|Alternative Title||Drought study in the Central and western Yunnan by tree-ring proxies|
|Place of Conferral||兰州|
|Keyword||树木年轮 云南中西部 无量山 云岭 密度年内波动 干旱变化|
本文选择青藏高原与云贵高原两大地形单元的结合部作为研究区，位置上属于云南的中西部，我们首先在更靠近云贵高原的云南中部无量山地区利用云南铁杉树轮宽度重建了过去140年干季(11月至4月)区域标准化降水蒸散指数SPEI1，这也是该地区的第一例树轮气候重建，据此分析了无量山地区干湿历史及其特征，并尝试进一步探究区域干湿变化与大尺度环流系统的关系；其次，利用无量山云南铁杉树轮中频繁而稳定出现的结构“异常”现象——年内密度波动(也称作伪轮、双轮等)，本文建立了年内密度波动年表，并分析其形成的环境因素及气候应用；最后，我们在更靠近青藏高原的横断山中南部云岭山脉，基于两个气候响应模式和响应时间更一致的云南铁杉年表，重建云岭地区春季干旱，并与周围地区基于树轮的水分重建进行对比分析。主要结论如下：无量山干季重建结果显示，1888-1908年、1948-1965年为持续的较干旱时期，1876-1887为持续的较湿润时期，1950s-1960s是重建期内最干旱的时期。1976-1997年是重建期内无量山地区气候干湿高频波动强度和幅度最大的时期，实际上这种气候波动增强可能从20世纪60年代早期就已开始，极端干湿年也集中出现在20世纪后半叶，近几十年云南中部气候变率增强、极端气候发生频率增加可能与全球变暖有关。云南干季干湿变化与南支槽的强弱有关，在大范围环流背景下，研究区气候则与NAO和AO有关，其中AO的影响可能更为显著,西风系统对该地区干季气候具有重要的影响。无量山树轮密度年内波动（Intra-annual density fluctuations, IADFs）主要发生于早材，4、5、6月持续的温度偏高，5月降水偏少以及7月降水偏多的气候模式十分有利于IADFs的形成，其中5月的高温少雨和7月的多雨气候起着关键作用。生长季前期尤其是5月的干旱胁迫，使得形成层活动减缓甚至停止，形成类似晚材的密度较大的细胞，而生长季中期（7月）气候适宜，形成层重新开始活动，继续早材的生长，最终导致早材内的密度波动。5月的干旱对IADFs的形成至关重要，而5月的雨量是云南雨季开始早晚的重要指示, 因此基于树轮的IADFs发生频率可作为历史上云南雨季开始时间的参考记录。基于两个云南铁杉、一个云南松轮宽年表的气候响应结果：可利用水分是云岭地区树木径向生长主要的限制因子。据此，将响应时间和模式更一致的两个铁杉年表合成为云岭区域年表，并重建了云岭地区4-5月PDSI。1953-1962，2005至今为区域持续的较干旱时期，而1856-1865，1874-1894则是持续的较湿润时期，湿润期都出现在20世纪之前，而干旱期都出现在1950s之后，区域气候整体存在变干趋势；云岭地区的极端干湿年份集中于1950s之后，区域干湿状况自1950s之后高频波动加剧，变幅增大，极端干湿年份的发生频率大大增加。这与云南中部的重建特征一致，表明与大范围升温有关的气候变率增大具有季节和区域的普遍性。与研究区周围基于树轮的水分重建序列对比显示，1960s以来，云岭干湿波动趋势与周围地区的一致性增强，但20世纪末至今横断山中北段和藏南地区均处于湿润状态，且这种湿润程度超过以往任何时期，而研究区及其所在的横断山偏南段却处于干旱状态，表明近几十年横断山南北对气候变化的响应程度可能存在差异。
Drought disaster has significant influence in economy、agriculture、social activity and ecology environment, as it is the most widely and serious natural disaster. Drought is the imbalance between water supply and demand, it is also stand out in semi-humid and humid area. There are frequent drought disasters in Southwest area, especially Yunnan province, southwest border of China. The drought disasters of Yunnan area are seasonal and regional because of its complex landform and the influence of monsoon system. At the background of global warming, it’s much meaningful to study the characteristics and trends of drought in this area. Since the meteorological data is too short to study the characteristics of drought in the long time scale, so the tree ring proxies with high resolution are considered to work it out. The spatial distribution of dendroclimatology studies is quite uneven, mainly focus on the northwest of Yunnan, but the rest of Yunnan is the study vacancy.
