Climate and Tree Growth

                                                                                                                                   

Introduction

 

            One important issue that concerns ecologists is the interaction between organism performance (e.g., growth and reproduction) and the environment where an organism lives. If an organism is placed in an environment where it cannot grow or reproduce, then it will not likely be a permanent or important member of the community there. Environmental tolerance of species ultimately determines the geographic range of species. For example, the snapping turtle is common from the eastern and central United States west to the Rockies. The natural range of this species does not extend into or beyond these mountains.

 

            Within the geographic range of a species there may be year-to-year variation in climate, which may in turn cause variation in performance. For most species, measuring year-to- year variation in performance is a laborious and slow task. This is especially true for animals, many of which tend to run and hide; they do not sit still while you count and weigh them. Fortunately, there are organisms that do sit still and also produce a long-term record of annual performance: trees. The widths of their annual rings can be used to determine how annual growth is affected by various factors.

                                                                             

            Annual rings in trees are obvious in stem cores or stem sections because the size of the water-conducting cells in wood changes depending on when the cells were produced. In spring, the cells are large but those produced through summer and fall get increasingly smaller until the end of the season, when growth ceases. A single annual ring thus represents the cells formed during one growing season.

 

            In this laboratory exercise, measurements will be taken from cross sections of white ash, Fraxinus americana, growing in southern Campbell County. These cross sections were cut at the end of the 1997 growing season). There may also be available cross sections from white oak (Quercus alba) which were cut at the end of the 2000 growing season; this tree came from the same southern Campbell County location. 

 

            After observing a cross-section of a tree, discuss in your group what factors might affect the width of the annual ring.

 

            Once you’ve chosen a factor to test your first step is to formulate a null hypothesis.

 

            You must now design your experiment using the tools available.  You should have a ruler, a dissecting microscope, and pins in addition to a cross-section of a tree..

 

            After you have collected your data you will need to analyze your data to see if a relationship exists.  Information on how to use SPSS is attached.
            In your group, each student should collect data on a different climatic variable and relate it to ring width. Observe plots of other students in the class. Try, by inspection, to determine which climatic variable explains the greatest amount of variation in ring width. Then proceed to the computer and follow the directions given in Instructions for Using the SPSS Statistical Program are on the page 24, calculating a value for R2. Record the results in the following spaces:

 

            Climatic variable                                     R2                  Type of relationship

 

                                                                                                                                            

 

                                                                                                                                            

                                                                       

                                                                                                                                            

 

                                                                                                                                            

 

 

Summary Questions

 

1. Based on the data in hand, what is the most important climatic factor affecting annual variation in tree growth?

 

 

 

2. In general, what combination of temperature and precipitation would give the smallest ring widths? Does your data support this?

 

 

 

 

3. One often hears that pollutants are inhibiting the growth of trees. Is there any evidence from this tree-ring chronology that growth is decreasing? Suggest an explanation for this.

 

 

 

 

4. What type of measurement might be taken to test the hypothesis that tree growth had declined during the last 10 years of the tree’s life? 

 

 

  1. What other factors might have affected tree ring growth?

 

 

 

 

  1. How could an experiment be designed to test the impact of temperature and rainfall on tree growth?