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Journal of Resources and Ecology >

2022 , Vol. 13 >Issue 1: 27 - 33

DOI: https://doi.org/10.5814/j.issn.1674-764x.2022.01.003

Ecosystems in Response to Global Change

Spatio-temporal Variation of Vegetation Ecological Quality and Its Response to Climate Change in Rocky Desertification Areas in Southwest China during 2000-2020

  • XU Lingling ,
  • QIAN Shuan , * ,
  • ZHAO Xiulan ,
  • YAN Hao
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  • National Meteorological Center of China Meteorological Administration, Beijing 100081, China
* QIAN Shuan, E-mail:

XU Lingling, E-mail:

Received date: 2021-07-21

  Accepted date: 2021-10-11

  Online published: 2022-01-08

Supported by

The National Natural Science Foundation of China(31700421)

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Abstract

Vegetation restoration is the primary task of ecological reconstruction and rocky desertification control in Karst areas. With vegetation net primary productivity and coverage as two key indicators, a vegetation ecological quality evaluation model was built based on meteorological and remote sensing data. Spatiotemporal variation of vegetation ecological quality index and its response to climate change in rocky desertification areas in Southwest China during 2000-2020 were also analyzed by using the difference method and linear trend method. The results showed that: (1) Vegetation ecological quality in rocky desertification areas in Southwest China showed a fluctuating upward trend during 2000-2020. In 2020, the vegetation ecological quality index reached 69.7, which was 19.9% and 9.3% higher than the averaged values for 2000 and 2000-2019, respectively, ranking the fourth highest since 2000. (2) Vegetation ecological quality of the rocky desertification areas in Yunnan, Guangxi and Guizhou provinces have been improved by 89.2%, 99.2% and 98.5%, respectively, from 2000 to 2020, with their vegetation ecological quality index values increasing by 0.5-0.75 per year in southeast Yunnan, most areas in Guizhou and northwest Guangxi. (3) Precipitation was an important meteorological factor affecting the vegetation ecological quality in rocky desertification areas. The vegetation ecological quality index in the northwest and central Yunnan rocky desertification areas has been rising slowly, but with localized declines at a yearly rate of nearly 0.25 caused by climatic warming and drying.

Key words： rocky desertification area; vegetation ecological quality; precipitation

Cite this article

XU Lingling , QIAN Shuan , ZHAO Xiulan , YAN Hao . Spatio-temporal Variation of Vegetation Ecological Quality and Its Response to Climate Change in Rocky Desertification Areas in Southwest China during 2000-2020[J]. Journal of Resources and Ecology, 2022 , 13(1) : 27 -33 . DOI: 10.5814/j.issn.1674-764x.2022.01.003

1 Introduction

Rocky desertification refers to a process that forms a landscape similar to that of a desert due to vegetation destruction, soil erosion, gradual exposure of rocks, and the decline or even loss of land productivity caused by unreasonable social and economic activities of human beings in Karst areas (Wang, 2002; Wang et al., 2004). Rocky desertification has been a serious ecological problem in Southwest China, mainly distributed in most of Guizhou, and parts of Guangxi, Yunnan, Sichuan, Chongqing and other provinces, with an area of about 5.0×105 km2 and a population of about 100
million; and this area represents a typical ecologically fragile area with the largest and strongest karst development in the world (Xiao et al., 2005). Among them, the combined Karst area in Yunnan, Guangxi and Guizhou provinces accounts for 65.3% of the total southwest Karst area, and the exposed carbonate area accounts for 31.2%, which is far higher than the average levels of other provinces (Su, 2002; Yang et al., 2007). The occurrence and development of rocky desertification are mainly composed of five interrelated processes: Vegetation degradation and loss, soil erosion, surface water loss, carbonate karst erosion and land biological productivity degradation (Li et al., 2010), of which surface vegetation degradation and loss is the most important feature and leading process (Luo et al., 2007). Therefore, vegetation restoration has been the primary task of ecological reconstruction and rocky desertification control in Karst areas (Li et al., 2002). To guide ecological restoration in rocky desertification areas, it is of great significance to accurately assess and monitor the distribution characteristics and spatiotemporal variation of vegetation in degraded Karst areas.
At present, a great deal of research has focused on ecological environment evaluation index in rocky desertification areas. Qin et al. (2011) proposed a geological evaluation system of rocky desertification based on rock, hydrology, geomorphology, climate, social economic cause indicators, soil and vegetation state indicators, bedrock bare rate, soil erosion rate and biological yield result indicators. Li et al. (2004) proposed a risk assessment system of rocky desertification based on lithology, landform, slope, population density and cultivated land rate. Li et al. (2009) established an evaluation system for the degree of rocky desertification occurrence based on quantifying and grading the indicators of slope morphology, dissolution landform morphology, bedrock bare rate, vegetation coverage rate, plant population, soil thickness and soil cover, soil erosion degree and land use type. In spite of the core role of vegetation in ecological restoration and rocky desertification control, indicators for vegetation ecological quality, its development and reversal changes on different spatiotemporal scales remain rare. Vegetation net primary productivity (NPP) and coverage are the two most basic variables for terrestrial plant community growth characteristics and vegetation ecological ecosystem services (Qian et al., 2020). Normalized difference vegetation index (NDVI), which can reflect the changes of surface vegetation coverage, biomass and ecosystem parameters to a certain extent, has become the preferred data source which has been widely used for vegetation change research (Zeng et al., 2014; Tian et al., 2017). In this study, a vegetation ecological quality index evaluation model for the rocky desertification areas in Southwest China was constructed based on meteorological, remote sensing and other multi-source data, and the spatiotemporal variation trend of the vegetation ecological quality index as well as its response to climate change during 2000-2020 were also analyzed. The aim of this study is to provide a new concept for vegetation ecological quality evaluation and monitoring, as well as scientific evidence for vegetation ecological reconstruction and objective rocky desertification control assessment in Karst areas.

