| Agricultural production is closely related to climate change. Under background of global warming, increasing political, economic and scientific attention has been drawn to the impacts of climate change on crop production. Rice is world’s major food crop, and serves as basic staple for more than half of the world’s population in Asia, Africa and Latin America. China is one of the world’s largest rice-producing countries, has the second largest area of rice cultivation and the highest rice production, and rice occupies more than 40% of the national cereals yields of China.Due to the spatial and temporal heterogeneity of climate, and the regionality and seasonality of rice planting, the climate change impacts on rice production is highly uncertainty. Agricultural land in China comprises less than 10% of world’s arable land area but feeds nearly 20% of the world population. The effects of climate change on rice production will be directly related to 1/5 of the world population’s food security. There is no doubt that food security will become a major problem in China at present and in the foreseeable future of next several decades. Food security is a major issue to be studied. In this study, by using quantitative method to analyze the influence of global warming on rice growth and production in China, we tried to provide scientific basis for determining the rice growing areas and arranging reasonable farming systems.Through observed and simulated climate and rice phenology data, using methods of statistical analysis like time series analysis, mathematical modeling and GIS technology, this study clarified spatial-temporal changes of rice growing season length and extreme temperature stress during different key growth stages of irrigated rice across mainland China. And by modifying the model of photosynthetic thermal productivity and adopting modified model, we estimated the historical and future rice yield and photosynthetic thermal productivity. Results of this study are presented as follows: (1) From 1961 to 2008, the change of safe growing season behaved periodically, with extending tendency after 1985. Rice safe growing season length in single rice region of Northeast and the mid-lower Yangtze River Valley, and double rice region of the mid-lower Yangtze River Valley and South China, had significantly increased with 9d/10a,6d/10a, 7d/10a and 7d/10a, respectively. Extending of growing season length in Northeast is because of the earlier safe sowing time and later mature time, and in the mid-lower Yangtze River Valley and South China is only due to the earlier of safe sowing time. Shifting of safe sowing time is more significant than that of mature time, and contributed greater to the change of growing season. Spatially, the longest safe growing season of rice appeared in Yunnan-Guizhou Plateau, and the shortest in the northern area of the Northeast China. Northeast and the mid-lower Yangtze River Valley had the most significant change trend in growing season length, with an increase of more than 10d.(2) From 1961 to 2008, low-and high-temperature stress during rice critical growth period in different planting regions distributed temporally and spatially. In general, global warming reduced low-temperature stress but did not increase high-temperature stress. Reduction of low-temperature stress mainly occurred in seedling and heading-flowering stage, no significant variation in booting. Among all planting regions, only single rice region of Northeast, double rice region of the mid-lower Yangtze River Valley and South China showed a decreasing trend of low-temperature stress, with LTSI reduction in seedling stage of 0.6/10a,1.2/10a and 1.6/10a respectively, in heading-flowering stage of 0.2/10a,0.3/10a and 0.4/10a respectively. The year when low-temperature stress starting reducing significantly appeared to be gradually later from the north to the south:during seedling stage low-temperature decreased since the mid-1980s in the Northeast and the mid-lower Yangtze River Valley, and did not drop until the 1990s in South China; during heading-flowering stage low-temperature decreased in the three regions started from the early of 1970s, the mid-1980s and the early of 1990s, respectively. No significant trends in HTSI were observed in all rice planting regions, while relatively high HTSI occurred in the mid-lower Yangtze River Valley during the 2000s. This relatively high HTSI did not suggest an increasing trend because it declined in the last two years. Through the analysis on the relation between extreme temperature stress and rice yield in county and provincial level, we founded a yield reduction of 291 kg ha-1 whenΔTSI increased by one unit in heading-flowering stage, and reduction of 165 kg ha-1 when happened in seedlings stage.(3) From 1961 to 2050, the change of photosynthetic thermal productivity behaved differently in time and space. The photosynthetic thermal productivity in single rice region of Northeast, rice region of the mid-lower Yangtze River Valley and double rice region of mid-lower Yangtze River Valley showed a significant increasing trend during 1961 to 2008. Double rice region of mid-lower Yangtze River Valley had the most significant tendency, with an annual increase of 150 kg ha-1; contrarily, the photosynthetic thermal productivity in the Sichuan Basin reduced 75 kg ha-1 each year significantly. From 2011 to 2050, the photosynthetic thermal productivity in all rice planting regions had shown a rising trend. This trend in single rice region of Northeast China, Sichuan Basin and Yunnan-Guizhou Plateau, and double rice region of the mid-lower Yangtze River Valley were significantly; with increasing trend under the A2 and B2 scenarios of 136 kg ha-1,79 kg ha-1,116 kg ha-1, 85 kg ha-1 and 91 kg ha-1,67 kg ha-1,118 kg ha-1,89 kg ha-1 each year, respectively. In space, from 1981 to 2050 the photosynthetic thermal productivity in the Northeast increased gradually from north to south, and in the South from the coastal area to the inland decreasing gradually. The highest productivity appeared in southwest Yunnan-Guizhou Plateau and parts of Southern China, while the lowest value in the northern part of northeast. Rice yield increased significantly from 1961 to 2008, but only had a small rise from 2011 to 2050. Spatially, rice yield had different distribution characteristics during the two periods. From 1981 to 2008, the rice yield in the Northeast increased gradually from surrounding area to middle annularly, and in the south from the coastal area to the inland decreasing gradually. The highest value appeared in Sichuan Basin. During the next 40 years, the rice yield in the Northeast increases from north to south, and in the south distributed zonally. The highest value appeared in southern part of northeast and eastern part of Sichuan Basin, while the lowest value in the two double rice regions.In conclusion, from 1961 to 2008, safe growing season extended generally and had obvious temporal and spatial characteristics. The forward shift of safe sowing time is more significant than backward shift of mature time, and contributed greater to the change of growing season length. During all critical growth periods, low-temperature stress reduced but high-temperature stress did not increase. Reduction of low-temperature stress mainly occurred in seedling and heading-flowering stage. The photosynthetic thermal productivity changes in planting regions were significantly different. It increased in single rice region of Northeast, rice region of the mid-lower Yangtze River Valley and double rice region of the mid-lower Yangtze River Valley, but decreased in Sichuan Basin. From 2011-2050 years, the photosynthetic thermal productivity in all planting regions had shown a rising trend. Rice yield increased generally, with an increasing trend from 1961 to 2008, but a smaller rise trend from 2011 to 2050. |
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