Karst ecosystem of Guangxi Zhuang Autonomous Region

constrained by geological setting: Relationship between

carbonate rock exposure and vegetation coverage

Cao Jianhua, Yuan Daoxian, Zhang Cheng, Jiang Zhangcheng

Karst Dynamics Laboratory, Ministry of Land and Resources; Institute of Karst Geology, Chinese Academy

of Geological Sciences, Qixing Road 40#, Guilin, Guangxi Zhuang Autonomous Region, 541004, China

e-mail: Jhcao@karst.edu.cn ABSTRACT

Karst ecosystem is restrained by karst environment, especially, by karst geological setting. In karst region,

factors impacting plant growth are mainly (1) soil erosion extremely faster than soil formation; (2) suffering

moisture-short in cyclicity owing to double-layer hydrogeological structure and (3) mineral elements

deficient or less available in soils.

With the ArcView3.2 as working platform, the data of percentage of carbonate rocks exposed and arbor, bush

and grass cover in each county in Guangxi Zhuang Autonomous Region were calculated. The results show

there has significant negative correlation between arbor coverage and percentage of carbonate rock by the

linear relation AC=-0.34PCR+33.75 with the correlation coefficient r=-0.75. And positive correlation can be

fitted between bush and grass coverage and percentage of carbonate rock by linear relation BC=0.25PCR-

0.45, GC=0.03PCR+0.99 with the coefficient r=0.71 and 0.49, respectively.

KEY WORDS: karst ecosystem; geological setting; vegetation evolution; Guangxi Zhuang Autonomous

Region, China

Introduction

Karst ecosystem is explained as the

ecosystem that is restrained by karst

environment (Yuan, 2001), especially, by karst

geological setting (Cao, 2003). Carbonate rock

is its material basement and its matter

migrating and energy transferring has own

particularities, such as soluble rock, calcium-

rich, and double-layer hydrogeological

structure, different from the other ecosystems

in the same climate zone. With the

ArcView3.2 as the working platform, this

paper uses GIS to determine the relationship

between the carbonate rock exposure and the

arbor, bush and grass coverage in Guangxi

Zhuang Autonomous Region (GZAR).

GZAR, covers 236 400 km2, is the typical

karst province in southwest China. A clear

shallow marine environment prevailed in

GZAR from Middle Cambrian to Late Triassic

in age and the carbonate rock strata is mainly

located in its middle part, and some of them in

its west and northeast part (Weng, 1988; Yuan,

1991). The thickness of carbonate rock strata is

more than ten thousand meters, and they derive

from Lower Palaeozoic group: Upper-Middle

Cambrian and Middle Ordovician series;

Upper Palaeozoic group: Devonian,

Carboniferous and Permian systems; and

Mesozoic group: Lower-Middle Triassic

series.

GZAR lies in subtropical monsoon climate

with annual average temperature of 17-23℃

and annual mean precipitation of 1100-

2700mm. The tower karst related to old and

rigid carbonate rock, develop well under the

condition with rainfall matching with high

temperature. And the magnificent karst

212 Cao Jianhua, Yuan Daoxian, Zhang Cheng, Jiang Zhangcheng

landscape result in notable tourism resources

(Zhu, 2000). However, the karst region of

GZAR is extremely fragile environment. The

irrational land-use and anthropogenic activities

easily cause the serious land degradation in

form of rocky desertification (Yuan, 1997;

Wang, 2004). Remote sensing data (TM

images obtained 2000) reveals that karst rocky

desertification land in GZAR covers 2,7·104

km2, account for 26,3% of the total karst area

(Tong et al, 2003).

Carbonate rock exposure and lithological map production

Using an officially published in 1:500

000 scale geological map as a data source,

a 1:500 000 scale digital geological map

has been constructed. Depended on the

lithological property and their thickness, a

carbonate rock distributive map has been

also produced from the geological map

(Fig.1). Types of carbonate rock

assemblage can be divided into three

groups: Continuous carbonate rock;

carbonate rock intercalated with clastic

rock; and carbonate/clastic rock

alternation. They are delineated and coded

as follows:

(1) Clastic rock, including chert,

metamorphic rock and igneous rock

(2) Carbonate rock

(21) Continuous carbonate rock: thickness

of carbonate rock >90%, no clear

clastic rock interbed.

