Short term changes in moisture content drive strong changes in Normalized Difference Vegetation Index and gross primary productivity in four Arctic moss communities

Climate change is currently altering temperature and precipitation totals and timing in Arctic regions. Moss communities constitute much of the understory in Arctic vegetation, and as poikilohydric plants moss are highly sensitive to timing and duration of moisture levels. Here we investigate the role of moisture content on NDVI, red and near-infrared reflectance, and gross primary productivity (GPP) of two sphagnum and two pleurocarpus moss community types during two separate drying experiments. For both experiments, blocks of moss were collected near Imnavait Creek, Alaska, saturated to full water capacity, and then allowed to air dry before being re-saturated. Drying of blocks was conducted in a translucent outdoor tent during the first experiment and under indoor climate-controlled conditions during the second. Community NDVI (experiment 1 and 2), and GPP (experiment 2) were measured at regular intervals during the dry-down and after rewetting. In both experiments, moss NDVI sharply declined between 80% and 70% moisture content for sphagnum moss communities (NDVI change = -0.17 to -0.2), but less so for pleurocarpus moss communities (NDVI change = -0.06 to -0.12). Changes in NDVI were largely the result of increases in reflectance in red wavelengths. Peak GPP for all community types in the second experiment (1.31 to 2.08 mu mo] m(-2)s(-1)) occurred at 80% moisture content and declined significantly as moisture content decreased. Rates of GPP continued to decline below 80% moisture content until near zero as moss reached a steady weight (air dry) over a period of 84 h, while NDVI values declined slowly between 70% hydration and fully air dry. Re-saturation caused NDVI to increase in both sphagnum (NDVI change = + 0.18 to + 0.23) and pleurocarpus (NDVI change = + 0.10 to + 0.17) communities. Only sphagnum communities showed GPP resuming (0.824 mu mol m(-2)s(-1)) after 24 h. The strong changes in NDVI and mismatch of moss NDVI values and GPP with moisture content fluctuations indicate that using NDVI as a proxy for productivity in Arctic vegetation communities may be problematic and underscores the need for quantification of moss community coverage, composition, and moisture content.