Ⅰ. INTRODUCTION
In South Korea, the temperature in the last 30 years has risen by 1.4 ℃ compared to the 20th century (1912–1941). Additionally, the summer is longer by 19 days and the winter is short by 18 days due to climate change over the past 106 years. RCP 8.5 is the pathway with the highest greenhouse gas emissions and represents the 90th percentile of the no-policy baseline scenario (Riahi et al., 2011). If the current greenhouse gas (GHG) emission trend is maintained (RCP 8.5), an increase in temperature of 6 °C and an increase in precipitation by 20.4% are expected. If efforts are made to reduce emissions (RCP 4.5), it was reported that an increase in temperature of 3.4 °C and an increase in precipitation by 17.3% are expected (Kim et al., 2012). Due to the ongoing climate warming, summer crops have been negatively affected due to high temperatures, while winter crops have been reported to increase productivity per area by the increasing temperature during the wintering and growing stages (Schauberger et al., 2017;Stöckle et al., 2018). Climate change generally is affected by the increase in temperature, as the crop cultivation area moves, invasive species will also increase rapidly, and influence observed yield fluctuations (Kim et al., 2012).
Italian ryegrass (Lolium multiflorum Lam.; IRG) is a representative winter forage crop that has a high feed value, excellent livestock palatability, and can be harvested from late April to mid-May of the following year after sowing the rice in South Korea (Kim, 1991;Barnes et al., 2003;Ji et al., 2018;Choi et al., 2018b). In particular, it is resistant to injury by moisture, so it is possible to cultivate paddy fields after harvesting rice (Arikado, 1964;Kim et al., 2015).
However, in the past, rye (Secale cereale) has been mainly grown because of its tolerance to the cold, and IRG was mainly cultivated in part of the southern region of Korea. (Heath, 1973;Hides, 1979;Choi et al., 2011). According to the forage cultivation status report, the domestic IRG cultivation area in 2005 was 12.5 thousand ha, which was very low (MAFRA, 2020). As a result of expanding forage seed supply by 2020, a total of 21 varieties, such as ‘Kowinerly’, ‘Greenfarm’, ‘Green call’, and ‘IR 601’, were developed through a research project at the National Institute of Animal Science and the cultivation area expanded to 165 thousand ha in 2019. IRG occupied 87% of the domestic winter forage cultivation area (Choi et al., 2011;Ji et al., 2011;MAFRA, 2020). New varieties developed in South Korea are cold tolerant and have high productivity, suitable for the climate of South Korea. For this reason, imported cultivars hard to cultivate in the northern regions of South Korea for low cold tolerance. With the development of new varieties and the cultivation technology of IRG, it has become possible to expand cultivation to the central and northern regions of Korea (Choi et al., 2011;Ji et al., 2011;Kim et al. 2015;Ji et al., 2019). Assessing the potential impact of climate change on food security in the future is great importance (Burke et al., 2010). In agriculture, it is possible to respond effectively to climate change by predicting crop productivity, quality, and damage to the crop (Reddy and Pachepsky, 2000). However, in order to reflect on the policy related with promotion of forage production, further research is required (Kim et al., 2012;Moon, 2013).
Kim et al. (2018) reported that under climate change, the suitability of IRG would increase up to 90% until the 2080s by using the EcoCrop model, and further studies are required to identify suitable areas for IRG cultivation using cold-tolerant varieties. Kim et al. (2019) found that climatic actors influence the productivity of IRG differently between upland and paddy fields by using multi-group structure equation modeling. In addition, IRG cultivation in paddy field conditions was more sensitive to autumn and next spring precipitation. Peng et al. (2016) studied a model for predicting IRG productivity according to climate factors using the K-fold cross-validation method in South Korea. However, there are few studies on predicting the changes in the suitable agro-climate zone of IRG introduced and domestic cultivars. Therefore, this study aimed to predict the IRG productivity changes of introduced and domestic varieties based on climatic factors that are highly correlated with productivity and identify suitable areas for IRG cultivation using the RCP 8.5 climate change scenario from the 1980s to 2100s.