(211) Continuous limestone

(212) Continuous dolomite

(213) Continuous mixed limestone/dolomite

(22) Carbonate rock intercalated with calstic

rock: thickness of carbonate rock 70-90%,

and clastic rock 30-10%.

(221) Limestone interbeded with clastic rock.

(222) Dolomite interbeded with clastic rock.

(23) Carbonate/clastic rock alternation:

thickness of carbonate and clastic rock,

accounting for 70-30% and 30-70%,

respectively.

(231) Limestone/clastic rock alternation.

(232) Dolomite/clastic rock alternation.

Based on the statistic results, the exposed

carbonate rocks mainly originate from the

Upper Palaeozoic group: Devonian,

Carboniferous and Permian system. Their

exposed area account for 33,5%, 33,9% and

25,4%, total 92,8% in total karst area (Fig.2).

Fig. 1. Distributive map of carbonate rock assemblage

types in GZAR. Means of the code of (1), (211), (212),

(213), (221), (222), (231) and (232) can be found in the

text.

Fig. 3 shows that the exposure of clastic rock

(1) covers 153 800 km2, 65,17% of the land

area of GZAR. The exposed strata mainly

include Lower-Middle Triassic mudstone,

siltstone, granite, tuff and liparite; Cretaceous

sandstone, mudstone, diorite, granite and tuff

breccia; and Silurian shale, siltstone, granite

and gneiss. Carbonate rock (2) covers 34,83%

of the total Province. And the limestone

assemblages occupies large proportion, it

(211)+(221)+(231) covers 63,06% of total

karst area, 21,99% of total Province.

Continuous mixed limestone/dolomite (213)

covers 32,28% of total karst area, 11,23% of

total Province. The dolomite assemblage

occupies very small proportion, it

(212)+(222)+(232) cover only 4,66% of total

karst area, 1,62% of total Province.

Karst ecosystem of Guangxi Zhuang Autonomous Region 213

Fig. 2. Exposed area of carbonate rocks from different

geological era (for explanation see text).

Fig. 3. Exposed area of different rock assemblages in

GZAR.

Relationship between carbonate rock exposure

and vegetation coverage

In order to determine the relationship

between the carbonate rock and vegetation

spatial distribution, the administrative map

with county’s border line is overlain on the

above carbonate rock lithological and arbor,

bush and grass coverage maps (the latter

extracted from TM images obtained in 2000,

Provided by Institute of Remote Sensing,

Chinese Academy of Sciences). Using the

Geoprocessing and Spatial extensions of

ArcView3.2, the mean ratios of arbor, bush,

grass and carbonate rock in each county can be

calculated.

Table1 and Fig. 4 show the statistic results: (1)

GAZR has 85 natural counties and the

carbonate rock can be found in 76 counties,

there are 45 counties with the percentage of

exposed carbonate rocks more than 30%.

During them, 8, 9, 13 and 15 counties with the

ratio of carbonate rock exposed >90%, 70-

90%, 70-50%, and 50-30%, respectively, and

they mainly distribute in the middle, southwest

and northeast GZAR; (2) The coverage, on an

average, of arbor, bush and grass of GZAR is

21,18%, 8,63% and 2,10%, respectively. If

karst county is defined as the county with the

ratio of carbonate rock exposure over 30%, the

average of arbor of karst counties and non-

karst counties is 12,43% and 31,45%, the latter

is 2,53 times of the former; In contrast, the

mean coverage of bush of karst counties and

non-karst counties is 14,60% and 1,61%, the

former is 9,07 times of the latter; and the mean

coverage of grass of karst counties and non-

karst counties is 2.64% and 1.34%, the former

is 1.97times of the latter.

