Climate change and its direct and indirect impacts on crops are major forces affecting the entire world (Piao et al., 2010). Crop production is greatly affected by changes in greenhouse gas concentrations, solar radiation, temperature, and available water patterns, which may have considerable consequences for potential and actual agricultural yields. Available agro-climate resources are the available parts of natural climate resources that can be directly absorbed and converted into dry matter and yield. As an important aspect of natural climate resources, variation in available agro-climate resources during the crop growing season could directly affect crop growth and grain yield under climate change (Bellon et al., 2011). Outside of the coolest regions, which currently have temperatures below the optimum range, rising temperatures might negatively affect crop production and shorten the time available for biomass accumulation (Supit et al., 2012). Crop yield reductions can be prevented by planting varieties that require more time to mature (Olesen et al., 2011; Liu et al., 2013a; Meng et al., 2016), which would increase the water requirement. Moreover, climate change can be a major threat to food stability by reducing crop yield and increasing interannual variations in yield (Challinor et al., 2014; Rippke et al., 2016). Due to the increased probability, frequency, and severity of possible extreme events, disasters have increased worldwide in the past 60 years and are expected to increase further (Meehl et al., 2000; Rosenzweig et al., 2001; Gabaldón-Leal et al., 2016). They can cause marked damage to crops, food systems, and local to global food security (Lesk et al., 2016). Therefore, among the potential impacts of climate change on crop production, extreme weather events are an important consideration, perhaps more so than average climate change (Katz and Brown, 1992; Rosenzweig et al., 2001; Guan et al., 2017).
Maize has been the main crop grown worldwide (Li and Wang, 2010), with 1.84 × 108 ha of harvest area and 1.02 × 109 tons of production in 2013. As the second largest maize producer and consumer, China accounted for 19.15% and 21.42% of the total maize harvest area and production, respectively, in the world in 2013 (FAO, 2014). In China, maize can be cultivated in most regions of the country and includes spring, summer, autumn, and winter maize. Based on the cultivated maize zones, the cultivated maize areas can be divided into six regions (Fig. 1): the North China spring maize region (NCS), the Huanghuaihai summer maize region (HS), the South-west China mountain maize region (SCM), the South China hill maize region, the Northwest China maize region, and the Tibet maize region. Among the six regions, NCS, SCM, and HS are the main maize cropping regions, encompassing more than 90% of the sowing areas in the country (Li and Wang, 2010). Under global warming, climatic conditions have been changing significantly in China. During the maize growing season, the average increase in maize growing degree-days was 67.3 °C day (10 yr)–1 during 1961–2007, with decreasing trends of precipitation and sunshine duration (Yang et al., 2011). Such changes can significantly affect maize cropping distribution (Liu et al., 2013b; Zhao et al., 2016), growth and development (Li Z. G. et al., 2014; Tao et al., 2014), and yield in China (Tao and Zhang, 2011; Cui et al., 2013; Meng et al., 2013, 2014; Wang J. et al., 2014; Wang X. H. et al., 2014; Tao et al., 2015). Furthermore, unexpected extreme events along with variations in climate resources have also been observed. In NCS, although significant warming trends have been observed during the past 30 years, the frequency of chilling damage for maize increased during the same period, and some severe freezing disasters occurred (Zhao et al., 2014) for unsuitable cultivar selections. In the past 50 years, increased in temporal and spatial drought events have also been observed with the variability in temperature and precipitation in both HS and SCM (Liu et al., 2014; Yang et al., 2015).
Investigating both the average amount and the stability of the available agro-climate resources could provide a scientific basis for understanding the effects of climate change on crops. The variations in the available agro-climate resources differ among different regions. Understanding the average amount and stability of these resources would allow suitable adaptations to be applied in each region. For example, cultivars with higher radiation and water use efficiencies could be planted in those areas with reduced available solar radiation and water resources, and appropriate agricultural management practices could be applied in areas with reduced available heating resources. In addition, more consideration should be given to extreme events in those areas with reductions in the stability of available agro-climate resources. To date, most studies have focused on the average changes in climate resources, and few have investigated the stability of climate resources. In this study, we collected historical climate data for the main maize cropping regions in China for the period 1981–2010 and examined the available solar radiation, heating, and precipitation resources indices during the maize growing season. The aims of this study include the following: (1) investigate the temporal and spatial variations in the average amount of maize-available agro-climate resources during the growing season in the three main cropping regions, and (2) understand the variations in the stability of maize available agro-climate resources in the three main cropping regions.2 Materials and methods 2.1 Study areas and historical climate data
Historical climate data from 331 meteorological stations in the main maize cropping regions during the period 1981–2010 were obtained from the China Meteorological Administration climate data-sharing service system (Fig. 1). The daily observed weather variables include maximum temperature, minimum temperature, average relative humidity, minimum relative humidity, wind speed, precipitation, and sunshine duration.
