Effects Of Climate Change And Extreme Events On Phenology And Yield Of Winter Rapeseed

Climate change has become a major threat to the global agricultural production.It was accompanied with rising temperature,increasing variability of precipitation and extremes.Rising population further stimulates the demand of oil.As the main source of edible oil,whether winter rapeseed production will face challenges or opportunities in typical planting regions of China in future is unclear.The projections of mean climate,extremes,phenology and yield of winter rapeseed could provide the feasibility to develop strategies and policies for farmers,breeders and policy makers,further to ensure the grain and oil security under climate change.In this paper,firstly,the climate data for 57 sites over 2021～2100 were statistically downscaled by the NWAI-WG method using 27 GCMs in the CMIP6 under four SSP of 1-2.6,2-4.5,3-7.0 and 5-8.5.Based on the observed（1961～2020）and projected（2021～2100）meteorological daily data,the mean climate and extreme temperture and precipitation indices during the winter rapeseed growth period were calculated.Secondly,the performance of DSSAT-CROPGRO-Canola model was evaluated site-specifically both in calibration and validation processes.Further,the winter rapeseed phenology and yield were simulated and their changes were investigated for the periods of 2021～2060 and 2061～2100 under four SSPs.The relationships between the yield and climate indices were revealed by pearson correlation analysis,multivariate linear regression and random forest models.The limiting factors of winter rapeseed production were identified in different subregions for future.Finally,the potential sowing window and optimal sowing date of winter rapeseed were proposed in order to reduce the risk of yield reduction caused by climate change.The primary conclusions of the paper are as follows:（1）The variations of mean and extreme climate indices spatially and temporally were revealed during the growth period of winter rapeseed from 1961 to 2100.Temperature and extreme hot events during the winter rapeseed growth period were expected to increase,contrastly the extreme cold events would decrease over 2021～2100.And the total precipitation was projected to increase slightly,which showed a wetting trend in the studied region.Specifically,compared with the historical period,the T_max,T_min and T_mean in winter rapeseed growth period were all projected to increase from 2021 to 2100 with a consistent trend.The differences between four SSP scenarios were much greater than the regions.Of which,the increment of T_mean varied from 2.3℃under scenario of SSP1-2.6 to 3.7℃under scenario of SSP5-8.5.The P_re would increase to different degrees,but the differences between SSP scenarios were small.The extreme temperature indices including TNn,TXx,ETR,i HD and TX90p in the growth period of winter rapeseed showed increasing trends from 2021 to 2100,while FD,i ID and TN10p showed the decreasing trends.As the radiation forcing,the number of sites with significant increasing trend gradually increased.However,the variation trends of extreme precipitation events were not significant in the future both in scenarios and periods.RX1day,RX5day,R10,R20 and SDII were expected to increase to a certain extent,while CDD varied in latitudes.CDD had increasing trend for most sites at the low latitudes,while contrastly in high latitudes.（2）The responses of phenology and yield for winter rapeseed to climate change were clarified based on DSSAT-CROPGRO-Canola model.The DSSAT-CROPGRO-Canola model had good performance on phenology and yield of winter rapeseed in YRB and HHR with RRMSE<11.7%.During the historical period,the anthesis and maturity date of winter rapeseed was earlier in the upper reaches of YRB（URY）and middle reaches of YRB（MRY）than in the lower reaches of the YRB（LRY）and Huang-huai region（HHR）.In the projections,the anthesis and maturity dates of winter rapeseed would be advanced（11～28 d）in the whole studied area,and they both gradually became late from south to north.The winter rapeseed yield would decrease by 0.8%,3.6%,and 2.9%in the subregion of URY,MRY,and LRY,while increase by 37.7%in the subregion of HHR over 2021～2060.Further,the increment of CO₂ concentration was expected to offset the reduction of winter rapeseed by 6～12%in YRB and contribute to enhanced productivity by 14～18%in the subregion of HHR.In 2061～2100,CO₂ effect is expected to offset the yield reduction more.（3）The effects of climate change and extremes on the yield of winter rapeseed were analyzed and the adaptive measure was put forward.From the random forest model,the importance ranks of mean and extreme climate indices for winter rapeseed yield varied in YRB and HHR.For the historical period,temperature is the most important climate index in YRB,while precipitation was front rank in HHR.From2021 to 2100,the importance of mean climate indices to winter rapeseed yield projection in YRB was ranked as CO₂>T_min>T_max>S_rad>P_re,while the order was CO₂>P_re>T_min>T_max>S_radin HHR.For extreme temperature and precipitation indices,they were less crucial than mean climate indices in winter rapeseed yield projections.In subregions of URY and MRY,there are four important extreme temperature indices including TX90p,i HD,i ID and TN10p.In subregions of LRY and HHR,FD was also important.The importance of extreme precipitation indices to winter rapeseed yield was higher in HHR than in humid YRB.In addition,the importance of R95p,R10 and R20 was higher than other extreme precipitation indices in each subregion.Compared with the current average sowing date,the potential sowing date window and optimal sowing date for winter rapeseed in YRB were expected to be delayed,while they would be mostly advanced in HHR.However,under the scenario of SSP3-7.0 and SSP5-8.5in 2061～2100,the potential sowing date window in HHR might be delayed.The results of this paper have theoretical and practical reference significance for rapeseed production adaptation to climate change.