Tree Rings as Temperature Proxies

Tree-ring series provide the most widely distributed and easily accessible archive of annually resolved proxy climate data. In regions with well-defined seasonal growth the annual growth rings of trees provide both chronological control and a continuous time series of proxy environmental variables. The year to year variability of the physical (e.g. width, density) and chemical properties of these annual rings provides potential proxies for the environmental factors that influence tree growth. Over the last 30 years dendroclimatology has become a major tool in the reconstruction of climates of the last millennium in many areas of the world (see Hughes, 2002 for a comprehensive review). The process of matching patterns of tree-ring width between series (crossdating) is fundamental to dendrochronology. It is used to verify the dating of tree-ring series and, in some cases, to demonstrate that the series are annual. For crossdating to be successful, the ring parameters must vary synchronously between series reflecting a common response to an external control. Although tree-growth is influenced by many factors, often only one is strongly and consistently limiting. Successful dendroclimate studies target sampling sites where a single climate parameter is, or is expected to be, the limiting factor to growth and therefore the primary control of interannual variability in ringwidth. The presence and strength of the common signal between tree-ring series within a site is a measure of the degree of climate control of growth: correlation between tree-ring series from different sites across a broad area almost inevitably reflects a common climate control as no other controlling factors vary systematically across these spatial scales. Trees growing at their range limits are often the most sensitive to climate variability whereas trees in mid-range or interior forest sites may show little interannual variability or be dominated by local signals related to forest dynamics. Classically, open-grown latitudinal and altitudinal treeline sites are most sensitive to temperature variations whereas lower treeline and/or forest border sites are moisture sensitive. However, growth at many treeline sites in arid mountains is sensitive to both precipitation and temperatures (e.g. Bristlecone pines). The key to dendroclimate research is careful site selection as the limiting factor to growth can also vary based on microsite considerations. Dendroclimate studies further assume that the growth-limiting factor at a site does not change over time and therefore contemporary tree-ring climate relationships can be used to reconstruct past climate conditions. Traditionally ring width (RW) is the primary variable utilized for dendroclimate work but more recently treering densitometry has also been used. In densitometry, precise thin sections are cut from cores, x-rayed and negatives scanned to produce a continuous, high resolution density trace orthogonal to the rings. Several ringwidth and density variables can be obtained from these scans. Maximum latewood density (MXD) reflects the thickening of cell walls at the end of the growing season and represents a narrower climatic window than more traditional ringwidth parameters. (Schweingruber, 1988). MXD in northern conifers is very strongly related to summer temperatures and has been used in several important recent temperature reconstructions. In recent years isotopic data have also been used to infer temperature but, at present, such data are not available in sufficient quantity for large scale temperature reconstructions.