Harold C. Fritts

Dendroecology: A Tool for Evaluating Variations in Past and Present Forest Environments

Publisher Summary This chapter reviews basis for some fundamental techniques, principles, and practices of dendroecology. Dendroecology refers to applications of dendrochronological techniques to problems in ecology. The important ecological problems for which dendroecological techniques are well suited include widespread outbreaks of herbivorous insects in forests, tree decline observed in forests of central and northern Europe and in some areas of the United States, and potential environmental changes brought about by the rising concentration of atmospheric CO 2 and other gases. A variety of structural characteristics of tree rings, such as width, wood density, and vessel size exhibit variability from one ring to the next. The principles and practices of dendroecology—namely, uniformitarianism, limiting factors, crossdating, standardization, variance of the mean and the signal-to-noise ratio, sample replication, tree and site selection, calibration and verification, and modeling are essentially the fundamental framework for understanding the discipline of dendroecology. It is noted that these principles are not laws or rules of nature but are well-tested best inferences based upon known facts at a particular time.

Multivariate Techniques for Specifying Tree-Growth and Climate Relationships and for Reconstructing Anomalies in Paleoclimate

Project: Dendroclimatic History of the United States, Contract: E-41-70(N) / 1970 Final Report Prepared for: Laboratory for Environmental Data Research, Environmental Data Service, Weather Bureau, Environmental Science Services Administration, United States Department of Commerce / November 9, 1970

Increasing atmospheric carbon dioxide: tree ring evidence for growth enhancement in natural vegetation

A response of plant growth to increased atmospheric carbon dioxide, which has been anticipated from laboratory data, may now have been detected in the annual rings of subalpine conifers growing in the western United States. Experimental evidence shows that carbon dioxide can be an important limiting factor in the growth of plants in this high-altitude environment. The greatly increased tree growth rates observed since the mid-l9th century exceed those expected from climatic trends but are consistent in magnitude with global trends in carbon dioxide, especially in recent decades. If correctly interpreted, these findings have important implications for climate studies involving tree ring observations and for models of the global carbon dioxide budget.

Relationships of Ring Widths in Arid‐Site Conifers to Variations in Monthly Temperature and Precipitation

Two multivariate techniques for evaluation of ring-width and climatic relation- ships are described. One technique employs multiple regression on principal components of climate. Response functions are obtained which express in mathematical form the relative effect of monthly temperature and monthly precipitation during a 14-month period on ring- width variations. Response functions are calculated for 127 coniferous tree sites in western North America. The other technique is a cluster analysis which is used to identify similarities among the response functions and classify them on the basis of their similarities and differ- ences. Higher-than-average precipitation most commonly results in higher-than-average growth, though on cold sites the effects of precipitation during the cooler parts of the year are some- times lacking or inverse. Precipitation is directly related to growth throughout the entire year for 32% of the sites, representing many that have been used in dendrochronology. In the remaining 68% of the sites the effects of precipitation vary from season to season. Tempera- ture is most commonly inversely related to ring width during autumn, spring, and summer. Inverse relationships occur in mid-winter for trees on the warmest sites and on certain high- altitude sites. However temperature effects are often direct ones, especially during winter and for many sites at high altitudes, high latitudes, or on north-facing slopes. Site factors appear most responsible for variations in the growth responses. Aspect of slope appears to be the most critical, followed by altitude and latitude. There are fewer differences between species than between factors of the site, although certain species such as bristlecone pine have a more or less unique growth response. Some speculations are offered on the effects of the sites and their microenvironments on biological processes linking the climatic variables and growth. The variety of ring-width responses to variations in climatic factors suggest that more physiological study of trees on extreme sites may reveal unique growth-environment rela- tionships that have not been observed on more optimum sites. The median percent of tree- growth variance accounted for by climate is approximately 60 to 65%. With such a high percentage of variation related to climate, ring-width variability, if adequately sampled, dated, and calibrated with climate, can be used successfully to estimate past climate, even though all linkages of cause and effect have not been demonstrated by physiological research. It is pro- posed for reconstruction of past climate that many ring-width chronologies be used with diverse tree-growth responses, because the differences can be handled by multivariate tech- niques and provide more information on various climatic factors than chronologies with the same growth response. It is also proposed that response functions have applications in the modeling of productivity in forest ecosystems. They can be used to ascertain the effects of climatic variation on tree growth and to help in extrapolating information gathered in one area to other tree sites.

