Arctic shrubs are the most widespread woody plants in the Far North and, like trees in lower latitudes, they form annual growth rings. These rings can be measured under a microscope to reveal the history of the past climates but also growth responses to the recent climate change that is vividly present in the Arctic. A new study shows how currently ongoing sea ice decline interacts with Arctic shrubs.

Arctic sea ice has been in steep decline over the last
two decades. Meanwhile, tundra shrub abundance has been increasing in many regions
of the Arctic. A new study published in the Proceedings of the
National Academy of Sciences of the United States of America, reveals that declining
Arctic sea ice extent has been associated with increasing shrub growth in some
regions of the Arctic and decreasing growth in other, generally drier,
regions.

An international team from eight countries was led by
Dr. Agata Buchwal from Adam Mickiewicz University in
Poznan, Poland. Research professor Bruce Forbes from
the Arctic Centre at the University of Lapland was a member of the group. Team
gathered 23 existing shrub-ring chronologies and investigated their relationship to
changes in sea ice extent, air temperature and precipitation. Their dataset included
birches and willows from Alaska, the Canadian Arctic, Greenland, Svalbard and
Siberia.

– Our synthesis represents a major collaborative effort
to disentangle one of the most complex issues in climate change research:
heterogeneity of tundra shrub dynamics across the Arctic, says Buchwal who launched
the synthesis during her Fulbright scholarship at the University of Alaska Anchorage
in 2015.  

While Arctic tundra
greening and browning have received increasing attention over the past decade, one
comparatively understudied area is the ultimate role of sea ice dynamics and decline
as drivers of terrestrial vegetation change. Sea ice decline is emerging as a
powerful driver of warming and associated precipitation changes across the Arctic,
and tundra shrubs are recognized as climatically sensitive proxies of Arctic
environmental change. However, sea ice – shrub interactions had not been studied at
the Pan-Arctic scale. 

– The comprehensive assessment
reported in our synthesis not only addresses that knowledge gap, but also, more
importantly, documents contrasting influences of declining sea ice on summer climate
and shrub growth at the biome scale, says Buchwal.

While the
majority of shrubs take advantage of warming induced by sea ice decline and increase
their growth, there is a remarkable group of shrubs that have progressively
decreased their growth during the period of sea ice decline. What drives these
divergent shrub growth responses to declining sea ice extent? Buchwal and team have
shown that regional changes in sea ice extent are highly coupled with changes in
local temperature and moisture availability. Specifically, sites with shrubs that
grew less with the declining sea ice extent were characterized by increasingly drier
conditions that hampered their growth. 

The team led by
Arctic Centre has been collecting shrub data in West Siberia since
2005. 

– We have now a chronology extending back more
than 100 years. Between 2011–2014 we needed high rubber boots to access mires and
creeks with tall willows on Yamal Peninsula. However, by summer 2017 these same
wetlands and waterways had dried out to the point that I was able to cross them in
sneakers without getting my feet wet. Nenets reindeer herders comments that they had
never seen the landscape so dry in their lifetimes, says Prof.
Forbes. 

Why we should care about the tundra shrubs? The
implications of increasing heterogeneity in shrub growth responses to sea
ice-induced changes in climate might be widespread, with local to global
consequences, including carbon uptake potential and albedo effects. While tundra
areas dominated by increasers have the potential to take up and store more carbon
from the atmosphere, areas dominated by decreasers might be areas of increasing
carbon loss to the atmosphere. 

– Tundra shrubs will not
announce the effects of climate change in the Arctic. Instead they patiently record
their responses to change in their growth rings. And it is our task to learn from
their records, says Buchwal.  

Publication online:
Divergence of Arctic shrub growth associated with sea
ice decline
Proceedings of the National Academy of Sciences of
the United States of America

More
information:

Agata Buchwal, 
Adam Mickiewicz University in Poznan
kamzik@amu.edu.pl
Twitter: @AgataBuchwal

Bruce Forbes
Research professor
Arctic Centre, University of Lapland
bruce.forbes(at)ulapland.fi
040 847 9202