Ⅱ. MATERIALS AND METHODS
1. Digital climate maps of climate change scenario in South Korea
In this study, digital climate maps with a 30 m grid resolution from the National Institute of Horticultural and Herbal Science were used. A 30 m grid resolution is suitable for agricultural use (Moon, 2013). The climate change scenario used was RCP 8.5, which belongs to the ‘Pessimistic IPCC RCP 8.5 climate scenario’ in the fifth assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (Table 1). A 30 m resolution climate change scenario map was created by applying microclimate technology to detailed climate change scenarios in South Korea with a resolution of 1 km by the Korea Meteorological Administration (KMA) (Yun, 2004;Moon, 2013). As shown in the schematic diagram of Fig. 1, it was analyzed using the 30 m resolution climate change scenario map.
2. Establishment of suitability classes and estimation of productivity for IRG cultivation
To estimate the productivity of the IRG, a comparative experiment on the adaptability results was analyzed using 136 results from 2013 to 2018. The weather data were collected from the Agricultural Weather Information Service (http://weather.rda.go.kr) of the Rural Development Administration. The statistical analysis of the Pearson correlation by SAS (9.4) was used to determine the climate factors related to the dry matter yield of IRG. The establishment of suitability classes of IRG was established using economic analysis according to the regression equation between the climate factor, which is highly correlated with productivity. The level of suitability classes of IRG was set in 4 classes: best suitable, suitable, possible, and low productivity.
IRG selected climatic factors that could affect overwintering (mean air temperature in December and January, minimum mean air temperature in January) as winter forage that were sown between September and October, and then obtained correlation with productivity. The correlation between climate factors, productivity, and the r-values of winter temperatures all showed a high correlation (Table 2). The mean air temperature and minimum mean air temperature in January showed a significant correlation (r = 0.64144, r = 0.67261, respectively) compared to other climate factors related to overwintering. Therefore, a regression formula was calculated to identify changes in the productivity of IRG cultivars using the relation with the data base (Table 2) collected based on the minimum mean air temperature in January, which is highly correlated with productivity (Fig. 2). Choi et al. (2018a) reported that the overwintering rate was high in coldtolerant varieties and showed a high correlation between productivity and the overwintering rate of IRG.
3. Establishment of suitability classes
The break-even point was set at the point where the gross income and managing cost were matched. The break-even point of IRG is 6 t ha-1 (low productivity), the slightly lower average productivity is between 6 and 7 t ha-1 (possible), and the upper average productivity is between 6 and 7 t ha-1 (suitable) of dry matter yield according to Choi et al. (2018a). The best suitable class is set to more than 9 t ha-1 of dry matter yield (Shin et al., 2012;Choi et al., 2018b). The suitability classes were calculated based on the minimum mean air temperature in January (Fig. 3). The best suitable class for all IRG varieties is more than -5 ℃, the suitable class is between -6 ℃ and -9 ℃, the possible class is between -10 ℃ and -12 ℃, and the low productivity class is below -13 ℃. Fig. 3 shows the suitability classes for the introduced and domestic IRG cultivars. When comparing varieties, the ratio of suitable and possible sites of domestic varieties was relatively high. Choi et al. (2000) found that the safe cultivation area of IRG was consistent with the area below the Hangang River, where the lowest minimum average temperature in January was -9 °C and altitude below 400 m. Kim et al. (2012) reported that the correlation between minimum mean air temperature in January and the productivity of IRG was higher than that of other indicators.
Ⅲ. RESULTS AND DISCUSSION
1. Prediction of IRG suitability classes according to climate change scenario
Fig. 4 shows the change in the IRG cultivation area according to the RCP 8.5 climate change scenario. The RCP 8.5 scenario is the most pessimistic outlook where there is no effort to reduce GHG emissions. According to the RCP 8.5 scenario, the radiative forcing exceeds 8.5 W/㎡ in 2100, the average temperature rises by 4.8 °C, and precipitation will increase by 6% (Park et al., 2014).
