Sensitivity of recruitment and growth of alpine treeline birch to elevated temperature

Abstract

Trees at temperature-limited alpine treeline are highly sensitive to temperature fluctuations. However, few studies have directly linked the recruitment and growth of juvenile trees to climate warming to investigate the processes underlying climate-induced alpine treeline shift. On the cold-humid Changbai Mountain of northeastern China, almost no tree recruitment occurred above the current treeline on the west side before 1985, whereas considerable recruitment has occurred since 1985. To investigate whether this recent recruitment was caused by warming temperatures, we established a 450 m × 50 m permanent plot above the current alpine treeline. We used dendrochronology to reconstruct the time series of recruitment of Betula ermanii juveniles. In addition, we used open top chambers (OTCs) to increase the growing season day-time temperature by c. 2.8°C and thereby to study the growth responses of juvenile trees to future warmer conditions beyond the current treeline. We found that tree recruitment increased substantially during a period of warmer summer temperatures since 1985, showing a high sensitivity of tree recruitment to temperature above the alpine treeline. The daily stem radial increment recorded by dendrometers was significantly (p ≤ 0.05) positively correlated with temperature but negatively correlated with soil water content, indicating no water limitation. Correspondingly, the increase in radial growth in OTCs was primarily a consequence of higher temperatures, especially in July. The regression slope between stem radial growth and temperature in the OTCs did not differ from that in ambient controls, indicating that the sensitivity of stem radial growth to temperature was not altered by warming. These suggest that future climate warming will induce increases in tree recruitment and growth above the alpine treeline. Our results revealed a direct link between temperature and recruitment and growth of juvenile trees, and that climate warming will likely cause forests to advance beyond the alpine treeline.