distributions from subalpine surface soils in the Dalaoling Forest Reserve, and reveals its implications for local climate conditions with respect to the altitude gradient. Well-preserved phytolith morpho-types, assemblages, and climatic indices were used to study the relationship between local vegetation and climate conditions. The phytolith classification system is mainly based on the characteristics of detailed morpho-types described for anatomical terms, which are divided into seven groups: long cells, short cells, bulliform cells, hair cells, pteridophyte type, broad-leaved type, and gymnosperm type. Phytoliths originating from the Poaceae are composed of Pooideae (rondel and trapeziform), Panicoideae (bilobate, cross, and polylobate), Chloridoideae (short/square saddle), and Bambusoideae (oblong concave saddle). Based on the altitudinal distribution of the phytolith assemblages and the indices of aridity (Iph), climate (Ic), and tree cover density (D/P), five phytolith assemblage zones have revealed the five types of climatic conditions ranging from 1,169 m to 2,005 m in turn: warm-wet, warm-xeric to warm-mesic, warm- xeric to cool-mesic, cool-xeric, and cool-mesic to cool-xeric. The Bambusoideae, Panicoideae, and Chloridoideae are the dominant vegetation at the lower-middle of the mountains, while Pooideae is mainly distributed in the higher mountains. The close relationship between phytolith assembleages and changes of altitude gradient suggest that vegetation distribution patterns and plant ecology in the Dalaoling mountains are controlled by temperature and humidity conditions. Our results highlight the importance of phytolith records as reliable ecoclimatic indicators for vegetation ecology in subtropical regions.

Keywords

central China / subalpine surface soil / phytolith records / vegetation / and climate change

Introduction

The Dalaoling Forest Reserve (DFR) is located within the western Hubei Province in central China, between the geographic coordinates 110°55 ' 00"‒110°57 ' 00 " E, and 31°03'00 "‒31°05 '00 "N. DFR covers a total area of 24,240 hm², of which 7,288 hm² lies in the virgin forests (Fig. 1). The study area is located at the northern bank of the Yangtze River. It overlooks the world’s biggest dam, Three Gorges Dam. The forest reserve is abundant in plant species. There occur 140 families, 500 genera, and 1,003 species, of which fern contributes 18 families, 35 genera, and 68 species; and seed plants occupy 122 families, 465 genera, and 935 species ( Wu et al., 1996).

The surface water system of the Dalaoling Mountains is directly attached to the Yangtze River. The pH of the surface water system is relatively stable which favors abundant biodiversity. The study area has a subtropical monsoon climate together affected by the southwest monsoon and southeast monsoon. The temperature ranges between ‒15°C and 28°C. Thus, the annual average temperature is about 6.5°C. Annual average precipitation and humidity are about 1,446.8 mm and 83.5%, respectively.

Vertical mountain climate change is significant because the valley is under the control of a typical subtropical climate, but the mountain area near the peak belongs to a warm temperate climate ( Shen et al., 2001). The vertical distribution of soil types is controlled by the altitude change: ① elevation of 960‒1,200 m, mountain yellow soil; ② elevation of 1,200‒1,500 m, mountain yellow-brown; ③ elevation of 1,500‒2,005 m, mountain brown soil ( Wu et al., 1996). Due to the vertical change of elevation and climate, vegetation types also display a vertical distribution pattern as follows: ① elevation of 800‒1,000 m, evergreen broad-leaved forest; ② elevation of 1,000‒1,700 m, evergreen and deciduous broad-leaved mixed forest; ③ elevation of 1,700‒2,005 m, deciduous broad-leaved and coniferous mixed forest ( Shen et al., 2000; Shen and Zhang, 2000).

