Abstract:

Tree growth at treelines is generally limited by low temperature so that warming tends to shift treeline upward. However, a global meta-analysis showed that treelines shifted upward at around 50% during the last century among the investigated treelines, and others kept static. Such evidence suggested that non-climate factors (such as interspecific competition) apart from climatic factors controlled treeline shift. The Tibetan Plateau hosts the highest treeline in the Northern Hemisphere, being a good locale for investigating climatic and biotic factors influencing treeline changes. As showed by our research, short and sparse shrub and grass acted as“tree nurseries”for recruitment, and benefitted treeline upward shift. Dense shrubs just above treeline inhibited tree establishment, and slowed upward movement of treelines. Climatic warming tended to promote the upward shift of alpine treelines at local and regional scales. However, upslope migration rates were controlled largely by interspecific interactions. Warming may not cause the upward shift of treelines if dense shrub dominate the above treelines.