近50年中国西北东部半湿润、半干旱过渡带气候变化及生态影响

以宝鸡、凤翔、千阳、陇县、西峰、环县、平凉、泾源、六盘山、固原市、西吉和海原等台站的气象数据为依据，对近50年来地处西北地区东部的半湿润、半干旱过渡带的气候要素进行了系统分析，发现区内各地气温有普遍升高的趋势。其中，冬季升温趋势更加明显。各地在20世纪90年代前后发生气温突变，以此为界，前后增加速度明显不同，此后的增温趋势明显加快。近50年以来，各地降水略有下降，主要是由夏季降水减少所引起，但是统计趋势不明显。受气温和降水变化综合影响，区域气候向变干和变暖的方向发展。根据Holdridge生命地带理论，初步分析了区域植被与气候之间的关系。计算表明，最近30年以来，西北东部半湿润、半干旱区过渡带的气候变化使区域植被也向干暖化类型变化。位于研究区中部的六盘山、泾源和西吉等地生命地带类型的变化都比较大。崆峒山(35°33′N,106°31′E)的圆柏(Sabina chinensis)树轮气候学分析表明，区域树木生长与前一年秋季降水呈显著正相关关系，与当年春季和夏季气温呈显著负相关趋势，说明生长季的缺水或高干燥度是树木生长的限制性因素。研究时段区域干旱频繁发生，使树木出现大量伪轮，为研究带来一定难度，但是也进一步验证了区域气候变化的干旱特征。干燥指数重建结果表明，研究区在20世纪20年代比较干旱，30年代是相对较为湿润的时期； 1940年和1960年左右较为干旱，但其间的干燥指数较低。60年代之后，区域干燥指数逐渐增加，表明干旱趋势日趋加重，与气象数据分析结果相一致。事实证明，研究区气候的干暖化变化对区域树木生长和生态系统具有非常不利的影响。

CLIMATE CHANGES AND THEIR ECOLOGICAL IMPACTS OF THE LAST 50 YEARS IN SEMIHUMID AND SEMIARID TRANSITIONAL ZONES OF THE EAST PART OF NORTH WEST CHINA

Climate elements of subhumid and semiarid transitional zones in North West China were systematically analyzed on the basis of meteorological data of the last 50 years collected from meteorological stations in Baoji，Fengxiang，Qianyang，Longxian，Xifeng，Huanxian，Pingliang，Jingyuan，Liupanshan，Guyuan，Xiji and Haiyuan. It is found that temperature in the study area shown a generally increasing trend during the last fifty years，which is more significant in winter. There was an inflexion near the 1990s: the temperature ascending rate after that time became obviously high. It is also found that precipitation in the study area shown a slightly decrease during the last fifty years，which is mainly due to the declination of summer precipitation，but its statistical trend is not significant. With these two trends，climate in the study area had a development to warming and drying during this period. Relationships between vegetation and the climate in the study area were analyzed by the Holdridge life zone theory. The results indicate that vegetations in these areas had changed into their dry­warm types during the last 30 years，especially in such areas as Liupan Mountains，Jingyuan and Xiji. The dendroclimatological analysis of Sabina chinensis in Kongtong Mountain(35°33′N,106°31′E)revealed a positive correlation between the tree growth and the precipitation in the autumn of the previous year and a negative correlation between the tree growth and the temperature in the current spring and summer，which means that shortage of water or aridity in the growing season should be a main constraint to tree growth. It should be noted that the frequent occurrence of aridity in the time interval studied produced lots of false rings. Although they bring some difficulties in research，they provide evidence for regional drying. The reconstruction of the arid index revealed the following times of climate change: relatively arid in the 1920s，relatively humid in the 1930s，arid around 1940 and around 1960，little humid between 1940 and 1960，and increasing arid after the 1960s. These results are in line with those results based on the meteorological data. Both revealed a warming and drying trend during the last 50 years in the study area. This trend has had adverse impacts on the ecosystem，especially tree growth.