Scenario Analysis on the Adaptation of Di ff erent Maize Varieties to Future Climate Change in Northeast China

XU YanhongGUO JianpingZHAO Junfang,and MU Jia

Chinese Academy of Meteorological Sciences,Beijing100081

Scenario Analysis on the Adaptation of Di ff erent Maize Varieties to Future Climate Change in Northeast China

Chinese Academy of Meteorological Sciences,Beijing100081

Based on gridded meteorological data for the period 1981–2100 from the RegCM3 regional model,the changing trends of climatic resources in Northeast China are analyzed,and the distributions of maize varieties are accordingly adjusted.In order to explore the e ff ects of di ff erent adaptation countermeasures on climatic productivity and meteorological suitability in the future,maize cultivars with resistance to high temperatures and/or drought are selected.The results show that,in the future,there is likely to be a signi fi cant increase in thermal resources,and potential atmospheric evaporation will increase correspondingly. Meanwhile,radiation is predicted to increase signi fi cantly during 2041–2070 in the growing season.However,changes in precipitation are unlikely to be sufficient enough to o ff set the intensi fi cation in atmospheric evaporation caused by the temperature increase.Water resources and high temperatures are found to be the two major factors constraining grain yield.The results also show that the warming climate will be favorable for maize production where thermal resources are already limited,such as in central and northern Heilongjiang Province and eastern Jilin Province;while in areas that are already relatively warm,such as Liaoning Province,climatic productivity will be reduced.The climatic productivity and the meteorological suitability of maize are found to improve when the planting of resistant varieties is modeled.The utilization of agricultural climatic resources through the adaptation countermeasures of maize varieties is to increase obviously with time.Speci fi cally,maize with drought-resistant properties will have a marked in fl uence on meteorological suitability during 2011–2070,with suitable areas expanding.During 2071–2100,those maize varieties with their upper limit of optimum temperature and maximum temperature increased by 2℃,or water requirement reduced to 94%,or upper limit of optimum temperature and maximum temperature increased by 1℃ and water requirement reduced to 98%,all exhibit signi fi cant di ff erences in climatic potential productivity,compared to the present-day varieties.The meteorological suitability of maize is predicted to increase in some parts of Heilongjiang Provine,with the eastern boundary of the “unavailable” area shifting westward.

climate change,Northeast China,variety adaptation countermeasure,Agro-Ecological Zone (AEZ)model,climatic productivity

1.Introduction

Over the last 100 years,the global average surface temperature has risen by almost 0.74℃,and the heating rate is twice as high in the second half of that period(Qin and Luo,2008).In the context of global warming,the annual mean surface temperature in China has increased signi fi cantly during the past 50 years.At the same time,the national mean temperature has increased by 1.1℃.This magnitude of increase in surface temperature is greater than that of the Northern Hemisphere and the globe(Ding et al., 2006),and the trend is expected to continue during the next 20–100 years(Tang et al.,2011).The costs and bene fi ts of climate change are not equally distributed around the world(Darwin et al.,1995).Developed countries can bene fi t from climate change through rising crop production while in developing countries production becomes limited;in other words,disparities in cereal production between the developed and develop-ing worlds will likely increase under global warming (Rosenzweig and Parry,1994).Climate change can have adverse e ff ects on agricultural production,and ultimately threatens food security.Therefore,studying how agricultural production can adapt under climate change is an important topic in order to achieve agricultural sustainable development.To date,a variety of adaptation options have been proposed as having the potential to reduce the vulnerability of agroecosystems to risks related to climate change(Smit and Skinner,2002),some of which are already well developed,such as the adjustment of sowing date,planting resistant varieties,and farm management(L¨u et al.,2010).

Northeast China is an important agricultural region and occupies a key strategic position in the grain market.The annual output of maize in this region is almost 40 million tons,accounting for approximately 30%of the total maize yield in China(Ma et al.,2008). On one hand,the warming climate serves to extend the growing season in Northeast China,which is conducive to an improvement in total grain production;on the other hand,the warmer and drier conditions present great challenges to maize production.For example, the grain- fi ll period of maize can shorten,meaning that kernel weight and ultimately yield can decrease. There have been many studies carried out that have focused on how maize varieties in Northeast China could adapt to future climate change,and the results have largely showed that early-maturing and mid-maturing varieties will be replaced by late-maturing varieties under a warming climate(Jia and Guo,2009;Zhao et al.,2009).Furthermore,in areas that are originally cooler,grain yields could bene fi t from a transition to new varieties;while in areas that are already relatively warm,such a transition may not have much of an effect(Yuan et al.,2012).

