<p id="d10156447e224">A famous American entomologist, Andrew D. Hopkins, estimated
in 1920 the progressive
delay in tree leaf-out with increasing latitude, longitude, and elevation, a phenomenon
referred to as “Hopkins’ bioclimatic law.” Here, based on massive ground observations
in the European Alps, we show that global warming has altered this law. In the early
1960s, the elevation-induced phenological shift (EPS) was approximately 34 days’ delay
for every 1,000-m increase in elevation, conforming to Hopkins’ bioclimatic law, whereas,
nowadays, this shift has reduced by 35% to 22 d⋅1,000 m
<sup>−1</sup>. Winter warming is likely to be responsible for this strong reduction
in the EPS
and future climate warming may strengthen this trend. Important consequences for the
functioning of mountain ecosystems are thus anticipated.
</p><p class="first" id="d10156447e230">One hundred years ago, Andrew D. Hopkins estimated
the progressive delay in tree leaf-out
with increasing latitude, longitude, and elevation, referred to as “Hopkins’ bioclimatic
law.” What if global warming is altering this well-known law? Here, based on ∼20,000
observations of the leaf-out date of four common temperate tree species located in
128 sites at various elevations in the European Alps, we found that the elevation-induced
phenological shift (EPS) has significantly declined from 34 d⋅1,000 m
<sup>−1</sup> conforming to Hopkins’ bioclimatic law in 1960, to 22 d⋅1,000 m
<sup>−1</sup> in 2016, i.e., −35%. The stronger phenological advance at higher elevations,
responsible
for the reduction in EPS, is most likely to be connected to stronger warming during
late spring as well as to warmer winter temperatures. Indeed, under similar spring
temperatures, we found that the EPS was substantially reduced in years when the previous
winter was warmer. Our results provide empirical evidence for a declining EPS over
the last six decades. Future climate warming may further reduce the EPS with consequences
for the structure and function of mountain forest ecosystems, in particular through
changes in plant–animal interactions, but the actual impact of such ongoing change
is today largely unknown.
</p>