Risk assessment of freezing injury during overwintering of wheat in the northern boundary of the Winter Wheat Region in China

Overview of the research region

Zunhua, Hebei province, China (N: 39°55″–40°22″, E: 117°34″–118°14″) was selected as the research region. It is located in the boundary of the Hebei province winter wheat-planting area, as well as by a northern boundary of the Northern Winter Wheat Region (Fig. 1). Additionally, it is the cold resistance identification test site of the NWWVRT in China. Therefore, it is of great significance to study the prevalence of winter wheat-freezing injury in this region.

Data sources of meteorological and freezing injury information

The data of daily maximum temperature, minimum temperature, average temperature, and precipitation used in this study were obtained from 1956 to 2016 from the surface weather stations in the China Meteorological Data Network (http://data.cma.cn). Information on historical winter wheat-freezing injury was also obtained from the China Meteorological Disaster Yearbook: Hebei volume (Zang, 2008) and related research (Dai et al., 2010). The dead tiller rate data of wheat were obtained from the NWWVRT from 2007 to 2016.

Formulation and test of the FII

Fit-test of FII and freezing injury degree

Existed studies (Fuller et al., 2007) mostly have used the dead plant rate (the ratio of the dead plant to the total plant) to determine the degree of wheat-freezing injury. However, this situation is limited to severe freezing injury. When freezing injury is mild, whole plant death rarely occurs, and it is the death of some tillers of the plant (Limin & Fowler, 2006). Therefore, the ratio of dead tillers to total tillers was used as the index to judge the degree of freezing injury in this study. To explore the relationship between the FII and the actual freezing injury degree, FII and data on the tiller death rate of Zunhua NWWVRT from 2007 to 2016 were used for univariate linear regression analysis (via IBM SPSS v.26.0).

Wavelet analysis

Wavelet analysis can indicate the periodic characteristics of time series information in different timescales, and it is better to show the change trend and qualitatively predict the future development trend (Kumar & Foufoula-Georgiou, 1993). In this study, the Morlet wavelet (Peng et al., 2013) was used to analyze the FII data from 1956 to 2016 (via MATLAB 2020b). As the time series data in this study were limited time series, the symmetry extension method was used to extend the two ends of the series to eliminate or reduce the possible “edge effect”. The Morlet complex wavelet function in the wavelet toolbox of MATLAB was then used to transform the extended data sequence, calculate the real part of the wavelet coefficients, and draw the real part diagram of the wavelet coefficients. The wavelet toolbox was further used to calculate the wavelet variance and to draw the wavelet variance and main period wavelet coefficient diagrams.

The starting and ending dates and the change trend of the overwintering period in each year

The starting and ending dates of the daily average temperature stably lower than 0 °C (i.e., the starting and ending dates of the overwintering period in each year) in each year were obtained from the five-day moving average method (Fig. 2). According to the change in the starting and ending dates of the overwintering period over the years, the starting dates were gradually delayed (Fig. 2A), and the fluctuation range was from the middle and late November to early December and was gradually smaller. These results indicate that the start date of the overwintering period was gradually stable in early December. Additionally, the end date was gradually advanced (Fig. 2B), and the fluctuation range was advanced from the middle and early March to the middle and late February, but the fluctuation range was large, indicating that the end date of overwintering was unstable.

Formulation and verification of the FII equation

Correlation between freezing disaster-causing factors

Information on the seven freezing disaster-causing factors in the overwintering period of each year was calculated, and a correlation test was conducted (Fig. 3). Results show that there was a certain correlation among disastrous factors, except for the “precipitation before overwintering”. These results indicated that a comprehensive variable can be constructed by the PCA method to reflect the information contained in the seven freezing disaster-causing factors.

Verification of FII values and wheat-freezing injury

The range-standardized values for seven freezing disaster-causing factors in each year were substituted into formula Eq. (4) to obtain FII values for each year (Fig. 4). Combined with the occurrence of wheat-freezing injury in history (Zang, 2008), our results showed that FII values were higher during the severe freezing injury year (the year indicated by the arrow in Fig. 4). These values better reflect the situation of historical freezing injury, which indicated that the FII can be used to reflect the actual situation of winter wheat-freezing injury.

On the basis of the cold resistance identification of the Zunhua NWWVRT, univariate linear regression was used to analyze the relationship between the dead tiller rate and FII values. Regression analysis showed that the dead tiller rate and FII value passed the 0.05 significance test (P = 0.014), indicating that a reliable functional relationship can be established between the two indicators. According to the histogram of the residual error of the regression analysis model used, the overall distribution of the residual error was normal. Combined with a P–P diagram, the regression model has a good fit (Fig. 5). The linear regression equation of univariate was calculated as follows: (5)${\displaystyle y=-34.826+65.511x,}$ where x and y represent the FII index and dead tiller rate, respectively.

