The Onset, Cessation, and Rate of Growth of Loblolly Pines in the FACE Experiment

• Activity Number: 117
• Type: Contributed
• Date/Time: Monday, August 9, 2004 : 10:30 AM to 12:20 PM
• Sponsor: Section on Statistical Consulting
• Author(s): Susanne Aref*+ and David Moore and Evan H. DeLucia
• Companies: Virginia Polytechnic Institute and State University and University of Illinois, Urbana-Champaign and University of Illinois, Urbana-Champaign
• Keywords: nonlinear regression ; segmented model

Abstract

The Duke Forest FACE (Free Air Carbon Dioxide Enrichment) experiment was set up to study what happens in a forest when the CO2 level is elevated as compared to a forest in an ambient environment. A part of the experiment concerns the impact of the elevated CO2 levels on the onset and cessation of growth of loblolly pine trees (Pinus taeda L.). The response variable was the basal area of about 200 trees in six experimental rings measured approximately monthly from 1996 to 2002. The question was how to determine the time of these events each year. Using nonlinear segmented regression models the rate and the duration of growth, and actual growth were determined from model estimates of onset and cessation of growth. Both the rate of growth and the yearly growth were significantly greater for elevated CO2 levels (at a 10% level). The magnitude and onset of growth, and the growth period did not appear to differ for the two CO2 levels, but did differ between categories or sizes of trees as well as between years. Since the growth of a tree is also based on the size of the tree, the relative basal area index was analyzed along with the basal area.