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Tibetan Plateau is the headwater region of many major Asian rivers and very susceptive to climate change. Therefore, knowledge about climate and its spatiotemporal variability in this area is very important for ecological conservation, water resource management and social development. The aim of this study was to reconstruct and analyze the hydroclimate variation on eastern Tibetan Plateau (ETP) over many centuries and explore possible forcing factors on regional hydroclimate variability. We used 118 tree-ring chronologies from ETP to reconstruct the gridded May-July Standardized Precipitation Evapotranspiration Index for the ETP over the last millennium. The reconstruction was developed using an ensemble point-by-point reconstruction method, and a searching region method was used to locate the candidate tree-ring chronologies. The reconstructions have nicely captured the spatial and temporal features of the regional drought variation. The drought variations in south and north of 32.5°N are notably different, which may be related to the divergence influence of North Atlantic Oscillation on the climate systems in the south and north, as well as differences in local climate. Spectral analysis and series comparison suggest that the drought variation in the northeastern Tibetan Plateau has been possibly influenced by solar activity on centurial and longer time scale. [ABSTRACT FROM AUTHOR]

Knowledge about the spatiotemporal tree growth variability and its associations with climate provides key insights into forest dynamics under future scenarios of climate change. We synthesized 17 tree-ring width chronologies from four tree species at the high-elevation sites in the southeast Tibetan Plateau (SETP) to study the regional tree growth variability and climate-growth relationships. Despite of diverse habitats and different physiological characteristics of these species, these tree-ring chronologies shared a significant common variance in SETP. An unprecedented increase in the shared variance is found along the latter half of the 20th century, coinciding with the enhancement of the frequency of extreme rings among chronologies. It is found that minimum winter temperature tends to be the dominant climate for trees in this region. The site-specific responses in cold (1965–1980) and warm (1990–2005) intervals by means of Fuzzy Cmeans (FCM) clustering reveal that the remarkable enhancement of growth synchrony among trees mainly occur in warm conditions. This is different from previous findings indicating that increased consistence among temperature sensitive tree rings in cold periods. This may be related to the reduced temperature sensitivity of regional tree growth as winter minimum temperature is lower than a certain threshold, which is in agreement with the “principle of ecological amplitude”. In addition, it is worth noting that precipitation in June have started to restrain the tree growth since the beginning of the 1980s, which is possibly an important contributor for synchronized growth among trees in SETP. [ABSTRACT FROM AUTHOR]

We established a Juniperus przewalski tree ring width chronology, based on tree ring cores collected from the A'nyêmaqên Mountains. Statistical analysis showed that the chronology was highly correlated with instrumental streamflow records from previous August to current July from the Tangnaihai Station in the upper reaches of the Yellow River, with a correlation coefficient of 0.656. Streamflow for the upper reaches of the Yellow River was reconstructed for the past 1234 years. Low flow periods for the 11-year averaged streamflow reconstruction were definite as lower than mean plus 1 standard deviation, and high flow periods were higher than mean minus 1 standard deviation. Over the past 1234 years, high flows occurred 18 times, and low flows occurred 12 times. The main low flow periods were identified as AD 1140-1156, AD 1295-1309, AD 1473-1500, and AD 1820-1847, and the main high flow periods were identified as AD 846-873, and AD 1375-1400. Extremely low streamflow over the reconstruction period was seen during the late 15th century, coinciding with a widespread drought phenomenon, which took place in the northeastern Tibetan Plateau over the same period. Reconstructed streamflow shows significant low-frequency variability, which is in line with drought variability of neighboring regions, as inferred from tree rings and other proxies. Multi-taper spectral analysis suggests the existence of significant periods of 2-5, 22, 35-38, 55-62, and 114-227 years, particularly significant for cyclic variations of years 159 and 36. [ABSTRACT FROM AUTHOR]

Although the contents of limiting elements in plants, such as nitrogen (N) and phosphorus (P), have been widely studied from subtropical to humid-temperate zones, the strategies used by coniferous species to allocation N and P in arid and semiarid forests remain unclear. In this study, samples of 545 leaves, 194 twigs, and 78 fine roots were collected from four coniferous species (Pinus tabuliformis, Picea wilsonii, Juniperus przewalskii, and Picea crassifolia) of three genera (Pinus , Picea, and Juniperus) in the northeastern Tibetan Plateau, and the contents of C, N, and P were analyzed. Two key parameters, namely the allometric exponent and coefficient of variation, were calculated to illustrate the relative investment of plants to N and P uptake and plasticity (variation of N and P), respectively. The contents of N and P and the N:P ratios were the highest in leaves, but their plasticity was the lowest. This confirmed the hypothesis that the leaves of coniferous species have a high content of limiting nutrients and homeostasis. At the regional level, the allometric exponent of N and P in leaves was 0.68, 0.74 in twigs, and 0.78 in fine roots, which is consistent with the results on a global scale. Thus, this invariant allometric relationship suggests the existence of an important mechanism that constrains the allocation of plant nutrients across broad environmental gradients. However, the allocation strategies for N and P shifted with the species, climate, and soil nutrients. Namely: their preferred nutrient uptake was P when the trees had a better nutritional status (semiarid environments, mean annual precipitations (MAP) > 300 mm), but the investment of N was strengthened when the habitat conditions become more severe (extremely arid environments, MAP <100 mm). Thus, our results can provide a novel perspective to understand the strategies of plant nutrient uptake in arid and semiarid forests. • Leaves of conifers have a higher content of limiting nutrients and lower plasticity. • Invariant allometric exponents of N and P exist among plant organs. • Allocation for N and P were shifted with the species, climate, and soil nutrients. • Allometric exponent can provide a novel indicator to predict plant nutrient uptake. [ABSTRACT FROM AUTHOR]