Maize can be planted in areas where the ≥10°C accumulated temperature (ACT10) exceeds 2100°C day (Gong, 1988; Liu et al., 2010). To select the study areas, the beginning and end dates of the period consecutively passing 10°C in each year for every meteorological station were defined by the 5-day moving averages method (Qu, 1991). The ACT10 during the consecutively passing 10°C period at each station with the 20th percentile during 1981–2010 was calculated as the cropping safety for maize. The inverse distance weighting (IDW) method in ArcGIS 10.0 software was used to interpolate the ACT10 values throughout each region (Zhao et al., 2014, 2016). Areas with an ACT10 greater than 2100°C day in the three main cropping regions were selected as the study areas (Fig. 1).2.2 Length of the maize growing season
In NCS and SCM, 99% and 89% of the maize cropping areas, respectively, were planted with spring maize, which was sown in the spring and harvested in the autumn. The climatologically safe planting and harvest dates for spring maize were determined by the beginning and end, respectively, of the period that is consistently above 10°C (Chen et al., 2012; Zhao et al., 2014, 2016). The maize growing season was defined as the period between the climatologically safe planting and harvest dates. In HS, the dominant cropping system is winter-wheat–summer-maize, and 99% of maize cropping areas were planted with summer maize. The growing season for winter wheat should be considered when deciding the planting dates and lengths of the maize growing season. Therefore, the observed planting dates and harvest dates for summer maize closest to each agro-meteorological station (Fig. 1) during 1981–2010 were compiled to determine the length of the summer maize growing season at each meteorological station in HS.2.3 Available agro-climate indices 2.3.1 Available solar radiation resources
Photosynthesis active radiation (PAR) is the available part of radiation that is intercepted and converted to accumulated dry matter and crop yield (Tollenaar and Aguilera, 1992). PAR data during the maize growing season were used to evaluate the solar radiation resources in the main cropping regions in China. Daily solar radiation at each station in the main cropping regions was calculated from daily extraterrestrial radiation (Rsi) using the Angstrom formula and the sunshine duration recorded at the stations (Allen et al., 1998; Zhao et al., 2015b). The available solar radiation resources during the maize growing season were calculated as the accumulated daily PAR values at the top of the canopy, which can be calculated based on daily extraterrestrial radiation (Sinclair and Muchow, 1999; Zhao et al., 2015b; Hao et al., 2016) and it is widely used in crop models (Lizaso et al., 2003).
where AcPAR (MJ m–2) is the available solar radiation resources during the maize growing season; n (day) is the length of the maize growing season; PARi (MJ m–2) is the PAR at the top of the canopy on day i; and Rsi (MJ m–2) is the extraterrestrial radiation on day i.2.3.2 Available heating resources
The daily thermal time (TT) is considered in maize simulation models to directly affect growth and yield. The daily TT was calculated by the formulas in the APSIM-Maize model (http://www.apsim.info/; Fig. 2), which has been demonstrated to simulate performances well in most regions of China (Chen et al., 2010a; Liu et al., 2012). The daily average value was derived from the daily maximum and minimum temperatures when daily TT was calculated (Wilson et al., 1995; Liu et al., 2013a). The accumulated daily TT values during the maize growing season were selected as the available heating resources in the main cropping regions in China (Zhao et al., 2015a).
where AcTT (°C day) is the available heating resources during the maize growing season; n (day) is the length of the maize growing season; and TTi (°C day) is the thermal time on day i.2.3.3 Available water resources
The effective precipitation (EP) is the fraction of the total precipitation as rainfall and snowmelt that is available to the crop and does not run off. In this study, EP was calculated following the U.S. Department of Agriculture Soil Conservation method, as presented below (Döll and Siebert, 2002; Ye et al., 2015). The summations of daily EP values during the maize growing season were calculated as the available water resources in the main cropping regions in China.
