The Influence of Herbaceous Weed Control and Seedling Diameter on Six Years of Loblolly Pine Growth—A Classical Growth Analysis Approach

Abstract

Abstract Classical growth analysis equations were applied to three loblolly pine (Pinus taeda L.) seedling ideotypes (determined by groundline diameter) that were subjected to either 2 years of complete herbaceous weed control (low weed abundance: LWA) or no herbaceous weed control (high weed abundance: HWA). For the first 2 growing seasons, seedlings of the LWA treatment had greater mean relative growth rate (RGR) and mean net assimilation rate (NAR) values. However, during years four through five, RGR and NAR were significantly lower in the LWA treatment than in the HWA treatment. The reduction in herbaceous weed interference appeared to reduce mean leaf area ratio (LAR) throughout the study period. Several trends in this data set suggest that growth analysis parameters may be confounded with size. The largest ideotype generally exhibited the smallest RGR, and RGR declined as trees increased in biomass. To remove the potential confounding of size, the basis of comparison was changed from trees of equal age to trees of equal biomass (total aboveground dry weight at the beginning of each growing season). This method resulted in a different interpretation of the results. For a given biomass, trees in the LWA treatment had greater RGR and LAR than those in the HWA treatment throughout the study period. Growth of seedling ideotypes within a given weed abundance treatment all fell along a single curve, suggesting that there was no apparent interaction between initial seedling diameter and herbaceous weed control. After the trees reached six kg biomass, the direct benefits from reduced herbaceous interference had apparently ceased. Initially, large differences in NAR were observed between treatments, but these differences diminished as trees approached 5 kg. In contrast, differences in LAR were initially small but increased as tree biomass increased. As trees increased in biomass, the partitioning of carbon into leaf area seems to be more responsible for the productivity gains associated with weed control. For. Sci. 37(2): 655-668.