| ABSTRACT: In this study, we investigated the response of trees growing at the cold margins of the boreal forest to climate variation in the 20th century. Working at eight sites at and near alpine and arctic treeline in three regions in Alaska, we compared tree growth (from measured tree ring-widths) to historical climate data to document how growth has responded to climate variation in the 20th century. We found that there was substantial regional variability in response to climate variation. Contrary to our expectations, we found that after 1950 warmer temperatures were associated with decreased tree growth in all but the wettest region, the Alaska Range. Although tree growth increased from 1900–1950 at almost all sites, significant declines in tree growth were common after 1950 in all but the Alaska Range sites. We also found that there was substantial variability in response to climate variation according to distance to treeline. Inverse growth responses to temperature were more common at sites below the forest margin than at sites at the forest margin. Together, these results suggest that inverse responses to temperature are widespread, affecting even the coldest parts of the boreal forest. Even in such close proximity to treeline, warm temperatures after 1950 have been associated with reduced tree growth. Growth declines were most common in the warmer and drier sites, and thus support the hypothesis that drought-stress may accompany increased warming in the boreal forest. |
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Figure 2. Growth trends of trees in three regions of Alaska.
The bars indicate the number of chronologies within which region that showed
significant positive trends in growth, significant negative trends in growth,
or no significant trend in growth from (a) 1901 to 1950 and
(b) 1950 to the end of the chronology (1997-1999).
Significance of growth trends was assessed using simple linear regression. (Click for Numerical Values, tab-delimited text or MS Word format). |
DATA:
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Download the tree ring data used in this study from the International Tree Ring Data Bank
at the WDC Paleo: | |||||||
| Site (download chronology or | measurements) | Lat. (° N) | Long. (° W) | Treeline Type | Azimuth | Tree Density | Elevation (m) |
| Alaska Range | |||||||
| Usibelli below treeline | (.rwl file) | 63° 54' | 148° 40' | Alpine | W | 542 ± 182 | 762 |
| Usibelli treeline | (.rwl file) | 63° 54' | 148° 40' | Alpine | W | 542 ± 182 | 762 |
| Canyon Creek | (.rwl file) | 63° 15' | 147° 47' | Alpine | NE | 422 ± 62 | 884 |
| Monahan Flats | (.rwl file) | 63º 16' | 147º 55' | Alpine | N | 201 ± 101 | 884 |
| Seward Peninsula | |||||||
| Fox River Bank | (.rwl file) | 64º 49' | 163º 42' | Arctic | NNE | N/A | 61 |
| Grasshopper Hill | (.rwl file) | 64° 55' | 163° 40' | Arctic | SE | N/A | 150 |
| White Mountains | |||||||
| Eagle Summit below treeline | (.rwl file) | 65° 30' | 145° 20' | Alpine | S | 225 ± 79 | 945 |
| Eagle Summit treeline | (.rwl file) | 65° 30' | 145° 20' | Alpine | S | 225 ± 79 | 945 |
| Twelvemile Summit below treeline | (.rwl file) | 65° 22' | 145° 56' | Alpine | NW | 93 ± 33 | 945 |
| Twelvemile Summit treeline | (.rwl file) | 65° 22' | 145° 56' | Alpine | NW | 93 ± 33 | 945 |
| Nome Creek below treeline | (.rwl file) | 65° 22' | 146° 37' | Alpine | SE | 175 ± 65 | 884 |
| Nome Creek treeline | (.rwl file) | 65° 22' | 146° 37' | Alpine | SE | 175 ± 65 | 884 |
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To read or view the full study, please visit the
Kluwer website. It was published in Climatic Change, Volume 52, 481-509, 2002. |
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Contact Us National Oceanic and Atmospheric Administration 23 July 2002
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