2 Material and method

2.1 Data

Monthly composite NDVI during 2000-2020 was obtained from the EOS/MODIS database of the United States with a spatial resolution of 1 km. Meteorological data, including daily average temperature, minimum temperature, maximum temperature, precipitation, relative humidity, wind speed, and sunshine hours during 2000-2020 were obtained from the National Meteorological Information Center, and these data were interpolated to the 1 km meteorological element grid data after monthly synthesis. Other basic data, such as vegetation type, land use, elevation and administrative boundaries were obtained from the National Basic Geographic Information Center.

2.2 Method

With vegetation net primary productivity and coverage as indicators, the vegetation ecological quality index evaluation model for the rocky desertification area was built. In this paper, only the key formulas are listed, and for more details see Qian et al. (2019, 2020).

2.2.1 NPP

Monthly and yearly NPP were calculated by the TEC model of terrestrial ecosystem carbon flux based on monthly surface meteorological data and monthly NDVI composite data of EOS/MODIS (Yan et al., 2015).
$NP{{P}_{ij}}=GP{{P}_{ij}}-{{R}_{ij}}$
$GP{{P}_{ij}}\text{=}{{\varepsilon }_{ij}}\times FPA{{R}_{ij}}\times PA{{R}_{ij}}$
$NP{{P}_{i}}=\underset{j=1}{\overset{n}{\mathop \sum }}\,NP{{P}_{ij}}$
In equations (1)-(3), NPPij, GPPij, Rij are net primary productivity, total primary productivity and respiratory consumption in month j of year i (gC m-2 month-1), respectively; εij is the actual utilization rate of light energy in month j of year i; FPARij is the proportion of photosynthetically active radiation absorbed by vegetation in month j of year i estimated from monthly NDVI; PARij is incident photosynthetically active radiation in month j of year i (MJ m-2 month-1), calculated according to sunshine hours and the ratio of incident photosynthetically active radiation to total solar radiation of 0.48; NPPi is vegetation net primary productivity in year i (gC m-2 yr-1); and n is the 12 months of year i.

2.2.2 Vegetation coverage

Monthly vegetation coverage was calculated by using monthly NDVI of EOS/MODIS, and the annual value was obtained by averaging the monthly values.
$FV{{C}_{ij}}=\left( NDV{{I}_{ij}}-NDV{{I}_{s}} \right)/\left( NDV{{I}_{v}}-NDV{{I}_{s}} \right)$
$FV{{C}_{i}}=\frac{\sum\limits_{j=1}^{n}{FV{{C}_{ij}}}}{n}$
In equations (4)-(5), FVCij is vegetation coverage in month j of year i; NDVIij is the NDVI composite data in month j of year i; NDVIs is the NDVI value when the pixel was pure soil, close to 0; and NDVIv is the NDVI value of a pure vegetation pixel, close to 1. In this study, NDVIs=0.05, NDVIv =0.95. FVCi is the annually averaged vegetation coverage in year i; and n is the 12 months of year i.

