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Tree Ring Data for Climate Reconstruction
and Indication of Shifting Climate Regimes

by Valerie Barber, Institute of Marine Science

Climate models predict that northern high latitudes will see an amplification of global warming from the greenhouse effect. Boreal forest ecosystems are sensitive to climatic and environmental shifts and may provide evidence of this warming; changes in the frequency of disturbance regimes in Alaskan boreal forests may already be in evidence. One of the focuses of my research is to investigate high-resolution (annual to decadal-scale) changes over the past several hundred years, a time frame highly relevant to the human perspective. Surprisingly, few studies have focused on this time period in Alaska. Two promising sources of this type of information are sediment cores from lakes with fast sedimentation rates, and tree rings. Recent work on finely spaced samples from cores collected in lakes of the PALE (Paleoclimates from Arctic Lakes and Estuaries) study indicates significant recent changes in the climate of interior Alaska. Conditions may have been much harsher in the early 1800s to mid-1800s. Tree ring studies offer a way of reconstructing climate prior to recorded meteorological data, which go back about 90 years in interior Alaska. Prior tree ring studies in interior Alaska have not focused on multiparameter climate reconstructions with annual resolution. My work focuses on utilizing multiproxy information from tree rings to reconstruct past changes in temperature and precipitation for interior Alaska over the past several hundred years and to look at how the general health of the forest stands have changed over time. In order to do this, the ring data must first be calibrated with the recorded meteorological data, which is the part of this study reported here.

The three different parameters of the annual tree rings (width, density, and ð13C isotope concentration) measured in this study produce distinctly different climatic information. Ring width is the parameter most commonly studied. Large, thin-walled cells are laid down early in the growing season when conditions are favorable for growth. These are followed by smaller, thick-walled and denser cells late in the growing season which form due to the onset of cooler temperatures, lack of soil moisture, and shorter days. The production of latewood cells terminates abruptly, followed by larger cells the following growing season. This abrupt termination at the end of one year and the beginning of the next year marks a ring boundary. Ring width is measured between two successive ring boundaries. In previous Alaskan studies, ring width has correlated with annual temperature.

Cell density is measured across the length and width of the tree ring, and there is intra-annual as well as inter-annual variation. Maximum density is the parameter that is typically measured and is usually correlated with summer temperatures here in Alaska. Wood density may be affected by factors other than those which affect tree ring width and may be more sensitive to environmental changes.

Stable carbon isotope ratio (ð13C) is correlated with moisture availability. This occurs because stomates remain open for longer periods when moisture is available, allowing for greater exchange of CO2 and selection of lighter carbon. Thus under moisture-limiting conditions or arid climates, reduced CO2 availability results in less discrimination against the heavier isotope and higher ð13C values.

The combination of width, density, and isotope analysis offers a powerful set of independent but mutually reinforcing tools for reconstructing climate because the effects of temperature and precipitation can be distinguished, and seasonal changes can be observed. A better understanding of how the boreal forests are affected by different climate parameters is necessary to predict response to future climatic changes.

The results from the calibration part of this study (1909-1981) show maximum latewood density is significantly cor- related with May and August temperature (0.557 and 0.691) and August precipitation (-0.464). Normalized May and August temperature is correlated with density (0.791, 0.912 with 5-year running mean). Wood ð13C ratios are negatively correlated with growth year precipitation (-0.488, -0.656 smoothed) and positively correlated (0.610, 0.857 smoothed) with May-August temperature. A combined index of normalized May-August temperature and yearly precipitation correlated with ð13C at 0.710 (0.844 smoothed). The results of this calibration indicate that there is significant climatically driven stress on the trees from this stand when summer temperatures are high and annual precipitation is low. This combination of circumstances has occurred more frequently since the mid-1970s than at any other time during the twentieth century.

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