Declining temperature and increasing moisture sensitivity of shrub
growth in the Low-Arctic erect dwarf-shrub tundra of western Greenland
Abstract
1. 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. 2. 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. 3. 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. 4. 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.