2.2.3 Vegetation ecological quality index evaluation model

The vegetation ecological quality index evaluation model was built with weighted NPP and vegetation coverage.
${{Q}_{i}}=100\times \left[ {{f}_{1}}FV{{C}_{i}}+{{f}_{2}}\left( \frac{NP{{P}_{i}}}{NP{{P}_{\max }}} \right) \right]$
In equation (6), ${{Q}_{i}}$ is the vegetation ecological quality index of year i, which is between 0 and 100; Qi =0, indicating vegetation coverage of 0 and NPP is 0; Qi=100, indicating vegetation coverage of 100%, and NPP reached the highest in the time period. FVCi is the averaged vegetation coverage of year i, NPPi is the annual NPP of year i, NPPmax is the annual highest NPP with the best climate conditions. f1 and f2 are weight coefficients with values of 0.5 (Qian et al., 2019).

2.2.4 Dynamic assessment of vegetation ecological quality

The interannual change of vegetation ecological quality between a current-year and a previous year or a perennial value was calculated by the difference method.
$\Delta {{Q}_{i1}}=\left( \frac{{{Q}_{i}}-\overline{Q}}{\overline{Q}} \right)\times 100\%$
$\Delta {{Q}_{i2}}=\left( \frac{{{Q}_{i}}-{{Q}_{i-1}}}{{{Q}_{i-1}}} \right)\times 100\%$
In equations (7)–(8), ${{Q}_{i}}$ is vegetation ecological quality index in year i, $\bar{Q}$ $$ ${{Q}_{i-1}}$ are the perennial mean value and the value in the previous year; $\text{ }\!\!~\!\!\text{ }\Delta {{Q}_{i1}}\Delta {{Q}_{i2}}$ are interannual changes of vegetation ecological quality between year i and the perennial and previous years, with $\Delta {{Q}_{i1}}\Delta {{Q}_{i2}}$< 0 indicating that vegetation ecological quality worsened and $\Delta {{Q}_{i1}}\Delta {{Q}_{i2}}$≥ 0 indicating vegetation ecological quality improved or remained the same.
The multi-year change trend of vegetation ecological quality was calculated by the linear trend method.
${{Q}_{i}}=a+b\times {{t}_{i}}$
$b=\frac{n\times \sum\limits_{i=1}^{n}{({{t}_{i}}\times {{Q}_{i}})}-\sum\limits_{i=1}^{n}{{{t}_{i}}}\sum\limits_{i=1}^{n}{{{Q}_{i}}}}{n\times \sum\limits_{i=1}^{n}{{{t}_{i}}^{2}}-{{\left( \sum\limits_{i=1}^{n}{{{t}_{i}}} \right)}^{2}}}$
In equations (9)–(10), $~{{Q}_{i}}$ is vegetation ecological quality index in year i, ${{t}_{i}}$ is the year, and i is yearly sequence. b is the multi-year change trend rate of the vegetation ecological quality, with b≥0 indicating vegetation ecological quality in a rising trend, and b<0 indicating vegetation ecological quality in a declining trend.

