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:
  1. What are the main seasonal climate drivers of C. tetragonashrub growth at the site and individual shrub level and are these stable over time?
  2. Are there differences in individual shrub growth responses to climate and are these related to micro-topographic positions?
  3. Is annual variation in branch initiation and mortality frequency related to variation in seasonal climate variables?
  4. Are area-wide, satellite-observed vegetation productivity, C. tetragona growth, branch initiation and mortality frequencies, and seasonal climate factors inter-related?