Phytoliths are an important and reliable tool for reconstructing local vegetation and climate ( Piperno, 1988, 1989, 2006). Grass phytoliths, in particular, offer a promising means of differentiating among grasses at the subfamily level and inferring subtle changes in palaeoenvironmental conditions ( Twiss, 1987, 1992; Fredlund & Tieszen, 1994, 1997; Alexandre et al., 1997; Lu and Liu, 2003). Grass phytolith indices have been successfully used to reconstruct humidity and aridity in marine sediments, grasslands, and tropical sediments ( Diester-Haass et al., 1973; Twiss, 1987; Fredlund & Tieszen, 1994, 1997; Alexandre et al., 1997; Barboni et al., 1999; Parker et al., 2004; Gu et al., 2008, 2012).

Up to date, there are few studies on how modern phytolith assemblages respond to subalpine climate change, which would provide important materials for natural reserve under current global change. This work seeks to explore the relationship between vegetation composition and elevation change by using phytolith records in the surface soil of subalpine forest. This study has demonstrated that the combination of three phytolith indices (Ic, Iph, and D/P) successfully reveals the local climatic and vegetation changes in subalpine forest areas.

Materials and methods

Sampling

The sampling of surface soils ranged from the elevation of 1,150 m to 2,005 m in the subalpine forest at the Dalaoling Forest Reserve (DFR). A total of 18 surface soil samples were used for phytoliths extraction (Fig. 1). The different samples with relevant elevation were selected based on the local climatic conditions and vegetation landscape. The settings of sampling sites for 18 surface soil samples and the altitudes, vegetation types, dominant plant species, and soil types were described while sampling (Table 1).

Phytoliths processing, identification and counting

The extraction of phytoliths in soils was conducted according to the method of Wang and Lu ( 1993). Twenty gram soil samples were dried, ground, sieved, and weighted. The ground samples were put into a sterile beaker and a 30% hydrogen peroxide (H₂O₂) solution was added. The mixture was stirred, rinsed, and centrifuged. The centrifuged sample was dried and ZnBr₂ was added, and the sample was centrifuged again to separate the supernatant from the other particles. Each sample slide was identified and counted at 400 × magnification under the Olympus BX51 microscope. A range of 250 to 300 grains of phytoliths were counted in each slide. The identification of phytoliths in surface soil is based on the morphological comparison with modern plant specimens and geological sediments ( Wang and Lu, 1993; Lu et al., 2006; Gu et al., 2008, 2012) in the study area, and the names of phytoliths refer to the International Code for Phytolith Nomenclature (ICPN) 1.0 protocol ( ICPN Working Group, 2005).

An aridity index (Iph) and a climate index (Ic) were obtained from phytolith statistics using the approaches of Diester-Haas ( Diester-Haass et al., 1973; Alexandre et al., 1997), and Twiss ( 1992). The aridity index, Iph, is a ratio of Chloridoideae to total Chloridoideae and Panicoideae phytoliths. The climate index, Ic, is a ratio of Pooideae to total Pooideae, Panicoideae, and Chloridoideae phytoliths, with high values corresponding to cool temperatures ( Twiss, 1992). The D/P is the ratio of dicotyledon versus Poaceae phytoliths (D/P) following Alexandre et al. ( 1997).

Results

Phytolith morphotypes and classification

The classification system used here follows the classification system of the phytoliths ( Wang and Lu, 1993; ICPN Working Group, 2005, Gu et al., 2008). Detailed descriptions for anatomical terms are divided into seven groups: long cells, short cells, bulliform cells, hair cells, pteridophyte type, broad-leaved type, and gymnosperm type. The first four phytolith types originate from Poaceae epidermis. Long cells are mainly composed of elongates. Short cells include rondel, saddle, cross, trapeziform, and bilobate. Bulliform cells consist of parallepipedal bulliform and cuneiform bulliform cells. Usually, short cells can be classified as the Pooideae (rondel and trapeziform), Panicoideae (bilobate, cross, and polylobate), Chloridoideae (short/square saddle), or Bambusoideae (oblong concave saddle) based on the previous research ( Twiss et al., 1969; Brown, 1984; Twiss, 1987, 1992; Mulholland, 1989; Wang and Lu, 1993; Fredlund and Tieszen, 1994; Piperno and Becker, 1996; Piperno and Pearsall, 1998; Gu et al., 2007, 2008) (Fig. 2).