where Pei (mm day–1) is the EP on day i, and Pi (mm day–1) is the total precipitation on day i; and AcEP (mm) is the available water resources during the maize growing season.2.4 Data analysis 2.4.1 Trends in growing season and available agro-climate resources
Linear regression analysis was used to calculate the trends in the climatological planting and harvest dates and available agro-climate resources. Statistical significance was determined by two-tailed t-tests after normality testing using the Kolmogorov–Smirnov (K–S) model (Li K. N. et al., 2014).2.4.2 Stability of available agro-climate resources
The stability of an available agro-climate resource can be defined as the degree of deviation of the resource from a standard value over a certain period. The coefficient of variation (CV), which is the ratio of the mean square error to the average, can reflect the degree of deviation for a dataset (Zhao et al., 2016; Guan et al., 2017) and was selected to indicate the stability of available agro-climate resources during the three study decades (1981–90, 1991–2000, and 2001–10). A higher CV indicates less stability in the data series (Dobermann et al., 2003).3 Results 3.1 Length of the maize growing season
In all three main maize cropping regions, the climatological planting dates were advanced, the climatological harvest dates were delayed (Fig. 3), and the length of the climatological growing season was prolonged during 1981–2010 (Table 1). In NCS, a significantly advancing trend [2.1 day (10 yr)–1, p < 0.01] was observed for climatological planting date, but the delay trend of climatological harvest date was non-significant [1.2 day (10 yr) –1, p > 0.05]. The average length of the climatological growing season was 157 ± 21 days, with a significant prolonging trend [3.3 day (10 yr) –1, p < 0.01] in NCS from 1981 to 2010. In HS, the advancing trends in planting date [1.1 day (10 yr) –1] and delayed harvest date [0.8 day (10 yr)–1] were non-significant (p > 0.05); in this region, the growing season for winter wheat was considered for establishing the planting and harvest dates. However, the average values for the growing season increased from 98 ± 7 days during 1981–90 to 102 ± 6 days during 2001–10, and a significant prolonging trend [2.0 day (10 yr) –1, p < 0.01] was found in this region. In SCM, the trends in climatological planting date [1.9 day (10 yr) –1] and harvest date [2.8 day (10 yr)–1] were significant (p < 0.05 for planting date and p < 0.01 for harvest date). The average length of the maize climatological growing season was prolonged significantly ( p < 0.01) by 4.7 day (10 yr) –1 during 1981–2010, with an average value of 249 days. Because of the complicated landforms in the region, the error bars were longer than those in the other two regions.
|Region||Average value (day)||Trend [day (10 yr)–1]|
|NCS||154 ± 23||156 ± 22||160 ± 19||157 ± 21||3.3**|
|HS||98 ± 7||100 ± 5||102 ± 6||100 ± 6||2.0**|
|SCM||246 ± 63||247 ± 64||255 ± 62||249 ± 63||4.7**|
|The symbol ** indicates statistically significant at p < 0.01.|
The available heating resources (TT) during the maize growing season generally decreased from south to north (2309.6°C day) in NCS, with a mean value of 1324.7°C day during 1981–2010 (Fig. 4 and Table 2). The average trend in TT was decreasing significantly [95.5°C day (10 yr)–1, p < 0.01] in NCS, with trends ranging from –85.0 to 343.1°C day (10 yr) –1, and these trends were higher than the average trends in the other two regions (Fig. 5 and Table 2). In HS, TT was higher in the central region than in the eastern and western portions, and the trends ranged from 396.3 to 1846.4°C day (10 yr)–1. The average trend in TT in HS was increasing significantly [57.7°C day (10 yr)–1, p < 0.01] and the lowest among the three regions. In SCM, TT decreased from southeast to northwest (0.3–5352.4°C day) during the maize growing season with a significantly increasing average trend [93.5°C day (10 yr) –1, p < 0.01].
The available water resources (EP) during the maize growing season generally decreased from southeast to northwest (52.7–262.9 mm) in NCS, with the average value of 158.3 mm. A decreasing average trend for EP [–5.3 mm (10 yr)–1] was found in NCS. In HS, EP decreased from 183.6 mm in the southern portion to 101.9 mm in the northern portion during the maize growing season. However, the average trend in EP increased by 3.0 mm (10 yr)–1, whereas it decreased in the other two regions. In SCM, EP was more than 340 mm in most of the areas and the average value was 376.0 mm. A decreasing average trend for EP [–5.8 mm (10 yr)–1] was found, with trends ranging from –27.2 to 19.1 mm (10 yr)–1.
