Age-dependent impacts of climate change and intrinsic water-use efficiency on the growth of Schrenk spruce (Picea schrenkiana) in the western Tianshan Mountains, China

Rising atmospheric CO2 (Ca) 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 Ca 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 CO2 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 CO2 fertilization effect. These age-dependent growth responses should be accounted for when assessing responses of forest dynamics to climate change.