Therefore,how to make full use of climatic resources to maximize maize yields is an important topic of research.Climatic productivity can not only reveal the relationship between crop’s growth and development,yield and climatic resources,but also help to discover the main yield-limiting factors and re fl ect resource utilization(Liu,2010).In the present study, we use daily meteorological data for the period 1981–2100,from simulations by RegCM3 under the A1B future-climate scenario,to quantitatively assess the contributions of di ff erent climate-change adaption options(e.g.,adjustment of maize variety layout,use of resistant varieties)to potential increases in maize productivity.Based on the results,we also discuss possible future development directions with respect to maize varieties in Northeast China.Furthermore,beyond Northeast China,the study provides a theoretical basis for agricultural adaptation options to climate change and the reasonable utilization of climatic resources for realizing high and stable maize yields.

2.Data and methods

2.1 Data

WechoseHeilongjiang,Jilin,and Liaoning provinces as our study areas.Meteorological data for Northeast China covering the period 1981–2100,as simulated by RegCM3 under the A1B future-climate scenario,are used.These data include daily average temperature,daily maximum and minimum temperature,daily total radiation,daily net radiation,daily average wind speed at 2 m above ground level,daily relative humidity,precipitation,etc.,and are available on a 0.25°×0.25°grid.Error correction for the gridded data was performed as detailed in Yuan et al. (2012).The growing seasons of maize for the period 1981–2010 are provided by the National Meteorological Information Center.

2.2 Methods

2.2.1Thermal index

Di ff erent maize cultivars require di ff erent amounts of cumulative temperature during the growing season(Gong,1988).Based on previous work, we divided maize varieties into four types:earlymaturing,mid-maturing,mid-late-maturing,and latematuring(Wang et al.,2011).Then,according to actual maize-growth data for Northeast China during 1981–2010,we established the statistical relationship between each variety type and its required cumulative temperature in corresponding stages of the grow-ing season. Jiayin agro-meteorological station was chosen as a typical station representing early-maturing varieties;Hailun agro-meteorological station was chosen as a typical station representing mid-maturing varieties;Changling and Harbin agro-meteorological stations were chosen as typical stations representing midlate-maturing varieties;and Wafangdian,Fuxin,and Zhuanghe agro-meteorological stations were chosen as typical stations representing late-maturing varieties. The resulting thermal index values for the di ff erent varieties in di ff erent stages of the growing season are detailed in Table 1.

Table 1.Cumulative temperature(℃ day)required by the di ff erent maize variety types in di ff erent stages of the growing season

2.2.2Resistant varieties

Like any other crop,maize grows more vigorously and accumulates more dry matter in a suitable environment.Under unsuitable conditions,the crop’s growth and development will be inhibited,and dry matter accumulation will be less.Therefore,we changed the basic temperature(Table 2)and water requirements to generate theoretical maize varieties adapted to future climate,and we then used these high-temperature and/or drought-resistant varieties to model the increase in climatic productivity.The nine proposed resistant varieties in the context of days required for growth remaining the same are detailed in Table 3.Use of these varieties as agricultural adaptation options to climate change was then evaluated.2.2.3Productivity

The Food and Agriculture Organization-Agro-Ecological Zone(FAO-AEZ)model is a commonly used method to calculate crop productivity under different climates.In di ff erent stages of the growing sea-son,maize requires di ff erent quantities of climatic resources. Therefore,in order to make the results more realistic,the growth period of maize was divided into the following four stages:sowingemergence,emergence-jointing,jointing-heading,and heading-maturity.Photosynthetic productivity,photosynthetic thermal productivity,and climatic productivity in these di ff erent stages of the growing season were calculated separately.Then,the crop potential productivity during the whole period was determined. The speci fi c quantities calculated were as follows:

Table 2.Basic temperature(℃)requirements of maize in Northeast China during di ff erent stages of the growing season

Table 3.Parameters of the modeled resistant maize varieties

a)Photosynthetic productivity

whereY0(kg hm?2)is the daily photosynthetic production of the reference crop(LAI=5;dry matter productivityYm=20 kg hm?2h?1),y0andyc(kg hm?2) are the dry-matter production on an overcast and clear day,respectively(Liu et al.,2001),Fis cloud coverage,Rse(MJ m?2)is the maximum e ff ective shortwave radiation on a clear day,andRsis the observed radiation(MJ m?2).

b)Photosynthetic thermal productivity

Photosynthetic thermal productivity is the yield determined mostly by sunlight and thermal resources. First,we calibratedy0andycof the reference crop by the values ofYmat di ff erent temperatures.