We choose the joint part of the Tibetan plateau and the Yunnan-Guizhou plateau as the study area, which located in the Central and Western Yunnan. Firstly, In the Wuliang Mountains, which close to the Yunnan-Guizhou plateau, we developed the first tree ring width chronology of this area and then reconstructed the Standardized Precipitation and Evapotranspiration Index(SPEI) in drought season(November to April) of the past 140 years, the characteristics of drought history and its connection with large scale circulation system are analyzed; Secondly, According to the frequent intra-annual density fluctuations(IADFs)(also called false ring, double ring and so on) of tree ring structure in the Wuliang Mountains, the first IADFs chronology of Yunnan was developed, and the environment trigger factors and climate application of the IADFs were further researched. Lastly, based on two Tsuga dumosa chronologies which response to climate change similarly, we reconstructed the spring drought history of the Yunling Mountains, which are near to the Tibetan Plateau, after that, we compared our drought reconstruction with other tree ring based hydroclimate reconstructions. The mainly conclusions are summarized as follows: The reconstruction of drought history in the Wuliang Mountains reveals that AD 1888-1908 and 1948-1965 are the continuous drought periods, while AD1950s-1960s is the most severe drought period during the past 140 years; AD1876-1887 is the continuous pluvial periods. AD1976-1997 has the maximum droughts and pluvial fluctuations in intensity and extent, actually this strengthen in climate variability may begin from the 1960s, all the extreme drought and pluvial years occur in the second half of 20th century. Recently decades, the climate variability of Central Yunnan shows an evident uptrend and the extreme climate events are increase in frequency and intensity. This phenomenon may be related to the rapid global warming. The hydroclimate of the Central Yunnan in drought season is influenced by the intensity of southern branch tough, as for the larger scale circulation, it is connected with North Atlantic Oscillation and Arctic Oscillation, the influence of AO is more significant, the westerly system has great impacts on study area. Intra-annual density fluctuations(IADFs) of tree ring in the Wuliang Mountains mainly appears in earlywood, The climate pattern of higher temperature in April、May and June, fewer precipitation in May and superior precipitation in July is much advantage in the formation of IADFs, high temperature and rainy climate in May with the addition of rainy July plays the key role of IADFs formation. The drought stress in earlier stage of growing season especially May slows down even stops the cambial activity, thus forms the latewood-like cells with greater density, In the middle stage of growing season, i.e. July, appropriate climate makes the cambial activity restart, earlywood growing continues, so there is a density fluctuation in a ring. The amount of precipitation in May indicates the start time of rainy season, For the reason of drought stress in May is important for IADFs formation, the frequency of IADFs may be a great proxy for timing the rainy season onset.According to the responses of the two Tsuga dumosa width chronologies and a Pinus yunnanensis width chronology to the climate change, we know that the available moist is the limiting factor for the radial growth of trees in the Yunling Mountains, we averaged the two Tsuga dumosa chronologies that have a similar response to climate, based on this composite chronology, the Palmer Drought Severity Index in April to May is reconstructed.AD 1856-1865 and 1874-1894 are the continuous pluvial periods, AD1953-1962 and 2005 to today are the continuous drought periods, all the pluvial periods occur in 20th century, while all the drought periods occur after 1950s, climate of this region is drying generally. In line with the climate characteristics of central Yunnan in recent decades, the droughts and pluvial variability of the Yunling area also increase in frequency and intensity since 1950s, and the extreme climate events occur frequently, this coherence may indicates that the widely warming increases climate variability among the seasons and regions. Comparisons of our drought reconstruction and other tree ring based hydroclimate reconstructions show that the coherence of hydroclimate fluctuations is strengthen between our study area and surrounding regions after 1960s,but their absolute condition is different even opposite from the end of 20th century to today, it may implies that there is a difference in response to climate change between the southern and northern Hengduan Mountains .
|陈瑶. 云南中西部干旱的树轮记录研究[D]. 兰州. 兰州大学,2017.|
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