Principles of Dendrochronology

as compiled by Henri Grissino-Mayer of the Laboratory of Tree-Ring Research at the University of Arizona, Tucson. Be sure to check out Henri's The Ultimate Tree-Ring page a great source for those interested in tree-ring data.

The Uniformitarian Principle

states that physical and biological processes that link current environmental processes with current patterns of tree growth must have been in operation in the past (after Fritts 1976). In other words, "the present is the key to the past," originally stated by James Hutton in 1785. However, dendrochronology adds a new "twist" to this principle: "the past is the key to the future." In other words, by knowing environmental conditions that operated in the past (by analyzing such conditions in tree rings), we can better predict and/or manage such environmental conditions in the future.

The Principle of Limiting Factors

states that rates of plant processes can occur only as fast as allowed by the factor that is most limiting. For example, if rainfall is the most limiting factor, then the amount of wood produced by a tree in any single year will reflect mostly the amount of rainfall that fell within that year.

The Principle of Aggregate Tree Growth

states that any individual tree-growth series can be "decomposed" into an aggregate of environmental factors, both human and natural, that affected the patterns of tree growth over time. For example, tree-ring growth in any one year is a function of an aggregate of factors:

the age related growth trend due to normal physiological aging processes.
the climate that occurred during that year the occurrence of factors within the forest stand (for example, a blow down of trees).
the occurrence of factors from outside the forest stand (for example, an insect outbreak that defoliates the trees, causing growth reduction).
random (error) processes not accounted for by these other processes.

Therefore, to maximize the desired environmental signal being studied, the other factors should be minimized. For example, to maximize the climate signal, the age related trend should be removed, and trees and sites selected to minimize the possibility of internal and external ecological processes affecting tree growth. Simple, right?

The Principle of Ecological Amplitude

states that species may grow, reproduce, and propagate across wide, narrow, or restricted ranges of habitats. For example, ponderosa pine (Pinus ponderosa) is the most widely distributed of all pine species in North America, growing in a diverse range of habitats (wet, dry, low elevation, and high elevation). Therefore, ponderosa pine has a wide ecological amplitude. Conversely, giant sequoia trees (Sequoiadendron giganteum) grow in restricted areas on the western slopes of the Sierra Nevada of California. Giant sequoia, therefore, has a narrow ecological amplitude. This principle is important because tree species useful to dendrochronology are often found near the margins of their natural range, such as white spruce trees (Picea glauca) near the upper latitudinal treeline.

The Principle of Site Selection

states that sites useful to dendrochronology can be identified and selected based on criteria that will produce tree-ring series sensitive to the environmental variable being examined. For example, trees that are especially responsive to drought conditions can usually be found where rainfall is limiting, such as rocky outcrops, or on ridgecrests of mountains. Therefore, a dendrochronologist interested in past drought conditions would purposely sample trees growing in locations known to be water-limited. Sampling trees growing in low-elevation, mesic (wet) sites would not produce tree-ring series especially sensitive to rainfall deficits. The dendrochronologist must select sites that will maximize the environmental signal being investigated.

The Principle of Crossdating

states that matching patterns in ring widths or other ring characteristics (such as ring density patterns) among several tree-ring series allow the identification of the exact year in which each tree ring was formed (after Kaennel and Schweingruber 1995). For example, one can date the construction of a building, such as a barn or Indian pueblo, by matching the tree-ring patterns of wood taken from the buildings with tree-ring patterns from living trees.

The Principle of Replication

states that the environmental signal being investigated can be maximized, and the amount of "noise" minimized, by sampling more than one stem radius per tree, and more than one tree per site. Obtaining more than one increment core per tree reduces the amount of "intra-tree variability", in other words, the amount of non-desirable environmental signal peculiar to only tree. Obtaining numerous trees from one site, and perhaps several sites in a region, ensures that the amount of "noise" (environmental factors not being studied, such as air pollution) is minimized.