Phytolith assemblages and indices

On the basis of phytolith identification and indices, five zones of phytolith assemblages have been discriminated and interpreted in terms of climate and major vegetation changes (Fig. 3).

Zone I (1,169‒1,272 m)

The first assemblage zone was marked by a gradual rise both in Ic (at its lowest point) and Iph indices. In terms of percentage, the broad-leaved trees dominated zone I (about 45%) and the percentage of the Poaceae group was medium and stable. The Panicoideae together with the Bambusoideae and Chloridoideae grasses were at maximum amounts, whereas the Pooideae morphotypes were rare. Pteridophyte and gymnosperm morphotypes were also present in different compositions. Phytolith assemblage and indices indicated a warm-mesic condition (Fig. 3).

Zone II (1,272‒1,371 m)

Like Zone I, Zone II is also marked by the domination of broad-leaved trees (about 45%) and the Poaceae morphotypes (about 35%). There was a gradual decrease in the Panicoideae, Bambusoideae, and Chloridoideae, but the Pooideae morphotypes increased slightly. Pteridophytes decreased slightly, but gymnosperm morphotypes showed slight growth and again remained significantly low. The Ic index increased with elevation, but Iph changed from the xeric to the mesic. Phytolith assemblage and indices indicate a change from a warm-xeric to a warm-mesic condition (Fig. 3).

Zone III (1,371‒1,700 m)

This zone is again dominaated by occurrences of broad-leaved trees and Poaceae morphotypes. The former decreased from 42% to 15%, but the latter increased from 30% to 36%. The percentage of gymnosperm morphotypes increased slightly from 3% to 12%. Notably the Poaceae and Pooideae morphotypes also increased gradually from 35% to 45% and from 7% to 15%, respectively, whilst the Panicoideae and Chloridoideae were getting rare. It was in this zone that the D/P was at its maximum value, which reflected the dominant coverage of dicotyledonous trees in the range of 1,370 m and 1,700 m. Ic increased abruptly and Iph fluctuated and continued to change from the xeric to the mesic. Phytolith assemblage and indices indicate a transition from a warm-xeric to a cool-mesic condition (Fig. 3).

Zone IV (1,700‒1,791 m)

There was a sharp increase in the abundance of gymnosperms varying from 11% to 25%, whilst broad-leaved trees decreased significantly from 17% to 6%. It is at this zone that the former approached the maximum abundance. The Pooideae morphotypes continued to rise from 15% to 20%. Ic continued to increase stably and Iph varied from the mesic to the xeric. Phytolith assemblage and indices indicate a transition from a cool-mesic to a cool-xeric condition (Fig. 3).

Zone V (1,791‒2,005 m)

Located at the highest altitude, this zone was marked by the dominant abundance of the Poaceae group varying from 50% to 60%. This was attributed to a notable increase in unciform hair cells, elongates, smooth, cuneiform hair cells, and rondels, whilst parallelepipedal bulliform cells, cylindrical polylobates, and oblong concave saddles decreased. In consequence, the Pooideae grass morphotypes, namely trapeziform sinuate and trapeziform polylobate, increased from 20% to 30%. The percentages of gymnosperms and pteridophytes declined abruptly (Fig. 3). The D/P index indicates that ligneous dicots keep stable and low at this zone. Ic continued to maintain steady growth, and aridity (Iph) varied from the mesic to the xeric. Like Zone IV, phytolith assemblage and indices indicate a transition from a cool-mesic to a cool-xeric condition (Fig. 3).