WhenYm≥ 20 kg hm?2h?1,

WhenYm＜20 kg hm?2h?1,

The calculation method for photosynthetic thermal productivity was as follows:

whereYmpis photosynthetic thermal productivity(kg hm?2).CLis the correction coefficient of LAI(Wang et al.,2008).The change in LAI is a single peak curve during the whole growing season and LAI generally reaches its maximum value in the fl owering stage of the growing season.Ympshould be corrected when LAI＜5.In this study,the values of LAI were obtained from Yuan and Guo(2010).CNis the correction coefficient of net dry-matter production.CNis 0.6 when the average temperature is＜20℃,and it is 0.5 when the average temperature is ≥ 20℃ (Zhao and Zhao,1988).CHis the harvest index,for which the value in this study is 0.55(Liu Wei et al.,2010). Finally,Gis the number of days in the growing season.

c)Climatic productivity

Climatic productivity is the highest per hectare yield obtained by radiation,temperature,and precipitation under the assumption that soil fertility and agro-technical measures are optimal for crop growth (Wang et al.,2003).The calculation method was as follows:

whereYp(kg hm?2)is the climatic productivity;f(p) is the moisture modi fi cation function;kyis the productivity response index,with its average value during the whole period being 1.25(Wang Xiufen et al.,2012);Tm(mm)is crop water requirement;kcis the crop coeffi cient obtained by vegetation fractional cover during di ff erent periods(Sun,2008;Tian et al.,2009);and ET0is the reference crop’s evapotranspiration computed by the Penman-Monteith model(Liu Yuan et al.,2010).

ETa(mm)is the actual evapotranspiration determined by the quantitative relation between available water(precipitation and previous soil water storage) and crop water requirement.ETawas calculated by taking 10 days as the unit of time:

whereSaandPais the soil water storage and precipitation,respectively,in the last 10 days(Zhao et al., 2011).

3.Results and analysis

3.1 Agro-climatic resources in Northeast China

Agro-climatic resources include thermal,water, and light resources.They re fl ect the in fl uence of climate change on agricultural production. Maize in Northeast China grows mainly over the period from May to September.The ≥ 10℃ day cumulative temperature and the sum of mean air temperature during that period can be used to re fl ect the heat conditions during the growing season(Ma et al.,2000). Likewise,total precipitation from May to September, probable evaporation,and aridity index can be used as indicators of drought.The aridity index is de fi ned as the ratio of probable evaporation and precipitation, meaning the lower the aridity index value is,the more humid the atmosphere is,and vice versa.To discuss light resources,solar radiation needs to be considered; changes in light resources can be expressed in terms of total radiation during the growing season.

Table 4 shows the predicted changes in climatic resources during 1981–2100,based on the RegCM3 data.As can be seen,the sum of mean air temperature from May to September and the ≥ 10℃ day accumulated temperature signi fi cantly increase.However, precipitation during the growing season increases less signi fi cantly in the model.Meanwhile,the rising temperature causes a continuous increase in atmospheric evaporation,showing a tendency toward an arid climate.Total radiation during the growing season from 1981 to 2100 also increases.In particular,the model predicts that total radiation will increase signi fi cantly during 2041–2070,but the rate of change will then slow during 2071–2100.

Table 4.Modeled changes in climatic resources during the maize growing season during 1981–2100

3.2 Climatic productivity after adjusting the maize variety distribution pattern

According to the di ff erent cumulative requirements of the di ff erent varieties of maize,we produced a theoretical distribution for maize in Northeast China for the period 1981–2100 and calculated the potential climate productivity on that basis.The results show that radiation has no signi fi cant in fl uence on photosynthetic thermal productivity and climatic productivity in most periods.Temperature and precipitation are the main meteorological factors a ff ecting climatic productivity for maize.Photosynthetic thermal productivity in Northeast China shows an S-shaped curve over the entire period(i.e.,1981–2100).Climatic productivity changes from 5921.3 to 15559.4 kg hm?2with large interannual variations.Photosynthetic thermal productivity is low during 1981–2010 (base period),and in one particular year(1993)the air temperature in most areas of China was lower than usual.This was a cold summer in the northeast region, and the photosynthetic thermal productivity reached its lowest value of 11895.5 kg hm?2.With the increase in temperature from 2011 to 2070,early-maturing varieties are gradually replaced by late-maturing varieties to make full use of the thermal resources,and the photosynthetic thermal productivity increases rapidly. However,when the temperature rises beyond the upper limit of optimum temperature for maize after 2071, the photosynthetic thermal productivity begins to decrease. Unsuitable water resource is an important factor limiting climatic productivity during the entire modeled period.Climatic productivity accounts for about 64.6%of the photosynthetic thermal productivity.