Fig. 4 (A) shows the IRG suitability classes for the average period of the past 30 years (1981–2010); the ratio of possible and low productivity areas was large in Gangwondo (North part of ROK), and the ratio of suitable and best suitable areas were relatively high in the central and southern regions. Choi et al. (2000) and Kim et al. (2012) reported that IRG can cultivate up to a minimum mean air temperature of -9 °C in January, and these results are consistent with this study. In the Gangwon area, where the altitude is high, there are many regions where the minimum mean air temperature in January falls below -9 °C. Fig. 4(F) shows the distribution of the period from 2051 to 2060, and it was found that the ratio of possible and low productivity areas decreased significantly. Fig. 4(J) shows the distribution of the period from 2091–2100, most of the area was classified as suitable and best suitable area. Choi et al. (2018a) reported that the correlation between the wintering rate and dry matter yield of cold-tolerant varieties is very high (r = 0.88), therefore, when cultivating IRG in the central and northern regions, select a cold-tolerant variety with a high over-wintering rate. The change in the IRG cultivation area was found to be 26.9% in the best suitable area between 1981–2010 but increased significantly to 88.9% between 2091–2100 due to global warming (Fig. 5).
2. Comparison of IRG suitability classes between domestic and introduced cultivars
Fig. 6 shows the suitability classes for IRG introduced and domestic varieties based on minimum mean air temperature (January) from the past to future decades under the RCP 8.5 scenario. In the average period of the past 30 years (1981– 2010), the ratio of suitable and best suitable area was relatively high in domestic varieties. Choi et al (2011) reported that ‘Kowinearly’ (IRG domestic variety) is a cold-tolerant variety that displays a consistent winter survival rate and productivity in the northern regions. Kim et al. (2015) reported that higher productivity yields were shown in domestically developed cold-tolerance cultivars such as ‘Hwasan 101’, ‘Kowineraly’. In the period from 2091 to 2100, most of the area was classified into suitable and best suitable area. From 2041 to 2050, the area ratio of IRG introduced cultivars was 3.4% for possible, 34.1% for suitable, and 62.1% for best suitable in 2041, but 1.3% for possible, 36.6 % for suitable, 62.1% for best suitable in 2050 (Fig. 7). KMA (2014) reported that the annual average temperature of the Korean Peninsula at the end of the 21st century is estimated to increase by more than 4 °C in the RCP 8.5 scenario. Additionally, it is predicted that all regions except Inje, Hongcheon, Wonju, Daewallyeong, and Jecheon will be included in the subtropical climate region at the end of the 21st century. Therefore, the difference between IRG domestic and introduced varieties disappears in the IRG suitability classes.
Ⅳ. CONCLUSIONS
The objective of this research was to predict IRG productivity changes of introduced and domestic varieties based on climate change factors and identify suitable areas for IRG cultivation using the RCP 8.5 climate change scenario. The minimum mean air temperature (r = 0.67261) in January showed the highest correlation with productivity. The ratio of possible and low productivity areas was large in Gangwondo, and the ratio of suitable and best suitable areas was relatively high in the central and southern regions in the past 30 years (1981–2010). The change in the IRG cultivation area was found to be 26.9% in the best suitable area between 1981–2010 but increased significantly to 88.9% between 2091–2100 under the RCP 8.5 scenario. When comparing the IRG suitability classes, IRG suitability classes of domestic and introduced cultivars, the ratio of suitable and best suitable areas was relatively high in the domestic varieties during the past 30 years (1981–2010). However, the difference between IRG domestic and introduced varieties almost disappears in the IRG suitability classes after the 2050s. These results showed that breeding with high adaptability and high productivity to a warmer climate condition will be more important than coldtolerant cultivars in the future. In addition, the results of this study can predict changes in IRG suitability classes between domestic and introduced cultivars according to the climate change scenario, but there are limitations in accurately predicting the productivity of IRG because the results may vary depending on other environmental factors. Therefore, it is necessary to develop a new model to improve the accuracy of prediction IRG productivity considering other environment factors in the future.
