Abstract
- Evergreen dwarf shrubs respond swiftly to warming in the cool and dry
High Arctic, but their response in the warmer Low Arctic, where they
are expected to be outcompeted by taller species under future warming,
remains to be clarified.
- Here, 12,528 annual growth increments, covering 122 years (1893-2014),
were measured of 764 branches from 25 individuals of the evergreen
dwarf shrub Cassiope tetragona from a Low-Arctic erect
dwarf-shrub tundra site in western Greenland. In addition, branch
initiation and mortality frequency time-series were developed. The
influence of seasonal climate and correspondence with fluctuations in
regional normalized difference vegetation index (NDVI), a
satellite-proxy for vegetation productivity, were studied.
- Winter temperatures were an important co-driver, summer temperatures
the main driver of growth. During past and recent warm episodes, shrub
growth diverged from summer temperatures. In recent decades, early
summer precipitation has become the main growth-limiting factor for
some individuals, likely through micro-topography-determined soil
moisture availability, and more than half of the shrubs studied became
irresponsive to summer temperatures. There was correspondence between
climatic drivers, C. tetragona growth and branch initiation
frequency, and satellite-observed vegetation productivity, suggesting
the area’s shrub-dominated tundra vegetation is limited by similar
climatic factors. Winter warming events were likely the predominant
cause of branch mortality, while branching increased after years with
poor growth and cooler-than-average summers.
- Synthesis These findings show that the erect dwarf-shrub tundra
in the Low Arctic has and will likely become less temperature- and
increasingly moisture-limited and that winter warming supports shrub
growth, but increased extreme winter warming event-frequency may
increase branch mortality and vegetation damage. Such counter-acting
mechanisms could offer an explanation for the vegetation stability
observed over large parts of the Arctic.