Spring phenology at different altitudes is becoming more
uniform under global warming in Europe.
Lei Chen, Jianguo Huang, Qianqian Ma, Heikki Hänninen, Sergio Rossi, Shilong Piao, Yves Bergeron.
Under current global warming, high-elevation regions are expected to experience
faster warming than low-elevation regions. However, due to the lack of studies
based on long-term large-scale data, the relationship between tree spring phenology
and the elevation-dependent warming is unclear. Using 652k records of leaf unfolding
of five temperate tree species monitored during 1951–2013 in situ in Europe,
we discovered a nonlinear trend in the altitudinal sensitivity (SA, shifted days per
100 m in altitude) in spring phenology. A delayed leaf unfolding (2.7 0.6 days per
decade) was observed at high elevations possibly due to decreased spring forcing
between 1951 and 1980. The delayed leaf unfolding at high-elevation regions was
companied by a simultaneous advancing of leaf unfolding at low elevations. These
divergent trends contributed to a significant increase in the SA (0.36 0.07 days
100/m per decade) during 1951–1980. Since 1980, the SA started to decline with a
rate of 0.32 0.07 days 100/m per decade, possibly due to reduced chilling at
low elevations and improved efficiency of spring forcing in advancing the leaf
unfolding at high elevations, the latter being caused by increased chilling. Our
results suggest that due to both different temperature changes at the different altitudes,
and the different tree responses to these changes, the tree phenology has
shifted at different rates leading to a more uniform phenology at different altitudes
during recent decades.