Climatic Data, Tree Ring Records

PAST CLIMATES CAN be reconstructed by the use of the relationship between the climate and tree-growth parameters (dendroclimatology). Dendroclimatology is a sub-discipline of dendrochronology, which is the analysis of tree rings, including the dating of annual rings and study of patterns of ring characteristics, such as width, density, and isotopie composition. Annual growth rings of trees are natural recorders of climatic conditions (a proxy variable).

Outside the tropics, in the temperate and boreal zones with a distinct growing season and a strong seasonality in either temperature or rainfall, diameter growth of trees is distinguished by the formation of tree rings. Each growing season, new cells are produced by the vascular cambium (a cell layer that separates bark from wood).

While those cells formed toward the outside of the cambial layer become part of the bark, cells formed inward build up rigid wood and are the building blocks of tree rings. The actual tree rings are formed by a change in growth characteristics of wood formation through the season. Wood cells formed at the beginning of the growing season are large, thin-walled, and low in density, and are called earlywood. Toward the end of the growing season, cells become smaller and more thick-walled, forming the latewood. Finally, when the growth season ends, growth stops and cells die, with no new growth appearing until the next spring. The ring boundary is the contrast in cell size between the small latewood cells and the large earlywood cells.

Diameter growth of trees in arid and semiarid environments strongly responds to changing soil water conditions and, thus, provides information on precipitation, while trees growing at high latitudes (poleward of 30 degrees), or high altitudes, are most sensitive to changing temperatures. In recent years, tree-ring studies have also been conducted in the tropical zone, where some tree species form distinct tree rings in reaction to dry and rainy seasons.

Analyses of tree rings have been carried out for several conifer species (such as spruce, pine, larch, fir, juniper, and cedar) and deciduous tree species (such as oak, beech, aspen, alder, birch, ash, and elm). [Page 250]However, deciduous species have not been studied as thoroughly as conifers, because of the more complicated structure and variability in their annual tree-ring growth patterns. Generally, tree species suitable for dendroclimatology are those that are sensitive to climate and show a variation in ring-to-ring growth, while those that lack ring variability are called complacent. Yet, tree-ring characteristics are not only influenced by a number of climate-related factors (soil and air temperature, precipitation or soil moisture, sunlight, wind, snow accumulation, or the length of the growing season), but also by non-climate-related factors such as soil fertility, atmospheric composition, slope gradient, fires, pests and diseases, grazing, logging, tree age, competition, genetic differences, and growth in previous years.

Moisture and a long growing season result in a wide ring, while drought and a short growing season results in a narrow ring.

Climate-related factors may alter cell enlargement and subsequent maturation of wood cells by regulating hormone activity or availability, by influencing net carbon gain via restricted photosynthesis, or by modifying metabolic pathways associated with the different growth processes. Variations in temperature and water availability can have direct effects on the rates or duration of cell expansion and wall thickening, and indirect effects on carbon availability and growth-regulator levels. Temperature represents the most critical factor at the beginning of the growing season, when sufficient water reserves are available. As the growing season progresses and soil moisture decreases, radial growth is increasingly exposed to stress due to water deficit. Adequate moisture and a long growing season will result in a wide ring, while drought and a short growing season will result in a narrow ring.

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