Tree-Ring Characteristics Along a Vegetation Gradient in Northern Arizona

Tree—ring samples of Pseudotsuga menziesii (Mirb.) Franco, Pinus ponderosa Laws., and Pinus edulis Engelm. for 1860 through 1962 were obtained from sites along a vegetational gradient, ranging from forest interior sites to semiarid lower forest border sites. Samples were analyzed using a stratified nested plot design. Near the lower forest border tree rings are narrowest, the variability in the relative ring—width response from year to year is greatest, and the variance in common or the correlation between radii and between trees is highest. This variability is more highly related to climatic fluctuations than the variability in highest. This variability is more highly related to climatic fluctuations that the variability in forest interior trees and may be attributed to longer and more frequent periods during which water stress is limiting to physiological processes in the trees. Hence, at the arid forest border tree—ring chronologies contain the most consistent, but variable, growth responses and provide the best record of climatic fluctuations. The semiarid lower forest border trees exhibit a high frequency of partial growth layers. Chronologies from the forest interior exhibit very few partial growth layers. Crossdating between both types of sites and among many individuals of different species makes absolute dating not only possible but exceedingly reliable. Such tree—ring analyses may have considerable application to the evaluation of ecological forest gradients.

Growth-Rings of Trees: Their Correlation with Climate

Many differences in the ring-width growth within a tree may be attributed to changing supplies of food and hormones. In moist sites or during periods of favorable climate, there may be sufficient food for the production of wide rings throughout the tree. But in dry sites or during years of low moisture and high temperatures, food competition within the tree is likely to be greater and the cambium at the base of the stem is likely to receive a limited food supply and may produce narrow rings. The cambium at the stem base depends upon the entire crown for food, hence ring growth reflects the trees ambient climate. But, the cambium in the top of the tree or in the upper branches depends upon a more restricted portion of the crown for its food and hormone supplies. The rings produced by the cambium vary greatly from branch to branch and are less reliable indicators of the climate surrounding the entire tree than rings at the tree base. Therefore, ring series at the base of trees in semiarid sites provide the most reliable, as well as the longest, record of macroclimatic variation. Tree-ring widths in certain coniferous species growing in semiarid sites appear to represent the integrated effect of climate on food-making and food accumulation in the crown throughout the 14 to 15 months previous to and including the period of growth. Trees in warm, low-elevation sites may utilize winter moisture most efficiently; trees in cool, high-elevation or more northern sites may utilize early summer and early autumn moisture most efficiently. But even with these differences, a significant amount of variance is found to be common among tree-ring series from a wide range of sites, species, and geographical areas in western North America (14, 19, 34), emphasizing a common dependence of ring widths on the gross regional patterns of precipitation and temperature. The remaining variance, which is not correlated among sites, may be attributed to local en-vironmental and climatic differences, to variability among and within trees, and to compounding effects of occasional fires, insect or other infestations, and recurring years of high seed production. It is evident that a large portion of the variability in ring-width patterns from semiarid sites in western North America does reflect differences in climate from year to year. If ring chro- nologies are derived from a number of trees in semiarid sites and if adequate corrections for age and trend are made, these chronologies may be used to reconstruct a first approximation of annual, or somewhat longer period, climatic fluctuations in the past (19).

Dendroclimatology and dendroecology

Abstract Dendrochronology is the science of dating annual growth layers (rings) in woody plants. Two related subdisciplines are dendroclimatology and dendroecology. The former uses the information in dated rings to study problems of present and past climates, while the latter deals with changes in the local environment rather than regional climate. Successful applications of dendroclimatology and dendroecology depend upon careful stratification. Ring-width samples are selected from trees on limiting sites, where widths of growth layers vary greatly from one year to the next (sensitivity) and autocorrelation of the widths is not high. Rings also must be cross-dated and sufficiently replicated to provide precise dating. This selection and dating assures that the climatic information common to all trees, which is analogous to the “signal”, is large and properly placed in time. The random error or nonclimatic variations in growth, among trees, is analogous to “noise” and is reduced when ring-width indices are averaged for many trees. Some basic facts about the growth are presented along with a discussion of important physiological processes operating throughout the roots, stems, and leaves. Certain gradients associated with tree height, cambial age, and physiological activity control the size of the growth layers as they vary throughout the tree. These biological gradients interact with environmental variables and complicate the task of modeling the relationships linking growth with environment. Biological models are described for the relationships between variations in ring widths from conifers on arid sites, and variations in temperature and precpitation. These climatic factors may influence the tree at any time in the year. Conditions preceding the growing season sometimes have a greater influence on ring width than conditions during the growing season, and the relative effects of these factors on growth vary with latitude, altitude, and differences in factors of the site. The effects of some climatic factors on growth are negligible during certain times of the year, but important at other times. Climatic factors are sometimes directly related to growth and at other times are inversely related to growth. Statistical methods are described for ascertaining these differences in the climatic response of trees from different sites. A practical example is given of a tree-ring study and the mechanics are described for stratification and selection of tree-ring materials, for laboratory preparation, for cross-dating, and for computer processing. Several methods for calibration of the ring-width data with climatic variation are described. The most recent is multivariate analysis, which allows simultaneous calibration of a variety of tree-ring data representing different sites with a number of variables of climate. Several examples of applications of tree-ring analysis to problems of environment and climate are described. One is a specification from tree rings of anomalies in atmosphere circulation for a portion of the Northern Hemisphere since 1700 A.D. Another example treats and specifies past conditions in terms of conditional probabilities. Other methods of comparing present climate with past climate are described along with new developments in reconstructing past hydrologic conditions from tree rings. Tree-ring studies will be applied in the future to problems of temperate and mesic environments, and to problems of physiological, genetic, and anatomical variations within and among trees. New developments in the use of X-ray techniques will facilitate the measurement and study of cell size and cell density. Tree rings are an important source of information on productivity and dry-matter accumulation at various sites. Some tree-ring studies will deal with environmental pollution. Statistical developments will improve estimation of certain past anomalies in weather factors and the reconstructtion of atmosphere circulation associated with climate variability and change. Such information should improve chances for measuring and assessing the possibility of inadvertent modification of climate by man.

