A technique to determine critical years in evaluating important environmental factors affecting tree growth in forest ecosystem

 

KIM, Eun-Shik and Young-sun Kim

Kookmin University

Department of Forest Resources

Seoul 136-702 Korea

Tel. : + (82 2) 910-4814

Fax. : + (82 2) 910-4809

e-mail address : kimeuns@kmu.kookmin.ac.kr

 

 

Abstract

 

As tree-rings are end-products of tree growth affected by many environmental factors, synchronous growth patterns represent the existence of large-scale environmental factors affecting tree growth simultaneously.   In this study, the potentials of crossdating tree-rings are examined by the determination of critical years to detect some of the large-scale environmental factors affecting tree growth simultaneously.  Tree-rings of tree species growing in the pine (Pinus densiflora Sieb. et Zucc.; Japanese red pine) forest on Mt. Namsan in central Seoul were used to determine the critical years and to examine the potentials to crossdate tree-rings to evaluate changes of tree growth in forest ecosystem affected by environmental changes.   In order to quantify the magnitude of changes in radial increment in tree-rings, a computational way of indexing technique was applied, which is used to determine critical years for annual growth of tree species in relation to environmental changes.  The synchronous patterns of tree growth observed in the tree-rings represent the existence of large-scale environmental factors affecting the growth of trees simultaneously, at least in the local scale.  Studied results provide us with new explanation on the relationships between environmental factors and tree growth in Korea as well as in this region.

 

 

Key Words: tree-rings, secondary growth, ecological variables, critical years, crossdating, environmental factors

 

 

 

INTRODUCTION

 

In temperate forests, tree-rings are formed annually as end-products of tree growth affected by many environmental factors.  As the tree growth is affected by so many environmental factors, it is very difficult to differentiate the effects of single environmental factor to tree growth and forest growth.  It is especially difficult to analyze the effects of such factors as global warming and climate change to forest growth because those factors are not so much variable as the other environmental factors.   Tree rings, the end-products of secondary growth of trees formulated in response to seasonal changes of environment, contain information on year-to-year amount of annual growth as well as growth history (Kim, 1988).   Their absolute and relative width as well as the quantity of the cells that makes up tree-rings provides us with a measure of the effects of succession, biotic and abiotic disturbances, climate and weather, and many other environmental factors which have influenced the growth.   Among many methodologies applied to tree-ring studies, crossdating of tree-rings, which allows the identification of the exact year in which each ring was formed (Fritts, 1976), is one of the most important premises for tree-ring related studies.   Crossdated tree-ring series show high cross-correlation among them.   If the growth of trees in a region fluctuates synchronously with each other for many years, this may not only indicates the existence of some environmental factors that affect tree growth broadly, but also provides us with a reasonable basis for subsequent study to find the factor that affects the synchronous fluctuation of tree growth.   Consideration on the selection of tree cores and the site as well as correct reading of them is important for better crossdating.  Therefore, the rejection of obviously useless cores is needed before the crossdating.   Cores are crossdated by the comparison of tree growth information using naked eyes, graphics of growth patterns, and/or computer programs.

 

The most important premises for applying the tree-ring studies to forest dynamics are 1) to find out the existence of critically good and bad years for tree growth, 2) to define them objectively and quantitatively, and 3) to find out the common factors that are responsible for the growth fluctuation.  Here, critically good and bad years for tree growth are defined as critical years.  Conventionally in tree-ring studies, critically bad years were evaluated from the skeleton-plot method, which researchers subjectively record the year that is characteristically narrow in tree-ring width on strips of paper with 2 mm divisions.  Usually this skeleton-plot method is useful to cross-date tree-rings in the area where tree growth is limited by a few factors and tree-ring width has low variability.  In the region where tree growth is limited by many factors and tree-ring width has high variability, it is difficult to apply this simple method to cross-date tree-rings, however.  Therefore, it is strongly needed to develop a objective and quantitative method to cross-date tree-rings in such areas as this region of temperate monsoon overcoming this problem.

 

This study was carried out to crossdate tree-rings using a relatively simple computer program and to determine critical years to crossdate tree-rings better, which ultimately provide potentials to better interpret tree growth and to ultimately evaluate the change of forest dynamics and biodiversity related to such environmental changes as global warming and climate change.

 

 

MATERIALS AND METHODS

 

Tree-rings of pine trees (Pinus densiflora Sieb. et Zucc.; Japanese red pine) and some hardwood trees including Mongolian oak (Quercus mongolica Fischer) collected from Mt. Namsan, central Seoul, Korea, were used to determine the critical years and to crossdate the tree-rings (Kim, 1993).

