Water Use Strategies Determine Divergent Growth Trends of Spruce and Juniper on the Southeastern Tibetan Plateau

Warming and increased CO2 levels are generally believed to enhance photosynthetic rates (A) and tree growth, especially in alpine treelines. However, this positive effect may be limited by moisture availability. Here we compare the radial growth and intrinsic water use efficiency (iWUE) of two widely-distributed evergreen needleleaf species, Tibetan juniper (Juniperus tibetica) and Balfour spruce (Picea balfouriana), at the same site near the treeline on the southeastern Tibetan Plateau using tree-ring width and dual isotopes (δ13C and δ18O). We observed that with a 70 ppm increase in atmospheric CO2 from 1954 to 2007, juniper radial growth was significantly stimulated, whereas spruce growth remained constant. Correlation analysis between tree-ring width and climate factors revealed that spruce was more moisture-sensitive than juniper. Although the increases in iWUE for juniper (22%) and spruce (26%) were similar, iWUE changes in spruce were more influenced by stomatal conductance (gs) rather than A, supporting that moisture availability inhibits spruce growth. Our study implies that predicting growth dynamics in alpine forests based on the growth-temperature relationship may be inadequate, as they are likely moisture-limited. It also suggests that increases in A may not be as widespread as previously thought, which helps improve forest productivity modeling.