Climatic productivity shows a south-north declining trend during the base period.Higher values are found in the southeast of Liaoning and the Tieling-Fushun area,and lower values appear mainly in the northwest of Jilin and the west of Heilongjiang.The maximum value is almost three times larger than the minimum value(Fig.1).Climatic productivity in the western areas during 2011–2040 is lower than that during the base period because of the increased potential evaporation and high-temperature weather.The results indicate that,during 2011–2040,spring maize could be planted in the Zhangguangcai and Laoye Mountains,where the climate is not currently(i.e.,in the base period)suitable for maize growth.The nongrowing areas will be reduced in the Changbai Mountains,and the climatic productivity will increase in the Xiao Higgan Mountains.After 2041,late-maturing varieties could be planted in most areas of Northeast China,and the climatic productivity largely increases in the east of Jilin and in most areas of Heilongjiang. Climatic productivity decreases by＞20%under climate warming and drying in western areas.Furthermore,the distribution of varieties in western areas would no longer need to be adjusted.Both high and low value areas are reduced.The results show that the climatic productivity in Liaoning Province,having previously had the highest values,will be lower than that in Jilin Province during 2071–2100;the disparities between these provinces will be narrowed.

With the plantable areas for late-maturing varieties shifting northward and enlarging in the east,the potential climatic productivity increases in the north and central Heilongjiang,and the east of Jilin.Meanwhile,climate productivity decreases in the southeast of Liaoning,the Changchun and Gannan-Harbin area, and Heilongjiang,as the climatic resources become mismatched.These results indicate that an increase in heat resources could favor agricultural production in areas that currently experience heat shortages.However,soil evaporation and plant transpiration wouldcontinue to increase under climate warming,such that precipitation is unlikely to meet crop water requirements without irrigation.The shift to a warmer and drier climate will therefore bring severe challenges to agricultural production,especially in overheated areas.Therefore,we need to consider agricultural adaptation options in order to increase the utilization of climatic resources against a background of climate change.

Fig.1.Climatic productivity of spring maize during the base period and its change(%)during 2011–2100 in Northeast China.

3.3 E ff ects of developing resistant varieties on climatic productivity

The warming and drying climate is an important factor that limits the potential of increasing yields in an overheated area.The current variety of maize is unlikely to be able to adapt to a future warmer climate. Based on adjusting the species distribution,our results indicate that developing resistant varieties would help increase climatic productivity(Fig.2),and the infl uence is closely related to the allocation of climatic resources. The extra output of high-temperatureresistant varieties(T1–T3)increases with time.The changes inYpof drought-resistant varieties(T4–T6) are consistent with that of the current variety(T0), whileYpof T4–T6 is higher than that of T0,with the extra output fl uctuating during 2011–2100.The ability of both high-temperature-and drought-resistant varieties(T7–T9)to adapt to climate change is better than that of T1–T6.High-temperature resistance increases climatic resources utilization to a large degree as the climate becomes warmer and drier during 2071–2100.The higher values ofYpfor T7–T9 compared to T0 show a signi fi cant increase.

3.3.1Variance analysis of climatic productivity of di ff erent cultivation patterns

Fig.2.Increases in climatic productivity resistant varieties compared to the current variety(T0)of maize.(a)T1–T0, T2–T0,and T3–T0;(b)T4–T0,T5–T0,and T6–T0;(c)T7–T0,T8–T0,and T9–T0.

