Effects of establishment silviculture on carbon and nitrogen isotope compositions and tree growth in a F1 hybrid pine plantation of southeast Queensland

Abstract

TThe effects of routine and luxury weed control and fertilization treatments on diameter, height, carbon (δ13C) and nitrogen (δ15N) isotope composition in foliage were assessed in an 8-year-old, F1 hybrid pine (Pinus elliottii var. elliottii Pinus caribaea var. hondurensis) plantation, on typical sandy soils in southeast Queensland. The aim of this research was to identify how weed control and fertilization treatments affect nitrogen nutrition and water use in F1 hybrid pine plantations, together with tree growth. The results have indicated that a lack of weed competition has shown greater potential for height and diameter at breast height (Dbh) increment and δ15N in the foliage. A strong correlation between growth and δ15N existed for all canopy positions, demonstrating that foliar δ15N may prove to be a better indicator of growth indices in the canopy than total foliar N concentrations alone. These differences could be in part due to discrimination against 15N by soil micro-fauna, during the processes of decomposition and mineralization and as a result of leaching of lighter 14N, where no weed cover exists. These results indicate that in an 8-year-old F1 hybrid pine plantation, luxury weed control treatments significantly affected foliar δ15N, but not foliar δ13C, nor foliar N or C concentrations. It is therefore concluded that luxury weed control treatments result in both greater height and Dbh increment and an increased pool of 15N abundance available for tree uptake as reflected in the foliage of 8-year-old F1 hybrid pine trees at this site