作 者 ：Zhou Guang-sheng and Zhang Xin-shi
Keywords:Regional potential evapotranspiration, Climate-vegetation classification, Thermal index, Moisture index, Global change,
Abstract:The study on climate-vegetation relationship is the basis for determining the re sponse of terrestrial ecosystem to global change. By means of quantitative analysis on climate-vegetation interaction, vegetation types and their distribution pattern could be corresponded with certain climatic types in a series of mathematical forms. Thus, the climate could be used to predict vegetation types and their distribution, the same is in reverse. Potential evapotranspiration rate is a comprehensive climatological index which combines temperature with precipitation, and could be used to evaluate the effect of climate on vegetation. In this respect, Holdridge life zone system has been drawing much attention and widely applied internationally owing to its simplicity. It is especially used in the assessment of sensibility of terrestrial ecosystems and their distribution in accordance with climate change and in prediction of the changing pattern of vegetation under doubled CO2 condition. However, Prentice (1990) pointed out that the accurancy of Holdridge life zone system is less than 40 % when it is used at global scale. The reason may be that the potential evapotranspiration calculated by Thornthwaite method, which is used in Holdridge life zone system, reflects the potential evapotranspiration from small evaporated area, while climate-vegetation classification is based on the regional scale. The authors try to establish a new climate-vegetation classification system based on the regional potential evapotranspiration. According to the following formula: where E designates regional actual evapotranspiration: Ep local potential evapotran-spiration: Epo, regional potential evapotranspiration. Ed can be calculated from Penman model or other models. E can be calculated from the following model: E=r · Rn (r2+Rn2+r · Rn) / (2) (r+Rn) · (r2+Rn2)where r designates precipitation (mm); Rn, net radiation (mm). Thus, Ep0 can be easily obtained. It is used as the regional thermal index (RTI) of climate-vegetation classification,and can be expressed as: RTl = Epo (3) Moisture index is another index of climate-veggetation classification. Usually, it can be expressed as the ratio between potential evapotranspiration and precipitation. However, this ratio can not reflect soil moisture, which is important for plant. The ratio between regional actual evapotranspiration and regional potential evapotranspiration is associated not only with climatic condition but also with soil moisture. So it can be used as the moisture index of climate-vegetation classification, and is defined as regional moisture index (RMI): RMI = E/Epo (5) Based on the average climatological data of 30 years from 647 meteorological observation stations in China. It was found that RTl could well reflect a regional thermal level. The values of RTI were less than 360 mm in cold temperate zone, 360～650 mm in temperate zone, 650～380 mm in warm temperate zone, 780～1100 mm in subtropical zone. And more than 1100 mm in tropical zone. RMI also reflects a regional moisture level very well. The values of RMI was less than 0.4 in desert area, 0.4～0.7 in grassland area and more than 0.7 in forest area. Thus, the climate-vegetation classification in China is established on the basis of the two indices: RTI and RMI. According to this model, the changing patterns of vegetation zones in China are given under the conditions of mean annual temperature in creasing by 2℃ and 4℃ and mean annual precipitation increasing by 20%. The results showed that the areas of forest and grassland would decrease, the vegetation zones would move northward and upward, and the area of desert would increase. The results also indicate that the Tibetan Plateau is an area highly sensitive to global change. It could be considered as an indicative or forewarning area for global change , and therefore, an area of great siginificance for monitoring and research. The possible beneficial effect of global change on China terrestrial ecosystems is that the plantation boundary will move northwards and upwards; and the disadvantageous effect is the expansion of desertification and the increase of instability in climatic conditions.
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