Is phosphorus limiting in a mature Eucalyptus woodland? : phosphorus fertilisation stimulates stem growth

Aims: Few direct tests of phosphorus (P) limitation on highly-weathered soils have been conducted, especially in mature, native Eucalyptus stands. We tested whether growth in a mature >80-year old stand of Eucalyptus tereticornis in Cumberland Plain Woodland was limited by P, and whether this P-limitation affected leaf photosynthetic capacity. Methods: P was added to trees at the native woodland site at 50 kg ha-1 year-1 in each of 3 years, and stem and leaf responses were measured. Results: Leaf P concentrations before fertilisation were < 1 mg g-1 and N:P ratios ranged between 16 and 23. Addition of 50 kg ha-1 year-1 of P increased leaf P concentration significantly (+50 %) compared to non-fertilised trees, for two but not for the 3 years. Despite higher leaf P in fertilised trees, photosynthetic capacity was unaffected. However, there was a 54 % increase in tree stem basal area growth during the first and second years of P fertilisation, statistically significant in the second year of the experiment. Conclusions: Our evidence shows that E. tereticornis is P-limited on Cumberland Plain soils. This has implications for forest responses to rising atmospheric [CO<inf>2</inf>], because photosynthesis in elevated [CO<inf>2</inf>] may become further constrained by required phosphate pools within the photosynthetic apparatus.