Tree rings and climatological information
Tree rings, also known as growth rings, are visible in the cross-sections of trees and provide valuable insights into both the age of a tree and the environmental conditions it experienced during its growth. Each ring consists of alternating bands of lighter early wood, produced during optimal growing conditions, and darker late wood, formed during less favorable conditions. The width of these rings correlates with the quality of the growing season, influenced by factors such as moisture, temperature, sunlight, and environmental stressors like pollution and disease. While tree rings are a reliable indicator of climatological conditions in temperate regions, their effectiveness diminishes in tropical areas where growth is continuous and less distinctly seasonal.
As climate change progresses, the growth patterns of trees are expected to be affected significantly. Increased levels of carbon dioxide (CO2) can enhance photosynthesis and water efficiency, potentially leading to wider growth rings. However, the long-term effects of these changes, particularly in older trees, remain uncertain. Additionally, rising temperatures may alter species composition in various ecosystems, while factors such as ozone and acid rain introduce complexities into tree health and forest ecosystems. Understanding these dynamics is crucial for assessing the impacts of climate change on forest biodiversity and overall ecosystem health.
Subject Terms
Tree rings and climatological information
Definition
Tree rings, also known as growth rings or annual rings, are visible in the woody stems of trees. A woody stem is composed of cells collectively known as secondary xylem. These cells are responsible for moving water and nutrients upward in the plant and provide support for the aboveground portion of the tree.
Tree rings are formed by the production of cells of different widths. When growing conditions are more optimal, as is typical in the earlier part of the temperate growing season, the diameters or width of the cells are large and the cells appear light to the naked eye. This growth is typically called early wood. During less optimal growing conditions or later in the season, the diameters of the conducting cells are smaller and more fibers for support are produced. Both fibers and the conducting cells produced in the later portion of the growing season (or under less desirable environmental conditions) are smaller in size and have thicker walls than do the cells in the early wood. Because the cells are smaller and the walls are thicker, the wood appears darker to the eye. This darker, denser wood is often called late wood. Bands of lighter, early wood alternating with bands of darker, denser wood creates the appearance of rings in a cross-section of a tree.
In temperate areas with distinctive seasons, trees typically produce one ring for each growing season. Scientists are able to determine the age of a tree by examining the number of rings in the trunk of the tree. Since the width of the ring correlates to the quality of the growing season, one can infer past climatological information by measuring the width of the growth rings. The information gathered represents localized conditions for that species in that particular environment. In tropical areas, the growth in trees is more continuous and the use of rings to determine age and climatic conditions is less reliable.
Numerous factors contribute to the overall quality of a growing season. Environmental factors include moisture, temperature, nutrient availability, sunlight, availability, wind, and pollution. Researchers have shown that the width of a ring in most trees is most closely associated with temperature and moisture availability. The age of the tree, competition with other organisms, herbivory, and disease can also influence the growth of a tree.
Significance for Climate Change
Changes in climate affect tree growth. Increasing levels of CO2 have been shown to enhance the rates of and improve water efficiency in some tree species. This enhancement, however, has been observed in laboratory and modified field environments, for relatively short periods of time (up to three years), on seedlings, under weed-free and insect-free conditions. It is uncertain whether the same result would occur under field conditions with older trees. Enhanced growth would potentially result in the accumulation of more wood through the production of wider rings.
With increasing CO2 concentrations and global warming, temperatures are increasing. It is likely that this temperature increase will affect the composition of tree species in some ecosystems. Ranges for many tree species are likely to be extended and will decrease for those species adapted to cooler environments. Temperature is likely to affect conditions such as fire, drought, wind, and ice, thus influencing the growth of trees.
In addition to atmospheric warming, warming of the soil will affect the growth of trees. Research has shown that root growth and root turnover is enhanced with elevated CO2 and temperatures. Much of the carbon in a tree is stored within its roots. Enhanced root growth might provide additional nutrients for the plant. In addition, as temperature increases, leaf litter decomposition also increases, which in turn increases the cycling of nutrients to the organisms growing in the soil.
Besides increases in CO2 and temperature, other climatic factors are increasing that could have a significant impact on tree growth. Two such factors are ozone and sulfur. Both have been studied extensively, with seemingly conflicting results. Trees have been shown to be susceptible to increasing levels of ozone and acid rain (the latter produced from sulfur in the atmosphere). This has been well documented in the laboratory, in modified field experiments, in individual field observations, and in highly polluted localized areas. A difference in susceptibility among tree species has also been documented. In mixed forests, some tree species are damaged by acid rain, ozone, and other pollutants, while other species in the forest remain seemingly unaffected.
It has been more difficult for researchers to link pollutants with the decline of entire forests. The most dramatic and best-documented example of the impact of acid rain on a forest is the devastation of the Black Forest in Germany. Clearly the potential for damage by pollutants exists and is a reality in some areas, but additional field research and careful monitoring are needed to understand forest ecosystems and the levels at which pollutants are significantly impacting the ecosystem.
Bibliography
Bell, J. N. B., and M. Treshow. Air Pollution and Plant Life. 2d ed. New York: John Wiley & Sons, 2002.
Karnosky, D. F., et al., eds. Air Pollution, Global Change, and Forests in the New Millennium. Oxford, England: Elsevier, 2003.
"How Tree Rings Inform Us ABout CLimate Change." Wilderness Society, 2024, wilderness-society.org/dendrochronology-how-tree-rings-inform-us-about-climate-change/. Accessed 21 Dec. 2024.
Raven, Peter H., Ray F. Evert, and Susan E. Eichorn. Biology of Plants. 7th ed. New York: W. H. Freeman, 2005.