Divergent tree radial growth at alpine coniferous forest ecotone and corresponding responses to climate change in northwestern China

Abstract

Global climate change is reshaping the geographical distribution of forests, especially in mountain regions. The alpine forest ecotone is expected to be affected first, due to the climatic limitations on tree growth, and is thus considered as an indicator of the forest succession to climate change. However, current studies involving tree radial growth and their responses to climate change at alpine forest ecotone remain scarce, posing challenges for predictions of forest dynamics and management strategies under different climate scenarios. Here, we studied 94 Chinese Pine (Pinus tabulaeformis; Pinaceae) and 86 Qinghai Spruce (Picea crassifolia; Pinaceae) trees along the alpine forest ecotone [2100–2400 m above sea level (a.s.l.)] to assess tree growth differences and corresponding responses to climate change in the Helan Mountains, northwestern China. The results revealed that growth rates of Chinese Pine at lowest altitudes and Qinghai Spruce at highest altitudes are strongly affected by water shortage and intraspecific competition, respectively, leading to the tree growth in these regions are inferior to those of other trees in the ecotone (the tree age at maximum width is about 20 years later than in the ecotone), especially for seedling and sapling stages. The responses of tree radial growth to dryness were broadly similar in each sample site, but wetness was more positive in trees at low altitudes than at high altitudes. Climate warming has inhibited the radial growth of Qinghai Spruce at lower distribution limits since 2005, while for Chinese Pine in the same area was not significantly affected by rising temperature, and even the upper limits of the Chinese Pine might migrate to higher altitudes as the calorific limit becomes higher. If climate warming continues, the above process may erode the Qinghai Spruce habitat, resulting in the transitional zone between Chinese Pine and Qinghai Spruce also moving to higher elevations.