Variations in climate since 1602 as reconstructed from tree rings

Abstract Spatial anomalies of tree-ring chronologies can provide information on high-frequency spatial anomalies in paleoclimate representing droughts, colder-than-normal intervals, and other synoptic-scale features. Examples are presented in which 65 tree-ring chronologies are calibrated with spatial anomalies in North American meteorological records of seasonal temperature and precipitation, and with sea-level pressure over the North American and North Pacific sectors. Multivariate transfer functions are obtained that scale and convert the past spatial variations in the tree-ring record into estimates of past variations in the meteorological record. Objective verifications of the reconstructions are obtained using independent meteorological observations for time periods other than those used in the calibration. Historical information or other proxy data from the 19th century are also used for verifying the decadal (or longer) and regional reconstructions and for identifying strengths and weaknesses of the various sources of information. The reconstructed winter and summer temperatures for the United States and southwestern Canada and winter precipitation for the Columbia Basin and California during the 17th through 19th centuries were found to differ from the 20th century means with large-scale variations evident. Extreme winters similar to 1976–1977 are also identified and found to be more frequent in the past, especially in the 17th century. The climatic reconstructions in this time domain are dominated by high-frequency, synoptic-scale fluctuations that can be interpreted as cyclonic-scale changes in atmospheric circulation. Such reconstructions may be useful for testing various climatic models and estimates developed primarily from 20th-century meteorological data against the longer estimated record for the 17th through 19th centuries.

TREE-RING EVIDENCE FOR CLIMATIC CHANGES IN WESTERN NORTH AMERICA

Abstract The relationships between climatic factors and fluctuations in dated tree-ring widths are statistically evaluated. A wide ring indicates that the years climate was moist and cool, and a narrow ring dry and warm. In general, ring width relates to a 14-month period from June through July but most tree-ring chronologies exhibit a closer relationship with autumn, winter, and spring moisture than with summer moisture. The climatic relationships for evergreen trees are attributed largely to the influence of environmental factors on photosynthesis and the accumulation of food reserves. Under abnormally dry and warm conditions, especially during the autumn, winter, and spring, little food is accumulated, new cells are formed more slowly during the growing period, and the resulting ring is narrow. Relative 10-yr. departures are calculated for the entire length of 26 tree-ring chronologies from western North America. Those portions after 1500 are used to map areas of high and low moisture. Periods of wide...

An assessment of the possible effects of volcanic eruptions on North American climate using tree-ring data, 1602 to 1900 A.D

Seasonal and annual temperature reconstructions derived from western North American semi-arid site tree-ring chronologies are used to examine the possible spatial response of North American climate to volcanic eruptions within the period 1602 to 1900. Low-latitude eruptions appear to give the strongest response. Cooling of the annual average temperatures in the central and eastern United States is reconstructed to follow volcanic eruptions with warming in the western states. The magnitude and spatial extent of the reconstructed cooling and warming varies seasonally. The warming that occurs in the west is strongest and most extensive in winter while the cooling in the east is most marked in summer. These results are based on reconstructed climate records which contain error terms unrelated to climatic factors. The suggested pattern of response to volcanic forcing is, however, supported by four independent temperature/proxy temperature series within the area of the temperature reconstructions. Additional support is provided by three independent series lying outside the area which suggest that the temperature spatial response may extend to the north beyond the area covered by the tree-ring reconstructions.