Spectral feature differences between shrub and grass communities and shrub coverage retri- eval in shrub-encroached grassland in Xianghuang Banner, Nei Mongol, China

doi:10.17521/cjpe.2016.0101

Abstract

Abstract:

Aims Shrub-encroached grassland has become an important vegetation type in China’s arid and semi-arid region. Our study objective is to explore the spectral features of shrub and grass communities, as well as their empirical relationships with shrub coverage. The quantitative estimation of shrub cover based on medium-resolution Landsat satellite imagery provides the practical basis for long term retrieval of large areas of shrub expansion in the grassland region. Methods Linear models and Multiple Endmember Spectral Analysis Model (MESMA) based on medium resolution Landsat satellite imagery were developed to quantify the shrub coverage in a shrub-encroached grassland region in Xianghuang Banner, Nei Mongol using the spectral features and their seasonal differences between the shrub and grass communities. Important findings Compared to Leymus chinensis and Stipa krylovii dominated grass communities, Caragana microphylla community had a higher normalized difference vegetation index (NDVI), modified red edge normalized difference vegetation index (mNDVI₇₀₅), and red edge slope. The red edge position of C. microphylla community shifted to longer wavelengths. The average and the maximum shrub coverage was 13% and 25%, respectively, in the shrub-encroached grassland based on both models. The correlation coefficient of determination (R²) and root mean square error (RMSE) of the linear model was 0.31 and 0.05, respectively. We found that the linear model based on seasonal differences of shrub and grass community was more suitable for retrieving shrub coverage in the study area from medium resolution imagery than the MESMA model that is based on mid-summer images.

http://jtp.cnki.net/bilingual/detail/html/ZWSB201610001

Key words: shrub encroachment, spectral features, shrub coverage, remote sensing, linear model, multiple endmember spectral mixture analysis model

Tao-Yu LIU, Xia ZHAO, Hai-Hua SHEN, Hui-Feng HU, Wen-Jiang HUANG, Jing-Yun FANG. Spectral feature differences between shrub and grass communities and shrub coverage retri- eval in shrub-encroached grassland in Xianghuang Banner, Nei Mongol, China[J]. Chin J Plant Ecol, 2016, 40(10): 969-979.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2016.0101

https://www.plant-ecology.com/EN/Y2016/V40/I10/969

Figures/Tables 8

Fig. 1 Geographic location of study area and the distributions of field sampling.

Fig. 2 Photos of 1 m × 1 m sampling plots and the auto-extracted pictures of vegetation coverage. A, Grass sample plot with a 34% vegetation coverage. B, Grass sample plot with a 34% vegetation coverage. C, Shrub sample plot with a 91% vegetation coverage. D, Shrub sample plot with a 32% vegetation coverage. E, F, G, H the vegetation coverage of A, B, C, D retrieved by the software, respectively (Vegetation coverage was colored green).

Fig. 3 Spectral features of different vegetation coverage between the shrub and grass communities. A, Spectra of Leymus chinensis with different vegetation coverage. B, Spectra of Stipa krylovii with different vegetation coverage. C, Spectra of Caragana microphylla with different vegetation coverage. D, Comparison of dominant shrub and grass communities’ spectra.

Table 1 Changes in spectral features with shrub and grass dominancy and vegetation coverage (mean ± SD)

1 m × 1 m小样方植被覆盖度 Vegetation cover within 1 m × 1 m plot	羊草 Leymus chinensis			克氏针茅 Stipa krylovii
1 m × 1 m小样方植被覆盖度 Vegetation cover within 1 m × 1 m plot	mNDVI₇₀₅	REP	NDVI	mNDVI₇₀₅	REP	NDVI
<10%	0.17 ± 0.01	705.22 ± 1.46	0.24 ± 0.02	0.19 ± 0.04	704.65 ± 4.20	0.26 ± 0.05
10%-20%	-	-	-	0.23 ± 0.03	702.81 ± 2.52	0.33 ± 0.06
20%-35%	0.35 ± 0.02	708.45 ± 2.32	0.49 ± 0.05	0.27 ± 0.04	702.62 ± 1.29	0.40 ± 0.06

Fig. 4 Linear models based on multi-temporal images and landscape of shrub-encroached grassland in different seasons. A, Linear model in the mid-summer. B, Landscape of shrub-encroached grassland in mid-summer. C, Linear model in early autumn. D, Landscape of shrub-encroached grassland in early autumn. NDVI, normalized difference vegetation index.

Table 2 Spectral features of Caragana microphylla by vegetation coverage (mean ± SD)

1 m × 1 m小样方植被覆盖度 Vegetation coverage within 1 m × 1 m plot	小叶锦鸡儿 Caragana microphylla
1 m × 1 m小样方植被覆盖度 Vegetation coverage within 1 m × 1 m plot	mNDVI₇₀₅	REP	NDVI
35%-50%	0.41 ± 0.06	716.15 ± 2.69	0.49 ± 0.07
50%-70%	0.51 ± 0.04	718.78 ± 2.63	0.63 ± 0.08
70%-90%	0.62 ± 0.09	722.94 ± 5.06	0.77 ± 0.11

Fig. 5 Predicted shrub coverage based on the linear model in shrub encroached grassland area in Xianghuang Banner.

Fig. 6 Endmember spectra of MESMA model and predicted shrub coverage in shrub-encroached grassland area in Xianghuang Banner. A, Endmember spectra in MESMA model. B, Shrub coverage distribution in shrub-encroached grassland in Xianghuang Banner.

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