3 Results

3.1 Analysis of vegetation ecological quality in 2020

The regional annual average temperature and precipitation in southwest rocky desertification areas in 2020 were 18.3 ℃ and 1314.0 mm, respectively, which were 0.3 ℃ higher and 74.7 mm more compared with their perennial mean values. In 2020, the vegetation ecological quality in southwest rocky desertification areas was 69.7, which was 5.9 higher than the perennial mean value. A total of 89.6% of the regional vegetation ecological quality belonged to the normal, better, and best ranks, and especially that of northeast Yunnan, Southwest Guizhou and north central Guangxi had significantly improved with vegetation ecological quality index of 10% higher (Fig. 1a). However, the vegetation ecological quality index in the northern and central parts of Yunnan was still 3%-10% lower, as the worse or worst ranks.
Fig. 1 Comparison of vegetation ecological quality index in 2020 with those of perennial (a) and 2019 (b) in rocky desertification areas of Southwest China
Compared with 2019, the annual average temperature was similar in 2020, but precipitation was 102.4 mm less, which led to vegetation ecological quality being slightly worse due to phased droughts (Fig. 1b). As shown in Fig. 2, precipitation during March to June in 2020 was 159-331 mm less than that in 2019, and located in Zunyi, Renhuai, Zhijin counties in northwest Guizhou, Suiyang, Fenggang, Yuping counties in northeast Guizhou, Yuqing, Guiding, Pingba, Longli and Changshun counties in central Guizhou (Fig. 2a). The situation was similar with precipitation in July, which was 236-505 mm less, and mainly located in Donglan County in northwest Guangxi, Yongfu, Guanyang, Guilin counties in northeast Guangxi, and Pingxian, Daxin counties in Southwest Guangxi (Fig. 2b). It is worth noting that precipitation in Huaping County in northwest Yunnan, Malong, Zhanyi, Fuyuan counties in northeast Yunnan, Xundian, Chengjiang counties in central Yunnan, and Qiubei County in southeast Yunnan was 143-205 mm less during March to June in 2020, which continued decreasing by 154-239 mm less in July. Meanwhile, the temperature in Yunnan was abnormally high in 2020, ranking the third highest since 1961, and the rainy season started 15 days later than normal, which resulted in serious phased droughts and vegetation ecological quality worsened.
Fig. 2 Precipitation differences in 2020 compared with 2019 during March to June (a) and in July (b) in rocky desertification areas of Southwest China

Note: Positive means precipitation enhancement; negative means precipitation reduction.

3.2 Changing trend of vegetation ecological quality during 2000-2020

The vegetation ecological quality index in rocky desertification areas of Southwest China showed a fluctuating upward trend during 2000-2020 (Fig. 3), which was relatively low from 2000 to 2012 and rose rapidly from 2013 to 2020. It reached 69.7 in 2020, ranking the fourth highest since 2000, which was 1.1 less than in 2016 (70.8), and slightly lower than that in 2019 (69.8) and 2017 (69.9). Compared with the values in 2000 and perennial mean in 2000-2019, it increased by 19.9% and 9.3%, respectively. The great improvement in vegetation ecological quality during 2000- 2020 in the southwest rocky desertification areas was mainly located in southeast Yunnan, most of Guizhou, and northwest Guangxi with vegetation ecological quality index increases of 0.5-0.75 per year on average (Fig. 4); while it increased slowly or even locally decreased in northwest and central Yunnan rocky desertification areas at a yearly rate of nearly 0.25.
Fig. 3 Changing trend of vegetation ecological quality index in rocky desertification areas of Southwest China during 2000-2020
Fig. 4 Spatial distribution of annual average change rate of the vegetation ecological quality index in rocky desertification areas of Southwest China from 2000 to 2020

Note: Positive means vegetation ecological quality improved; negative means vegetation ecological quality worsened.

From the perspective of provinces, there was significant vegetation ecological quality improvement during 2000- 2020 in 98.5% of the Guizhou rocky desertification areas, and the vegetation ecological quality index has increased from 54.5 in 2000 to 69.4 in 2020, for an annual average increase of 0.7, ranking the third highest since 2000, but slightly lower than in 2016 and 2019. There was vegetation ecological quality improvement in 99.2% of the Guangxi rocky desertification areas, and the vegetation ecological quality index reached 74.5 in 2020, which was 12.9 higher than in 2000 (61.6), for an average increase of 0.6 per year, ranking the fifth highest since 2000. Since 2000, 89.2% of the regional vegetation ecological quality has been improved in Yunnan rocky desertification areas, with the vegetation ecological quality index increasing by 0.4 every year, reaching 65.6 in 2020, and ranking the third highest since 2000, but slightly lower than in 2016 and 2018.
As shown in Fig. 5, the regional annual average temperature in southwest rocky desertification areas has experienced a general uptrend from 2000 to 2020, with most of Yunnan and central Guizhou warming sharply at an average of 0.5-0.8 ℃ or even 0.8-1.0 ℃ every 10 years, while it rose more gradually in northern Guizhou and most of Guangxi with an average of about 0.3-0.5 ℃ every 10 years (Fig. 5a). The annual precipitation in the southwest rocky desertification areas during 2000 to 2020 was characterized by a highly variable spatial distribution. Precipitation showed an increasing trend in most of Guizhou, Guangxi and southeast Yunnan with an annual average increase of 5-10 mm, even reaching 10-20 mm yr-1 in northeast Guizhou, which provided favorable meteorological conditions for vegetation restoration and ecological quality improvement. Meanwhile, it showed decreasing trends in northwest and eastern Yunnan at an average of more than 2 mm yr-1 (Fig. 5b), which led to intensified drought in spring and adverse conditions for the fragile vegetation ecological restoration. Taking the typical drought years of 2009-2012 in Yunnan as an example, precipitation in the four consecutive years was seriously low and most areas suffered especially severe drought in 2010 (Yang et al., 2012; Lu et al., 2013) with a vegetation ecological quality index only 56.4, which was the lowest among the results for the recent 21 years.
Fig. 5 Spatial distribution of annual average temperature and precipitation averaged changing rate in rocky desertification areas of Southwest China from 2000 to 2020.

