Warming acts through earlier snowmelt to advance but not extend alpine community flowering

Large-scale warming will alter multiple local climate factors in alpine
tundra, yet very few experimental studies examine the combined yet
distinct influences of earlier snowmelt, higher temperatures and altered
soil moisture on alpine ecosystems. This limits our ability to predict
responses to climate change by plant species and communities. To address
this gap, we used infrared heaters and manual watering in a fully
factorial experiment to determine the relative importance of these climate
factors on plant flowering phenology, and response differences among plant
functional groups. Heating advanced snowmelt and flower initiation, but
exposed plants to colder early-spring conditions in the period prior to
first flower, indicating that snowmelt timing, not temperature, advances
flowering initiation in the alpine community. Flowering duration was
largely conserved; heating did not extend average species flowering into
the latter part of the growing season but instead flowering was completed
earlier in heated plots. Although passive warming experiments have
resulted in warming-induced soil drying suggested to advance flower
senescence, supplemental water did not counteract the average species
advance in flowering senescence caused by heating or extend flowering in
unheated plots, and variation in soil moisture had inconsistent effects on
flowering periods. Functional groups differed in sensitivity to earlier
snowmelt, with flower initiation most advanced for early-season species
and flowering duration lengthened only for graminoids and forbs. We
conclude that earlier snowmelt, driven by increased radiative heating, is
the most important factor altering alpine flowering phenology. Studies
that only manipulate summer temperature will err in estimating the
sensitivity of alpine flowering phenology to large-scale warming. The
wholesale advance in flowering phenology with earlier snowmelt suggests
that alpine communities will track warming, but only alpine forbs and
graminoids appear able to take advantage of an extended snow-free
season.