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

•Subalpine environments favor establishment and growth of birch not conifers.•Current climate change favors hardwood trees rather than conifers in Jiuzhaigou.•Broadleaf trees remain for a longer period than the usual in current forests.•Forest ecosystems are affected by a combined natural and anthropogenic factors. 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 2900m and 3200m 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 (600m2) with deep soil (>15cm depth). In contrast, the site characteristics required for successful establishment of the two evergreen conifers A. faxoniana and P. purpurea are soil >9cm deep and slopes