Using seven well-replicated Qilian juniper (Sabina przewalskii Kom.) ring-width chronologies developed at Zongwulong and Shalike Mts. in the northeastern part of the Qaidam Basin annual precipitation from previous July to current June in the recent 1000 years was reconstructed for Delingha. The reconstruction can capture 63.1% of precipitation variance and the equation was stable over time. For the reconstructed precipitation, wet periods occurred in AD1520-1633 and 1933-2001, whereas dry intervals in 1429-1519 and 1634-1741. In addition, the magnitude in precipitation variation was lower before 1430 with about 15 mm, but it increased to 30 mm during the period of 1430 to 1850. After 1850, the precipitation variance decreased again. In contrast to the increase in temperature, a decrease in annual precipitation was evident since the 1990s. The agreement in low-frequency variation between the reconstruction and the glacier accumulation and particulate content in Dunde ice cores during the recent several hundred years suggested that the precipitation reconstructed in this study was rather reliable, and represented a regional signal. This 1000-year reconstruction could benefit our understanding of climatic variation in decadal to century-scale in this region, and provide basic data to climate models and to prediction of future climate in the 21st century. Keywords: northeastern part of the Qaidam Basin, tree-ring width series of Qilian juniper, precipitation reconstruction in 1000 years.

Birch (Betula) trees and forests are found across much of the temperate and boreal zones of the Northern Hemisphere. Yet, despite being an ecologically significant genus, it is not well studied compared to other genera like Pinus, Picea, Larix, Juniperus, Quercus, or Fagus. In the Himalayas, Himalayan birch (Betula utilis) is a widespread broadleaf timberline species that survives in mountain rain shadows via access to water from snowmelt. Because precipitation in the Nepalese Himalayas decreases with increasing elevation, we hypothesized that the growth of birch at the upper timberlines between 3900 and 4150 m above sea level is primarily limited by moisture availability rather than by low temperature. To examine this assumption, a total of 292 increment cores from 211 birch trees at nine timberline sites were taken for dendroecological analysis. The synchronous occurrence of narrow rings and the high interseries correlations within and among sites evidenced a reliable cross‐dating and a common climatic signal in the tree‐ring width variations. From March to May, all nine tree‐ring‐width site chronologies showed a strong positive response to total precipitation and a less‐strong negative response to temperature. During the instrumental meteorological record (from 1960 to the present), years with a high percentage of locally missing rings coincided with dry and warm pre‐monsoon seasons. Moreover, periods of below‐average growth are in phase with well‐known drought events all over monsoon Asia, showing additional evidence that Himalayan birch growth at the upper timberlines is persistently limited by moisture availability. Our study describes the rare case of a drought‐induced alpine timberline that is comprised of a broadleaf tree species.

Physiological and ecological mechanisms that define treelines are still debated. It has been suggested that the absence of trees above the treeline is caused by low temperatures that limit growth. Thus, we hypothesized that there is a critical minimum temperature (CTmin) preventing xylogenesis at treeline. We tested this hypothesis by examining weekly xylogenesis across three and four growing seasons in two natural Smith fir (Abies georgei var. smithii) treeline sites on the southeastern Tibetan Plateau. Despite differences in the timing of cell differentiation among years, minimum air temperature was the dominant climatic variable associated with xylem growth; the critical minimum temperature (CTmin) for the onset and end of xylogenesis occurred at 0.7±0.4 °C. A process-based modelling chronology of tree-ring formation using this CTmin was consistent with actual tree-ring data. This extremely low CTmin permits Smith fir growing at treeline to complete annual xylem production and maturation and provides both support and a mechanism for treeline formation.

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.

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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.

Aging is a universal property of multicellular organisms. Although some tree species can live for centuries or millennia, the molecular and metabolic mechanisms underlying their longevity are unclear. To address this, we investigated age-related changes in the vascular cambium from 15- to 667-y-old Ginkgo biloba trees. The ring width decreased sharply during the first 100 to 200 y, with only a slight change after 200 y of age, accompanied by decreasing numbers of cambial cell layers. In contrast, average basal area increment (BAI) continuously increased with aging, showing that the lateral meristem can retain indeterminacy in old trees. The indole-3-acetic acid (IAA) concentration in cambial cells decreased with age, whereas the content of abscisic acid (ABA) increased significantly. In addition, cell division-, cell expansion-, and differentiation-related genes exhibited significantly lower expression in old trees, especially miR166 and HD-ZIP III interaction networks involved in cambial activity. Disease resistance-associated genes retained high expression in old trees, along with genes associated with synthesis of preformed protective secondary metabolites. Comprehensive evaluation of the expression of genes related to autophagy, senescence, and age-related miRNAs, together with analysis of leaf photosynthetic efficiencies and seed germination rates, demonstrated that the old trees are still in a healthy, mature state, and senescence is not manifested at the whole-plant level. Taken together, our results reveal that long-lived trees have evolved compensatory mechanisms to maintain a balance between growth and aging processes. This involves continued cambial divisions, high expression of resistance-associated genes, and continued synthetic capacity of preformed protective secondary metabolites.

Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) represent different tree functional types, which can be found extensively in northwestern China. The former is drought-tolerant, whereas the latter is hygrophilous and shade-tolerant. We compared their intrinsic water-use efficiency (iWUE, inferred from carbon isotopic discrimination, d 13 C, in their wood) as a function of atmospheric CO 2 concentration, [CO 2 ], and climate. d 13 C of spruce was consistently about 1:23& higher than that of juniper in semi-arid areas but was lower in arid areas. This difference was stable over time and demonstrated strong cross-correlations between species, although some subtle high-frequency (2 or 3 years) variations existed in both species, suggesting that regional climate may control carbon isotope discrimination. The C i =C a ratio (the [CO 2 ] values in leaf intercellular and the atmosphere, respectively) of the juniper increased steadily over time, whereas that of the spruce showed a long-term downward trend. IWUE increased at all sites over the 150-year study period, mainly caused by increasing [CO 2 ]. The relationship between iWUE and [CO 2 ] reveals that the spruce was more sensitive than the juniper to increasing [CO 2 ], suggesting a species-specific adaptation to long-term environmental changes. Correlations between the high-frequency variations in stable carbon discrimination (D) and climate indicate similar intra-site responses to climate in both species, but different response strengths. Overall, complex interactions of temperature and moisture on stable carbon discrimination during current growth seasons of both species were environmental-determined. Regulation of gas exchange and reduced transpiration may influence water and energy budgets directly; therefore species-dependent responses of trees to elevated CO 2 should be considered in future research on global plant physiological ecology.