 

To describe the current conditions of tree growth, tree cores sampled from 40 trees at 3 sites were taken using the point-sampling methods on Mt. Namsan, central Seoul, Korea.   The sites were randomly selected considering the representativeness and the difference of the vegetation types which are closely related to the aspects (Kim, 1993).   While the stands of northern and western slopes, where pine trees are growing as isolated islands on the ridges of the mountain, are mainly dominated by the deciduous tree species, southern slopes are mainly dominated by the pine trees.   On the eastern slopes, pine trees are showing a generally suppressed growth by the other taller trees.   Sampled tree-rings were carefully prepared and precisely examined using a computer-aided Tree-Ring Measuring System at Kookmin University.

 

Critical years that trees showed synchronously good or bad growth were determined using the SAS program (SAS Institute, Inc., 1990).   In the SAS program, the ratios of good and bad growth were calculated using the equations,

 

            yi = (xi-xi-1)*100/xi-1, (when xi >= xi-1),

 

            yi = (xi-xi-1)*100/xi, (when xi < xi-1),

 

              where, xi  : tree-ring increment of the i-th year,

                     xi-1 : tree-ring increment of the (i-1)-th year, and

                     yi  : ratio of the i-th year's growth evaluated

 

   The ratios of the growth were categorized as follows:

             GGG  : increased growth with the ratio, yi >= 300%

       GG  : increased growth with the ratio, 200% <= yi < 300%,

       G  : increased growth with the ratio, 100% <= yi < 200%,

         +  : increased growth with the ratio, 50% <= yi < 100%,

          -  : decreased growth with the ratio, -50% >= yi > -100%,

         B  : decreased growth with the ratio, -100% >= yi > -200%,

         BB  : decreased growth with the ratio, -200% >= yi > -300%,

             BBB  : decreased growth with the ratio, yi,<= -300,

 

    here, no sign was allocated with the ratio, -50% < yi < 50%.

 

Here, critical year is defined as the year when the tree-ring width was critically increased or decreased compared to that of previous year.   Although there is no quantitative criterion to determine the critical years, the increasing rates of more than 100%, 200%, and 300% in growth were used as the criteria to evaluate critical years of good growth, and the decreased rates of decreased increment compared to actual increment in the current year of less than 100%, 200%, and 300% in growth were used as the criteria to evaluate critical years of bad growth in this study.   Critical years provide dendroecologists with the potential to crossdate the tree-rings.

 

The author simulated daily soil moisture contents using daily inputs of temperature and precipitation data for the last 82 years (Kim, 1992).   Computer simulation model, BROOK, developed by Federer and Lash (1978), made it possible to simulate the daily soil moisture deficit in this region.

 

 

RESULTS AND DISCUSSION

 

Determination of critical years

 

When the author checked the fluctuating growth patterns, it was interesting to observe that the trees on Mt. Namsan showed a rather synchronous growth patterns, which indicates that

Table 1. Determination of critical years for tree growth

    Name                                                                

     of  Qm01  Qm02  Qm03  Qm04  Qm05  Qm06  Qm07  Qm08  Qm09  Qm10  Qm1  Tree                                                                

Year                                                                     

 1944       +                                                           

 1945                                                                    

 1946       +                                                             

 1947                                                                    

 1948       -                                                            

 1949                                                                    

 1950       -                                                     +      

 1951                                                             -      

 1952                                                                    

 1953                                                                     

 1954       B                                                            

 1955       +                                                            

 1956       -                                                            

 1957       +                                                     B      

 1958       G     -                                               -      

 1959            GG                                                      

 1960                                                             G      

 1961             -    BB                                   +     +      

 1962             -     +                                   -            

 1963       B     +     G          -                        +            

 1964             -                                                      

 1965       +    BBB    B          -                                     

 1966       +     +     +                 B                       +      

 1967                              +                                     

 1968                                                                   -

 1969             G                             -                 -     +

 1970                                     B     +                         

 1971                                                                    

 1972                                                                    

 1973                              -      +                       -      

 1974                                                             +      

 1975       -                      -                  G                  

 1976             B     B                                         B      

 1977       +                 -                       -                  

 1978       B           -     B                       -                  

 1979       G     +     G     G           -                              

 1980                         +    +      G           +           +      

 1981                                                 -                  

 1982                                           -                        

 1983                   -                                         +      

 1984                              B            +           +            

 1985                   +                                         -      

 1986                                                                    

 1987       +     -           B                             -             

 1988       -                 -                                          

 1989       +     +           G                                         +

 1990                         +    G                        +            

 1991       -                                               -            

 1992                                                                   -

Note: The first two characters in the name of tree, i.e., Qm, Pd, Fr, Sa, Kp, and Rs stand for Quercus mongolica, Pinus densiflora, Fraxinus rhynchophylla, Sorbus alnifolia, and Robinia pseudoacacia, respectively.
Table 1. Determination of critical years for tree growth (continued)

