Effects of land cover and phenology changes on the gross primary productivity in an Artemisia ordosica shrubland

doi:10.17521/cjpe.2020.0387

Abstract

Abstract:

Aims We aimed to examine how changes in vegetation cover and phenology affect the trend of gross primary productivity (GPP) in an Artemisia ordosica shrubland in the Mau Us Desert during the first two decades of the 21th century.

Methods We used the vegetation photosynthesis model (VPM) in combination with remote sensing data (MODIS) to simulate GPP dynamics during 2005-2018. Eddy covariance (EC) measurements of GPP (GPP_Flux) were used to parameterize and validate the VPM model. The “derivation and threshold” approach was used to determine the start (SOS) and the end of the growing season (EOS), as well as the length of the growing season (LOS) for each year. Ordinary least squares (OLS) regression was used to examine the variations in temperature, normalized differences vegetation index (NDVI), and GPP over time. OLS, multiple, and partial correlation analyses were used to test the relationships among temperature, NDVI, phenology, and GPP.

Important findings (1) Modeled GPP well captured the dynamics of GPP_Flux, whereas the MODIS product (MOD17A2H) significantly underestimated GPP_Flux. (2) NDVI, annual maximum NDVI(NDVI_max), and annual GPP all showed an increasing trend during 2005-2018, indicating the role of vegetation recovery in promoting GPP. (3) SOS showed an advancing trend (2.1 d·a^-1) and EOS showed a delaying trend (1.5 d·a^-1), and therefore both SOS and EOS contributed to the increasing trend of LOS (3.6 d·a^-1). (4) Annual GPP was higher with greater LOS (6.44 g C·m^-2·a^-1 for 1 day increase in LOS). (5) Increases in vegetation cover and growing season length explained 79% and 57% of the variations in GPP, respectively. (6) Increases in vegetation cover played a more important role than growing season extension in promoting GPP.

Key words: gross primary productivity, vegetation index, phenological phase, climate change, vegetation photosynthesis model, Mau Us Desert

YUAN Yuan, MU Yan-Mei, DENG Yu-Jie, LI Xin-Hao, JIANG Xiao-Yan, GAO Sheng-Jie, ZHA Tian- Shan, JIA Xin. Effects of land cover and phenology changes on the gross primary productivity in an Artemisia ordosica shrubland[J]. Chin J Plant Ecol, 2022, 46(2): 162-175.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2020.0387

https://www.plant-ecology.com/EN/Y2022/V46/I2/162

Figures/Tables 9

Fig. 1 Overview of the Artemisia ordosica shrubland at the Yanchi ecology research station of Mau Us Desert study site in Ningxia.

Fig. 2 Comparison of flux observation gross primary productivity (GPP)(GPPFlux) and vegetation photosynthesis model (VPM) prediction GPP (GPPVPM) and MOD17A2H product GPP (GPPMODIS) in Mau Us Desert in Ningxia from 2016 to 2018. Each data point represents the daily average value. The flux observation GPP in 2012-2015 was used for model parameterization, and the flux observation GPP in 2016-2018 was used for model validation.

Fig. 3 Temporal dynamics of normalized difference vegetation index (NDVI) and gross primary productivity (GPP) in Mau Us Desert in Ningxia from 2005 to 2018 (A), as well as annual mean NDVI, vegetation photosynthesis model (VPM) simulated annual GPP (GPPVPM), and flux tower observed annual GPP (GPPFlux) linear trend (B).

Fig. 4 Relationship between vegetation photosynthesis model (VPM) simulated annual gross primary productivity (GPP) and annual mean normalized differences vegetation index (NDVImean)(A), annual max NDVI (NDVImax)(B) and vegetation photosynthesis model (VPM) simulated GPPmax (C) in the Mau Us Desert in Ningxia from 2005-2018.

Fig. 5 Interannual variation in the start date of the growing season (SOS), the end data of the growing season (EOS), the length of growing season (LOS) and average annual temperature, annual precipitation during 2005-2018 in the Mau Us Desert in Ningxia. SOS, EOS and LOS were delineated by normalized difference vegetation index (NDVI) and simulated gross primary productivity (GPP). Dashed lines are the 14-year linear trend of SOS, EOS, LOS, temperature and precipitation.

Fig. 6 Relationship between annual gross primary productivity (GPP) and the start of the growing season (SOS)(A, D), the end of the growing season (EOS)(B, E), and the length of the growing season (LOS)(C, F) in Mau Us Desert in Ningxia from 2015 to 2018.

Table 1 Standardized regression coefficients for annual max gross primary productivity (GPPmax), annual max normalized differences vegetation index (NDVImax), and the multiple linear regression equations of gross primary productivity (GPP)(g C·m-2·a-1)

变量 Variable	标准回归系数 Standardized regression coefficients	回归方程 Equations	R²	p
GPP_max, LOS_GPP	0.85, 0.01	GPP = -170.92 + 103.95GPP_max + 0.70LOS_GPP	0.85	<0.01
GPP_max, LOS_NDVI	0.75, 0.22	GPP = -373.42 + 91.51GPP_max + 2.01LOS_NDVI	0.86	<0.01
NDVI_max, LOS_GPP	0.81, 0.10	GPP = -385.59 + 1766.93NDVI_max + 0.24LOS_GPP	0.76	<0.01
NDVI_max, LOS_NDVI	0.87, 0.03	GPP = -467.32 + 1642.28NDVI_max + 0.96LOS_NDVI	0.77	<0.01

Fig. 7 Relationship between annual gross primary productivity (GPP) and multiplication of the length of growing season (LOS) and GPP (LOSGPP·GPPmax), (LOSNDVI·GPPmax), multiplication of LOS and normalized differences vegetation index (NDVI) (LOSGPP·NDVImax), (LOSNDVI·NDVImax) in the Mau Us Desert in Ningxia from 2005 to 2018.

Table 2 Comparision of phenology result in this study and previous studies

研究区域 Study area	生长季开始日期 (年序日) Start of the growing season (day of the year)	生长季结束日期 (年序日) End of the growing season (day of the year)	生长季长度 Length of the growing season (d)	研究时段 Study period	数据来源 Data resources	分辨率 Spatial resolution (km)	文献来源 Source of literature
宁夏毛乌素沙地 Mau Us Desert in Ningxia	109-142	278-305	136-194	2005-2018	MODIS: NDVI	0.5	本研究 This study
	111-144	267-310	129-199	2005-2008	GPP_VPM	0.5	本研究 This study
	90-156	245-323	125-200	2001-2013	MODIS: NDVI	0.25	Wang et al. 2015
	80-160	270-325	135-243	1982-2015	AVHRR LTDR-V4: NDVI	5	Zhu et al. 2018
陕甘宁交界 Shaanxi-Gansu- Ningxia junction	80-160	290-330	130-250	1999-2010	SPOT-VGT: NDVI	1	Wei et al. 2014
宁夏毛乌素沙地 Mau Us Desert in Ningxia	109-124	281-305	157-193	2012-2018	通量塔 Flux Tower: GPP	0.22	本研究 This study

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