karst ecosystem of guangxi zhuang autonomous region...
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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
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ck a
ssem
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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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