International Polar Year 2007–2008 - WMO

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IPY 20 07–20 08
line zones have been documented across the PPS
study sites; still, the advancing zones are dominant.
Several IPY projects showed major changes in
ecosystems within particular environments. In the
Yukon North Slope, one of the northern regions of
Canada where climate warming has been most rapid,
ArcticWOLVES project studies detected changes
in the abundance of many species. Abundance
of savannah sparrows and peregrine falcons has
increased, but abundance of Baird’s sandpipers and
gyrfalcons has decreased. Many rough-legged hawk
and peregrine falcon nests are now failing as mud
cliffs collapse due to increased rates of permafrost
melt. The new northern occurrences of at least five
species of butterflies were confirmed. Advancement
in the onset of laying for many avian species was also
detected. These observations add significantly to the
recent review of species range and ecosystem process
changes (Post et al., 2009). In sub-Arctic Sweden, the
BTF team recorded changes in the location of the tree
line and also changes in the structure of the tree line in
that aspen trees had recently replaced mountain birch
in many areas (Van Bogaert et al., 2010b).
Both ArcticWOLVES and “Back to the Future” projects
found that climate change acted as a driver of change
directly and indirectly through complex interactions
among species. ArcticWOLVES studies demonstrated
conclusively that predation played a dominant role in
the structuring and function of arctic ecosystems and
that many animal populations are strongly impacted,
and sometimes driven, by predator-prey interactions.
In parallel, “Back to the Future” studies showed that
the interaction between two sub-Arctic tree species
(mountain birch and aspen) was driven largely by
an invertebrate herbivore of one that responded to
climate, and the moose herbivore of the other species
(Van Bogaert et al., 2010). Similar conclusions have
been reached in a recent review of changes in arctic
ecosystems (Post et al., 2009).
It has been known for some time that changes
in ecosystems can be sudden, even catastrophic,
in contrast to ongoing gradual changes. Examples
are forest fires and rain on snow events that have
decimated ungulate populations. During extreme
winter warming events, temperatures increase rapidly
to well above freezing (e.g. a change from -20°C to
+5/+10°C in 24 hours) and may remain so for a week-
long period. Such warming events can result in near
complete snow thaw across large regions. Return
of freezing temperatures can also be rapid, leaving
ecosystems, unprotected due to a lack of snow cover,
exposed to extreme cold. Exposure to extreme cold
can damage vegetation either directly (through
freezing or winter desiccation) or indirectly through
ice encasement by re-freezing of melted snow. These
events are of considerable concern for indigenous
reindeer herders in the sub-Arctic as winter warming
events may cause harsh grazing conditions, limit food
supply and, consequently, incur large economic costs
through the necessity for additional feeding. However,
ecosystem response to extreme winter warming
events has received little attention.
Simulation of such events within the IPY project
ENVISNAR at the Abisko Scientific Research Station
using infrared heating lamps and soil warming
cables has revealed that (especially) evergreen dwarf
shrubs show large delays in phenology, reproduction
and even extensive shoot mortality in response
to extreme winter warming (Bokhorst et al., 2008).
Physiological measurements taken during the
simulations have demonstrated that plants will initiate
spring-like development after only three to four days
of exposure to ~5°C. This breaks winter dormancy/
winter hardening and leaves the plants vulnerable to
the returning cold following the warming event. Such
findings from the simulation study have recently been
supported by consistent evidence from a naturally
occurring extreme winter warming event that
occurred in northwestern Scandinavia in December
2007 (Bokhorst et al., 2009). During the following
summer extensive shrub mortality was observed.
Vegetation “health”, assessed through remote
sensing, showed a 26% reduction in NDVI across 1400
km 2 compared to the previous year (Fig. 2.9-6). This
reduction indicates a significant decline in either leaf
area or photosynthetic capacity at the landscape scale
(as illustrated by the IPY GOA and ABACUS projects).
These impacts of extreme winter warming are in
sharp contrast to the observed greening of the Arctic
through shrub expansion considered to be caused by
summer warming in other regions.
Overall, the full potential impacts of increased frequency
of extreme winter warming events on Arctic
ecosystems could be considerable in terms of ecosystem