Stem diameter growth rates in a fire-prone savanna correlate with photosynthetic rate and branch-scale biomass allocation, but not specific leaf area

Plant growth rates strongly determine ecosystem productivity and are a central element of plant ecological strategies. For laboratory and glasshouse-grown seedlings, specific leaf area (SLA; ratio of leaf area to mass) is a key driver of interspecific variation in growth rate (GR). Consequently, SLA is often assumed to drive GR variation in field-grown adult plants. However, there is an increasing evidence that this is not the general case. This suggests that GR - SLA relationships (and perhaps those for other traits) may vary depending on the age or size of the plants being studied. Here we investigated GR - trait relationships and their size dependence among 17 woody species from an open-canopy, fire-prone savanna in northern Australia. We tested the predictions that SLA and stem diameter growth rate would be positively correlated in saplings but unrelated in adults while, in both age classes, faster-GR species would have higher light-saturated photosynthetic rate (A(sat)), higher leaf nutrient concentrations, higher branch-scale biomass allocation to leaf versus stem tissues and lower wood density (WD). SLA showed no relationship to stem diameter GR, even in saplings, and the same was true of leaf N and P concentrations, and WD. However, branch-scale leaf:stem allocation was strongly related to GR in both age groups, as was A(sat). Together, these two traits accounted for up to 80% of interspecific variation in adult GR, and 41% of sapling GR. A(sat) is rarely measured in field-based GR studies, and this is the first report of branch-scale leaf:stem allocation (analogous to a benefit:cost ratio) in relation to plant growth rate. Our results suggest that we may yet find general trait-drivers of field growth rates, but SLA will not be one.