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The onset of xylogenesis plays an important role in tree growth and carbon sequestration, and it is thus a key variable in modelling the responses of forest ecosystems to climate change. Temperature regulates the resumption of cambial activity, but little is known about the effect of water availability on the onset of xylogenesis in cold but semi-arid regions. The onset of xylogenesis during 2009-2014 was monitored by weekly microcoring Juniperus przewalskii trees at upper and lower treelines on the north-eastern Tibetan Plateau. A logistic regression was used to calculate the probability of xylogenic activity at a given temperature and a two-dimensional reverse Gaussian model to fit the differences between the observed and estimated days of xylogenesis onset at given temperatures and precipitation within a certain time window. The thermal thresholds at the beginning of the growing season were highly variable, suggesting that temperature was not the only factor initiating xylem growth under cold and dry climatic conditions. The onset of xylogenesis was well predicted for climatic thresholds characterized by a cumulative precipitation of 17.0 ± 5.6 mm and an average minimum temperature of 1.5 ± 1.4 °C for a period of 12 d. Xylogenesis in semi-arid regions with dry winters and springs can start when both critical temperature and precipitation thresholds are reached. Such findings contribute to our knowledge of the environmental drivers of growth resumption that previously had been investigated largely in cold regions without water shortages during early growing seasons. Models of the onset of xylogenesis should include water availability to improve predictions of xylem phenology in dry areas. A mismatch between the thresholds of temperature and moisture for the onset of xylogenesis may increase forest vulnerability in semi-arid areas under forecasted warmer and drier conditions.

A series of studies have shown that temperature triggers the onset of xylogenesis of trees after winter dormancy. However, little is known about whether and how moisture availability influences xylogenesis in spring in drought-prone areas. Xylogenesis was monitored in five mature Qilian junipers (Juniperus przewalskii) by microcore sampling from 2009 to 2011 in a semi-arid area of the north-eastern Tibetan Plateau. A simple physical model of xylem cell production was developed and its sensitivity was analysed. The relationship between climate and growth was then evaluated, using weekly wood production data and climatic data from the study site. Delayed onset of xylogenesis in 2010 corresponded to a negative standardized precipitation evapotranspiration index (SPEI) value and a continuous period without rainfall in early May. The main period of wood formation was in June and July, and drier conditions from May to July led to a smaller number of xylem cells. Dry conditions in July could cause early cessation of xylem differentiation. The final number of xylem cells was mainly determined by the average production rate rather than the duration of new cell production. Xylem growth showed a positive and significant response to precipitation, but not to temperature. Precipitation in late spring and summer can play a critical role in the onset of xylogenesis and xylem cell production. The delay in the initiation of xylogenesis under extremely dry conditions seems to be a stress-avoidance strategy against hydraulic failure. These findings could thus demonstrate an evolutionary adaptation of Qilian juniper to the extremely dry conditions of the north-eastern Tibetan Plateau.

Extremely old trees are valued and revered by residents in many countries. Yet information about the ages and locations of the oldest living trees is rarely available, especially at the national level. We compiled a series of dendrochronological datasets of the age and location of very old trees (≥1000 years old) across China, and found that there are at least 98 ancient living trees exceeding 1000 years of age in the country. The oldest living individual we identified was a Qilian juniper (Juniperus przewalskii) tree that, as of 2009, was determined to be 2230 years old. The oldest trees occurred most frequently in remote, high-elevation areas.

Moisture conditions, especially those that occur as multi-decadal anomalies, have profound impacts on society, especially in arid and semi-arid regions. However, the lack of high-resolution climate data for the first millennium CE greatly limits our understanding of how moisture variations have influenced history. Here, we present an 1882-year (134–2015 CE) tree-ring chronology developed from Qilian juniper ( Juniperus przewalskii Kom.) growing in the western Qilian Mountains, northwest China. The tree-ring index correlates significantly with the May–June self-calibrating Palmer Drought Severity Index (sc-PDSI) and can be used to reconstruct May–June moisture variations since 241 CE. The reconstruction reflects moisture conditions at the annual to multi-decadal time scales over the past two millennia. During the period from the third to eighth centuries, there were prominent interdecadal fluctuations, with the third century and the late fifth century being the wettest and driest periods during the reconstruction, respectively. The transition from the wet third century to the dry fifth century corresponded with key events in Chinese history, namely the demise of the Western Jin Dynasty and the chaotic Southern and Northern Dynasties. Thus, our reconstruction provides new evidence for the potential linkage between abnormal climate conditions and social changes in ancient times.