Table 1. Summary results of carbonate rock exposure and arbor, bush and grass coverage county as the information unit

Vegetation mean coverage(%) Carbonate rock exposure (%)

>90 90-70 70-50 50-30 30 <10

arbor 4.37 7.31 10.7 20.87 25.49 34.31

bush 23.07 18.06 15.41 7.72 3.57 0.66

grass 3.53 2.45 2.44 2.48 1.71 1.67

104km

2 Area of Rock exposure (km

2)

Code o

f ro

ck a

ssem

bla

ge

Code o

f ro

ck a

ssem

bla

ge

214 Cao Jianhua, Yuan Daoxian, Zhang Cheng, Jiang Zhangcheng

Fig. 4. Distributive features of grass, bush, arbor and carbonate rock exposure in each county, GZAR

Seen from Fig. 5, there has significant

negative correlation between arbor coverage

(AC) and percentage of carbonate rock (PCR)

exposure by the linear relation

AC=-0,34PCR+33,75 with the correlation

coefficient r=-0,75. And positive correlation

can be fitted between bush and grass coverage

(BC and GC) and percentage of carbonate rock

exposure by linear relation BC=0,25PCR-0,45

and GC=0,03PCR+0,99 with the coefficient

r = 0,71 and 0,49. Without anthropogenic

disturbance, the vegetation evolution is

commonly from grass community, grass-bush

community to arbor community. Therefore, it

is suggested that the vegetation development

constrained by the karst geological setting in

GZAR. i.e. it implies the plant growth and

community evolution is slower in karst area

than that in non-karst area.

Karst ecosystem of Guangxi Zhuang Autonomous Region 215

Fig. 5. Coverage of arbor, bush

and grass as a function of

percentage of carbonate rock

exposure in each county, GZAR.

Discussion

Slow pedogenic process and severe soil

erosion giving the bad condtion for plants to

live and grow

Under the warm-humid subtropical climate

condition, the pedogenic process and soil

formation has two-stage in karst region (Ji,

2004): firstly, carbonate rock dissolution and

soluble matter (calcium, magnesium and other

soluble component) largely leaching out;

secondly, chemical weathering of insoluble

residues (mainly argillaceous substances) and

pedogenic process. Therefore, the soil

formation and its thickness are closely related

to the content of insoluble material contained

in the parent carbonate rocks. In GZAR, the

parent carbonate rocks are old and mainly

originate from Upper Palaeozoic group:

Devonian (D), Carboniferous (C) and Permian

(P) systems. As the result of study, the mean

contents of insoluble matter contained in

216 Cao Jianhua, Yuan Daoxian, Zhang Cheng, Jiang Zhangcheng

D,C,P carbonate rocks are 2,28%, 2,32% and

3,73%, respectively (Zhang, 1979). It is

estimated that one meter thick soil layer should

last 250-850 ka (Yuan, 1988). It is greatly

slower than that in non-karst regions, such as

mudstone, mud-shale, granite and tuff as

parent rock in the same climate zone.

Photo1. Rocky desertification with very thin soil layer in the depression after karst vegetation destroyed, Pinguo, GZAR.

Photo 2. Many sinkholes in the low cropland and rainwater difficult to remain in epikarst zone, Masahn, GZAR.

On the other hand, carbonate rock in GZAR

is very hard with low porosity. The mean

porosities of D, C, P limestone and dolomite

are 0,64%, 0,73%, 1,79% and 2,58%, 2,63%,

3,75%, respectively (Yuan, 1991). Limestone

is the dominant rock in GZAR karst region.

