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Rising atmospheric CO 2 ( C a ) can increase tree radial growth by increasing intrinsic water-use efficiency (iWUE). However, the effects of age on the response remain unknown, particularly for forests in remote areas such as China’s Tianshan Mountains. Here, we assessed age-dependent growth trends of Picea schrenkiana using tree-ring data from low and high elevations and two detrending methods to detect robust long-term trends. We developed age-dependent relationships between basal area increment (BAI) and iWUE based on size-class isolation and separated the contributions of climate variables and iWUE to radial growth. The juvenile trees showed growth increases over time, versus growth reductions in two older age classes. An overall negative trend existed for combined data from all age classes and elevations. iWUE increased in response to increasing C a for trees in all age groups at low and high elevations, with higher values in near-mature and mature trees (100 yr < age ≤ 160 yr) at the lower sites. However, age affected radial growth and its responses to climate and iWUE. For juvenile trees (≤60 yr), BAI increased during the study period, temperature in May-July and iWUE all have positive effects on the radial growth with significantly high contribution variance from iWUE. The smaller hydraulic constraints in juvenile trees may permit higher photosynthetic rates, reduced climatic sensitivity, and increased growth by CO 2 fertilization. For older trees, BAI decreased, possibly due to stomatal closure caused by warmer temperatures and decreasing hydraulic efficiency with increasing tree age; thus, increased iWUE could not outweigh environmental stresses. The trees at high and low elevations responded consistently to environmental changes, but with different response intensity. Thus, climate and cambial age together determined whether increased iWUE could increase radial growth and the potential CO 2 fertilization effect. These age-dependent growth responses should be accounted for when assessing responses of forest dynamics to climate change. [ABSTRACT FROM AUTHOR]

Tree ring δ 13 C as a climate proxy is widely used for palaeoclimate research, however, its temporal stability response to the climate change remains unclear under more than one limited factors on tree growth. Here, we used a millennium tree-ring δ 13 C chronology combining two annual-resolution δ 13 C chronologies since 1800 from long-lived Qilian juniper ( Sabina przewalskii ) to assess its instability of the climate signal in the northeastern Qinghai–Tibetan Plateau. Tree-ring δ 13 C chronologies were strongly correlated with the regional mean April to August temperature from 1956 to 2008, but the associations were absent within the period 1901 to 1955 values in the CRU TS dataset. Comparison of the millennium-long δ 13 C series with reconstructed Asian temperatures also demonstrated that the δ 13 C chronology exhibited climate signal temporal instability. Substantial oscillations were revealed using a frequency-dependent analysis and 51-year running correlation analysis from the millennium-long tree-ring δ 13 C and δ 18 O series. Dual-isotope approach indicated that stomatal limitations created a statistical significant positive correlation between tree-ring δ 13 C and δ 18 O, but photosynthetic rate may be dominant when the correlations were not significant. Our results suggest that tree-ring δ 13 C series in the northeastern Qinghai–Tibetan Plateau is responded instability to temperature variations in the past 1000 years. [ABSTRACT FROM AUTHOR]