Analysis of spatio-temporal variation of crop yield in China using stepwise multiple linear regression
Introduction
China must make strategic decisions on enhancing food production and ensuring food security for 1.4 billion people, and those decisions will have a large effect on agriculture and land use. Over the past 60 years, crop yield in China has increased dramatically, on average by 125 kg ha−1 y−1 (FAO, 2019). For example, the average annual yield increases are 2.5, 1.7 and 3.1 % for maize, rice and wheat, respectively (National Bureau of Statistics of China, 2019). However, there are large spatio-temporal variations of crop yields in China, among others due to climate and soil variation (Chen et al., 2011). Overall, the grain production in eastern China was higher than that in the west of China (Wang et al., 2018a). Therefore, understanding the temporal and spatial variation of crop yield across China is crucial for national food availability and food security.
Many studies were conducted on analyzing the spatio-temporal variation of crop yields in China. High-yield maize was mainly produced in Heilongjiang, Jilin and Liaoning provinces in northeast China, with average yields of 1.07, 1.38 and 1.52 Mg ha−1 in 1961 and increasing rates of 54 kg ha−1 y−1, 81 kg ha−1 y−1 and 67 kg ha−1 y−1 over the past 60 years, respectively (Guo et al., 2017). Rice yield increased by 23 kg ha−1 y−1 since 2000. The highest rice yield was produced in central China with 7.07 Mg ha−1 in 2015; while the lowest rice yield was in south China, with 5.85 Mg ha−1 in 2015 (Wang et al., 2018b). Wheat yield varied from 6.33 Mg ha−1 in north China to 14.80 Mg ha−1in south China. It also increased from west to east along the same latitude (Lv et al., 2017). Sunflower yield was steady from 1985 to 2008 and increased slowly afterwards; in 2015, productive sunflower was mainly distributed in north and central China (Gansu 3.67 Mg ha−1, Hebei 2.82 Mg ha−1, Xinjiang 2.81 Mg ha−1, Inner Mongolia 2.74 Mg ha−1, Ningxia 2.56 Mg ha−1) (Fu et al., 2019).
In order to improve crop yields, exploring the causes of spatio-temporal yield variation is helpful to design policies. Important factors of agricultural production and its variation include technology, genetics, climate, soil, field management practices and associated decisions such as fertilizer application, tillage and crop hybrid selection, irrigation management, row spacing, planting date and depth (Kukal and Irmak, 2018). On a global scale, over 21 % of yield variation could be explained by agro-climatic variation (Iizumi and Ramankutty, 2016). Climatic factors, especially their annual variation, exhibit a stronger overall linkage to changes in late paddy rice yield of China than technological factors (Wang et al., 2016). The warming trend increased rice yield in northeast China and soybean in north and northeast China; however, it decreased maize yield in seven provinces in the central and northeast regions and wheat yield in three provinces (central and northeast China) (Tao et al., 2008). Thus, the relationship between climatic factors and yield is scale-, location- and crop-dependent. Large-scale statistical data and regional climate datasets are important for investigating general response patterns of crop yields to climate change and variation.
Most studies on yield variation addressed either one specific crop (Deng et al., 2019) or main grain crops (Iizumi and Ramankutty, 2016). In such case, the result does not reflect the integral productivity for all crops. Moreover, existing research mostly focused on the influence of climatic factors on yield (e.g. Kukal and Irmak, 2018). Few studies aimed at explaining the yield variation using agricultural, environmental and economic factors, let alone explore the importance of these factors. As a consequence, a better understanding of the contributions of agricultural, environmental and economic factors on yield and yield variation is needed. A provincial-scale evaluation of the spatio-temporal variation for aggregate yield over a long-term period could provide a general overview and the required high-level information for decision makers.
A disadvantage of analysis of aggregate yield variation is that different crops may have very different yields. For instance, the average yield of tomatoes, rice and maize per unit area is quite different and only evaluating the aggregate yield would not recognize these differences. To account for this, it is also useful to run separate analyses for different crop categories and for individual crops. Such analyses do not suffer from the problem that yields from different crops are aggregated, but an important drawback is that the analysis has to be done for many different crop categories, individual crops and perhaps even for different crop varieties. This may yield too detailed and too much information, thus obscuring general patterns. Decision and policy makers in particular need integrated information that show general patterns and trends.
In this research, we collected agricultural, environmental and economic factors that may influence the spatio-temporal variation of yield, and analyzed their correlation with the aggregate yield of all crops in 31 provinces in China, from 1978 to 2015. This study aims at: a) analyzing the space-time patterns of yield aggregations at multiple levels in China with large yield datasets; b) constructing empirical models of different yield aggregations using stepwise linear regression; c) exploring the major explanatory variables of spatio-temporal variation of different yield aggregations and their relative importance; d) analyzing the temporal and spatial patterns of the regression residual (difference between observations and predictions) for the models for yield aggregated over all crops, staples crops, cash crops and three individual crops (maize, rice and wheat).
Section snippets
Yield and explanatory variables
We collected yield data from 1978 to 2015 of 31 provinces of China (excluding Hong Kong, Macao and Taiwan; Chongqing started from 1997). For potatoes, the data were from 1982 to 2015, while watermelon data were from 1996 to 2012. To assess the effect of fertilization policies on temporal variation of yield at provincial scale, we distinguished three fertilization periods: (a) high-yield fertilization (1978–1995), (b) balanced fertilization (1996–2005), (c) soil-test based fertilization
Results
Because of space limitations, this section mainly focuses on the results of the first level modelling (aggregate yield over all crops), and presents fewer results for the level 2 (staples and cash crop categories) and level 3 (individual crops) analyses. More detailed results for levels 2 and 3 are provided in Appendix B.
Spatial and temporal variation of yield
The marked increase of aggregate yield in China between 1978 and 2015 (Fig. 2a) is likely caused by improved crop varieties and increased fertilizer application. However, the average annual growth rates in Heilongjiang, Guizhou, Beijing, Qinghai and Jilin were smaller than those in other provinces. In Heilongjiang and Jilin, constraints from shallow topsoil resulting from long-term continuous cropping, severe black soil loss due to soil erosion and insufficient organic matter input led to
Conclusions
This study analyzed temporal and spatial variation in crop yield aggregations at provincial level in China from 1978 to 2015. Stepwise multiple linear regression was used to explore the relationships between crop yield and agricultural, environmental and economic explanatory variables. The temporal and spatial patterns of yields were different for different levels of crop aggregations. Most of the models had an R-square larger than 0.6, except for rice, potato and cotton. AMP, soil and economic
Funding
This research was supported by the National Key Research & Development Program of China (No. 2016YFD0200101), the National Natural Science Foundation of China (No. 31972515) and the Earmarked fund for the China Agriculture Research System (No. CARS-09-P31).
CRediT authorship contribution statement
Yingxia Liu: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing, Visualization. Gerard B.M. Heuvelink: Methodology, Software, Resources, Data curation, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Zhanguo Bai: Conceptualization, Software, Investigation, Resources, Data curation, Writing - review & editing, Visualization, Supervision,
Declaration of Competing Interest
None.
Acknowledgements
The authors thank Luis de Sousa (ISRIC-World Soil Information, The Netherlands) for kindly explaining the SoilGrids parameters and two anonymous reviewers for constructive comments that greatly improved this work.
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