Phloem Sap and Wood Carbon Isotope Abundance (δ13C) Varies with Growth and Wood Density of Eucalyptus globulus under Nutrient Deficit and Inform Supplemental Nutrient Application

Eucalyptus globulus, commonly known as blue gum or southern blue gum, is a tall, evergreen tree endemic to southeastern Australia. E. globulus is grown extensively in plantations to improve the sustainability of timber and fibre production across Australia. Sustainable forest management practices necessitate the consideration of 'off-site' carbon and ecological footprints. Pursuing optimal supplemental nutrient application and maximum growth rates is therefore critical to the establishment of a sustainable timber and fibre production industry. Biological indicators that can predict growth responses are therefore of extreme value. We investigated the carbon isotope abundance of wood cellulose (delta C-13(cel)) in E. globulus to determine potential relationships with the carbon isotope abundance of phloem sap (delta C-13(phl)) where the trees were subjected to different level of nutrient availability. This study also sought to determine the effect of nutrient additions on the growth of the E. globulus and to quantify the relationship between the volumetric growth of wood and delta C-13(cel). Phloem sap and wood cores were collected from trees within study plots which were subjected to seven nutrient treatments over a two-year period in a monoculture E. globulus plantation in South Australia. Phloem sap was collected using the razor blade technique and wood cores were collected using a stem borer. The carbon isotope abundance (delta C-13) of phloem sap and wood grown in the radial direction of the stem were determined. The basic and dry densities of wood were determined, and their relationships with phloem and wood delta C-13 were established. The delta C-13(phl) was significantly correlated with delta C-13(cel). The relationship between delta C-13(cel) and the wood density of the respective wood sections was significant but did not consistently show the same pattern. There was no significant variation in basic density observed along the radial direction of the stem wood of the short-rotation E. globulus trees. A positive correlation was observed between delta C-13(cel) and the wood basic density, but the relationship was not consistent along the radial direction of the stem. However, positive correlations were observed between delta C-13(cel) and the air-dry density of respective wood sections. The relationship between phloem and wood delta C-13 and the relationship between delta C-13 and wood density along the radial direction of the stem needs to be considered while monitoring forest growth under nutrient- and water-limited conditions.