What was done
Using data obtained from the dominant stands of loblolly pine plantations growing at 94 locations spread across the southeastern United States, the authors employed mean height measurements made at three-year intervals over a period of 15 years to calculate a site index related to mean growth rate for each of the five three-year periods, which index would be expected to increase monotonically if growth rates were being enhanced above "normal" by some type of monotonically-increasing growth-promoting factor.

What was learned
Westfall and Amateis report that "mean site index over the 94 plots consistently increased at each remeasurement period," which would suggest, in their words, that "loblolly pine plantations are realizing greater than expected growth rates," and, we would add, that the growth rate increases are growing larger and larger with each succeeding three-year period.

What it means
As to what could be causing the monotonically increasing growth rates of loblolly pine trees over the entire southeastern region of the United States, the authors say that in addition to rising atmospheric CO2 concentrations, "two other likely factors that could affect growth are temperature and precipitation."  However, they report that a review of annual precipitation amounts and mean ground surface temperatures showed no trends in these factors over the period of their study.  They also suggest that if increased nitrogen deposition were the cause, "such a factor would have to be acting on a regional scale to produce growth increases over the range of study plots."  Hence, they are partial to the aerial fertilization effect of atmospheric CO2 enrichment explanation.

Westfall and Amateis also note that "similar results were reported by Boyer (2001) for natural stands of longleaf pine, where increases in dominant stand height are occurring over generations on the same site."  It is our opinion that the results of both studies could well be manifestations of the Greening of the Earth phenomenon that has been postulated by Idso (1982, 1986,1995) to result from the ongoing rise in the air's CO2 content, which is described in the various subsections of the Biospheric Productivity section of our Subject Index.

References
Boyer, W.D.  2001.  A generational change in site index for naturally established longleaf pine on a south Alabama coastal plain site.  Southern Journal of Applied Forestry 25: 88-92.

Idso, S.B.  1982.  Carbon Dioxide: Friend or Foe?  IBR Press, Tempe, Arizona, USA.

Idso, S.B.  1986.  Industrial age leading to the greening of the Earth?  Nature 320: 22.

Idso, S.B.  1995.  CO2 and the Biosphere: The Incredible Legacy of the Industrial Revolution.  Department of Soil, Water & Climate, University of Minnesota, St. Paul, MN.