     Name                                                      

      of  Pd01 Pd02 Pd03 Pd04 Pd05 Pd06 Pd07 Fr01 Sa01 Kp01 Rs01

 Year Tree                                                      

 1944                                                           

 1945                                                           

 1946                        +                                  

 1947                                       -             +     

 1948                                      GG             -     

 1949                                       +                   

 1950                  B    B                             -     

 1951        -        BBB  BB               B             +     

 1952        +             GGG                                  

 1953                       -                                   

 1954                 GG    +               B             +     

 1955        B   BBB   G    +               G                   

 1956             -    -                    -                   

 1957        B              -                                   

 1958        +    +    B                    +                   

 1959        G   GG         -               B                   

 1960             +    +                    +             -     

 1961             -    +    +              GGG                   

 1962             +   BB                   BBB            +     

 1963                  G   BB              GG                   

 1964                  G    G               G                   

 1965                 BB   BB          B   BBB                   

 1966                 GGG  GGG         G   GG                   

 1967                  G    G          +    G                   

 1968        -                   -    BBB                 +     

 1969                 BB    B    -     G   BBB            -     

 1970                  +    -    G     G    G             +     

 1971                  +        BB     -    +                   

 1972        +         B        BB     B    -                   

 1973                      GGG        GGG  GG        -          

 1974                  +        GG     -    +        G          

 1975        -                                                  

 1976                 BBB  BBB  BB         BBB            +     

 1977        B    -    GG   G          +                  -    -

 1978                  +    +    G     -                      GG

 1979        +         G    +   GG         GGG       +          

 1980                  -        BB          B                 GG

 1981                       -          B    B                   

 1982             G    +         +     G   GGG                 B

 1983                  G    G    G     G   GG        +         B

 1984                                  B   BBB                  

 1985             G    -         B          +             +     

 1986             B    -    -   BB                   +          

 1987                  +    +   GG     G             -         -

 1988                  -         -    BB   BBB       +          

 1989                       -                                   

 1990        G              G          +                        

 1991        -    B                        GGG            +     

 1992        G    +    G    +    G     +   GGG                 +

Note: The first two characters in the name of tree, i.e., Qm, Pd, Fr, Sa, Kp, and Rs stand for Quercus mongolica, Pinus densiflora, Fraxinus rhynchophylla, Sorbus alnifolia, and Robinia pseudoacacia, respectively.

there are some factors that affect the growth of trees simultaneously (Kim, 1988).  Another thing to point out is that there are certain years that trees showed very good or bad growth synchronously, which is the critical year.   By examining the environmental factors in these years, we can interpret which are the major environmental factors that affect the growth of tree species significantly.   Based on the records of tree growth, Kim (1994) selected critical years as was shown in the rightmost column of the Table 1.   The year that more number of trees are apparent in the Table indicates that the trees have shown either better or worse growth in the year.

 

In this study, absolute ratios evaluating tree growth of the current year in relation to that of the previous year were calculated and the ratios were compared with each other.   When tree growth ratios were examined, many trees showed abrupt growth changes in radial increment, which was indicated by 'GG', 'GGG', 'BB', and 'BBB' signs in the Figures.   Using the years when tree showed abrupt growth change, crossdating can be carried out.   Although, at this point, it is difficult to mention the range to crossdate tree-rings in species and in geographical location, this study clearly shows that it is possible to crossdate tree-rings of the trees on Mt. Namsan in central Seoul, Korea. Based on the results, it is possible to mention that the years of 1974, 1979, and 1989 are evaluated as good years for tree growth, while the years of 1988, 1976, and 1986 are evaluated as bad years for tree growth.  Among the years, the year of 1992 can be evaluated as the best year, while the year of 1976 can be evaluated as the worst year.