Note: Positive means temperature or precipitation in increasing tendency; negative means temperature or precipitation in decreasing tendency.

4 Discussion

In our study, the vegetation ecological quality in rocky desertification areas of Southwest China showed a fluctuating upward trend from 2000 to 2020, which was consistent with existing studies. Wu et al. (2007) found that the vegetation coverage and biomass of areas with different levels of rocky desertification in Southwest Guizhou both showed gradually increasing trends with rates ranging from 50% to 360% during the process of ecological restoration, and the rocky desertification in Southwest Guizhou was basically controlled. Luo et al. (2021) also drew the conclusion that the environment of southwest rocky desertification areas was showing an overall trend of improvement from 2000 to 2015 based on normalized difference vegetation index, net primary productivity, surface albedo and slope indexes. Rocky desertification is the result of natural and human factors. Since 2000, with the implementation of ecological measures, such as the returning farmland to forest or grassland project and the biogas energy project, etc., the frequency and degree of human activities have been reduced, and favorable external conditions have been created for ecological restoration in rocky desertification areas. The relationship between annual precipitation and vegetation ecological quality was one focus in our study, and the results showed that vegetation ecological quality was usually higher in the years with good water conditions. Shi et al. (2012) found that the distribution of rocky desertification areas in Qiannan Prefecture was in good agreement with drought frequency, and severe droughts as well as rapid transitions from drought to flood would both aggravate vegetation degradation or rocky desertification development. Increasing precipitation was beneficial for vegetation coverage, biomass enhancement and vegetation ecological restoration. However, if the rainfall was heavy and concentrated, it could also affect the evolution and development of rocky desertification through severe surface runoff scouring and soil erosion. Further study on the impact of precipitation on rocky desertification is needed.
Vegetation restoration is the important premise for solving the environmental problems of rocky desertification in Karst areas, with plant species diversity restoration and development as the central components (Liu et al., 1999). Wu et al. (2007) selected vegetation coverage, biomass, species diversity, community evenness, ecological dominance, and species richness indexes as indicators to reflect the ecological restoration status in the comprehensive management of different levels of rocky desertification. In the vegetation ecological quality index evaluation model of our study, only vegetation net primary productivity and coverage factors were considered, and indexes related to biodiversity were ignored. This limitation should be strengthened in future work by vegetation community surveys in small areas to make the evaluation results more accurate and objective.

5 Conclusions

Rocky desertification is one of the most prominent ecological and environmental problems in Southwest China, and vegetation restoration is the primary task of ecological reconstruction and rocky desertification control in Karst areas. With vegetation net primary productivity and coverage as two key indicators, a vegetation ecological quality index evaluation model was established based on meteorological and remote sensing data. The spatiotemporal variation of vegetation ecological quality and its response to climate change in the rocky desertification areas in Southwest China during 2000-2020 were also analyzed by using the difference method and the linear trend method. Main conclusions were as follows:
(1) Vegetation ecological quality in rocky desertification areas in Southwest China showed a fluctuating upward trend during 2000-2020. In 2020, the vegetation ecological quality index reached 69.7, which was 19.9% and 9.3% higher than the average values of 2000 and 2000-2019, respectively, ranking the fourth highest since 2000.
(2) The vegetation ecological quality of rocky desertification areas in Yunnan, Guangxi and Guizhou has improved by 89.2%, 99.2% and 98.5%, respectively, from 2000 to 2020, with the vegetation ecological environments of Southeast Yunnan, most of Guizhou and Northwest Guangxi significantly improved and their vegetation ecological quality index increased by 0.5-0.75 per year.
(3) Precipitation was an important meteorological factor affecting vegetation ecological quality in rocky desertification areas. The vegetation ecological quality index in northwest and central Yunnan rocky desertification areas has been rising slowly, but declined locally at a yearly rate of nearly 0.25 caused by climate warming and drying.

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