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.