Xuemei Shao

Temporal trends and variability of daily maximum and minimum, extreme temperature events, and growing season length over the eastern and central Tibetan Plateau during 1961–2003

[1] Daily and monthly maximum and minimum surface air temperatures at 66 weather stations over the eastern and central Tibetan Plateau with elevations above 2000 m were analyzed for temporal trends and spatial variation patterns during the period 1961 - 2003. Statistically significant warming trends were identified in various measures of the temperature regime, such as temperatures of extreme events and diurnal temperature range. The warming trends in winter nighttime temperatures were among the highest when compared with other regions. We also confirmed the asymmetric pattern of greater warming trends in minimum or nighttime temperatures as compared to the daytime temperatures. The warming in regional climate caused the number of frost days to decrease significantly and the number of warm days to increase. The length of the growing season increased by approximately 17 days during the 43-year study period. Most of the record-setting months for cold events were found in the earlier part of the study period, while that of the warm events occurred mostly in the later half, especially since the 1990s. The changes in the temperature regime in this region may have brought regional-specific impacts on the ecosystems. It was found that grain production in Qinghai Province, located in the area of prominent warming trends, exhibited strong correlations with the temperatures, although such relationships were obscured by the influence of precipitation in this arid/semiarid environment in juniper tree ring records. In western Sichuan Province under a more humid environment, the tree growth ( spruces) was more closely related to the changing temperatures.

Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference

Twenty-three published proxy temperature series over China spanning the last 2000 years were selected for an uncertainty analysis in five climate regions. Results indicated that, although large uncertainties are found for the period prior to the 16th century, high level of consistency were identified in all regions during the recent 500-years, highlighted by the two cold periods 1620s-1710s and 1800s-1860s, and the warming during the 20th century. The latter started in Tibet, Northwest and Northeast, and migrated to Central East and Southeast. The analysis also indicates that the warming during the 10-14th centuries in some regions might be comparable in magnitude to the warming of the last few decades of the 20th century which was unprecedented within the past 500 years. Citation: Ge, Q.-S., J.-Y. Zheng, Z.-X. Hao, X.-M. Shao, W.-C. Wang, and J. Luterbacher (2010), Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference, Geophys. Res. Lett., 37, L03703, doi: 10.1029/2009GL041281.

Annual Precipitation Variation Inferred From Tree Rings Since A.D. 1770 For The Western Qilian Mts., Northern Tibetan Plateau

Abstract A long-term perspective of the recent climate change on the Tibetan Plateau is hampered by a lack of sufficiently long weather records. Here we describe a tree-ring based reconstruction of annual (prior July to current June) precipitation for the western Qilian Mts., northern Tibetan Plateau. This reconstruction accounts for 54.9% of the variance in instrumental precipitation data from 1935 to 2003. It shows distinct dry periods in 1782–1798, 1816–1837, 1869–1888 and 1920–1932, matching in general with local historical archives and other climatic proxy data on the northern Tibetan Plateau. Our research provides a background for evaluating hydroclimatic changes in the past two hundred years in a vulnerable arid region on the northern Tibetan Plateau.

Dendroclimatic temperature record derived from tree-ring width and stable carbon isotope chronologies in the middle qilian mountains, China

ABSTRACT Using long-lived Qilian juniper (Sabina przewalskii Kom.) in the middle Qilian Mountains, the temperature variations in the last 1000 yr were reconstructed. We find that the annual growth ring width and δ13C series mainly reflect variations in regional temperature. Except in May, warmer temperatures indicate greater growth over the period from December to April, and δ13C values in tree-rings are higher for years with higher temperature. The notable features in the temperature reconstruction are the occurrence of the Little Ice Age from A.D. 1600 to 1880 and the abrupt warming over the end of past millennia. The comparison of our chronology to a Northern Hemispheric temperature proxy shows that our tree-ring data will facilitate intercontinental differentiation of large-scale synoptic climate variability.

Dendroclimatic evaluation of climate-growth relationships of Meyer spruce (Picea meyeri ) on a sandy substrate in semi-arid grassland, north China

Abstract. Dendroclimatic techniques were used to assess the climate–growth relationships of refugial Meyer spruce (Picea meyeri Rehd. et Wils.) on a sandy substrate in semi-arid grassland of north China. Statistical analysis of the tree-ring data showed a mean series intercorrelation of 0.47, a signal-to-noise ratio of 14.44, and a mean sensitivity of 0.18, indicating suitability for climatic analysis. Radial growth was positively correlated with precipitation in February and May of the current year, and in September of the preceding year. However, radial growth of Meyer spruce also correlated negatively with mean monthly temperature in current May, of which mean maximum temperature explained most of the observed variation. In addition, radial growth negatively correlated with solar radiation over most of the year. Rainfall appeared to be the dominant growth-limiting factor in this semi-arid grassland, with temperature and solar radiation being of lesser importance. This study suggests that Meyer spruce in this stand is promising for dendroclimatic and ecological studies because of good cross-dating characteristics and high sensitivity to climate.

