Recent (1986-2006) Vegetation-Specific NDVI Trends in Northern Canada from Satellite Data
Keywords: northern, vegetation, community, lichen, vascular, remote sensing, Landsat, AVHRR, NDVI, trends
AbstractRecent northern vegetation changes caused by climate warming have been well documented, using experimental plot warming to examine vegetation-specific changes and satellite image data to examine overall trends. Previous remote sensing efforts have employed the Normalized Difference Vegetation Index (NDVI) from AVHRR, whose 1 km to 8 km pixel size is too large for examination of broad scale vegetation-specific responses because of mixing within the pixel footprint. In this paper, we reconcile differences between field- and remote sensing-based approaches by using both medium-resolution (30 m) and coarse resolution (1 km) data to study 20 years of vegetation-specific responses to northern climate warming (1986 to 2006). Trends are compared among vegetation communities from two separate Landsat classifications in Canada’s eastern and western forest-tundra transition zone, as well as a 1 km AVHRR database recently developed over Canada. A comparison of absolute trends among mapped vegetation communities revealed lichen-dominated communities consistently exhibiting lower trends than those dominated by vascular plants, with both exhibiting increasing NDVI. Our results and those obtained from experimental warming suggest that the magnitude difference in NDVI increase between lichen and vascular vegetation is related to increasing vigor and biomass of vascular vegetation, in contrast to physiological impairment of lichen due to the short-term secondary effect of temperature on moisture. In the longer term, succession from lichen to vascular is likely responsible for the small observed NDVI increase over lichen-dominated regions. The fact that both Landsat and AVHRR exhibited similar relative vegetation-specific trends in NDVI suggests that this phenomenon may be widespread in the North.