Growth response analysis after early control of woody competition for 14 loblolly pine plantations in the southern U.S.

Only a few growth and yield programs allow users to model the effects of hardwood competition on yields from pine plantations. Several of these programs were developed with the assumption that reducing hardwood competition would consistently produce a Type 2 growth response where pine volume gains increase over time. However, the actual response is not always a Type 2 response. To determine growth response types resulting from woody control treatments, plot volume data were analyzed from 14 trials (on 13 sites) measured over a period of 2 decades (The COMProject). The “age-shift” method of growth analysis and regression analyses were used to classify the types of responses. After 20 years, stand volumes of loblolly pine (Pinus taeda L.) were increased after woody control at 13 of 14 trials when compared to no controls. At four trial locations the age gain ranged from 0.7 to 1.6 years and the growth response was classified as a pseudo-Type 1 response (i.e., pine growth was increased while the total above-ground biomass of the mixed-stand was not altered by the species shift). At nine trial locations a true Type 2 response was observed (i.e., increase in total above-ground biomass of the pine dominated mixed-stand) and the age gain ranged from 0.9 to 5.1 years. At a site in Louisiana, woody control on two similar blocks resulted in a reduction in both early and mid-rotation pine volumes (termed a Type E response) while two other blocks resulted in an early reduction that changed to a mid-rotation volume increase (termed a Type F response). Thus, four types of growth response were associated with woody control treatments.