Woods Hole Research Center

Vegetation response to climate warming in high latitudes has typically been viewed as one in which the vegetation growth, or greenness, increases under the influence of increased CO₂ concentrations in the atmosphere and associated extension of the growing season. The photosynthetic gains conferred by the modified climate offset respiratory losses associated with higher nighttime temperatures. Indeed, satellite observations of high latitude boreal forests indicated that this was the case – vegetation “greening” occurred between 1982 and 1991. Analysis by Woods Hole Research Center scientists suggests, however, that this trend has not continued uniformly in time or space. Rather, findings indicate that while tundra vegetation experienced an increase in both peak photosynthesis and growing season length between 1982 and 2008, many forested areas actually experienced a decline in photosynthetic activity and showed no systematic change in growing season length over the same period. Climatic warming occurred across the entire region, but the change in the forest response indicates that long-term changes may not be predictable from initial, short-term observations. Fire disturbance has also increased with the warming but does not explain the decline in forest photosynthetic activity.

Satellite-derived map of photosynthetic trends across the circumpolar region from 1982 through 2008. The colors correspond to the magnitude of the productivity trend over time. The average of growing season values are identified as significantly negative (browns/reds) or significantly positive (greens).

These results were the first showing that high latitude forests may be in decline due to warming and drying, which is consistent with climate model predictions for the region. Declining productivity trends have implications for the direction of feedbacks to the climate system. For example, the scientific community thought that increased greening might provide a negative feedback on CO₂ concentrations, reducing CO₂ via increased photosynthesis, which would moderate climate warming. WHRC results suggest that this may not be the case in boreal North America. Rather, forests appear to be in decline. These results emphasize the importance of longer-term synoptic observations of arctic and boreal biomes. The results are important because they describe how large areas of forest are changing and how those changes are related to climate. The results are increasingly supported by a variety of field studies, including tree rings that show systematic changes in vegetation in response to climate variability.

Growing season (June, July and August) photosynthetic activity from tundra and forest areas compared to temperature observations. The satellite data extend from 1982 to 2003 and the temperature data from 1982 through 2000. The dotted lines are statistical fits to the data. Note the close correspondence of increasing temperature and photosynthetic activity in tundra areas, and how they diverge in forest areas.