Rice cultivation may occur in elevations exceeding 2,500 m in Lijiang and adjacent counties in Yunnan and Sichuan Provinces.In the vicinities of the said county,the upper limit of rice lies in the neighbourhood of 2400m,while a cultivar of the crop,heigu,does complete its life cycle every year at a height of 2,660m in Yongning Ba of Ninglang county.Rice is grown above 2500m both in Madeng Ba of Jianchuan county and Pantianguo of Weixi county in Yunnan, and the same is true of Yienyun county of Sichuan Province. Lijiang and adjacent counties constitute a part of the“Region of Transversal Mountain Ranges”.This is probably the region in which rice production has extended to the highest altitudes in China.Eastward or westward from this region, the cultivation of this crop is restricted to considerably lower elevations. For example,to the west of the“Region of Transversal Mountain Ranges”, a farm in Yigong (2250m,30°19′N,94°51′E) of Bomi county in Xizang had conducted experimentation with rice cultivation for six years from 1970 to 1976 and failed.In no year had the crop produced a yield.In some cases,there might be ears each with one to two or a few mature grains. It has been fairly well established that low temperatures over a period of 15 days or longer (from 10 days before florescence to the end of pollination) are critical to yield formation.Air temperature and sunshine seem to function complementarily for reasons not yet definitely known.In general,if maximum air temperature falls below 23℃ in a sunny day,pollinatlon of many cultivars would not proceed normally.If it rises above 25℃ or 27℃,fertilization of most cultivars will not be adversely affected and is likely to pave the way for normal maturity. The impact of minimum air temperature may also be of some importance.With overcast sky,daily mean air temperature below 20℃ or 21℃ usually casts negetive influence to pollination and maturation.To identify a locality suitable for rice cultivation,it is essential to check whether there exists a period of 15 days or more in which mean maximum temperature exceeds 23℃ or 25℃ with fairly ample sunshine.If data of maximum temperature are not available,mean temperature higher than 21℃ or 20℃ may be taken as an approximate equivalent. although this will give less accurate result. With this general idea in view,analyses of the climatic factors governing the upper limits of rice cultivation in South China have been made. The limit for rice-culivation is not high in regions of the world close to the equator.For instance,rice cultivation in the Philippines almost nowhere exceeds 4300 feet (1312m),the crop is non-existing in places like Baguio (1510m). The possible and economic cultivation limits drop still much lower in innertropical reigons of Indonesia where geoecological conditions do not allow any rice-cultivation above 1500m.The upper limits of rice cultivation in Phillipines and Indonesia are much lower than that of Lijiang.Why? The answer probably lies in the absence of a period of sufficient length with a mean maximum temperature above 25℃ or 27℃ with suitable sunshine and rainfall localities above 1500m in the Philippines and Indonesia because of the long cloud cover and high precipitation. Explanation may also be sought from the low temperature tolerance of different strains.Heigu is the known cultivar most resistant to low temperature stress.The lowest temperature it can tolerate during heading and blossoming is about 4 or 5℃ lower than that of the crop in eastern China. The highest upper limits of rice cultivation in the Transversal Ranges is closely correlated with climatic conditions originated under specific geographic environment of the region.The huge barrier,formed of the great QinghaiXizang Plateau in the north and high mountain ranges 2000-3000m or even higher above sea level in the east,has effectively blocked the influence of cold air advection all the year round.Owing to the uplift of the Qinghai-Xizang Plateau and the Transversal Mountain Ranges,most of the areas receive sufficient solar radiation the year round,except the southern and eastern fringes.