Nutrient concentration, resorption and lifespan: leaf traits of Australian sclerophyll species

1. Most plants withdraw nutrients from leaves as they age, and redeploy them elsewhere in the plant. The proportion of nutrients resorbed and the residual nutrient concentration in senesced leaves are different but complementary indices of nutrient conservation via this process. A major spectrum of strategic variation runs from plant species with typically long leaf lifespan (LL), high leaf mass per area (LMA), low leaf nutrient concentrations, and low photosynthetic capacity, to species with the opposite characteristics. It is unknown to what extent either facet of resorption covaries with the LL-LMA spectrum. 2. Green-leaf and senesced-leaf N and P concentrations were quantified for 73 evergreen species from four sites in eastern Australia (nutrient-rich and nutrient-poor sites in each of two rainfall zones). Leaf nutrient concentrations in green and senesced leaves were negatively correlated with LL across all species and at most sites, especially if N-2-fixing species were excluded from analyses involving leaf N. 3. Proportional resorption did not differ with soil nutrients, as has been found elsewhere, nor was it correlated with LL. Green-leaf and senesced-leaf nutrient concentrations were lower for species on poorer soils. A simple model was described in which the proportion of resorbed vs soil-derived nutrients deployed in new leaves is set by the relative cost of nutrients from the two sources. The model provides a prospective explanation for the observed differences between species from nutrient-rich and nutrient-poor habitats. 4. The results from this study provide support for the argument that selection to minimize nutrient losses has affected the residual nutrient concentration in senesced leaves, rather than proportional resorption per se. Further, variation among species in residual nutrient concentration was correlated with one of the key spectra of strategic variation between plant species, the leaf lifespan-LMA axis of variation.