Discussion

Previous studies have demonstrated that Iph and Ic indices have significant potential as climatic indicators for palaeoenvironmental interpretation ( Twiss, 1987, 1992; Barboni et al., 1999; Parker et al., 2004). Mean annual precipitation together with mean annual temperature and altitude have a remarkble influence on the spatial distributions of vegetation and phytolith types in the study area. Our results are consistent with distribution patterns of modern soil phytolith types in China. Bulliform cells, oblong concave saddle, bilobate, and cross and square saddles are mainly in the tropical, subtropical, and warm temperate areas, whereas rondel, trapeziform, elongate, and unciform hair cell (point-shaped) are mainly in the subalpine and cool temperate areas ( Wang and Lu, 1993; Lu et al., 2006)..

Phytoliths assemblages reflect the climate condition of vegetation (especially the Poaceae grasses) in the Dalaoling subalpine forest. Zone I is marked by a warm-wet condition with the presence of Panicoideae, Bambusoideae, and broad-leaved trees. Zone II is a little cooler and drier than Zone I with the presence of Pooideae, Panicoideae, Chloridoideae, Bambusoideae, and broad-leaved trees. Zone III is marked by a transition from a warm-dry to a cool-wet condition, with declining temperature related to the mixture of Panicoideae, Bambusoideae, Pooideae, and the declining number of broad-leaved trees. Zone IV is dominated by a transition from a cool-wet to a cool-dry condition, with the dominant presence of Pooideae and gymnosperms. Zone V is dominated by the transition from a cool-wet to a cool-dry condition, with the presence of rising Pooideae and decreasing gymnosperms. Phytolith assemblages are sensitive to elevation and vegetation change that is related to climate condition change. Zones I-III considered together show a pattern of a warm-temperate environment corresponding to the evergreen and deciduous broad-leaved forest, mountain yellow soil, and brown-yellow soil. Zones IV-V considered together reflect a pattern of a cool-temperate environment, corresponding to the deciduous broad-leaved and coniferous mixed forest, and mountain brown soil. In contrast to macroscopic vegetation classification, classification using phytolith assemblages shows more detailed variations related to the local micro-climate which cannot be reflected by zonal vegetation. Additionally, phytolith records from the soil samples mostly originate from the Poaceae, which demonstrates that the Poaceae are subtile to the local environment. It should be noted that the occurrence of mountain brown soil and brown-yellow soil is related to the uplift of the mountain and outcrop of red paleosols developed in South China in response to the enhanced East Asia Summer Monsoon since the Mid-Pleistocene ( Hong et al, 2010, 2013; Gu et al., 2013).

Our results show that ligneous dicots dominated the lower-middle of the mountains, but Poaceae and gymnosperms have a high proportion in the high mountains (Fig. 3). On the other hand, the Bambusoideae, Panicoideae, and Chloridoideae are dominant at the lower-middle of the mountains, but the Pooideae dominated the high mountains due to the change of temperature and humidity. The Poaceae compositions and plant ecology are under the control of the temperature and precipitation conditions that are related to the elevation change.

Conclusions

The phytolith records from modern soils in the Dalaoling subalpine forest (Central China) provide an excellent framework for researching the interaction between phytolith, altitude, and local vegetation change. Our results revealed that phytoliths remaining in the soil can reflect the vegetation change related to the elevation of the mountain. The phytolith assemblage zones can reflect the plant ecology of the Poaceae living in the subalpine forest area. The cold group of phytoliths such as long cells, unciform hair cell, and Pooideae types were found to increase with higher elevation; but the warm group of phytoliths such as the Bambusoideae, Panicoideae, and Chloridoideae, and bulliform cells decreased with increasing elevation. The Bambusoideae, Panicoideae, and Chloridoideae are dominant at the lower-middle of the mountain, but the Pooideae dominated at high elevation due to the change of temperature and humidity. Five phytolith assemblage zones have revealed the five types of climatic conditions ranging from 1,169 m to 2,005 m in turn: warm-wet, warm-xeric to warm-mesic, warm- xeric to cool-mesic, cool-xeric, and cool-mesic to cool-xeric. Our research has demonstrated that vegetation composition and plant ecology are under the control of climate conditions that are related to the elevation change.

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