Asymmetric effects of daytime and nighttime warming on alpine treeline recruitment

Trees at their upper range limits are highly sensitive to climate change,
and thus alpine treelines worldwide have changed their recruitment
patterns in response to climate warming. However, previous studies focused
only on daily mean temperature, neglecting the asymmetric influences of
daytime and nighttime warming on recruitments in alpine treelines. Here,
based on the compiled dataset of tree recruitment series from 172 alpine
treelines across the Northern Hemisphere, we quantified and compared the
different effects of daytime and nighttime warming on treeline recruitment
using four indices of temperature sensitivity, and assessed the responses
of treeline recruitment to warming-induced drought stress. Our analyses
demonstrated that even in different environmental regions, both daytime
and nighttime warming could significantly promote treeline recruitment,
however, treeline recruitment was much more sensitive to nighttime warming
than to daytime warming, which could be attributable to the presence of
drought stress. The increasing drought stress primarily driven by daytime
warming rather than by nighttime warming would likely constrain the
responses of treeline recruitment to daytime warming. Our findings
provided compelling evidence that nighttime warming rather than daytime
warming could play a primary role in promoting the recruitment in alpine
treelines, which was related to the daytime warming-induced drought
stress. Thus, daytime and nighttime warming should be considered
separately to improve future projections of global change impacts across
alpine ecosystems.