The climatic signals that directly affect the trees can be registered by xylem during its growth. If the timings and duration of xylem formation change, xylogenesis can occur under different environmental conditions and subsequently be subject to different climatic signals. An experimental design was applied in the field to disentangle the effects of age and size on xylem phenology, and it challenges the hypothesis that the timings and dynamics of xylem growth are size-dependent. Intra-annual dynamics of xylem formation were monitored weekly during the growing seasons 2013 and 2014 in Chinese pine ( ) and Qilian juniper ( ) with different sizes and ages in a semi-arid region of northwestern China. Cell differentiation started 3 weeks earlier in 2013 and terminated 1 week later in 2014 in small-young pines than in big-old pines. However, differences in the timings of growth reactivation disappeared when comparing the junipers with different sizes but similar age. Overall, 77 days were required for xylem differentiation to take place, but timings were shorter for older trees, which also exhibited smaller cell production. Results from this study suggest that tree age does play an important role in timings and duration of growth. The effect of age should also be considered to perform reliable responses of trees to climate.

Although cambial reactivation is considered to be strongly dependent on temperature, the importance of water availability at the onset of xylogenesis in semi-arid regions still lacks sufficient evidences. In order to explore how environmental factors influence the initiation of cambial activity and wood formation, we monitored weekly cambial phenology in Qilian juniper (Juniperus przewalskii) from a semi-arid high-elevation region of northwestern China. We collected microcores from 12 trees at two elevations during the growing seasons in 2013 and 2014, testing the hypothesis that rainfall limits cambial reactivation in spring. Cambium was reactivated from late April to mid-May, and completed cell division from late July to early August, lasting 70–100 days. Both sites suffered from severe drought from January to April 2013, receiving < 1 mm of rain in April. In contrast, rainfall from January to April 2014 was 5–6 times higher than that in 2013. However, cambial reactivation in 2014 was delayed by 10 days. In spring, soil moisture gradually increased with warming temperatures, reaching 0.15 m3/m3 before the onset of xylogenesis, which may have ensured water availability for tree growth during the rainless period. We were unable to confirm the hypothesis that rainfall is a limiting factor of cambial reactivation. Our results highlight the importance of soil moisture in semi-arid regions, which better describe the environmental conditions that are favorable for cambial reactivation in water-limited ecosystems.

Key messageTemperature was the dominant climatic factor that controlled Juniperus przewalskii growth before 1988. However, the limiting effect of temperature on trees gradually weakened after 1988. Treeline shifted upward since 1890, especially during the 1930–1949. The saplings of the species died due to the warming.Forest mortality triggered by drought and treeline dynamics due to the regional warming has been widely reported. In the eastern Qilian Mountains, northwestern China, sapling of Juniperus przewalskii was found dead in the high elevation at the fringe of its spatial distribution. Tree-ring widths were measured, and climatic records were related to the tree-ring width to understand the sapling’s death and the mechanism of the treeline dynamics in this region. Results showed that tree-ring widths were positively correlated with the temperatures of previous year’s September to current year’s February. However, this correlation weakened in the late twentieth century as the mean annual temperature increased. Treeline shifted upwards, and the regeneration of J. przewalskii trees peaked during 1930–1949, of which most grew at higher elevations. Sapling trees (less than 2 m tall) started to die after 1959 and the death rate peaked 2000–2004, which lagged the warming induced drought. Drought stress induced by rising temperatures explained the death of the sapling trees. This study sheds light on the impacts of climate changes for tree growth, forest dynamics, and tree mortality, and provides insights that will be useful to forest managers for managing vulnerable arid and semi-arid forests.

