Introduction
Arctic and alpine areas are warming at faster rates than other areas,
partly because of decreased albedo after advanced snowmelt , and
feedback mechanisms related to the decline in Arctic sea-ice extent .
The Arctic tundra biome is divided into five bioclimatic subzones, which
are defined by summer temperatures and dominant plant growth forms in
each zone . The vertical tundra vegetation structure varies from a
discontinuous layer of very short plants in the coldest zone to
multi-layered, complex canopies in the warmest zone. As a consequence of
the rapid warming, these vegetation zones are expected to shift
northward and into higher elevations. Plant height has already increased
over the past thirty years throughout the tundra biome, largely caused
by species turnover due to immigration of taller species from warmer
areas . This is at least partially related to the increase in
productivity and abundance of deciduous shrub species . Such an increase
in canopy height, because of increased abundance of tall shrub species
may lead to an increase in competition for light and a decrease in dwarf
shrub abundance, particularly in the Low- and Sub-Arctic tundra .
However, the potential of deciduous shrubs to increase with summer
warming, likely depends on soil moisture availability . Deciduous shrub
growth has increased rapidly with recent warming in warmer wetter areas,
while it has declined in dry tundra areas in recent decades, possibly in
relation to diminishing sea-ice extent . In the cooler and drier High
Arctic, evergreen dwarf shrub growth has increased rapidly with recent
warming and leaf size and plant height of evergreen dwarf shrubs
increases to long-term experimental warming . However, it remains
unknown whether evergreen dwarf shrubs, which are generally better
adapted to drier conditions and are found on well-drained, relatively
dry soils , can respond with increased growth to the amplified warming
in the already warm Low Arctic.
The satellite-based normalized difference vegetation index (NDVI) is
generally regarded as a proxy for biomass and productivity of
terrestrial vegetation. NDVI analyses have revealed long-term greening
trends over large parts of the tundra biome in the northern hemisphere
in recent decades . Nonetheless, a significant percentage of Arctic
ecosystems appears to be stable and the mechanisms behind this stability
are poorly understood . Interannual variation in NDVI in the Siberian
Arctic and Arctic-alpine northwest North America have been related to
interannual variation in shrub growth and summer temperatures. This
suggests a possible link between summer warming-induced Arctic greening
and shrub growth. Although many tundra areas have experienced greening
over the past decades, other regions have experienced browning events in
recent times . Such browning events have been linked to the occurrence
of extreme warm spells in winter, which result in snowmelt, often
accompanied by rain-on-snow events . These may result in the encasement
of plants in ground-ice or leave plants exposed to subsequent periods of
severe frost without insulation by a sufficiently deep snow layer .
Extreme winter warming and rain-on-snow events, the frequency of which
is likely to increase in the future , can damage vegetation and lower
its productivity in the subsequent summer . Single events have been
linked to branch mortality of several shrub species, includingCassiope tetragona . Still, shrub branch-mortality and
branch-initiation frequency time-series and possible links with climate
and NDVI have not been studied thus far.
Here, the climate-growth relationships in the evergreen dwarf shrubCassiope tetragona are studied at a Low-Arctic site in western
Greenland. The species is found throughout the Arctic, but is most
prevalent in the High Arctic . The following research questions were
addressed:
- What are the main seasonal climate drivers of C. tetragonashrub growth at the site and individual shrub level and are these
stable over time?
- Are there differences in individual shrub growth responses to climate
and are these related to micro-topographic positions?
- Is annual variation in branch initiation and mortality frequency
related to variation in seasonal climate variables?
- Are area-wide, satellite-observed vegetation productivity, C.
tetragona growth, branch initiation and mortality frequencies, and
seasonal climate factors inter-related?