Regression analysis results also showed that the larger the FII value was, the higher the dead tiller rate was and the more severe the degree of freezing injury was. It showed that the FII value was a good representation of the degree of freezing injury and had high applicability.

Analysis on the change trend and causes of the FII values

As the FII values can reflect the degree of freezing injury, it is of practical significance to analyze the change trend of FII values in 60 years. As the result shows, the interannual variation range of the FII values in Zunhua is 0.485–1.566 from 1956 to 2016, with the maximum value from 1968 to 1969 and the minimum value from 2014 to 2015 (Fig. 4). From the results, the amplitude of the FII between 1956 and 1986 was larger than 1987 to 2016, and there are many peaks during this period, which indicated that wheat-freezing injury occurred frequently in this period. By contrast, from 1987 to 2016, the amplitude of the FII was small, and the frequency of freezing injury was less. Observed the change trend of the FII curve during this period, 1995–1996 and 2009–2011 had the two largest peaks where severe freezing injury occurs. From overall view, the interannual variation trend of the FII in Zunhua showed a significant downward trend (R² = 0.7412) in the 60-year period from 1956 to 2016, but the interannual fluctuation range within whole period was large, with a coefficient of variation value of 27.2%. At the end of the curve, the fluctuation range of the FII increased, which indicates that the risk probability of freezing injury has also increased.

For the change of the overwintering period in 60 years, the delay in the starting time and advance in the ending time of the overwintering period (Fig. 2) led to the shortening of the overwintering period (Fig. 6A). The extreme minimum temperature (Fig. 6B), average temperature (Fig. 6C), and negatively accumulated temperature (Fig. 6D) during the overwintering period also showed an increasing trend. This result show that the gradual warming of the overwintering environment of winter wheat, which is conducive to safe overwintering. Correspondingly, the change in the FII over the years showed a decreasing trend (Fig. 4). Although the climate was gradually warming in the general environment, the frequency of extreme climate increased recently, and the risk of wheat-freezing injury increased.

Wavelet analysis

Three periods, including short (0–10a) (“a” means the timescale “year”), medium (11–20a), and long periods (21–35a), were identified in the evolution process of the FII (Fig. 7A). The long period (21–35a) spanned the whole timescale, showing obvious seasonal changes in abundant–withered–abundant, with peaks in 1963, 1985, and 2007, respectively. The periodicity of the other two periods was not particularly significant, but the characteristic of the short period (0–10a) was more obvious than that of the medium period (11–20a). In the wavelet variance curve (Fig. 7B), the fluctuation energy of the FII time series also has a clear peak on the timescale of 33a, which indicates that the periodic oscillation around 33a was the strongest. This was the first main cycle of the variation. Similarly, a low peak value on the timescale of 4a was also observed, which is the second main cycle of annual variation. Additionally, the average period of change was approximately 40 years on the 33a timescale, and it has experienced approximately 1.5 abundant–withered periods (the red curve in Fig. 7C). On the 4a timescale, the average period of change was approximately five years, which has experienced approximately 12 abundant–withered periods (the blue curve in Fig. 7C).

As a whole, the real part of the wavelet was not completely closed at the right end (Fig. 7C), which indicates that it was still in the period of high risk of freezing injury. Additionally, the right end value of the first principal period curve of the periodic wavelet coefficient graph did not drop below zero, and the second principal period was at its peak, which indicates that the fluctuation energy of the freezing injury probability was strong in a short term. This result also indicates that the risk of wheat-freezing injury probability was high.

Occurrence and evolution of freezing injury and its climatic characteristics

Because the FII is a better representation of the real freezing injury situation (Figs. 4 and 5), the occurrence and evolution law of winter wheat-freezing injury in Zunhua in the past 60 years can be analyzed according to the change trend in FII. From the change in FII within 60 years, the freezing injury of winter wheat in the first 30 years showed the characteristics of heavy degree, high frequency, and large interannual fluctuation, indicating that the climate in winter before the mid-1980s was harsh and that climate change was unstable (Zhu et al., 2014). The low temperature and absolute value of the negatively accumulated temperature during the overwinter period were also large, and the frequency of extreme low temperature was high. These conditions were not conducive to winter wheat, which further led to a decrease in population density and yield reduction (Livingston & Swinbank, 1962; Fowler et al., 1996).

After the mid-1980s, the occurrence degree and frequency of freezing injury decreased (Tao et al., 2006), which was proposed to be caused by global warming (Zhai et al., 1999). In this study, the delay of the starting date and the advance of the ending date of the winter wheat overwinter period in Zunhua for the past 60 years led to the shortening of overwinter time. The negatively accumulated temperature, average temperature, and extreme minimum temperature of the overwinter period also showed an increasing trend (Fig. 6), which further confirmed the inference that climate warming reduced the risk of wheat-freezing injury.

