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The effect of warming-induced drought stress on tree growth has been frequently discussed, and a decline in the growth of forest and increased mortality has been reported for many areas of the world. A hiatus in the global warming trend began around the year 2000, but how tree growth has responded to this warming slowdown remains unclear. Here we present a study of the recent impacts of climate change on Qinghai spruce (Picea crassifolia) growth using 38 tree-ring records collected from the Qilian Mountains in northwestern China. Results indicate that there was a clear increase in Qinghai spruce growth during the recent warming hiatus. Similarly, regional vegetation growth also improved as indicated by NDVI variations, which is closely associated with regional climate changes. The mean air temperature increased dramatically during the period 1980–2001, whereas precipitation decreased slightly. This combination of warmer temperatures and reduced precipitation created drought conditions that limited the growth of Qinghai spruce during this period. Since 2001, the warming trend has slowed, whereas precipitation has clearly increased, creating moist conditions as reflected by the higher drought index (SPEI). A decline in drought pressure, coupled with an increasingly long growing season, might be the main reason for the recent improvement in the growth of Qinghai spruce in the Qilian Mountains. Our results suggest that the growth of Qinghai spruce is very sensitive to climate change. A better understanding of future climate changes and their likely impact on tree growth should be of significant interest to forest managers.

Ongoing climate changes posed significant threats to forest growth, structure and ecological functions. However, in our field survey in the east Qilian Mountains, we found that growth condition of Qinghai spruce under climate changes was better in anthropogenic management forest than disturbance-free stands. Accordingly, we suggested a hypothesis that the environmental change induced suppression in Qinghai spruce growth could be mitigated by anthropogenic management. Such a hypothesis was tested by comparing Qinghai spruce growth and population dynamic patterns between anthropogenic-managed forests and disturbance-free ones. We found that Qinghai spruce radial growth persistently declined since the late 1950s in the disturbance-free forests, while the radial growth of anthropogenic-managed trees almost kept steady-going trends since the 1970s in which anthropogenic-management appeared. Besides, some anomalous growth-declining years occurred in disturbance-free forests did not appear in anthropogenic-managed forests in the same year after anthropogenic-management emerged. Moreover, missing ring and tree mortality took place in disturbance-free stands and became increasingly frequent after the mid-1980s. The frequencies and proportions of missing rings and tree die-off were much lower in anthropogenic-managed forests than disturbance-free ones. Additionally, the occurrence of Qinghai spruce radial growth decline and increased mortality in recent time in our disturbance-free forests could be well explained by global-change-type drought in conjunction with elevated temperature and warming-related pest outbreaks. These results would be of fundamental significance for the further understanding of the changing forests, which could provide new clues for forest management and conservation in the future.

Drought is the primary limiting factor for tree growth in arid and semi-arid regions, where it can greatly influence xylem anatomical traits and induce multiple responses to wood development. However, the mechanisms driving xylogenesis and intra-annual density fluctuations (IADFs), as well as the seasonal dynamics of wood development in water-limited environments, are still unclear. In this study, we assessed the kinetics of xylem development weekly in Chinese pine (Pinus tabuliformis) and Qinghai spruce (Picea crassifolia), at two altitudes in the Helan Mountains, semi-arid China. Results indicate that summer drought in June inhibited cambial division and reduced xylogenesis, resulting in only 40% of the xylem developing before August. The onset of abundant precipitation in mid-July reactivated radial growth. This second growth extended the growing season by one month and partly compensated for the negative effect of the drought on growth. E-type IADF were characterized by narrow cells with thick walls, formed during July and early August. The duration of cell enlargement explained 57% of variance in tracheid diameter, showing that it played an important role in determining cell size. In contrast, changes in the cell wall thickness were mainly driven by the rate of secondary wall thickening, which explained 65% of the variance. This study reveals the potential mechanisms for xylem development and E-IADF formation under summer drought in semi-arid China, provides evidence for the plasticity and resistance of Chinese pine and Qinghai spruce under varying environmental conditions, and contributes to our understanding of likely future trends in forest productivity under climate changes.

