Increasing NDVI values in northern Alaska: studies that mix shrub density, spectral and CO2 exchange measurements

Delineating the mechanisms and consequences of changes in tundra landscapes is central to predicting the functional ecology of Alaska in the 21st Century. Evidence has been mounting during the last decade that shrub communities are expanding in the Arctic and Normalized Difference Vegetation Index (NDVI) values which measure surface greenness are rising. Several studies have suggested that NDVI increases are being driven by increases in shrub abundance. While it is clear that NDVI has increased across vegetation types, it is not clear that NDVI values are increasing in moist acidic tundra (MAT), the most extensive vegetation type in arctic Alaska and the one most likely to be changed by global warming. The MAT is important to large mammal herbivores such as caribou which provide subsistence for indigenous people. The focus of this research was to determine what rising NDVI values actually mean in the MAT. The degree to which tundra community composition affects NDVI is still very poorly understood. In order to clarify the role of shrub encroachment per se as opposed to other functional groups in driving increases in NDVI, we measured functional group composition in moist acidic tundra in conjunction with hand-held measures of NDVI and direct CO2 exchange measurements to explicitly link spectral properties, shrub, graminoid and bryophyte density and trace gas feedbacks to atmospheric chemistry. Point frame data shows a shrub coverage of Betula nana (Dwarf Birch) and Salix pulchra (Diamond Leaf Willow) combined of 5% to 35% in MAT. Our results seem to indicate that high shrub density (>30%) corresponds to peak season NDVI values greater than .75 whereas low shrub density correspond to values below .65 (R2=.66). Furthermore, NDVI is closely correlated with canopy leaf area and greater leaf area is associated with higher rates of gross and net ecosystem CO2 uptake.