New patterns of establishment and growth of Picea, Abies and Betula tree species in subalpine forest gaps of Jiuzhaigou National Nature Reserve, Sichuan, southwestern China in a changing environment

Temperature increase in the southeastern Tibetan Plateau in the past thirty years is high but its effects on tree dynamics of subalpine coniferous forests are not well understood. This study focuses on tree establishment and growth in 26 subalpine forest gaps between 2900 m and 3200 m in Jiuzhaigou National Nature Reserve, Sichuan, China. Normally Abies faxoniana and Picea purpurea dominate the canopy of subalpine forests in the reserve. Our study examined tree species establishment and growth, environmental conditions in these gaps, and the potential future changes in forest composition in the reserve. Seedlings, saplings and mature trees were counted, measured and cored to assess species’ numbers, age, and growth parameters. Root structure was also studied. Gap environmental conditions were recorded and climate changes examined. For a wide range of differently aged gaps, numerous seedlings, saplings and even mature canopy trees of Betula albo-sinensis , a deciduous broadleaf tree, were observed. B. albo-sinensis was most abundant in gaps with the following characteristics: (1) steep slopes (>30°), (2) areas with thin soil (<5 cm depth) or (3) large areas (>600 m2) with deep soil (>15 cm depth). In contrast, the site characteristics required for successful establishment of the two evergreen conifers A. faxoniana and P. purpurea are soil >9 cm deep and slopes <30°. B. albo-sinensis has shallower roots with smaller root:shoot ratio than the conifer species. Increasing temperature, declining cloud cover and increasing high rainfall incidences are increasing habitats that favor establishment and growth of B. albo-sinensis and inhibit the establishment and growth of A. faxoniana and P. purpurea in subalpine areas of Jiuzhaigou, southwestern China.