In NCS, the available radiation resources (PAR) during the maize growing season showed a similar distribution as TT, with an average value of 1640.9 MJ m–2, and the average PAR significantly increased by 20.9 MJ m–2 (10 yr)–1. The smallest values of PAR (903.0–1081.5 MJ m–2) during the maize growing season were found in HS, and the average value decreased by 11.6 MJ m–2 (10 yr)–1, which was an opposite trend to that observed in the other two regions. In SCM, PAR decreased from 3047.4 MJ m–2 in the southwest to 387.0 MJ m–2 in the northeast. The average PAR was significantly increased by 25.2 MJ m–2 (10 yr)–1 (p < 0.05) in this region.
|Available agro-climate resource||Average||Trend (per 10 yr)|
|North China spring maize region|
|Thermal time (°C day)||2309.6||25.3||1324.7||343.1||–85.0||95.5|
|Effective precipitation (mm)||262.9||52.7||158.3||50.2||–18.6||–5.3|
|Photosynthesis active radiation (MJ m–2)||2121.3||494.8||1640.9||357.6||–124.2||20.9|
|Huanghuaihai summer maize region|
|Thermal time (°C day)||1846.4||396.3||1645.2||158.9||5.9||57.7|
|Effective precipitation (mm)||183.6||101.9||130.9||22.3||–15.4||3.0|
|Photosynthesis active radiation (MJ m–2)||1081.5||903.0||963.3||32.5||–61.2||–11.6|
|Southwest China mountain maize region|
|Thermal time (°C day)||5352.4||0.3||2229.1||300.8||–184.2||93.5|
|Effective precipitation (mm)||717.7||152.6||376.0||19.1||–27.2||–5.8|
|Photosynthesis active radiation (MJ m–2)||3047.4||387.0||1950.8||173.3||–103.1||25.2|
Similar temporal trends in the stability of TT were found in NCS and HS. Compared with the CV of TT during 1981–90, higher CVs of TT were found in the other two decades, indicating poor stability. The poorest stability for TT was during 1991–2000, which exhibited the highest CV values (Fig. 6). In SCM, the average CVs of TT increased from 0.12 during 1981–90 to 0.16 during 2001–10, representing a decreasing trend in stability. Among the three regions, HS showed the highest average stability of TT (average CV of 0.04); the average CVs were 0.12 and 0.13 in NCS and SCM, respectively, over 30 yr.
In all three regions, the CVs of EP were higher than those of TT and PAR during 1981–2010, with the greatest difference found in HS. In NCS and HS, the CVs of EP were the highest during 1991–2010 and lowest during 2001–10. However, in SCM, the average CV values of EP increased slightly, from 0.12 during 1981–90 to 0.13 during 2001–10. Among the three regions, HS showed the poorest stability for EP during 1981–2010, with an average CV of 0.25; SCM showed the highest stability, with average value of 0.13.
Compared with the average CVs of TT and EP, the average CVs of PAR were lower in all three regions. In NCS and HS, the CVs of PAR were the highest during 1991–2010. However, the lowest values for the CV of PAR were found during 1981–90 in NCS but during 2001–10 in HS. Meanwhile, an opposite temporal trend for the average CV of PAR was found in SCM, where the CV of PAR was the lowest during 1991–2010. Among the three regions, HS showed the highest stability for PAR during 1981–2010, with an average CV of 0.06, whereas the average value in both NCS and SCM was 0.08.4 Discussion
Consistent with several previous studies (Chen et al., 2012; Liu et al., 2013a; Zhao et al., 2014), the results of the present study showed that the length of the climatological growing season for maize has been prolonged significantly in NCS under climate change (Fig. 3). The observed growing season has shown the same trend (Chen et al., 2012; Li et al., 2012; Zhao et al., 2015a). In HS, because of the winter-wheat–summer-maize cropping system and the decreased length of the growing season for winter wheat (Li et al., 2016), there has been an increase in the length of the growing season for maize. In SCM, the length of the climatological growing season for maize has been prolonged significantly according to the same calculation methods depending on temperature used for NCS. Similar trends have been also reported in previous studies (Dai et al., 2011).
The available agro-climate resources are parts of the climate resources that can be directly used by crops as energy for growth (Trnka et al., 2011). Under global warming, temperature has increased significantly in higher latitude regions (IPCC, 2014). In this study, the increasing trend of TT in NCS was stronger than the trends in both HS and SCM (Fig. 5). This may be due to the fact that most of the meteorological stations in NCS are located in higher latitude regions than those in the two other regions (Fig. 1). Mid- and late-maturing cultivars with longer growing season and higher yield potential can be widely planted in NCS (Zhao et al., 2014). However, because most of the maize fields were rain-fed in the three cropping regions, decreased EP in NCS and SCM may increase the risk of drought (Liu et al., 2014; Zhao et al., 2014), with negative effects on maize yield (Dong et al., 2015), especially in the presence of increased heating resources. Irrigation management and drought-tolerant cultivars might offset the negative effects of drought (Hao et al., 2016).