Tree transpiration plays a determining role in the water balance of forest stands and in water yields from forested catchments, especially in arid regions. This study describes direct measurements of tree sap flow by heat pulse velocity method, and calculations of stand transpiration and canopy conductance, during the 2011 and 2012 growing seasons, in an 80- to 120-year-old Picea crassifolia stand in a natural forest. The experiment was carried out on Qilianshan Mountain, located in the upper Heihe River Basin, in the arid region of Northwest China. It was found that stand transpiration (E-c) ranged from 1.0 mm d(-1) to 2.4 mm d(-1), and from 1.2 mm d(-1) to 2.4 mm d(-1), during the 2011 and 2012 growing seasons, respectively. Total E-c was 195.2 mm, 219.6 mm during the 2011 and 2012 growing seasons, respectively. And total Ec during the 2012 growing season was 12.5% higher than during the 2011 growing season. Based on the half-hourly temporal scale, canopy conductance was between 0.3 mm s(-1) and 51.3 mm s(-1), and between 1.2 and 57.0 mm s(-1), during the 2011 and 2012 growing seasons, respectively. Canopy conductance was sensitive to variations in climatic variables such as vapor pressure deficit (D), air temperature (T), wind speed (W) and global short-wave radiation (R). It is also evident that an exponential decay function, including D and T, accounted for 81% and 19% of the variations in canopy conductance, respectively. These results was not only providing the basis for more detailed analyses of water physiology and growth of Qinghai spruce trees for the later application of a climate-driven process model, but also might have implications for forest management in Qilianshan Mountains. (C) 2014 Elsevier B.V. All rights reserved.

Canopy interception has large spatial variability which complicates measurements and achievement of spatial representation. In the present study, we measured gross rainfall and throughfall from June 2011 to October 2012, and leaf area index (LAI), plant area index (PAI), and spatial locations of trees within Picea crassifolia forest in Qilian mountains of northwestern China. Spatial variability of canopy interception and related factors, and the minimum number and locations of collectors were analyzed by statistical techniques. The results show that spatial variation of canopy interception has a significant relationship with PAI, but not with LAI. This indicates that PAI is more appropriate parameters for a canopy interception model than LAI. Based on the relationship between canopy interception loss and PAI, the minimum number and locations of collectors were estimated. In this study, 10–12 collectors yielded representative throughfall in the forest, but these collectors required to be placed at the sites of mean values of PAI.

In order to accurately estimate whole-tree water use and individual tree transpiration, it is important to have reliable information on radial patterns of sap velocity and responses of sap flow to local environmental conditions. Therefore, variation in sap flow and environmental conditions was investigated in a mature Qinghai spruce (Picea crassifolia) stands during the growing season of 201 1 at the Pailougou watershed, in Qilianshan Mountain, located in the upper Heihe River Basin, in the arid region of Northwest China. Daily sap flow was measured by the heat-pulse technique on nine trees during the growing season. It was found that the highest daily sap flow velocity in sap flow radial distribution was at 20 mm sapwood depth, and that from 10 mm to 20 mm sapwood depth, the daily sap flow velocity gradually increased, whereas from 20 mm to 40 mm sapwood depth, sap flow velocity gradually diminished. A simple Gaussian regression model for the radial distribution of sap flux velocity was formulated, which explained 92% of the radial profile variation of sap flow velocity. Sap flow velocity was heightened by increasing the global short-wave radiation (R, W m(-2)), vapour pressure deficit (D, kPa), and air temperature (T, degrees C) when R < 800 W m(-2), D < 1.4 kPa, and T < 18.0 degrees C. It is, however, inherently difficult to establish firm relationships between sap flow velocity and R, D, and T because of the complex crown environment. The correlation of daily sap flow velocity to soil moisture content on a clear day was fitted by a logistic regression. We conclude that measurement of radial flow pattern provides a reliable method of integrating sap flow from individual measuring points to the whole tree. And D, R, T and soil moisture had varying influences on sap flow velocity in the Qinghai Spruce. (C) 2013 Elsevier B.V. All rights reserved.