Consequently, on the surface, they bring about

tall and steep tower karst landforms with

relative height 50-250 m. Carbonate rocks

dissolution residues and soil particles are

easily carried by water moving from slope to

lowlands. Even more, there are dolines,

sinkholes and foot-caves in the lowland and

depressions connecting with the underground

conduits and subterranean rivers. Large

quantities of soil particles, especial topsoil of

cultivated land in lowland or slight slope lost

to underground, finally, deposit and

accumulate in caves or move to surface water

bodies. Small quantities of soil remain in the

Karst ecosystem of Guangxi Zhuang Autonomous Region 217

rock fractures usually with 10-30 cm thick. It

is estimated, from the suspended substance in

GZAR karst surface rivers, that the rate of soil

erosion is 56-129 t·km-2

·a-1

(He, 2000). The

rate soil erosion is very faster than that of soil

formation. In regional scale, there are thin soil

layer and insufficient mineral nutrient to

support plant to live and grow well. If the

human activities to destroy the fragile karst

plant communities, the karst terrain easily

become the rocky desertification (Photo1),

moreover, they are very difficult to rehabilitate

(Zhang et al., 2002).

Double-layer structure of karst hydrogeology

lead plant to suffer moisture-short in cyclicity

Long-term karst process and heterogeneous

water-bearing media result in the double-layer

structure of karst hydrogeology. On the

surface, the epikarst zone is estimated to

regulate 8% of total natural water resources in

southwest karst area (Chen, 2003). The karst

ground water resources occupies 66% in total

water resources in GZAR (Cao, 2004).

Because the rainwater, especially following the

deforesting, can quickly enter into

subterranean through dolines, sinkholes and

foot-caves (Photo 2). It is short-time for

rainwater remaining in epikarst zone.

Consequently, the plants will suffer moisture-

short when it is no rain over one week. The

plants living on the upper part of hill are more

serious. This may be the reason for the

biomass of forest community in the upper part

of hill only about 60% of that in lower part of

hill (Zhou, 2001). Deng et al. (2004) observe

the leaves of Cyclobalanopsis glauca living

top of the hill have the features for adaptation

to water stress with denser epidermis cell and

stomata with thicker wall and small stomata

index, in comparison with that in lower part of

hill in Nongla peak cluster area, Mashan

county, GZAR.

Karst geochemical background impacted

some mineral nutrient elements in soil to

release Tyler et al. (1998) research the mineral

elements content in seed and leaf of thirty-five

herbaceous plants growing on both limestone

and silicate soils in southern Sweden. The

results indicate that concentration of Rb and

Co in seeds of plants originating from

limestone soil are, on an average, about half of

those from silicate soil, while Ca and Mo

concentration are higher. Mean seed

concentration of K, Mn, and Zn are only a

third to a half of mean leaf concentration.

Preliminary investigation on both limestone

soil and silicate soil whose parent rock derive

from Upper-Middle Devonian series limestone

and sandstone-shale in Guilin Yajie Karst

Experimental Site, were carried out.

Concentration of Ca, Mg, Fe, Mn, Cu, Zn in

soils and plants (Pine sp.) are analyzed. The

total contents of Ca, Mg, Fe, Mn, Cu, Zn in

limestone soils are 1.5-4.5times higher than

those in silicate soils. Concentrations of liable

elements of Fe, Mn, Cu, Zn in limestone soils

are only 25%-75% of those in silicate soils,

however, the liable Ca and Mg in limestone

soils are 3,3 and 1,8 times higher.

Concentration of pine stems is keep positive

correlation with the liable element

concentrations in soils. It is suggested that

some mineral elements are deficient or less

available in limestone soils for plant growth.

Acknowledgement

The authors are most grateful for Dr Chen

Zhihua who work in Chinese Geological

University (Wu han) for providing the digital

geological map of GZAR and also give great

thanks to Dr Liu Mingliang and Wang

Shaoqiang who work in Institute of Remote

Sensing, Chinese Academy of Sciences for

providing the digital maps of arbor, bush and

grass coverage in GZAR. The research work

was jointly supported by National natural

Science foundation of China (Nos. 40372116

and 90202016), the ability-renovation project

of Science and Technology Department,

GZAR.

218 Cao Jianhua, Yuan Daoxian, Zhang Cheng, Jiang Zhangcheng

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