Recent reduction in the frequency of frost accounts for most of the increased growth of a high elevation spruce forest in northwestern China

Abstract

Key message: Using process-based models in combination with dendrochronological measurements provides a way to explain recent increased tree growth in northwestern China. Abstract: Dendrochronological studies of tree rings in a 250-year-old Qinghai spruce (Picea crassifolia) forest in the Qilian Mountains of northwestern China indicate a 60 % sustained increase in tree-ring growth between 1980 and 2009 compared with any time since 1785. Over the same period, the maximum, minimum, and average temperatures all increased by nearly 2 °C during the growing season (May through September), the frequency of frost decreased 18 days, precipitation remained unchanged, while atmospheric concentrations of CO2 increased by 48 ppm. To explain how the changes in climatic variables might cause the increase in tree growth, we parameterized a process-based growth model (3-PG, physiological processes predicting growth) with values from the literature and performed a series of sensitivity tests. The results of our analysis indicated that a reduction in frost frequency during the growing season, which allows stomata to remain open, enhanced gross photosynthesis by 42 %. Up to a 20 % increase in PG could be attributed to rising atmospheric CO2 between 1980 and 2009, with half of this attributed to increased light interception from a simulated 0.4 increase in canopy leaf area index. The increase in average and maximum temperatures had little direct effect on gross photosynthesis with the optimum temperature set between 9 and 10 °C. Indirectly, the increase in monthly average minimum temperature during the growing season, although small, crossed a threshold that reduced the impact of frost. Our analyses show the value of combining dendrochronological measurements with a process-based model to gain a more holistic understanding of how environmental factors interact to affect tree growth.