Variations of stomatal density and carbon isotope values of Picea crassifolia at different altitudes in the Qilian Mountains

The stomatal characteristics, length and dry weight as well as stable carbon isotope composition (δ13C) of needles and tree rings of Qinghai spruce (Picea crassifolia) at different altitudes in the Qilian mountains were investigated. The results showed that stomatal density, distribution pattern on epidermis, and length and dry weight of needles all increased with altitude below 3,000 m. In contrast, these parameters all decreased with increasing altitude above 3,000 m. Furthermore an assay of tree rings showed that near 3,000 m in altitude was the optimum zone for growth and development of Qinghai spruce. Although atmospheric CO2 concentration influences stomatal density, the effects of many environmental factors, such as temperature, light and rainfall, could obscure the effects of changes in atmospheric CO2 concentration on stomatal density at altitudes higher than the optimum for growth. The correlation of stomatal density with atmospheric CO2 concentration and δ13C of Qinghai spruce needles was significant below 3,000 m, but was insignificant above 3,000 m altitude. The δ13C value of needles, however, was negatively correlated with atmospheric CO2 concentrations. Therefore, the influence of altitude should be considered in research on plant physiological ecology and the relationship of stomatal density with δ13C value or atmospheric CO2 concentration.