Under the scenario of a growing mismatch of climatic factors,to slow the downward trend of climatic productivity,we need to enhance resistance from thepoint of view of the basic temperatures and water requirements.In order to further analyze how much and to what extent the basic temperature and water requirements would need to be adjusted to make the di ff erence between the current variety and the resistant varieties statistically signi fi cant,we perform a variance analysis of climatic productivity of the current variety and the resistant varieties(Table 5).As can be seen from the results,during 2071–2100,the climatic productivity of T2 and T3 is signi fi cantly different from T0,but the di ff erence between T2 and T3 is not signi fi cant.The climatic productivity of T4 and T5 is not signi fi cantly higher than that of T0.When the water requirement is reduced to 94%(T6),the climatic productivity is signi fi cantly higher than that of T0 during 2011–2100.Both high-temperature-and drought-resistant varieties largely decrease the adverse e ff ects of the warming/drying climate.The di ff erence between T7 and T0 is signi fi cant during 2071–2100, T8 is signi fi cantly di ff erent from T0 after 2041,and the climatic productivity of T9 is signi fi cantly di ff erent from T0 after 2011.

Table 5.Variance analysis of climatic productivity of resistant maize varieties

3.3.2Distribution of suitable meteorological conditions for di ff erent cultivation patterns

Climatic productivity is the highest biomass yield obtained by the full utilization of thermal,water,and light resources,and the value can be used to re fl ect the suitable grade of meteorological conditions in a particular region.In this study,climatic productivity is classi fi ed into fi ve groups through cluster analysis (Table 6):unavailable(value 1),relatively unavailable(value 2),relatively available(value 3),available (value 4),and most available(value 5).

According to the variance analysis,the di ff erence between the resistant varieties T2,T6,and T9 and the current variety in terms of meteorological suitability was analyzed and the results are shown in Fig.3. The most available areas for T0 during 2011–2040 are mainly in the east of Liaoning and central Jilin;the available areas are in central Liaoning,east to Dunhua (except the area around the Changbai Mountains), central Jilin,and eastern Heilongjiang;the relatively available areas are in southwestern Liaoning,the areas west to Changchun,and the areas south to Humain and western Heilongjiang;a relatively unavailable area is around Songnen Plain;and the regions in the Changbai Mountains and northern Heilongjiang, with their scarce heat resources,are classi fi ed as un-available for maize growth.The di ff erence between T2 and T0 is not signi fi cant in terms of the distribution of meteorological suitability in the period 2011–2040.Meanwhile,the available area for T6 expands: the western boundary grows to Fuxin from Xinmin and central Liaozhong,the northern boundary extends northward in Heilongjiang,meteorological suitability increases in Qinan and central Suiling,and the western boundary of the relatively unavailable area shifts westward by 0.4°of longitude.Climate suitability for T9 increases the most,with the northern boundary of available area moving from 44.86°to 45.88°N.

Table 6.Classi fi cation criterion of meteorological suitability based on climatic productivity of T0

Fig.3.The distribution of meteorological suitability for di ff erent maize varieties T0,T2,T6,and T9.

The suitability of meteorological conditions decreases due to increasing temperature and less precipitation during 2041–2070.The most available and available areas for T0 decrease,while the relativelyavailable and relatively unavailable areas increase. Meteorological suitability increases less when only the basic temperature requirements are adjusted;when the crop water requirement is reduced,the climatic suitability rises.The northern boundary of the most available area shifts from 44.20°to 44.58°N,the available area increases,and the relatively unavailable area decreases in Heilongjiang.Climatic resources utilization will improve after adjusting the basic temperature and water requirements,but the di ff erence between T9 and T6 in terms of the distribution of meteorological suitability is not signi fi cant.

The climatic suitability for T0 during 2071–2100 is higher than that during 2041–2070,with precipitation increasing.High temperature is also the main factor limiting the increase of potential productivity during 2071–2100,suggesting that enhancing the tolerance of maize to high temperatures could increase the climatic productivity in this period.The available area in Huachuan and Jixian in Heilongjiang Province for T2 is larger than T0,and the available area is reclassi fi ed as the most available in southern Yichuan. The climatic suitability for T6 changes markedly in Heilongjiang:the available area expands,and the relatively available areas transform into available areas in southern Huanan,southern Huachuan,and in parts of southern Jixian.The boundary of the available area shifts westward.The available area for T9 further expands;the relatively available areas transform into available areas in Hulin and Mishan;the available area expands to central Huanan and Jixian;the boundary of the most available area advances westward;and the boundary of the relatively unavailable area shifts westward by about 1.3°of longitude.

Overall,the results suggest that enhancing stress tolerance in maize would be bene fi cial for climatechange adaptation through an increase in climatic resources utilization.

