Cross-species relationships between seedling relative growth rate, nitrogen productivity and root vs leaf function in 28 Australian woody species

1. Seedling relative growth rate (RGR) is often decomposed into the product of specific leaf area (leaf area per leaf mass, SLA), net assimilation rate (rate of mass increase per unit leaf area per unit time, NARa) and leaf mass ratio (ratio of leaf to total dry mass, LMR). Commonly, most cross-species variation in RGR is accounted for by variation in SLA, while no general relationships occur between RGR and either NARa or LMR. NARa can be factored into the product of leaf nitrogen productivity (rate of mass increase per unit leaf nitrogen per unit time, LNP) and leaf nitrogen concentration (area basis, LNCa). In this way the influence on RGR of leaf nitrogen - how it is displayed, and how it is utilized - can be investigated. 2. Seedlings of 28 Australian woody dicot species were grown under controlled, favourable conditions. Variation in SLA, LNP, LNCa and LMR explained c. 44%, 22%, 19% and 15% of variation in RGR, respectively. SLA and LNP were positively associated with RGR, while LNCa was negatively associated with RGR. LNP and LNCa were negatively correlated, the counteracting trends between RGR and each of these attributes resulting in no relationship between RGR and NARa. It is argued that this phenomenon may be widespread and may contribute to the inconsistency of reported relationships between NARa and RGR. 3. The functional balance between leaves and roots can be described in terms of mass allocation and morphology (static ratios or allometric coefficients) or, alternatively, in terms of leaf 'activity' (NAR) and root 'activity' (nitrogen uptake rate, NUR). At any given time most species allocated greater mass to leaves than to roots, but species with low RGR tended to be partitioning a greater ongoing proportion of new biomass to the roots rather than to the leaves, resulting in a proportionally greater increase in root surface compared with leaf surface over time. Nitrogen uptake rate was correlated with leaf and whole-plant nitrogen concentration, but not with other attributes. While it is clear that root and leaf functions must be co-ordinated (and thus in balance) for growth to occur, there is little evidence that this balance varies systematically with RGR across all species.