 

It was interesting to check that, among the 40 samples of tree-rings, the growth patterns of some trees were different from those of the other trees, which can be ascribed to false reading of tree-rings, possibly based on the missing rings or false rings.   This procedure can be used as a good device to check the correctness of the reading of tree-rings.   While the potentials using this index are promising to check the validity of the reading, further studies on the year-to-year indices are needed to interpret them ecologically meaningfully.

 

Drought conditions and their effects

 

Based on the results of the simulation of the BROOK model, summary table for the occurrence and the period of drought in each year since 1961 is presented in Table 2.   When the occurrences of the drought and the critical year were compared with each other, there were several years that tree growth was negatively related.   For example, the years of 1988, 1982, 1979, 1965, and 1977 can be categorized as very dry years.   Except in the year of 1979, trees showed negative growth in those years when droughts occurred.   It is also worthwhile to note that the trees grew well in the years of 1970, 1967, 1974, and 1992, when no drought occurred.   Ten years among the 32 year period have shown either positive or negative growth synchrony with the fluctuation of soil moisture conditions.   These conclusively indicate that tree growth is strongly affected by the occurrence of drought.   In other words, drought is one of the major factors that give strong and negative impacts upon the growth of trees on Mt. Namsan (Kim, 1994).

 

When the relationships between the drought and tree growth were examined statistically, generally, trees showed negative correlations to the length of drought of the current year and the previous year (Kim, 1994).   However, the mountain cherries on Mt. Namsan showed a positive correlation to the length of drought of the previous year.   In addition, trees showed high correlations to the lowest moisture condition of the current year and the previous year.

 

Here, it is worthwhile to note that the correlation coefficients between the growth of trees and the drought vary by species.   While some of these differences can be explained by the difference of the growth strategy of the tree species, some of them can be attributed to the mere chances.   These should be explained by the ecophysiological studies on the growth of tree species.   In this paper, it is suffice to point out that soil moisture stress is one of the major factors that should be explained prior to describe any effects or damages that are caused by the air pollutants in natural conditions.   On the effects of air pollutants to tree growth, the readers are referred to Kim(1994) for more details.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 2. Relationship between drought and critical years for tree growth

 

Year

Number of Days for Severe Drought   

The Lowest Moisture Condition                  of the Year (mm)

Year That Tree Growth May Have Been Negatively Affected  

  1961     

  1962     

  1963     

  1964     

  1965     

  1966     

  1967     

  1968     

  1969     

  1970     

  1971     

  1972     

  1973     

  1974     

  1975     

  1976     

  1977     

  1978     

  1979     

  1980     

  1981     

  1982     

  1983     

  1984     

  1985     

  1986     

  1987     

  1988     

  1989     

  1990     

  1991     

  1992     

  0

  4

  5

  0

 35

  0

  0

  1

  9

  0

  0

  3

  0

  0

  0

  0

 27

 10

 68

  1

  0

 56

  2

  0

  0

  0

  8

107

  0

  0

  0

  0

 B(-)*      

                 

                 

 B(-)*      

                 

 B(++)** 

 B(-)*      

                 

 D(+)       

 D(-)        

                 

 B(++)** 

 D(+)       

 P(+)D(-)

 B(---)***                 

 D(--)      

 B(+)*     

 B(+++)***                 

 P(-)D(+)

                 

 D(---)     

 B(++)** 

 B(--)**   

 D(+++)  

 B(---)***                 

 B(+)*     

 B(---)***                 

 D(++++)

 P(+)       

 B(--)**   

 P(++++)***

             

                 

                 

             

                 

 no drought               

                 

                 

 no drought               

                 

                 

                 

 no drought               

                 

                 

             

                 

                 

                 

                 

             

                 

                 

 no drought               

                 

                 

             

                 

                 

                 

 no drought               

 

 Note : Notation of the tree symbol using an alphabet

                 P: Pine trees; D: Deciduous trees; and B: Both categories of the trees.

 

 

CONCLUSIONS

 

Tree-rings are end-products of tree growth affected by many environmental factors.   Synchronous growth patterns represent the existence of large-scale environmental factors affecting the growth of trees similarly.   In this study, the potentials of crossdating tree-rings are examined by the determination of critical years.   Tree-rings of tree species including pine trees growing on Mt. Namsan in central Seoul were used to determine the critical years and to examine the potential to crossdate tree-rings.   These results provide us with new explanation on the relationships between the environmental factors and the growth of trees growing on Mt. Namsan in central Seoul, Korea.

 

 

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