Prediction of litchi flower induction in South China region based on the CMIP6 climate model

【Objective】Litchi is an important economic fruit crop in the tropics and subtropics of Southern China, playing a vital role in promoting local specialty industries and rural revitalization. In China, litchi is primarily distributed in the low-latitude regions of South China and is extensively cultivated in coastal areas, with the production period extending from March to August. Due to the diverse climate and terrain, the layout of litchi cultivation in China exhibits a varied development trend and has resulted in the formation of several key production areas. Under the trend of global warming, insufficient accumulation of low temperature and extreme high temperature are important factors leading to the instability of litchi flowering and low yield. In recent years, the flowering rate of medium-late maturing litchi varieties in the low-latitude regions of South China has generally been low, resulting in a pronounced al-ternative-bearing years with high and low production. Therefore, in-depth studies on the prediction of litchi flower induction under future climate scenarios can help to plan in advance for the selection and breeding of climatically suitable varieties and rationally adjust the planting layout and other strategies to cope with the climate change.【Methods】In this study, we selected the early-maturing variety Feizixiao and the medium-late maturing varieties Seedless litchi and Ziniangxi as test subjects. We investigated the climatic factors influencing the induction of litchi flowering in planting bases across six cities and counties (Haikou, Chengmai, Danzhou, Baisha, Qionghai, and Lingshui), over a span of five consecutive years (2019—2023). To identify the key climatic factors affecting litchi flower formation, we calculated chilling accumulation for low temperature thresholds of 10 ℃, 12 ℃, 15 ℃, 18 ℃, and 20 ℃, as well as heat accumulation for high temperature thresholds of 25 ℃, 28 ℃, and 30 ℃. We then constructed a florescence climate index model, correlating it with the litchi flower formation rate for both early-maturing and medium-late maturing varieties. Additionally, we selected the average ensemble of four climate models (CanESM5, FGOALS-g3, GFDL-CM4, and IPSL-CM6A-LR) from CMIP6 to assess the spatial and temporal evolution characteristics of the commercial cultivation limits and flower formation induction of litchi under two climate scenarios, comparing the base period with future projections in the South China region.【Results】Early-maturing variety‘Feizixiao’needs little chilling for flower induction, and low temperature below 20 ℃ can promote induction, while medium-late maturing varieties Ziniangxi and Seedless litchi need much chilling, and effective low temperature is below 15 ℃ for induction, and the high temperature of 25 ℃ or above has a counteracting effect on the induction of flower formation. Based on the induction of accumulated chilling and harmful heat accumulation, we constructed the litchi florescence climate index (FCI), which is strongly correlated with the flower formation rate, and it can be used as a reference for the measurement of the degree of difficulty in the flower formation of litchi. In the baseline period (1979—2014), extreme low temperatures above 0 ℃ were concentrated in southern Yunnan, south- central Guangdong and Guangxi, Southern Fujian, Hainan island and Sichuan Basin. These regions represented the current main areas for commercial litchi cultivation in China, with 25 ℃ serving as the demarcation line. There is a significant temperature difference between the southern and northern regions. The 0 ℃ dividing line is projected to shift northward in future periods, leading to a gradual fragmentation and reduction of areas experiencing sub-0 ℃ temperatures. The SSP_585 scenario evolves more rapidly compared to the SSP_245 scenario, and by 2080, the potential planting areas will encompass all regions except for the high- altitude areas in the northwest. In future climate scenarios, extreme high temperatures exhibit a significant upward trend. The warming amplitude of the SSP_585 scenario is notably faster and more pronounced. By 2080, extreme high temperatures in the coastal areas of South China under the SSP_585 scenario are generally expected to exceed 27 ℃, with temperatures nearing 30 ℃ along the northwestern and southern coasts of Hainan. January is a crucial time for litchi as it transitions from the induction stage to the differentiation stage. The significant increase in extreme high temperatures expected in January will greatly heighten the risk of disrupting litchi flower formation. In the baseline period, the region experiencing challenging flowering conditions was limited to Hainan Island. As time progresses, this area will gradually migrate to higher latitudes, with the most significant northward expansion of difficult flowering conditions projected for 2080 and 2100 under the SSP_585 scenario, reaching approximately 25° N latitude, and covering almost all of the central and southern regions of Guangdong and Guangxi.【Conclusion】The pattern of winter temperature distribution in South China is expected to change significantly under future climate scenarios, with the 0 ℃ isotherm shifting northward and the boundaries of litchi cultivationexpanding to higher latitudes. Over time, the cumulative cold required for flower formation in low-latitude regions will gradually decrease, while harmful cumulative heat will increase. As a result, the stability of flower formation in medium-late maturing litchi varieties in the coastal low-latitude regions will be at a greater risk. In contrast, large production areas for early-maturing and extra-early maturing litchi varieties are likely to emerge. The selection and breeding of low-cold-demanding extra-early-maturing litchi varieties, along with the scientific and rational planning of high- cold- demanding medium- and late-maturing litchi, will be the focus of many scholars and fruit growers in the coming decades.