Trends in Satellite-Observed Circumpolar Photosynthetic Activity from 1982 to 2003: The Influence of Seasonality, Cover Type, and Vegetation Density

Abstract

Time series analyses of a 22-yr record of satellite observations across the northern circumpolar high latitudes were conducted, and trends in vegetation photosynthetic activity were assessed using a series of statistical tests. The results indicate that most of the northern circumpolar high latitudes (>85%) showed no significant trend in vegetation activity despite systematic climate warming during the period of analysis. Of the areas that did change, many showed the expected trends in ?greening? of vegetation activity. There were, however, significant differences in the magnitude and even in the direction of trends when stratified by vegetation type and density. Tundra areas consistently and predominantly showed greening trends. Forested areas showed declines in activity (?browning?) in many areas, and these were systematically higher in areas with denser tree cover?whether deciduous or evergreen, needle- or broad-leafed. The seasonality of the trends was also distinct between vegetation types, with a divergence in trends between late spring and early summer (positive) versus late summer (negative) portions of the growing seasons in forested areas. In contrast, tundra and other predominantly herbaceous areas showed positive trends in all portions of the growing season. These results confirm recent findings across the high latitudes of North America and are supported by an increasing array of in situ measurements. They indicate that the boreal forest biome might be responding to climate change in previously unexpected ways, and point to a need for an expanded observational network, additional analysis of existing datasets (e.g., tree rings), and improvements in process models of ecosystem responses to climate change.