This is where we present the results of our tree ring research.  We took width measurements of the rings of each tree and used them to calculate the annual basal area increment.  Basal area is the cross-sectional area of a tree taken at the breast height diameter measurement (dbh).  We measured tree diameter at a standard height of 1.37 meters above the ground.

To calculate Basal Area, you divide the diameter by two to get the radius; you square the radius, and then multiplying by
3.14 (Pi) -- its your basic equation for finding the area of a circle.  To find the annual incremental growth in basal increment
for Year X, you use the following equation:

 X-(X-1),   where X is the basal area at year X and X-1 is the basal area of the tree measured up to year previous to X.
 (X-1)

(units are cm and cm²).

Calculating basal area increment gives us one year's basal area growth.  This value is then expressed as a percentage of the overall basal area up to the previous year.  It allows us to standardize growth rates and make comparisons among trees and between sites.  To peruse these analyses, please scroll downward.
 

Average basal area increment (%) of the three species on the dry site shows comparable patterns.  Four years of incremental increases in annual percentage growth between 1981 and 1985 (with the exception of 1983 for sugar maples) were followed by three years of declining growth through 1988.  For the sugar maples, the decrease in 1988 was probably due to the (1988) attack by pear thrips (other links to pear thrips).  Growth since 1988 has fluctuated considerably for sugar maples and especially hickories; while the pattern of white oak growth has remained relatively stable.  In general, white oaks demonstrated less robust growth in comparison to sugar maples and hickory species on the dry upland site.  This is curious because they are typically a dry site species.

To see the average basal area increment data click here

A comparison of average basal area increment (%) of the three species on the mesic site shows that  annual growth for white oaks is smaller than either sugar maple or hickory.  For most years, the difference in growth between white oaks and the other two species is greater at the mesic site than at the xeric site.

White oaks tend to track hickory growth better than sugar maple growth.  The data draw attention to two potentially significant years:

1981:  marked decrease in incremental growth for white oak and hickories, possibly due to defoliation by gypsy moths.
1985:  marked increase in incremental growth for white oak and hickories.

To see the average basal area increment data click here

Both mesic and xeric sites show a strong correlation in hickory growth since 1981.  Following three years of incremental increases in basal area up to 1985 at the mesic site or five years of increasing growth at the xeric site, hickories at both locations grew less and less until 1988, the year of least growth.  From 1988 to the present, growth has fluctuated by up to two percentage points between years.  In the past twelve years, growth has not reached pre-1985 levels.

Significant years:

1988:  The year of least growth at both sites.
1981:  Growth at the mesic site decreased by more than a percentage point, while hickories grew more at the xeric site.

To see the hickory tree ring data click here.

Significant years:

1985:  Both sites recorded high growth levels which were not matched again except in 1992.

To see the white oak tree ring data click here

Sugar maple cores are non-porous and harder to read, therefore our data may reflect measurement errors. The overall patterns at both wet and dry sites, however, suggest declining incremental growth since 1990, though it is difficult to determine from the data if one site demonstrates better growth than the other.  The one notable exception is 1988 which recorded a significant decrease in annual growth at the xeric site.  This date corresponds with an outbreak of pear thrips.  Why did the pear thrips outbreak appear to affect the dry site and not the wet site?

To see the sugar maple ring data click here
 

"The Regression Digression"

• As trees get older, their growth in diameter usually slows down.

• As this forest grows, individual trees require more growing space forcing them to compete with one another.  The forest may be stagnating (although this usually only occurs in single aged monospecific stands).