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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

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Great Basin bristlecone pine (Pinus longaeva) at 3 sites in western North America near the upper elevation limit of tree growth showed ring growth in the second half of the 20th century that was greater than during any other 50-year period in the last 3,700 years. The accelerated growth is suggestive of an environmental change unprecedented in millennia. The high growth is not overestimated because of standardization techniques, and it is unlikely that it is a result of a change in tree growth form or that it is predominantly caused by CO2 fertilization. The growth surge has occurred only in a limited elevational band within Ϸ150 m of upper treeline, regardless of treeline elevation. Both an independent proxy record of temperature and high-elevation meteorological temperature data are positively and significantly correlated with upper-treeline ring width both before and during the high-growth interval. Increasing temperature at high elevations is likely a prominent factor in the modern unprecedented level of growth for Pinus longaeva at these sites.climate change ͉ dendrochronology ͉ Great Basin ͉ tree rings ͉ treeline B ackground. Bristlecone pine (Pinus longaeva) is notable for its individual trees that attain great age, for its use in the calibration of the radiocarbon timescale, and for its role in providing an element in millennial-scale multiproxy reconstructions of temperature. The ring-width chronologies from long-lived bristlecone pine are annually resolved and can reach back thousands of years, making these high-resolution multimillennial proxy records of climate a rare and valuable resource in paleoclimatology. Uppertreeline bristlecone pine site locations are cold for much of the year and can be extremely dry during the summer growing season. As a result, these high-elevation tree-ring series contain some information on moisture availability, but they also bear an important imprint of temperature variability, so that both types of signal may be present in records from the upper treeline (1-7). There are interannual responses to precipitation variations at all elevations, including some degree of high-frequency variability related to extreme drought conditions at the upper treeline (8), although the variability related to precipitation is more pronounced at lower elevations (1, 9). Conversely, the main decadal to multidecadal ring-width variability at treeline locations may be related more closely to temperature than to precipitation (10). Despite the challenges in using these natural archives of climate successfully, we argue that it is worthwhile to make considerable effort to achieve the best possible use of this concentration of long annual records.

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We examined relationships between tree ring-width and climate at 232 sites around the circumpolar boreal forest to explore variability in two types of response to temperature: a browning response characterized by inverse correlations between growth and temperature, and a greening response characterized by positive correlations between growth and temperature. We used moving-window correlation analysis for eight 30-year time windows, lagged by 10 years, to characterize the climate response at each site from 1902 to 2002. Inverse growth responses to temperature were widespread, occurring in all species, all time periods, and in nearly all geographic areas. The frequency of the browning response increased after 1942, while the frequency of the greening response declined. Browning was concentrated in five species (Picea abies, Picea glauca, Picea mariana, Picea obovata and Pinus banksiana), and occurred more frequently in the warmer parts of species' ranges, suggesting that direct temperature stress might be a factor. In some species, dry sites were also more likely to experience browning; moisture stress might thus be an additional explanation in some cases. As inverse responses to temperature are widespread, and occur in a broad array of species, there is unlikely to be any single explanation for their occurrence.

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