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

The effects of routine and luxury weed control and fertilization treatments on diameter, height, carbon (δC) and nitrogen (δN) 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 δN in the foliage. A strong correlation between growth and δN existed for all canopy positions, demonstrating that foliar δN 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 N by soil micro-fauna, during the processes of decomposition and mineralization and as a result of leaching of lighter N, 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 δN, but not foliar δC, 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 N abundance available for tree uptake as reflected in the foliage of 8-year-old F1 hybrid pine trees at this site. Introduction In 2005 there were approximately 5,500 hectares of F1 hybrid pine established by FPQ (Forestry Plantations Queensland 2006). The F1 hybrid pine is the predominate plantation pine grown in the subtropics of southeast Queensland. How carbon (C) and nutrient assimilation is affected by silvicultural practice is a continually developing field of research (Binkley et al. 2004). It is known that successful plantation establishment is attributed to appropriate weed control and fertilization practices at early plantation establishment (Smethurst and Nambiar 1989; Wagner et al. 2006). The effects of silvicultural practices on C assimilation, nutrition and water use have been reviewed by various researchers (Vitousek and Matson 1985; Woods et al. 1992). This paper purports to investigate if and how weed control and fertilization practices affect total foliar C and N concentrations and δC and δN in F1 hybrid pine foliage as indexes of water (Ehleringer et al. 1993) and N uptake (Lajtha and Michener 1994). Materials and methods Foliage was collected from an 8-year-old F1 hybrid pine plantation site known as Experiment 350GYM, located in Toolara State Forest, southeast Queensland, Australia. The site has a subtropical climate with dry winters and wet summers with an approximate annual rainfall of 1256 mm. Soil types throughout the study area are alluvial and vary from grey to yellow podzolics and earths. The experiment is a randomized complete block design. For the purposes of this investigation four treatments have been selected. Specifically the four treatments selected were: 1 Routine weed control and routine fertiliser (RWC+RF); 2 Luxury weed control and routine fertiliser (LWC+RF); 3 Routine weed control and luxury fertiliser (RWC+LF); 4 Luxury weed control and luxury fertiliser (LWC+LF). Fertiliser treatments The routine fertiliser treatment included an application of 50 kg/ha of phosphorus (P) as Mono-ammonium phosphate (MAP) 10 weeks after planting. The MAP was applied at the rate of 226 kg/ha and was reported to provide 10% elemental N and 21.9% of P. In addition to the 50 kg/ha of MAP, the luxury fertiliser treatment included a special mix of 5 kg/ ha Cu, 5 kg/ha Zn, and 5 kg/ha B applied as a basal dressing and 50 kg/ha of potassium (K) in the form of Muriate of Potash applied at 120 g/tree at a distance of approximately 20 cm from each tree. No extra N fertiliser was applied. Weed control treatments Pre-planting weed control was applied with a tractor mounted multiboom delivering 7.2 l/ha glyphosate and 10 l/ha simazine. Woody weed control was also carried out 6 months prior to planting with D50 and grazon applied at 10 l/ha and 1.5 l/ha respectively. Routine weed control treatment was applied in accordance with routine practice at approximately at 6, 12 and 16 months. Luxury weed control was carried out every two months in the first 6 months with sprays thereafter varying from 3-5 months. All luxury weed control treatments were completed by the third year after planting. Growth data Growth data including tree diameter at breast height (Dbh) and height were assessed for their responses to the selected treatments in 2004. Foliage samples Foliage samples were collected in 2006 from each of the five canopy positions from 4 selected trees in each plot by taking fully expanded leaves. Foliage at each position was composited to make one sample. The positions collected were 1. north side top position; 2. north side mid section; 3. north side inner most needles; 4. south side mid section; and 5. north side lower section. Samples were refrigerated until returning to the laboratory and then dried in an oven at between 40-55 ̊C for five days. After drying, samples were grounded to a fine powder. Approximately five mg of the ground material was then pelletized and analyzed for total C, N and isotope compositions, on a GV Isoprime, Mass Spectrometer, Manchester UK. Statistical analysis All data were analyzed using SAS version 9.1, using a general linear model (GLM) with a least squared means, post-hoc analysis, and with a factorial ANOVA and least significant difference, posthoc analysis in Statistix 8. The objective was to ascertain the significance of the effects of the treatments on growth and the selected foliar parameters. Correlations were carried out between foliar parameters at each canopy position and tree growth, and between the foliar parameters. Results Dbh An ANOVA analysis for Dbh indicated that there were significant main effects between treatments. Results indicated that LWC+RF (15.3 cm) was greater than LWC+LF (14.9 cm) and that RWC+RF (12.3 cm) was greater than RWC+LF (12.2 cm). No significant interaction between weed control and fertiliser treatments existed (P<0.05). Height An ANOVA analysis for height indicated that there were significant main effects between treatments. LWC+RF (10.02 m) was greater than LWC+LF (9.62 m) and RWC+RF (8.72 m) was greater than RWC+LF (8.54 m). No significant interaction between treatments occurred (P<0.05). Total C and δC There were no significant differences in foliar C and δC between treatments (P<0.05). There were significant differences in foliar δC among the canopy positions (Figure 3). -30.5 -30 -29.5 -29 -28.5 -28 Canopy