The Qilian Mountains, located in northwest China and serving as a crucial water recharge area, have exhibited significant regional differences in precipitation patterns in recent decades. However, the limited temporal coverage of instrumental data has hindered a deep understanding of hydroclimate variations and regional differences. Further investigation into their long-term spatial and temporal precipitation characteristics is urgently needed. In this study, a new tree-ring-width chronology spanning 1743 years was established in the central Qilian Mountains using Qilian juniper (Juniperus przewalskii Kom.) samples. Significant correlations were found between the tree-ring indices and precipitation during both the growing and pre-growing seasons. Based on these correlations, annual precipitation from August of the previous year to July of the current year was reconstructed. The reconstruction model successfully explains 34.5% of the variation in precipitation during the calibration period. The analysis of the reconstructed series reveals notable interannual to multi-decadal dry–wet variability during the period from 614 AD to 2016 AD. The mid- to late-15th century emerges as the longest-lasting dry period, while the last decade stands out as the wettest. Comparative analysis with other precipitation reconstructions in the eastern and western Qilian Mountains reveals that regional drought events tend to be more pronounced and enduring. Low-frequency fluctuations on decadal to century scales show distinct wet and dry periods in the 12th–18th centuries in both the eastern and western parts of the Qilian Mountains, with weaker fluctuations in subsequent centuries. However, the central part of the Qilian Mountains exhibits opposite trends, possibly due to the complex interactions of multiple circulation systems.

Juniperus przewalskii Komarov, an endemic conifer in the high-altitude mountains of the Qinghai–Tibet Plateau, develops its cones in a synergistic manner with the oligophagous pest Megastigmus sabinae Xu et He (Hymenoptera: Torymidae), forming a highly specialized interaction system. However, the physiological adaptation mechanisms underlying this interaction remain unclear. Feeding by M. sabinae larvae significantly induced defense responses in J. przewalskii cones. During the early instars (2nd~3rd) of M. sabinae larvae, infested J. przewalskii endosperms upregulated protein content (48.91%; 3rd instar), significantly enhanced peroxidase (POD) activity (71.10%; 2nd instar), and specifically enriched coumarins and cinnamaldehyde derivatives (2nd instar) to increase M. sabinae larvae metabolic costs. In later instars (4th~5th) of M. sabinae larvae, the infested endosperms downregulated starch content (29.69%; 4th instar), increased phenylalanine ammonia-lyase (PAL) activity (57.34%; 4th instar), and accumulated steroid derivatives to suppress larvae development. Conversely, M. sabinae larvae demonstrated unique adaptive strategies: maintaining high levels of glutathione S-transferase (GST) as an antioxidant defense system during early instars and upregulating the level of digestive enzymes in later stages to overcome host multi-layered defenses. Juniperus przewalskii counters pest infestation through dynamic nutrient modulation, temporal activation of protective enzymes, and a multi-layered chemical defense network. The adaptation of M. sabinae larvae appears to involve the developmental regulation of detoxification and digestive enzyme levels. This study provides novel insights that enrich coevolution theory in alpine ecosystems.

The climate in the source region of the Yangtze River, Yellow River, and Mekong River is of great research interest because of its sensitivity to global change and its importance in regulating water resources to densely populated and vast areas downstream. A five-century long record of spring (May–June) for the Palmer Drought Severity Index (PDSI) was reconstructed for this region using tree-ring width chronologies of Qilian juniper (Juniperus przewalskii Kom.) from five high-elevation sites. The reconstruction explained 46% variance in the PDSI during the instrumental period 1955–2005. The reconstructed PDSI showed that the occurrence of dry extremes became frequent during the last century relative to the previous four centuries. The standard deviation of the reconstructed PDSI in the 100-year window showed that the recent century held apparent high values of standard deviation in the long-term context. Sustained droughts occurred in periods 1582–1631, 1737–1757, 1772–1791, 1869–1891, 1916–1939, and 1952–1982, whereas relatively wet intervals were observed in 1505–1527, 1543–1564, 1712–1736, 1792–1816, 1852–1868, 1892–1915, and 1983–2008. Notably, in the context of the past five centuries, the study region showed an increased inter-annual variability in the recent century, suggesting an intensified hydroclimatic activity possibly associated with global warming. Moreover, through diagnostic analysis of atmospheric circulation, we found that the negative phase East Asian–Pacific teleconnection pattern may be likely to trigger drought in the study region.