Since the beginning of the 21st century, climate instability has increased (Tian et al., 2016). Additionally, extreme climate events occurred frequently (Hänninen, 1991), and the probability of winter wheat-freezing injury increased (Fig. 4). Our results found that the occurrence of freezing injury in Zunhua over the past 60 years showed a periodic change, and the small period was nested in the large period. At present, FII curve is at the end of a long period-positive peak and the top of the short period-positive peak (Fig. 7C), indicating that the probability of wheat frost damage in this area in the future will be relatively high. Therefore, to avoid severe freezing injury of winter wheat, it is necessary to select wheat varieties with strong cold resistance in future production processes, which can greatly reduce the impact of periodic freezing injury on grain production.

Guidance of risk assessment of freezing injury to agricultural production

Our results showed that the probability of freezing injury in Zunhua was high in the short term (Fig. 7C). On the basis of these results, the following problems in the production process should be paid attention to: (1) the climate in the edge area of the original planting strong winter wheat cannot meet the needs for wheat complete the vernalization process because of the gradual warming of the climate. Previous studies have shown that the wheat variety structure in northern boundary of Winter Wheat Region in China was gradually changing from a strong winter to winter or semi-winter (Zheng, Ge & Hao, 2002; Wang et al., 2013), which will increase the risk probability of seedlings having freezing injury when they encounter extreme low temperature weather (Paulsen & Heyne, 1962). Therefore, growers should try selecting varieties with stronger winter characters to reduce the adverse effects caused by climate change. (2) Owing to the delay in the starting date of the wheat-overwintering period (Fig. 2A), the sowing time should be appropriately late according to the actual climate conditions to prevent the vigorous growth of seedlings before overwintering (Sadras & Monzon, 2006). Simultaneously, irrigation should be timely and sufficient before winter to avoid freezing injury caused by drought (Fischer, 1985). When breeding new winter wheat varieties, cold resistance should also be screened (Zheng et al., 2015a; Zheng et al., 2015b).

Suggestion on identification of cold resistance of varieties

Owing to global warming, cold resistance identification of new varieties in a field will result in a situation where the dead plant and dead tiller rate of all varieties are zero (Wang et al., 2008). Here, it was impossible to make an objective evaluation on the cold resistance of all varieties. Therefore, to fully reflect on the cold resistance of various varieties, the occurrence probability of freezing injury can be improved by the following ways: (1) sowing all tested varieties in advance to form exuberant seedlings before overwintering, which can increase the probability of freezing injury of seedlings, or decrease planting density, which will increase the probability of freezing injury (Fowler & Limin, 2004). (2) Due to the north boundary of Winter Wheat Region is constantly moving north, the area with higher latitudes and worse wintering environment can also be added as a new cold resistance identification test point to fully reflect the cold resistance ability of various varieties(Chen et al., 2011; Yang, Liu & Chen, 2011).

Deficiency and suggestion of establishing the FII equation

The occurrence of winter wheat-freezing injury is an extremely complex process (Craufurd & Wheeler, 2009). The seven freezing disaster-causing factors selected in this study for formulating a FII equation cover effective information such as the water condition before overwintering, the intensity, duration, and cumulative effect of low temperature, as well as the effective precipitation during overwintering. The FII can reflect the occurrence degree of winter wheat-freezing injury through the test. Our results showed that, the higher the FII value is, the more severe the freezing injury would be. In addition to the selected factors causing freezing injury, the occurrence of freezing injury is affected by wheat genetic factors (Yan et al., 2004; Fu et al., 2005), duration and thickness of snow cover (Yoshida et al., 1998), duration and speed of wind (Porter & Gawith, 1999), and cultivation management methods (Hassan et al., 2021). For example, during the overwintering period of 2012–2013, wheat seedlings were covered with snow for approximately one month, which effectively protects seedlings; hence, the freezing injury degree of seedlings was less, but the FII value was higher. Thus, more meteorological factors related to wheat-freezing injury can be included in the formulation of a FII. However, it should also be noted that the increase in data collection make this model more complex, which is not conducive to the operation of users.

Additionally, soil moisture condition has a great impact on wheat overwintering as drought aggravates the occurrence of freezing injury (Al-Issawi et al., 2013). The content of soil moisture during overwintering directly affects the occurrence probability of wheat-freezing injury (Alireza et al., 2021). We speculate that taking soil moisture content as the water condition factor of FII equation probably achieve more accurate prediction effect. In conclusion, the establishment of the FII equation needs to be further improved.