SEASONAL CAMBIAL ACTIVITY OF RELICT CHINESE PINE AT THE NORTHERN LIMIT OF ITS NATURAL DISTRIBUTION IN NORTH CHINA - EXPLORATORY RESULTS

SUMMARY The seasonal dynamics of cambial activity of Chinese pine (Pinus tabulaeformis Carr.) was studied at the northern limit of its natural distribution in a semi-arid grassland ecosystem in North China. We collected micro-cores from five even-aged trees at weekly intervals from April 15 to September 16, 2006. Cell division in the cambial zone of these trees started within the third week of May. In June and July the rate of xylem cell production was highest and around mid-August cell division ended. However, cell-wall formation was not yet completed around mid-September. The cell-division period appears to coincide with the time of highest monsoon precipitation and of above 0 °C daily minimum temperature. Moreover, the cambium activity period seems to be in agreement with the time of the highest number of sunshine hours/day. These preliminary results are important for dendroclimatic studies with Chinese pine in North China and can furthermore be used as a benchmark for future monitoring activities in ecologically similar areas.

Response and dendroclimatic implications of δ13C in tree rings to increasing drought on the northeastern Tibetan Plateau

The stable carbon isotope composition (delta(13)C) of tree rings in a climate-sensitive region can provide a retrospective view of changes in environment and climate. Here, we report on the development of the first annually resolved delta(13)C tree ring chronology obtained from natural forests on the northeastern Tibetan Plateau. Climate data show a warming trend and more frequent droughts occurring in the research region since the 1970s. The isotope record of Qinghai spruce (Picea crassifolia) spans the period 1890-2002 with a general decreasing trend over the last century followed by an abrupt increase in delta(13)C over the last decade. The stable carbon discrimination against heavier atmospheric carbon (Delta(13)C) is negatively correlated to May temperature and positively correlated with June-July precipitation. The regional Palmer drought severity index (PDSI) correlated significantly with Delta(13)C series after 1960, whereas this relationship was not stable over the period 1933-1960. However, much stronger correlations were observed between the high-frequency anomalies in annual Delta(13)C and PDSI in June and July during the period 1933-2002. The temporal stability analysis revealed trends in the response to drought stress affecting trees Delta(13)C linked to climatic warming. Intrinsic water-use efficiency increased by 7.7% in Qinghai spruce in response to increased severity of regional drought during the 1990s compared to the average of the previous decade. Our preliminary results suggest that carbon isotope in certain tree taxa growing on Tibetan Plateau may be an effective proxy for reconstructing regional drought.

Relationships between tree increment, climate and above-ground biomass of grass: a case study in the typical steppe, north China

Meyer spruce (Picea meyeri Rehd. et Wils.) is a relict conifer, and Chinese leymus (Leymus chinensis [Trin.] Tzvel.) is a dominant grass in typical steppe of the Xilin River Basin, northern China. Herein, we evaluated the relationships between tree-ring width index of Meyer spruce, climate and above-ground biomass of Chinese leymus, and reconstructed above-ground biomass of Chinese leymus from 1955 to 1994 using tree rings. Both Meyer spruce and Chinese leymus exhibited significant positive (P < 0.05) growth response to May precipitation during the current growing season. In addition, the growth of Chinese leymus was significantly correlated (P < 0.05) with the sum of current July and August precipitation. Two predominant linear relationships between ring width and annual grass production were found, and each corresponding to different precipitation patterns in this region. With reference to seasonal precipitation patterns, the above-ground biomass of Chinese leymus from 1955 to 1994 was reconstructed using two models derived from tree-grass growth relationships. The comparison of climate-growth relationships between relict Meyer spruce and dominant Chinese leymus pointed to the different adaptation strategies of two ecosystem elements to the semi-arid steppe climates. The reconstruction of grass production could reduce the gap in our knowledge of the past dynamics of typical steppe, allowing verification of model estimates of natural climate fluctuations. This may be also an important step towards understanding the response of different ecosystem components to future climate change in the typical steppe, and delivering the baseline reference for a sound steppe management plan.

Relationships between tree growth and NDVI of grassland in the semi-arid grassland of north China

This study explores the relationships between the Normalized Difference Vegetation Index (NDVI), recorded above‐ground grass biomass and tree‐ring width index of relict Meyer spruce (Picea meyeri Rehd. et Wils.) forest in the typical steppe, north China. The average NDVI in May, June and August derived from an area of 0.5°×0.5° shows a large correlation with measured above‐ground production, indicating that NDVI can reflect the approximate variability of above‐ground biomass in the typical steppe. The integrated NDVI from 20 May to 10 July also exhibits high agreement with tree‐ring width series of Meyer spruce from 1982 to 1994, which is attributed to their common response to precipitation in the previous August–October and current May. This study provides a basis for linking remotely sensed NDVI of grassland to tree growth in semi‐arid grassland.

A 3585-YEAR RING-WIDTH DATING CHRONOLOGY OF QILIAN JUNIPER FROM THE NORTHEASTERN QINGHAI-TIBETAN PLATEAU

This article documents the development of a precisely dated and well-replicated long regional tree-ring width dating chronology for Qilian juniper (Juniperus przewalskii Kom.) from the northeastern Qinghai-Tibetan Plateau. It involves specimens from 22 archeological sites, 24 living tree sites, and 5 standing snags sites in the eastern and northeastern Qaidam Basin, northwestern China. The specimens were cross-dated successfully among different groups of samples and among different sites. Based on a total of 1438 series from 713 trees, the chronology covers 3585 years and is the longest chronology by far in China. Comparisons with chronologies of the same tree species about 200 kin apart suggest that this chronology can serve for dating purposes in a region larger than the study area. This study demonstrates the great potential of Qilian juniper for dendrochronological research.