• NDVI increase related to extreme temperature rise • Extreme temperature controls the beginning of vegetation growth • The delayed response of vegetation to extreme precipitation occurred late in growth • The effect of extreme temperatures on vegetation depends on moisture condition • Extreme temperatures intensity and NDVI are projected to increase in the future Understanding how extreme climates affect vegetation is of great importance for the protection of ecosystems. However, most of previous studies focused on the impact of average climate change on vegetation, and few on the effects of extreme climate on vegetation change. Such studies are particularly scare in arid and semiarid mountainous areas where vegetation is relatively homogenous and ecosystems are more vulnerable to climate change. In this study, we analyzed the vegetation response to extreme climate change in the Qilian Mountains, northeastern Tibetan Plateau, employing 11 extreme climate indices and the normalized difference vegetation index (NDVI), and further explored possible future trends in NDVI and extreme climate indices using detrended fluctuation analysis (DFA). The results indicated that NDVI in the Qilian Mountains showed an overall increasing trend from 1982 to 2015, with a rate of 0.004 per decade; 38.11% of the region showed a significant increasing trend and only 3.34% showed a significant decreasing trend. The vegetation areas with a significant increasing trend were dominated by grassland, cropland, and forest. The extreme climate indices accounted for approximately 29.8% of the change in NDVI, with the extreme precipitation intensity index (Rx1day) contributing the largest amount (17.1%), followed by the extreme temperature intensity index (TNx; 12.7%). Compared with extreme temperatures, the interannual variation in extreme precipitation has a greater impact on vegetation growth in the Qilian Mountains because moisture is the main climatic factor that limits vegetation growth in this region. There was a significant time-lagged effect of climate factors on vegetation growth in general, with a lag up to three months. The beginning of vegetation growing season was controlled by temperature-related extreme indices, but not directly by precipitation-related indices, and the cumulative effect of extreme precipitation and the time lag of NDVI's response appeared in the later part of the season. The DFA results showed that the extreme temperature intensity, extreme precipitation intensity, and the NDVI are expected to continuously increase in the future. [ABSTRACT FROM AUTHOR]

• We monitored sap flow for 16 spruces and 14 junipers at 4 arid to semi-arid sites. • Sap flow is almost completely controlled by VPD except for spruce in the arid site. • Increasing site aridity drives a shift towards more anisohydric stomatal behavior. • Spruce exhibits stronger stomatal regulation of transpiration than juniper. • Transpiration may increase first and then decrease from semi-arid to arid forests. Climate change is altering regional aridity and species composition in dryland ecosystems. Understanding the effects of long-term increasing aridity and species-specific stomatal behaviors on transpiration is therefore important for water-resource forecasting. To assess the effects of site aridity levels and species on stand transpiration rate (E s), we monitored sap flux density (J s) and relevant environmental parameters for 16 isohydric Picea crassifolia (spruce) and 14 anisohydric Juniperus przewalskii (juniper) trees over three growing seasons at four arid to semi-arid high-elevation sites on the Tibetan Plateau. Our results show that E s was about 5–9 times higher in semi-arid sites than in arid sites, and about 6–9 times higher in spruce than in juniper. Spruce exhibited stronger stomatal regulation of transpiration than juniper. Soil water supply strongly promoted J s only for spruce in the arid environment (R2 = 0.23), while at the other sites, J s was mostly controlled by atmospheric vapor pressure deficit (VPD) (R2 > 0.82). However, increasing site aridity greatly reduced the sensitivities of both J s and canopy conductance to VPD , especially for juniper. We expect that in the transition from semi-arid to arid alpine forests, E s will initially rise due to increasing VPD. However, in the long term, there may be a stronger decline in E s since both the sensitivity of J s to VPD and the stand sapwood area will decrease. These findings could be used to reduce the impacts of climate change on water resources in high-elevation drylands. [ABSTRACT FROM AUTHOR]

While temperature is known to be an important factor determining cambial phenology in cold and humid climates, how it interacts with other factors in determining the onset and end of xylogenesis is still not fully understood. Here, we analyzed phenological traits related to observed cambial phenology and xylogenesis over the course of two extreme hydroclimatic years across six sites that span the spatial distribution of Qilian juniper ( Juniperus przewalskii Kom.) in the northeastern Tibetan Plateau. Cambial phenology and xylogenesis were assessed weekly or biweekly from the microcores of 30 trees. We found that the onset of xylogenesis significantly correlated with annual mean and minimum temperatures, regardless of the moisture conditions, resulting in an advance of the onset of xylogenesis of 10.1 days °C −1 . In contrast, the end of xylogenesis was related to both maximum temperatures and drought conditions. The contrast in drought conditions over the two growing seasons made the importance of the water balance on the end of xylogenesis quite clear. During the extremely dry conditions of 2013, xylogenesis ended up to 30 days earlier than during 2012, a wetter and cooler year. Our results indicate that strong shifts in water availability during the growing season are crucial to the rate of wood production. Also, the significant correlation between the total number of xylem cells and both the duration and rate of wood production, highlights how changes in the timing of xylogenetic processes and the growth rate can have a strong influence on the future growth and performance of J. przewalskii . Our findings indicate that while warming promotes an early growing season and, potentially, a longer growing season, warming and decreased moisture availability has a strong influence on tree growth and productivity in cold and arid regions. [ABSTRACT FROM AUTHOR]