PAR is the basic energy for crop photosynthesis. Through the processes of chlorophyll synthesis and photosynthesis and photosensitive regulatory mechanisms such as phototropism and photoperiodic activity, crops convert radiant energy into chemical energy (Udo and Aro, 1999). Decreased PAR in NCS and SCM could decrease maize photosynthesis rates, reduce dry matter accumulation, and eventually reduce grain yields. Cultivars with higher radiation use efficiency that are planted at suitable planting density might be help offset the effects of decreased PAR (Zhao et al., 2015b; Hao et al., 2016).
In addition to the variability in the average amounts of the available agro-climate resources, the stability of climatic conditions also influences agricultural production (Isik and Devadoss, 2006; Zhao et al., 2016; Guan et al., 2017). Compared with the stability of the agro-climate resources during 1981–90, the stability of all resources decreased during 1991–2000 and 2001–10. Increased fluctuations of climatic conditions negatively impact the suitability of the climate for maize growing, increase disaster risk, and decrease final yield (Zhao et al., 2016). In NCS, with the significantly increased temperature, the planted areas for hybrids with a longer growing period and higher yield potential were extended. In Addition, the risk of freezing damage increased due to the poor stability of the heating resources. In HS, a wide range of irrigation water supplies would be needed to adapt to the inter-annual climate variation (Chen et al., 2010b), which also affects final grain yield (Wu et al., 2008). According to China’s National Assessment Report on Climate Change, heating resources will increase and climate fluctuations will be more intense in the whole country in the future. Insufficient available water resources will be the main challenge, and irrigation will be an essential measure to improve maize yield (Liu et al., 2017). Moreover, more consideration should be given to the extreme events caused by more intense climate fluctuations.
In this study, variations in available agro-climate resources in the main maize cropping regions in China were revealed, which may be used to provide scientific guidance for addressing the effects of climate change on maize. However, there are some limitations to this study. Farmers across China and other countries are currently adapting to climate change, particularly in terms of changing the timing of cultivation and the selection of cultivars (Olesen et al., 2011; Wang et al., 2012; Liu et al., 2013a; Li et al., 2016). Changing cultivars might change the indices for calculating the available climate resources, which were held constant in this study. Furthermore, the selection of cultivars depends on breeding technology, governmental policies, the grain market, and extreme weather conditions (Ye et al., 2015). In this study, the theoretical growing season for spring maize was only affected by heating resources in NEC and SCM, whereas soil moisture should also be considered in crop production (Zhao et al., 2015a). In practice, it is impossible for farmers to use the entire theoretical growing season and all of the available agro-climate resources (Zhao et al., 2015a). For multiple cropping systems in HS and SCM, the total output and resource use efficiency for the cropping systems should be considered in future research (Zhao et al., 2014, 2016).5 Conclusions
Knowledge of the average amount and stability of available agro-climate resources for maize in the main cropping regions of China under climate change is essential for farmers and advisors to optimize cropping choices and develop adaptation strategies under limited resources. However, most studies have focused on the average changes in climate resources, and few have investigated the stability of climate resources. In this study, we collected historical climate data for the main maize cropping regions in China for the period 1981–2010 and examined the available solar radiation, heating, and precipitation resource indices during the maize growing season.
During 1981–2010, the climatological growing season for maize was significantly prolonged in all three main cropping regions in China [3.3, 2.0, and 4.7 day (10 yr)–1 in NCS, HS, and SCM]. However, the spatial and temporal patterns of the available agro-climate resources differed among the three regions. The available heating resources for maize increased significantly in all three regions, with increasing trends in NCS [95.5°C day (10 yr)–1] and SCM [93.5°C day (10 yr)–1] being stronger than the trend in HS [57.7°C day (10 yr)–1]. Decreasing trends in available water resources were found in NCS [5.3 mm (10 yr)–1] and SCM [5.8 mm (10 yr)–1], and an increasing trend was observed in HS [3.0 mm (10 yr)–1]. Increased available heating resources but decreased water might increase the risk of drought in NCS and SCM. Irrigation managements and drought-tolerant cultivars might help offset the negative effects of drought. In contrast, increasing trends in available radiation resources were found in NCS [20.9 MJ m–2 (10 yr)–1] and SCM [25.2 MJ m–2 (10 yr)–1], whereas a decreasing trend was found in HS [11.6 MJ m–2 (10 yr)–1]. In HS, cultivars with higher radiation use efficiency and planted at suitable planting density might be selected to adapt. Compared with the stability of the agro-climate resources during 1981–90, the stability of all three resource types decreased during 1991–2000 and 2001–10 in the three cropping regions. More consideration should be given to the extreme events caused by more intense climate fluctuations. The results can provide guidance in the development of suitable adaptations to climate change in the main maize cropping regions in China.
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