How weed control and fertilisation influence tree physiological processes and growth at early establishment in an exotic F 1 hybrid pine plantation of subtropical Australia

Purpose This study investigated how nitrogen (N) nutrition and key physiological processes varied under changed water and nitrogen competition resulting from different weed control and fertilisation treatments in a 2-year-old F 1 hybrid ( Pinus elliottii Engelm var. elliottii × P. caribaea var. hondurensis Barr. ex Golf.) plantation on a grey podzolic soil type, in Southeast Queensland. Materials and methods The study integrated a range of measures including growth variables (diameter at ground level (DGL), diameter at breast height (DBH) and height (H)), foliar variables (including foliar N concentration, foliar δ 13 C and δ 15 N) and physiological variables (including photosynthesis (A n ), stomatal conductance (g s ), transpiration ( E ), intrinsic water use efficiency (WUE i ) ( A /g s ) and xylem pressure potential (Ψ XPP )) to better understand the mechanisms influencing growth under different weed control and fertilisation treatments. Five levels of weed control were applied: standard (routine), luxury, intermediate, mechanical and nil weed control, all with routine fertilisation plus an additional treatment, routine weed control and luxury fertilisation. Relative weed cover was assessed at 0.8, 1.1 and 1.6 years after plantation establishment to monitor the effectiveness of weed control treatments. Soil investigation included soil ammonium (NH 4 + -N), nitrate (NO 3 − -N), potentially mineralizable N (PMN), gravimetric soil moisture content (MC), hot water extractable organic carbon (HWETC), hot water extractable total N (HWETN), total C, total N, stable C isotope composition (δ 13 C), stable N isotope composition (δ 15 N), total P and extractable K. Results and discussion There were significant relationships between foliar N concentrations and relative weed cover and between tree growth and foliar N concentration or foliar δ 15 N, but initial site preparation practices also increased soil N transformations in the planting rows reducing the observable effects of weed control on foliar δ 15 N. A positive relationship between foliar N concentration and foliar δ 13 C or photosynthesis indicated that increased N availability to trees positively influenced non-stomatal limitations to photosynthesis. However, trees with increased foliar N concentrations and photosynthesis were negatively related to xylem pressure potential in the afternoons which enhanced stomatal limitations to photosynthesis and WUE i . Conclusions Luxury and intermediate weed control and luxury fertilisation positively influenced growth at early establishment by reducing the competition for water and N resources. This influenced fundamental key physiological processes such as the relationships between foliar N concentration, A n , E , g s and Ψ XPP . Results also confirmed that time from cultivation is an important factor influencing the effectiveness of using foliar δ 15 N as an indicator of soil N transformations.