Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance

Abstract

We analyzed trends in a time series of photosynthetic activity across boreal North America over 22 years (1981 through 2003). Nearly 15% of the region displayed significant trends, of which just over half involved temperature-related increases in growing season length and photosynthetic intensity, mostly in tundra. In contrast, forest areas unaffected by fire during the study period declined in photosynthetic activity and showed no systematic change in growing season length. Stochastic changes across the time series were predominantly associated with a frequent and increasing fire disturbance regime. These trends have implications for the direction of feedbacks to the climate system and emphasize the importance of longer term synoptic observations of arctic and boreal biomes.climate change ͉ environmental change ͉ remote sensing ͉ trend analysis

Temperature and Snow-Mediated Moisture Controls of Summer Photosynthetic Activity in Northern Terrestrial Ecosystems between 1982 and 2011

“…However, the timing, magnitude and relative importance of these climate controls varies substantially between and within biomes, consistent with differences in climate responses between plant functional types observed in earlier studies [15,31]. Simple correlations suggest that summer temperature is the dominant control of summer anomalies of photosynthetic activity in the cool boreal and Arctic regions, whereas spring and summer soil moisture availability is the main control of summer photosynthetic activity across the continental interiors in the northern temperate grasslands and to a lesser extent in croplands ( Figure 5).…”

“…Earlier studies have reported an apparent shift towards increased moisture limitation of vegetation productivity with continued warming since the the early 1990s across much of the temperate and boreal ecosystems [4,15,21]. Our results indicate that the interannual association between climate (temperature, moisture and snow) and NDVI variability has not changed over time ( Figure 7) and thus there is no evidence for consistent regional-scale temporal shifts from temperature to moisture limitation with recent warming on a year-to-year basis and at the grid box level.…”

“…The most extensive vegetation browning has occurred in the North American boreal forests in Canada and Alaska [22]. Most of these regional reductions in summer NDVI have been attributed to increasing drought stress with hotter summers, an extended period of evaporation and enhanced atmospheric evapotranspiration demand associated with rising temperatures [15,16,21,22]. This effect is referred to as "temperature-induced drought stress" [16,23].…”

“…The most recent studies have found that after the initial increasing trend in NDVI (i.e., greening) observed in the first decade of the record [5,20], longer and hotter growing seasons appear to have resulted in reductions of summer NDVI (i.e., browning) in the drier and more continental regions of the boreal forest since the 1990s [15][16][17]21]. The most extensive vegetation browning has occurred in the North American boreal forests in Canada and Alaska [22].…”

“…The analysis of the global satellite record of Normalized Difference Vegetation Index (NDVI) imagery since 1981 (e.g., [14]) has shown that increases in vegetation photosynthetic activity have been stronger in Eurasia than in North America and more sustained in the colder tundra biome than over the boreal forests [15][16][17][18][19]. The most recent studies have found that after the initial increasing trend in NDVI (i.e., greening) observed in the first decade of the record [5,20], longer and hotter growing seasons appear to have resulted in reductions of summer NDVI (i.e., browning) in the drier and more continental regions of the boreal forest since the 1990s [15][16][17]21].…”