4.Conclusions and discussion

The impact of cultivar-based adaptation measures on increasing the potential productivity of maize under a warmer and drier climate was quantitatively evaluated by using the FAO-AEZ model.The results show that:

(1)Heat resources are likely to change signi ficantly in Northeast China under climate warming, with the ≥ 10℃ day cumulative temperature and the sum of temperature from May to September increasing.Precipitation in the growing season shows a nonsigni fi cant increasing trend with a large degree of interdecadal fl uctuation.Total radiation is predicted to increase signi fi cantly during 2041–2070.Thermal conditions are found to improve over Northeast China as the climate warms,but precipitation may not compensate for the increasing evapotranspiration,the aridity index could increase,and the climate is likely to be generally warmer and drier.

(2)The increase in heat resources could bring favorable conditions for agricultural production in Northeast China.In the model’s results,the planting boundaries for di ff erent patterns extend eastward and northward.The instability in precipitation leads to unstable climatic productivity.The climatic productivity changes from 5921.3 to 15559.4 kg hm?2;because the computation models are di ff erent from others,this value is lower than that in a previous study (Yuan et al.,2012).The increased value of photosynthetic thermal productivity is not enough to o ff set the negative e ff ects of lower water suitability,and the increase in climatic productivity is limited during the study period.However,we fi nd great potential for increased climatic productivity in the future if accompanied by irrigation.Maize climatic productivity gradually improves due to climate warming in places where heat is originally insufficient.Meanwhile,the increase of heat resources has an adverse e ff ect on maize growth and development in places that are already relatively hot,especially in Liaoning Province,resulting in a decline of climatic productivity.The change in climatic productivity in the southwestern area is opposite to the changes in the southeast and northwest,and the disparities between high and low values will be narrowed.

(3)As the climate becomes warmer and drier,enhancing the stress tolerance of maize could increase its productivity and the climatic resources utilization effectively.The warmer and drier the climate becomes, the greater the increase will be in terms of the rangeof production potential of resistant varieties,especially high-temperature-resistant varieties,whose productivity is found to rise obviously with time.The combined e ff ect of high-temperature-and droughtresistant varieties on increasing productivity is better than high-temperature-or drought-resistant varieties only.The suitability of meteorological conditions is graded on the basis of the production potential values,and the results show that the “available” area could expand by enhancing stress tolerance,while the“unavailable” area may shrink.

(4)Solar radiation is an important resource affecting agricultural production because it has a direct in fl uence on photosynthetic productivity,and thus ultimately climatic productivity as well.If we use the base radiation and the simulation from RegCM3 as the initial conditions to compute the potential productivity,the changing trend ofYpis similar,and the di ff erences in values are not signi fi cant.The results show rich solar resources in Northeast China,suggesting that it is not the main factor limiting agricultural production.This is consistent with the previous study by Wang Ming et al.(2012).

Crop physiological characteristics during di ff erent stages of the growing season are considered.Growth and development as well as yield production are regarded as dynamic processes,and meteorological factors a ff ecting crop growth and yield are comprehensively analyzed in the AEZ model,which has been widely applied internationally in theoretical studies. Actual maize production is also a ff ected by soil,agricultural techniques,socioeconomics,natural disasters etc.In this study,only light,heat,and water resources are taken into consideration to calculate the climatic productivity,i.e.,it is an ideal output.By taking other factors a ff ecting agricultural production into account,the calculated values of climatic productivity could be more accurate.

The abilities of three kinds of resistant varieties to adapt to climate change are evaluated in our study. We assume that the upper limit of optimum temperature and the upper limit of temperature would increase by 1,2,and 3℃,respectively,in these hightemperature-resistant varieties,but this assumption is not based on future temperature-change scenarios. How to better design the basic parameters of temperature requirements of resistant varieties will be an important focus of our work in the future.In addition, the degree of stress tolerance to high temperature and drought is regarded as the same in di ff erent regions. Actually,climatic resources vary on the regional scale, and thus the main factors restricting climatic productivity are often di ff erent in di ff erent areas.How to determine the range of optimum temperatures of high-temperature-and drought-tolerant varieties in di ff erent regions needs to be further studied.

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(Received July 10,2013;in fi nal form February 20,2014)

Supported by the China Meteorological Administration Special Public Welfare Research Fund(GYHY201106020)and National Natural Science Foundation of China(31371530).

?Corresponding author:gjp@cams.cma.gov.cn.

?The Chinese Meteorological Society and Springer-Verlag Berlin Heidelberg 2014